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Citation: Salnikova, D.I.; Nikiforov,
N.G.; Postnov, A.Y.; Orekhov, A.N.
Target Role of Monocytes as Key Cells
of Innate Immunity in Rheumatoid
Arthritis. Diseases 2024,12, 81.
https://doi.org/10.3390/
diseases12050081
Academic Editor: Maurizio Battino
Received: 30 March 2024
Revised: 21 April 2024
Accepted: 22 April 2024
Published: 25 April 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
diseases
Review
Target Role of Monocytes as Key Cells of Innate Immunity in
Rheumatoid Arthritis
Diana I. Salnikova 1, * , Nikita G. Nikiforov 2,3,4 , Anton Y. Postnov 3and Alexander N. Orekhov 2,3,5
1Laboratory of Oncoproteomics, Department of Experimental Tumor Biology, Institute of Carcinogenesis,
Blokhin N.N. National Medical Research Center of Oncology, 24 Kashirskoe Highway, 115522 Moscow, Russia
2
Laboratory of Angiopathology, The Institute of General Pathology and Pathophysiology, 8 Baltiyskaya Street,
125315 Moscow, Russia; nikiforov.mipt@googlemail.com (N.G.N.); ano.inat@mail.ru (A.N.O.)
3
Laboratory of Cellular and Molecular Pathology of Cardiovascular System, Federal State Budgetary Scientific
Institution “Petrovsky National Research Centre of Surgery”, 3 Tsyurupa Street, 117418 Moscow, Russia;
anton-5@mail.ru
4Center for Precision Genome Editing and Genetic Technologies for Biomedicine, Institute of Gene Biology,
Russian Academy of Sciences, 34/5 Vavilova Street, 119334 Moscow, Russia
5Institute for Atherosclerosis Research, Osennyaya Street 4-1-207, 121609 Moscow, Russia
*Correspondence: dianasalnikova08@yandex.ru
Abstract: Rheumatoid arthritis (RA) is a chronic, systemic, and inflammatory autoimmune condition
characterized by synovitis, pannus formation (with adjacent bone erosion), and joint destruction. In
the perpetuation of RA, fibroblast-like synoviocytes (FLSs), macrophages, B cells, and CD4
+
T-cells—
specifically Th1 and Th17 cells—play crucial roles. Additionally, dendritic cells, neutrophils, mast
cells, and monocytes contribute to the disease progression. Monocytes, circulating cells primarily
derived from the bone marrow, participate in RA pathogenesis. Notably, CCR2 interacts with CCL2,
and CX3CR1 (expressed by monocytes) cooperates with CX3CL1 (produced by FLSs), facilitating the
migration involved in RA. Canonical “classical” monocytes predominantly acquire the phenotype
of an “intermediate” subset, which differentially expresses proinflammatory cytokines (IL-1
β
, IL-6,
and TNF) and surface markers (CD14, CD16, HLA-DR, TLRs, and
β
1- and
β
2-integrins). However,
classical monocytes have greater potential to differentiate into osteoclasts, which contribute to bone
resorption in the inflammatory milieu; in RA, Th17 cells stimulate FLSs to produce RANKL, triggering
osteoclastogenesis. This review aims to explore the monocyte heterogeneity, plasticity, antigenic
expression, and their differentiation into macrophages and osteoclasts. Additionally, we investigate
the monocyte migration into the synovium and the role of their cytokines in RA.
Keywords: monocyte subsets; macrophages; innate immunity; rheumatoid arthritis; chemokines;
proinflammatory cytokines
1. Introduction
RA is a chronic, systemic, and autoimmune disease. The mechanisms of this condition
are still being investigated. The illness is characterized by an inflammatory process in
the synovial joints, which is called synovitis [
1
]. Moreover, RA is accompanied by the
hyperplasia of the synovial tissue. The disease is complicated by the destruction of the
cartilage structure and the formation of pannus [
2
]. Throughout the course of the pathology,
bone erosion occurs and predominantly surrounds the peripheral synovial joints. The
clinical symptoms of RA include pain, edema development, and tenderness of the synovial
joints located symmetrically and peripherally [
3
]. A great deal of progress has been made in
the treatment of RA with antagonists of proinflammatory cytokines, such as tumor necrosis
factor (TNF), interleukin-6 (IL-6), and IL-1
β
. However, the disease remains refractory
in some patients. Studies have demonstrated that prostaglandins, leukotrienes, reactive
oxygen species, nitric oxide, lipoxins, and platelet-activating factors as small-molecule
Diseases 2024,12, 81. https://doi.org/10.3390/diseases12050081 https://www.mdpi.com/journal/diseases
Diseases 2024,12, 81 2 of 20
inflammatory mediators play an important role in RA development. Such compounds
assist in inducing, maintaining, or reducing inflammation and could therefore be potential
therapeutic targets [1].
Innate and adaptive autoimmune cells promote the continuation of RA pathogenesis.
Among them are B cells and CD4
+
T cells, which include the T helper 1 (Th1), Th2, and
Th17 cells. Moreover, monocytes/macrophages and resident macrophages participate
in the disease development [
4
]. In addition, synovial cells and FLSs contribute to RA.
Furthermore, dendritic cells (DCs), neutrophils, mast cells, and immunoregulatory cells,
for example, regulatory T (Treg) cells, are involved in the complex interaction that leads
to the preservation of chronic inflammation [
5
]. The above-mentioned cells stimulate the
formation of the modulators of local inflammation, such as cytokines, chemokine ligands,
and the appropriate receptors. These modulators take part in synovial inflammation and
destruction [6].
Monocytes are a type of blood cell containing a horseshoe-shaped nucleus in their
cytoplasm. They relate to the mononuclear phagocyte system and are much smaller than
macrophages, which are in turn much smaller than multinucleated osteoclasts. Mono-
cytes originate from bone marrow precursors [
7
]. Together with lymphocytes, monocytes
were previously known as agranulocytes. Moreover, monocytes are defense cells of the
second line after neutrophils in an innate immune system and are able to phagocytose
foreign particles. They have the capability of differentiating into their own subsets and
macrophages based on the necessity of the microenvironment. Additionally, monocytes
produce cytokines and act as antigen-presenting cells (APCs) [8].
Monocytes differ from other autoimmune cells due to several reasons. First of all, they
are capable of triggering, maintaining, and increasing the activity of inflammation in the
synovial joints in RA. Moreover, chemokine receptors CCR2 (CD192) of CD14
+
monocytes
and CX3CR1 (fractalkine receptor, GPR13) of CD16
+
monocytes interact with the corre-
sponding ligands CCL2 (MCP-1) and CX3CL1 (fractalkine) released from FLSs, and this
process contributes to the migration of monocytes from circulation and their recruitment
into the RA synovium [
9
]. These chemoalkine receptors represent the differentiation mark-
ers of monocytes. A disintegrin and metalloproteinase domain-containing protein 10 and
products of FLSs such as granulocyte–macrophage colony-stimulating factor (GM-CSF),
TNF, and IL-1
β
promote monocyte migration. Next, monocytes are characterized by the
increased expression of chemokine receptors and antigens on their cell surface [
6
]. Among
them are CD14, CD16, toll-like receptors (TLRs), human leukocyte antigen DR (HLA-DR),
and
β
1- and
β
2-integrins that are adhesion molecules. Moreover, they produce proinflam-
matory cytokines such as TNF, IL-1
β
, and IL-6. The differentiation of monocytes leads
to the development of classical, intermediate, and non-classical monocyte subsets [
10
].
The intermediate monocytes are prevalent in RA, and they undergo differentiation into
proinflammatory M1 macrophages. Proinflammatory macrophages are induced by TNF,
GM-CSF, interferon (IFN)-
γ
, and lipopolysaccharide (LPS). M1 macrophages produce
proinflammatory cytokines TNF, IL-1β, IL-6, and IL-12 and stimulate inflammation in the
synovial joints [
11
]. Furthermore, classical monocytes expressing CD14
++
and CD16
−
have
relatively greater potential to differentiate into osteoclasts and induce the erosion of the
surrounding bones in the synovial joints in RA [
12
]. Osteoclastogenesis is the process by
which osteoclasts are differentiated. Th17 cells trigger the formation of the receptor activa-
tor of the NF-
κ
B (RANK) ligand (RANKL) by FLSs, which contributes to the differentiation
of monocytes into osteoclasts [
13
]. Osteoclastogenesis is increased by cytokines such as
TNF, IL-1β, and IL-6 [12,14].
The overview reveals the formation, classification, and specific markers of monocytes,
their functioning as APCs and progenitors of macrophages and osteoclasts, the expression
of chemokine receptors, and the production of cytokines in RA.
Diseases 2024,12, 81 3 of 20
2. Monocytes in RA
2.1. Development of Monocytes
Monocytopoiesis, the process of monocyte production, originates from human stem
cells, predominantly found in the bone marrow. Monocytes are part of the mononuclear
phagocyte system and act as key innate immune cells in the organism. The process of mono-
cytopoiesis occurs at a stage of embryonic development via definitive hematopoiesis. The
ventral wall of the aorta is a site for the aorta gonad mesonephros emergence following three
weeks of pregnancy in women and at the E-7 stage in mice. The aorta gonad mesonephros
is the first location in the embryo where monocytes appear [
15
]. Hematopoietic stem cells
differentiate into monoblasts under the influence of various cytokines, including GM-CSF
and macrophage colony-stimulating factor (M-CSF). Hematopoietic stem cells undergo
various stages of multipotent progenitor transformation into monocyte/macrophage and
DC progenitors. These progenitors lose the ability to generate granulocytes and either
give rise to “common monocyte progenitors” restricted to monocytes and their progeny
or are directed to a common DC progenitor [
15
]. The subsequent stages involve further
maturation and differentiation, giving rise to promonocytes and finally monocytes. These
differentiated monocytes are released into the bloodstream, where they execute crucial
functions such as phagocytosis, antigen presentation, and cytokine secretion, playing a piv-
otal role in immune surveillance and tissue homeostasis. Moreover, monocyte production
happens in the liver, thymus, and spleen of the fetus. Since the late period of embryonic
development, the formation of monocytes relocates to the bone marrow, which remains the
main site for the monocyte production in adults and older people [16].
2.2. Human Monocyte Subsets and Their Role in RA
The expression of CD14 and CD16 markers on the cell surface of monocytes was
used to identify and categorize the subsets of monocytes. According to the classification
developed by Ziegler-Heitbrock et al., three types of monocyte subsets are presented in
humans [7].
2.2.1. Classical Monocytes
Primarily, there are classical monocytes, which have high expression of the CD14
marker and, more importantly, do not express the CD16 marker. Furthermore, they pro-
duce the cytokines TNF, IL-1
β
, IL-6, and IL-10 in high concentrations. Several forms of
inflammatory stimulus are capable of eliciting the cytokine expression from monocytes. For
instance, this process occurs as a consequence of the LPS stimulation and activation of the
immune complex [17]. This type of monocyte undergoes differentiation into intermediate
monocytes, inflammatory macrophages, and osteoclasts in the regions of inflammation.
Classical monocytes function as phagocytic scavenger cells and promote synovial inflam-
mation and the osteoclastic erosion of bones. CD16
−
, but not CD16
+
, monocyte subsets
represent circulating osteoclast progenitors that differentiate into osteoclasts [
12
]. Conse-
quently, classical monocytes are able to be the circulating predecessors of osteoclasts in
erosive RA.
2.2.2. Intermediate Monocytes
The second group refers to intermediate monocytes, which are characterized by the
high expression of the CD14 marker and diminished expression of the CD16 marker. The
number of these monocytes is highly upregulated in the peripheral blood and synovia in
RA. They secrete TNF, IL-1
β
, and IL-6 in high quantities in the synovial joints in RA [
18
].
The differentiation of intermediate monocytes results in inflammatory monocyte subsets
and proinflammatory M1 macrophages. They maintain the inflammation of the synovial
joints and enhance the disease development. Additionally, intermediate monocytes lead
to the direct activation of Th17 cells in the inflamed synovial joints and their expansion.
The amount of these monocytes increases in the blood circulation over the course of the
RA pathogenesis [
19
]. The enhanced level of classical and intermediate monocytes in an
Diseases 2024,12, 81 4 of 20
untreated RA patient is a prognostic factor for the decline or absence of susceptibility to
methotrexate therapy [20].
2.2.3. Non-Classical Monocytes
In addition, there are non-classical monocytes that have diminished expression of
the CD14 marker and high expression of the CD16 marker. Non-classical monocytes
preferentially associated with the endothelial wall engage in ”patrolling behavior” urgently
in cases of tissue injury [
21
]. This process provides an early inflammatory response. Non-
classical monocytes stimulate the adhesion of monocytes in the microvessels of joints.
Resident anti-inflammatory M2 macrophages represent another product of monocyte
differentiation and help to resolve inflammation. Among the monocyte subsets, the non-
classical ones proliferate the least [
10
] and undergo senescence to a greater degree [
22
].
They are capable of acquiring a senescence-associated secreted phenotype (SASP) that
transforms into a proinflammatory type and causes inflammation. Purchasing a phenotype
occurs due to age-related chronic inflammation called inflammaging [23].
2.2.4. Disease-Modifying Antirheumatic Drugs
The prevalence of circulating monocytes subsets, particularly the CD16 expressing
subsets, is associated with RA progression and assists in estimating treatment effectiveness,
for example, in the case of disease-modifying antirheumatic drugs (DMARDs). Com-
monly prescribed DMARDs include methotrexate, sulfasalazine, and hydroxychloroquine.
Methotrexate inhibits dihydrofolate reductase, an enzyme that converts dihydrofolic acid
into tetrahydrofolic acid. This enzyme, in turn, is a donor of one-carbon groups in the
synthesis of purine nucleotides and thymidylate required for DNA synthesis. As a re-
sult, methotrexate inhibits DNA synthesis and repair, cell mitosis, and, to a lesser extent,
affects RNA and protein synthesis [
24
]. Sulfasalazine dissociates into 5-aminosalicylic
acid in the connective tissue of the intestinal wall, and 5-aminosalicylic acid promotes
the anti-inflammatory properties of sulfasalazine and sulfapyridine [
25
]. Sulfapyridine
is a competitive antagonist of para-aminobenzoic acid. This drug stops the synthesis of
folate in the cells of microorganisms and causes antibacterial activity. Hydroxychloroquine
seals lysosomal membranes and prevents the exit of lysosomal enzymes. Moreover, this
drug disrupts DNA replication, RNA synthesis, and hemoglobin utilization by erythrocytic
forms of plasmodium. Hydroxychloroquine weakens the activity of proteolytic enzymes,
which are protease and collagenase. Additionally, this drug decreases the activity of leuko-
cytes and the chemotaxis of lymphocytes. DMARDs help to prevent joint deformity and
functional impairment [26].
2.3. Murine Monocyte Subsets in RA
Murine monocytes are subdivided into Ly6C
++
CD43
+
, LY6C
++
CD43
++
, and Ly6C
−
types. The first one is an analogue of human classical monocytes. Another type resembles
the intermediate monocytes. Both of these monocyte types trigger synovial inflammation
under sterile conditions. The third type is an equivalent of non-classical monocytes and
manages the progression and termination of sterile synovial inflammation [
27
]. In the
beginning, Ly6C
−
monocytes develop into proinflammatory M1 macrophages, which
induce and maintain the inflammation in the synovial joints. Further, M1 macrophages are
exposed to polarization and acquire an alternative phenotype, M2, that suppresses synovial
inflammation [28].
2.4. Monocyte Markers
Monocytes are characterized by numerous antigen expression types on their cell
surface, with some alterations in dependence on the monocyte subset. These antigens
manage RA development [8].
Diseases 2024,12, 81 5 of 20
2.4.1. Markers of Double-Positive Monocytes/Macrophages
Double-positive monocytes/macrophages were revealed in human and rat periph-
eral blood. These cells express both CD4 and CD8. In addition, they possess activated
phenotypes expressing M1 and M2 markers, producing proinflammatory cytokines and
contributing to chronic inflammation. Double-positive monocytes/macrophages may be
transferred to the inflammation area and be involved in the immune response of Th1
type [
29
]. These cells promote synovial hyperplasia and facilitate joint damage in RA
(Figure 1).
Diseases 2024, 12, x FOR PEER REVIEW 5 of 21
2.4. Monocyte Markers
Monocytes are characterized by numerous antigen expression types on their cell sur-
face, with some alterations in dependence on the monocyte subset. These antigens manage
RA development [8].
2.4.1. Markers of Double-Positive Monocytes/Macrophages
Double-positive monocytes/macrophages were revealed in human and rat peripheral
blood. These cells express both CD4 and CD8. In addition, they possess activated pheno-
types expressing M1 and M2 markers, producing proinflammatory cytokines and contrib-
uting to chronic inflammation. Double-positive monocytes/macrophages may be trans-
ferred to the inflammation area and be involved in the immune response of Th1 type [29].
These cells promote synovial hyperplasia and facilitate joint damage in RA (Figure 1).
Figure 1. Double-positive monocytes/macrophages, expressed markers, proinflammatory (IL-1β,
IL-6, TNF, M-CSF, and GM-CSF) cytokines, role in RA. CD, cluster of differentiation; IL, interleukin;
TNF, tumor necrosis factor; GM-CSF, granulocyte–macrophage colony-stimulating factor.
2.4.2. Clusters of Differentiation
CD14 represents the LPS receptor, which is able to be linked to an LPS-binding pro-
tein. A CD14 interaction with an LPS–LPS binding protein complex initiates signaling
pathways with the involvement of TLR-4. Further, TLR-4 triggers the formation and se-
cretion of the proinflammatory chemokine IFN-γ inducible protein 10 and the cytokines
TNF, IL-1β, and IL-6 [30]. CD16 designates the immunoglobulin (Ig) Fc-gamma receptor
III (FcγRIII), which has a specific phagocytic activity. CD16
+
monocytes, mostly the inter-
mediate ones, are more abundant than non-classical monocytes in RA [19]. IgG-containing
immune complexes, for instance, containing anti-citrullinated protein antibody (ACPA),
provoke a higher level of TNF in the case of FcγRIII/CD16
+
hyperexpression on CD14
++
monocytes in RA. Magnetic microbeads for negative and positive selection with the use
of anti-CD14 and anti-CD16 monoclonal antibodies allow separating the CD14 and CD16
markers [31]. CD56 is an adhesion molecule in neural cells. CD56
+
monocytes mainly be-
long to classical monocyte subsets and spread in RA. Under the influence of LPS stimula-
tion, CD14
bright
CD56
+
monocytes secrete greater amounts of TNF, IL-10, and IL-23. Anti-
TNF therapy, for example, with the use of etanercept, diminishes the level of these cyto-
kines [17]. Monocytes as well as myeloid series cells express the myeloid identification
marker CD115 [32].
Figure 1. Double-positive monocytes/macrophages, expressed markers, proinflammatory (IL-1
β
,
IL-6, TNF, M-CSF, and GM-CSF) cytokines, role in RA. CD, cluster of differentiation; IL, interleukin;
TNF, tumor necrosis factor; GM-CSF, granulocyte–macrophage colony-stimulating factor.
2.4.2. Clusters of Differentiation
CD14 represents the LPS receptor, which is able to be linked to an LPS-binding protein.
A CD14 interaction with an LPS–LPS binding protein complex initiates signaling pathways
with the involvement of TLR-4. Further, TLR-4 triggers the formation and secretion of
the proinflammatory chemokine IFN-
γ
inducible protein 10 and the cytokines TNF, IL-1
β
,
and IL-6 [
30
]. CD16 designates the immunoglobulin (Ig) Fc-gamma receptor III (Fc
γ
RIII),
which has a specific phagocytic activity. CD16
+
monocytes, mostly the intermediate ones,
are more abundant than non-classical monocytes in RA [
19
]. IgG-containing immune
complexes, for instance, containing anti-citrullinated protein antibody (ACPA), provoke a
higher level of TNF in the case of FcγRIII/CD16+hyperexpression on CD14++ monocytes
in RA. Magnetic microbeads for negative and positive selection with the use of anti-CD14
and anti-CD16 monoclonal antibodies allow separating the CD14 and CD16 markers [
31
].
CD56 is an adhesion molecule in neural cells. CD56
+
monocytes mainly belong to classical
monocyte subsets and spread in RA. Under the influence of LPS stimulation, CD14
bright
CD56
+
monocytes secrete greater amounts of TNF, IL-10, and IL-23. Anti-TNF therapy, for
example, with the use of etanercept, diminishes the level of these cytokines [
17
]. Monocytes
as well as myeloid series cells express the myeloid identification marker CD115 [32].
In addition to ACPAs, antibodies targeting various post-translational modifications
such as carbamylation [
33
], acetylation, and malondialdehyde are found in RA. Approx-
imately half of the ACPA clones in RA patients react with carbamylated antigens. Some
antigens even show reactivity to acetylated peptides. The cross-reactivity may be due to
similar electron density distributions at key recognized residues. Anti-malondialdehyde
antibodies, although not cross-reactive with citrullinated antigens, can activate osteoclasts
Diseases 2024,12, 81 6 of 20
similar to ACPAs [
34
]. As a result, targeting antigens from different protein modifications
trigger various pathways. Co-existing mechanisms and overlapping protein residues com-
plicate the understanding of the key protein modifications for specific autoantibody effects.
Various protein modifications can co-exist in the same tissues, cells, proteins, and even
at the same peptide epitope. This co-existence explains the presence of autoantibodies
targeting various post-translational modifications in RA and their similar functional effects.
Colasanti T. et al. (2020) described the detection of antibodies targeting homocysteiny-
lated alpha 1 antitrypsin in RA patients seropositive for the “classical” ACPA and RF.
Alpha 1 antitrypsin is a protease inhibitor protein of the serpin superfamily and protects
tissues from the effects of many enzymes secreted by inflammatory cells. Variation in the
alpha 1 antitrypsin gene is associated with increased ACPA production, which leads to RA
progression and joint destruction [35].
2.4.3. HLA-DR and Co-Stimulatory Molecules
The HLA-DR marker refers to the group of MHC II (CD68) molecules. HLA-DR acti-
vates CD4
+
cells via interaction with T cell receptors [
36
]. Intermediate monocytes produce
a higher level of HLA-DR in comparison with other monocyte subsets in RA [
37
]. Fur-
thermore, intermediate monocytes secrete a significant amount of TNF through Pam3Cys
activation and binding to TLR2/TLR1. Classical monocytes increase the expression of HLA-
DR over a period of low RA activity. Monocytes of the synovial joints are characterized by
higher expression of HLA-DR than monocytes of the peripheral blood. Additionally, DCs
demonstrate HLA-DR expression to a high degree and participate in antigen presentation
to CD4
+
T cells. HLA-DR interacts with an antigen and passes the first signal to the T cell
receptor on a CD4
+
T cell. Co-stimulatory molecules CD80 (B7-1) and CD86 (B7-2) are
expressed on the monocyte surface and work as a second signal. To sum up, both signals
stimulate CD4
+
T-cells and enable adaptive autoimmune responses that happen with the
involvement of Th1 and Th17 cells in RA. The expression of HLA-DR and CD80 is enhanced
on the intermediate monocyte surface in the RA synovial joints, and this process contributes
to the activation of IL-17+and IFN-γ+CD4+T-cells under conditions of cocultivation [38].
Moreover, activated Th1 cells promote the expression of HLA-DR and CD80 on the surface
of the monocytes in RA.
2.4.4. Toll-like Receptors
TLRs are expressed on the surface of innate immune cells, for instance, monocytes,
macrophages, DCs, and neutrophils. TLRs identify inflammatory tissues, microbial prod-
ucts, and cellular debris that remains after cell necrosis. Lipoteichoic acid and peptidoglycan
recognition activate TLR2. LPS identification provokes TLR4 functioning [
39
]. Moreover,
signal transduction through TLR2 and TLR4 is stimulated by molecules of heat shock
protein 60, hyaluronic acid, and fibronectin products that are synthesized endogenously.
The TLR level intensifies in resident and infiltrating cells in the RA synovium. Furthermore,
TNF secretion is enhanced by increasing the TLR2 expression in CD16
+
macrophages of
blood and synovial origin in RA. The production of TLR2 grows with the assistance of
classical and intermediate monocytes in the RA peripheral blood and synovial joints. All
three monocyte subsets demonstrate a high TLR9 level on their surface independently of
blood or synovial localization. TLR2 and TLR9 promote the early secretion of proinflamma-
tory cytokines TNF and IL-1
β
[
40
]. Thereby, they induce inflammation that is maintained
by their other products, such as the cytokine IL-6 and the chemokine macrophage inflam-
matory protein-1. Various levels of these proinflammatory factors are correlated with the
involvement of TLR agonists. High expression of TLR3 and TLR4 is observed on fibroblasts
in the RA synovial joints. In particular, continuous inflammation and joint damage cause
this secretion [39].
Monocytes express the genes of type 1 interferons, especially in response to TLR
signaling. Roelofs M.F. et al. (2009) described the co-expression of synovial TLR3/7
expression with IFN-
α
, IL-1
β
, and IL-18, but not with TNF, IL-12, or IL-17 [
41
]. The
Diseases 2024,12, 81 7 of 20
stimulation of TLR3/TLR7 on monocytes, monocyte-derived DCs, or synovial fibroblasts
contributed to the secretion of type I IFN. However, no biologically active IL-1
β
or IL-
18 could be revealed. Type I IFN-
α
increased the TLR3/7 mRNA expression, whereas
IL-1
β
and IL-18 did not. In spite of the fact that the mRNA level of TLR4 remained
unchanged, IFN-
α
enhanced the response to TLR4 agonists, a phenomenon that was clearly
more marked in patients with RA. To conclude, type I IFNs are highly co-expressed with
TLR3/TLR7 in RA synovium. They enhance TLR3/TLR7-mediated cytokine production
and TLR4-mediated responses [41].
2.4.5. β1- and β2-Integrins
The migration of circulating monocytes proceeds after their adhesion into the vascular
endothelium.
β
1- and
β
2-integrins are expressed in high concentrations on monocytes
in RA and participate in the adhesion of monocytes to activated endothelial cells. The
β
1 subfamily of integrins includes very late antigen-4 (
α
4
β
1) and -5 (
α
5
β
1) that bind
to fibronectin and vascular cell adhesion molecule-1 in the endothelium and provoke
monocyte adhesion [
42
].
β
2-integrins take part in the formation of CD11
a,b,c
(CD18) com-
plexes in RA, which are mostly presented by CD11
b
molecules and associated with the
ligands in the endothelial lining. Moreover, intercellular adhesion molecule-1 (CD54),
-2 (CD102), and -3 (CD50) cause monocyte adhesion to endothelial cells. Inflammatory
monocytes/macrophages migrate into the synovial joints in RA and result in local inflam-
mation [43].
2.4.6. A Proliferation-Inducing Ligand
Two separate forms of a proliferation-inducing ligand (APRIL) are enhanced in the pro-
cess of RA progression. The first form is a soluble one, which refers to the TNF superfamily
and is secreted by myeloid cells, for instance, DCs, monocytes, macrophages, activated
T cells, and B cells. Soluble APRIL operates as a proinflammatory cytokine and binds to
receptors of two types. The first one is a transmembrane activator, calcium modulator
and cyclophilin ligand interactor receptor (TACI), which is produced in B cells [
44
]. The
second one is a B cell maturation antigen receptor (BCMA) that is secreted in plasma cells.
The second form of APRIL is a receptor on the cell surface that is highly produced by the
above-mentioned myeloid cells, including all the circulating monocyte subsets in RA. In
the process of binding to TACI and BCMA, a surface form of APRIL functions as a ligand
or receptor and provokes the formation and viability of B cells and plasma cells in RA. As a
result, a B cell-mediated autoimmune response is provided [45].
2.4.7. Sialic Acid Binding Ig-like Lectin
Siglec-1 is a representative of the sialic acid binding Ig-like lectin family. These proteins
undergo upregulation by the circulating monocytes and macrophages in RA, and their
concentration rises with the development of RA and increasing level of C-reactive protein,
erythrocyte sedimentation rate, and IgM-rheumatoid factor. Singlec-1 operates as an
autoantigen, takes part in a cell-to-cell contact of autoimmunity, and induces inflammation
in RA [46].
2.4.8. Transmembrane TNF
Transmembrane TNF is overexpressed on the monocyte surface in RA. Etanercept is
a TNF inhibitor that binds to transmembrane TNF. Consequently, the anti-inflammatory
cytokine IL-10 and soluble decoy receptors are produced. Thus, reverse signaling through
transmembrane TNF occurs in monocytes and represents a prognostic factor for the phar-
macological effect of anti-TNF agents in RA [
47
]. CD147 is an extracellular matrix metallo-
proteinase inducer, which promotes the production of matrix metalloproteinases (MMPs)
by monocytes, for instance, MMP2, MMP3, and MMP9. MMPs provoke the dissolution of
extracellular matrix, which leads to the destruction of synovium in RA. CD147 is hyperex-
pressed in CD14
+
monocytes in the RA peripheral blood and synovial joints. Infliximab
Diseases 2024,12, 81 8 of 20
is a TNF inhibitor that is supposed to block the CD147 secretion on the surface of CD14
+
monocytes in RA [48].
2.4.9. Other Monocyte Markers
α
-Enolase is an anti-citrullinated autoantigen for anti-citrullinated protein autoanti-
bodies and is also expressed on the surface of monocytes/macrophages in RA. The main
function of
α
-enolase is to initiate the secretion of proinflammatory cytokines, for example,
TNF, IFN-
γ
, prostaglandin E2, and IL-1
α
/
β
[
49
]. Proteinase-activated receptor-2 is a mem-
ber of the G protein receptor family and is produced by monocytes and T lymphocytes. The
expression of proteinase-activated receptor-2 is directly proportional to the RA activity. In
addition, the C-reactive protein and erythrocyte sedimentation rate concentrations increase
over the course of the disease. Proteinase-activated receptor-2 initiates the production of
the proinflammatory cytokine IL-6 [
50
]. Allograft inflammatory factor-1 is produced in the
cytoplasm of monocytes/macrophages, lymphocytes, and synovial fibroblasts. Its function
is to initiate and preserve inflammation in the synovial joints in RA. Hyperexpressed allo-
graft inflammatory factor-1 participates in the proliferation of inflammatory macrophages
and activated T lymphocytes in the RA synovium [
51
]. Moreover, 50–60% of CD14
+
CD16
++
non-classical monocytes have been shown to express the 6-sulfo LacNAc (SLAN) antigen,
which is an O-linked glycosylated variant of P-selectin glycoprotein ligand-1 recognized
by specific monoclonal antibodies, including macrophage-derived chemokine 8 and anti-
D-dimers 1 and 2 [
52
,
53
]. In clinical studies, Hofer Th.P. et al. (2015) showed a fivefold
depletion of SLAN-positive monocytes in patients with chronic inflammation [
54
]. SLAN is
one of the monocyte markers stimulating the differentiation of non-classical monocytes into
anti-inflammatory M2 macrophages, which, in turn, provide phagocytosis and resolution
of inflammation.
Monocytes of the peripheral blood are progenitors of macrophages, and both of them
mediate inflammation. Cardiovascular complications of RA are similar to synovitis and
subclinical atherosclerosis, which have an inflammatory mechanism in the endothelium.
Altered levels of the following receptor expression in circulating monocytes cause RA,
accompanied by atherosclerotic complications, for instance, localized in a coronary artery.
Specifically, low secretion of the scavenger receptor (CD36) is observed [
55
]. Moreover, a
comorbidity is characterized by the high production of the calcium-sensing receptor and
low-density lipoprotein-related receptor protein-1. The functional IL-25 receptor is secreted
on the monocyte surface in RA and interacts with the cytokine IL-25, which is produced
mostly by T cells. As a result, proinflammatory cytokine formation through the p38 mitogen-
activated protein kinase (MAPK)-dependent Soc-3 pathway occurs. Additionally, this
process signifies that IL-25 performs a negative control of monocyte-associated autoimmune
disorders [56].
2.5. Cytokines
The cytokines participating in the RA pathways are subdivided into two types: proin-
flammatory and anti-inflammatory cytokines. The first type is responsible for the occur-
rence and development of synovial inflammation [
57
]. Conversely, the functioning of
anti-inflammatory cytokines leads to a reduction in the inflammatory process. The cytokine
environment of the RA synovium is formed by the cells of two groups. The first one
includes resident synovial cells. The second group is represented by innate and adaptive
autoimmunity cells. A higher secretion of cytokines is provided in the RA synovium to a
greater extent by monocytes/macrophages and synovial fibroblasts than by T cells. The
cytokines are involved in all the stages of RA development. Moreover, they unite the
processes of immune regulation and inflammation that mediate the clinical symptoms and
pathogenesis of RA [58].
Diseases 2024,12, 81 9 of 20
2.5.1. Proinflammatory Cytokines
IL-2 and IFN-
γ
are the main proinflammatory cytokines of Th1 cells. IL-2 stimulates
the maturation and viability of T cells. IFN-
γ
initiates the functioning of HLA-DR. Moreover,
IFN-
γ
localized in the synovial joints in RA triggers the development of inflammatory M1
macrophages [
59
]. IL-17 is a pleiotropic cytokine secreted by Th17 cells. This cytokine
maintains the synovial inflammation. In addition, IL-17 participates in the activation
of monocytes and promotes their migration. Furthermore, the Th17 cytokine stimulates
the formation of cytokines by monocytes. There is a synergistic effect between IL-17
and TNF participating in the distribution of RA processes. Moreover, TNF increases
osteoclastogenesis, as well as between IL-17 and IL-1
β
. These advantageous interactions
lead to the activation of fibroblasts in the synovial joints, resulting in the production of
proinflammatory cytokines and MMPs. Additionally, the Th17 cytokine provokes the
erosion in the RA synovial joints. IL-17, in addition to TGF-
β
, IL-1
β
, and IL-23, increases
osteoclastogenesis in RA [60].
TNF is mostly secreted by activated synovial FLSs and inflammatory macrophages in
RA. Moreover, this cytokine is secreted by intermediate monocytes, T cells, and B cells. TNF
participates in the distribution of RA processes. Moreover, TNF increases osteoclastogenesis
in synovial inflammation. Infliximab is an effective anti-TNF agent that demonstrates the
central role of TNF in RA development [
61
]. The B cell activator factor is a representative
of the TNF family. Along with a soluble form of APRIL secreted by peripheral blood
mononuclear cells, the B cell activator factor is able to be linked to B cells, resulting in B
cell proliferation. Next, plasma cells are developed from autoreactive B cells and lead to
the formation of RF and ACPA. Consequently, a B cell-mediated response occurs in RA,
particularly in the early stages. Atacicept is an antagonist of APRIL and the B cell activator
factor and is undergoing clinical trials at the moment [62].
In addition, FLSs release other proinflammatory cytokines in the RA synovial joints,
for example, GM-CSF, IL-1
β
, IL-6, and IL-18. M1 macrophages ordinarily produce high
concentrations of TNF, IL-1
β
, IL-6, and IL-12. With the secretion of TNF, Il-1
β
, and IL-6,
monocytes stimulate an inflammatory process in the RA synovium. Adhesion molecules
β
1-
and
β
2-integrins are expressed on the monocyte surface under the effect of proinflammatory
cytokines. Intermediate monocytes are prevalent among the monocyte subsets that form
the proinflammatory cytokines Il-1
β
, IL-6, and TNF in RA. Moreover, classical monocytes
secrete the above-mentioned cytokines in accordance with the TLR agonists functioning [
63
].
The cytokine IL-18 is released by mononuclear cells in RA and is able to synergize with
Il-1
β
, IL-12, and IL-15. Next, these molecules trigger the IFN-
γ
formation by activated
synovial T cells and mediate the realization of the Th1 response. Moreover, IL-18 is capable
of acting as a straight proinflammatory cytokine in RA and causing the secretion of TNF
and IL-1βunder the control of macrophages [64].
NF-kB signaling plays an important role in RA pathogenesis. This statement is con-
firmed by the NF-kB participation in the production of macrophage inflammatory protein-
1a and the chemokine IL-8 [
65
]. Additionally, IL-15 stimulates neutrophils and delays the
apoptosis of FLSs and endothelial cells over the period of synovial inflammation. Synovial
fibroblasts are the main cells producing IL-23 in RA. Moreover, monocytes secrete IL-23
and initiate Th17 cell distribution and IL-17 expression in the RA synovial tissue. TNF,
IL-1
β
, and IL-32 trigger inflammation in the murine synovium that is supposed to indicate
their proinflammatory activity. IL-33 is a cytoplasmic representative of the IL-1 family and
is expressed in activated monocytes and fibroblasts in the RA synovium. IL-33 undergoes
emission from injured or necrotic cells and participates in the inflammation process. For
instance, this protein controls the formation of mast cells. Furthermore, it manages the
secretion of Th2 cytokines, such as IL-4, IL-5, and IL-13. Next, IL-33 promotes Th2-regulated
pathological conditions [66].
Diseases 2024,12, 81 10 of 20
2.5.2. Anti-Inflammatory Cytokines
IL-1Ra is an IL-1 receptor antagonist formed by monocytes in RA. IL-1Ra and IL-
10 are anti-inflammatory cytokines; therefore, their antagonistic action presupposes the
decline and termination of inflammation. Nevertheless, IL-10 and TGF-
β
prevail among
the products of Breg cells, Treg cells, and suppressor cells of myeloid origin. Il-4 and IL-10
are also secreted by Th2 cells. Moreover, Th2 cells enhance IL-1Ra expression. In addition,
M2 macrophages secrete IL-10 and contribute to wound healing, tissue remodeling, and
inflammation arrest. IL-10, as an anti-inflammatory cytokine, decreases inflammation.
Therefore, a decrease in IL-10 production leads to the development of the inflammatory
process and its transformation into a chronic form. IL-10 and M-CSF added to the cultivation
medium promote monocyte survival, growth, and differentiation into macrophages [
67
].
Furthermore, monocytes express the cytokines IL-11, IL-19, IL-20, and IL-24. Additionally,
the products of monocyte secretion are GM-CSF, G-CSF, and oncostatin M [68].
2.6. Chemokine Receptors
Chemokines binding to their receptors on the monocyte surface cause the return and
migration of monocytes from the blood into the synovial tissue. CCR2 and CX3CR1 are
chemokine receptors that are located on monocytes and trigger their chemotaxis. The first
receptor is overexpressed on classical CD14
++
CD16
−
monocytes and binds to CCL2, which
is a monocyte chemoattractant protein [
69
]. The second receptor undergoes high expression
on predominantly intermediate CD14
++
CD16
+
monocytes and also non-classical CD14
+
CD16
++
monocytes in RA and interacts with CX3CL1. IL-1
β
cytokines and TNF induce
the synovial fibroblasts to produce mainly CCL2, CX3CL1, and CCL20 in RA. CCL20,
or macrophage inflammatory protein-3 alpha, is a different monocyte chemoattractant.
Moreover, the c-Jun N-terminal kinase, extracellular signal-regulated kinase, and phos-
phatidylinositol 3
′
–kinase signaling pathways participate in the chemoattractant secretion
by the Th17-associated cytokine IL-17. In addition, IL-17 has an indirect influence on
monocyte migration [70].
Chemokine receptors undergo constitutive expression on peripheral blood and syn-
ovial monocytes/macrophages, mediate the monocyte chemotaxis, and maintain their
level. The migration of peripheral blood monocytes to the synovial tissue during the in-
flammatory process promotes the increased concentration of chemokine receptors on their
surface in RA [
6
]. Therefore, the regional inflammation and monocyte preservation in the
synovium are defined by the overexpression of chemokine receptors on the RA monocytes.
The increased expression of CCR1, CCR2, and CCR4 receptors is provided on peripheral
blood monocytes, whereas synovial monocytes demonstrate high production of CCR3 and
CCR5 in RA. Furthermore, the circulating monocytes secrete CCR9, which interacts with
the CCL25 chemokine, or thymus-expressed chemokine. This binding results in monocytes’
development into macrophages in RA. The CCR7 chemokine receptor is secreted by not
only B lymphocytes, CD4
+
T lymphocytes, DCs, and FLSs but also circulating monocytes in
RA. CCR7 binds to the CCL19 and CCL21 ligands that are characterized by hyperexpression
in RA. The regulation of CCR7 and CCL19 is directly proportional to the RA progression.
Moreover, the CCL19 concentration in plasma indicates the radiographic development of
joint destruction. The DMARD therapy maintains the normal level of both components of
the CCR7/CCL19 system [26].
3. Monocyte Functions in RA
The RA etiopathogenesis and its sensitivity to therapy are defined by genetic pre-
disposition, environmental factors, infections, and the decline of autoimmune regulation.
Frequent inflammation induces the development of autoimmune conditions. The patho-
genesis of RA involves the activation and polarization of monocytes into proinflammatory
M1 macrophages within the joint environment [
71
]. In the arthritic joint, monocytes are
recruited by proinflammatory cytokines such as TNF, IL-1, and IL-6, which are produced
by resident cells, including FLSs and macrophages. These cytokines promote the activation
Diseases 2024,12, 81 11 of 20
of monocytes through the binding of their receptors, leading to the upregulation of adhe-
sion molecules and chemoattractant receptors on the monocyte surface. As a result, the
monocytes adhere to the endothelium of blood vessels and migrate into the synovial tissue.
Activated monocytes express increased levels of cell surface markers, such as CD80 and
CD86, which facilitate interactions with T cells and promote antigen presentation [
72
]. M1-
like macrophages are characterized by the production of proinflammatory cytokines and
reactive oxygen species, which contribute to tissue damage and perpetuate inflammation in
the joint. Furthermore, M1-like macrophages express high levels of surface markers, such
as CD14 and CD16, which enhance their phagocytic and antigen-presenting capabilities.
3.1. Monocyte Interactions with Other Cells
Cells of innate and adaptive immunity work in cooperation to cause and maintain
an inflammatory process in RA. The main cells taking part in this signaling are DCs,
monocytes/macrophages, FLSs, CD4
+
T cells (Th1, Th2, and Th17 cells), B cells, mast
cells, and neutrophils. When immunoregulatory cells are not capable of attenuating acute
inflammation, a chronic form of the disease develops. DCs, macrophages, and FLSs are
activated at the beginning of RA pathogenesis [73].
Circulating monocytes are precursors of macrophages. Additionally, monocytes are
able to operate as APCs and activate T cells. Moreover, they are classified as monocyte-
derived DCs due to their capacity to behave as classical DCs [
7
]. Monocyte-derived DCs
perform hyperexpression of IL-6 and IL-23. Tolerogenic monocyte-derived DCs are another
type of DC that are capable of triggering the formation of Foxp3
+
Treg cells in RA. Myeloid-
derived suppressor cells (MDSCs), Breg cells, and Treg cells control and decrease the
inflammatory process [74].
Among CD4
+
T cells, the role of the Th1 and Th2 cells in the RA progression was ini-
tially revealed. The cytokine IL-12 mediates the participation of monocytes/macrophages
in the polarization of CD4
+
Th1 cells. Th2 cells and their cytokines mainly decrease Th1
activation. Intermediate monocytes prevail in the RA peripheral blood and synovium and
are supposed to be the main monocyte subsets that modulate the Th17 cell functioning.
Th17 cells were investigated as effectors in RA because of the ability of their pleiotropic
cytokine IL-17 to synergize with TNF. IL-17A triggers the secretion of proinflammatory
cytokines IL-1
β
, IL-6, and TNF in macrophages through the AP-1, NF-kB, and MAPK
pathways [
60
]. IL-1
β
, IL-6, and IL-23 initiate the activation and control the polarization of
Th17 cells. Activated T lymphocytes take part in cell-to-cell contact and trigger the secretion
of TNF and IL-1
β
. Moreover, T lymphocytes release cytokine inhibitor secretory IL-1Ra in
monocytes/macrophages [75].
Mast cells are a source of small-molecule inflammatory mediators. Neutrophils un-
dergo chemotaxis into the RA synovial joints, mostly in the early period of inflammation,
and release inflammatory mediators and enzymes. Over the period of joint effusion,
the level of neutrophils predominates in the synovial fluid in comparison with the syn-
ovium [76].
A cytotoxic T-lymphocyte-associated antigen 4 (CTLA4) is a receptor for co-stimulatory
molecules and is expressed on the surface of activated T cells [
77
]. CTLA4 binds with a high
avidity to CD80 (B7-1) and CD86 (B7-2) on APCs. They are expressed on the APC surface
and interact with T cell co-stimulatory receptor CD28 through signal 2 in order to activate
the T cells. Moreover, CTLA4 promotes CD80 and CD86 transport to the CD28 receptor
located on T cells for further activation of these co-stimulatory molecules [
72
]. CTLA4-Ig is
a CTLA4 inhibitor and is registered as Abatacept. A blocking effect of Abatacept indicates
a cell-to-cell contact mechanism of antigen presentation via T cell-monocyte/macrophage
cooperation [77].
Moreover, a surface form of APRIL secreted by monocytes promotes cell communica-
tion with B cells and plasma cells that produce BCMA and TACI [
44
]. In addition, FLSs
secrete soluble APRIL and, consequently, release BCMA. Furthermore, FLSs and synovial
macrophages express CXCL16 and CCL20 chemokines, which interact with CCR6
+
mem-
Diseases 2024,12, 81 12 of 20
ory T cells. This communication, along with IL-15 cytokines, mainly contributes to CD4
+
T cell migration and their involvement in the synovial tissue. Moreover, the enhanced
transport of circulating monocytes into the RA synovial joints stimulates the participation
of CXCR6
+
T cells in the RA progression. Additionally, the CCL20 expression by activated
monocytes occurs in the synovium of mice. Stimulated monocytes secrete CCL20, which
contacts the CCR6 chemokine receptors, a product of Th17 cells. This interaction increases
the CCL20 involvement in the synovial tissue. To sum up, cooperation between mono-
cytes/macrophages, DCs, B cells, Th1, and Th17 cells is related to a positive feedback loop
and has an effect on the continuation of inflammation in the synovial joints [60].
FLSs are the resident cells presenting in the intimal lining of the synovium along with
resident macrophages, which are synoviocytes of type A. FLSs are highly proliferated in
the synovial tissue, thus stimulating the hyperplasia of the synovium and damage to the
joints. FLSs express RANKL and participate in the differentiation of osteoclasts and bone
erosion [
78
]. CCL2 and CX3CL1 are FLS chemokines and behave as chemoattractants for
monocytes/macrophages. Moreover, the corresponding MMPs eliminate the extracellular
matrix. IL-6, IL-18, TNF, and GM-CSF, as the FLS cytokines, promote the stimulation of
innate and adaptive immune cells [61].
Treg cells primarily decrease inflammation and regulate adaptive immunity. The Treg
cells are determined as CD4
+
CD25
+
CD127
low
FOXP3
+
cells. There are two ways in which
they cooperate with APCs and effector T cells. The first one is direct communication through
cell-to-cell contact. The second way occurs with the help of anti-inflammatory cytokines, for
instance, TGF-
β
, IL-4/IL-13, and IL-10. Furthermore, IL-10, as a potent anti-inflammatory
cytokine, influences both the innate and adaptive immune cells [
5
]. CD8
+
FoxP3
+
Treg cells
induced by anti-CD3 monoclonal antibodies decrease Th1 and Th17 cell activation through
P38 phosphorylation inhibition and halt RA inflammation [
74
]. Monocytes promote the
FoxP3 expression in CD8
+
FoxP3
+
Treg cells through direct cell contact. IL-10 decreases the
Th17 cell amount and the IL-17 cytokine generation while simultaneously increasing the
Foxp3
+
Treg cell development. Thus, IL-10 modifies the ratio of Th17 cells and Treg cells in
the CD4
+
T cell population. Activated monocytes/macrophages are able to have both a
positive and negative effect on the Treg cell phenotype and function, with the predominance
of a negative one [74].
MDSCs are tumor-associated cells and take part in the formation of tumor-induced
T regulatory cells and T cell anergy. There are two subsets of MDSCs. The first one is
presented by monocytic MDSCs. Consequently, monocytes can be expanded and polarized
towards MDSCs [
13
]. The second group includes granulocytic MDSCs. Both of these sub-
sets suppress Th1 and Th17 cell proliferation. Moreover, they secrete IL-10 and contribute
to the expansion of Treg cells in mice. To sum up, MDSCs possess immunoregulatory
activity and repress RA synovial inflammation [4].
3.2. Monocyte Transport to the RA Synovial Tissue
Monocytes are the important innate effectors in the progression of RA. Circulating
monocytes undergo chemotaxis and infiltration into the synovial tissue and stimulate
synovial inflammation. Intermediate monocytes, and, to a high extent, the classical ones,
are constantly involved in the local synovial inflammation development. This statement is
confirmed by the increased presence of both of them in the RA synovium as well as the
declining concentration of classical monocytes in the RA peripheral blood and a slightly
enhanced number of intermediate monocytes there. M-CSF promotes the differentiation of
classical monocytes into the intermediate subset, which indicates the heterogeneous charac-
ter of the monocyte subsets and their possible polarization into their own other type [
13
].
The cooperation between CCL2/CCR2 and CX3CL1/CX3CR1 contributes to the migration
of circulating monocytes and their functioning in the RA synovium. CCL2 is released by
bone marrow mesenchymal stem cells and precursor cells. The CCR2/CCl2 complex on
monocytes triggers their recruitment from the bone marrow into the bloodstream. The
CX3CL1 production by FLSs is supported by disintegrin and metalloproteinase domain-
Diseases 2024,12, 81 13 of 20
containing protein 10 and disintegrin; therefore, they mediate the monocyte migration in
RA [79].
3.3. Differentiation of Monocytes into Macrophages
In RA, monocytes are replaced by circulating monocytes, which constantly enter
the synovium to support the local inflammation, although their part among the synovial
macrophages is small. GM-CSF is released by activated synovial fibroblasts under the influ-
ence of IL-1
β
and TNF and provokes the survival of monocytes more than their polarization
into macrophages [
67
]. The human stem cells from the fetal liver and erythro-myeloid
predecessors supply the resident macrophages over the period of embryogenesis. However,
circulating monocytes supply the recruited inflammatory macrophages when it is necessary,
for example, in acute synovial inflammation. When the niche in the synovial tissue is free
or unsaturated, macrophage development from circulating monocytes occurs. When there
is no available niche, the macrophages are self-replenished by their own turnover. Interme-
diate monocyte subsets (CD14++ CD16+) are the prevailing monocytes in the RA synovial
tissue, and they mainly differentiate into inflammatory M1 macrophages [
30
]. Synovial
macrophages are located in the sub-lining and lining layers at the cartilage–pannus junction.
The activated macrophages of the intimal lining release cytokines and provoke articular de-
struction. The rheumatoid factor autoantibodies and antigens form IgG-containing immune
complexes, which stimulate the macrophages. Furthermore, innate immune cells, T cells,
and fibroblasts secrete cytokines that have an impact on the stimulation of macrophages
via cell-to-cell contact. The involved TLR-2 and TLR-4 activate macrophages and maintain
their level in RA [39].
There are two major phenotypes of macrophages that are differentiated from mono-
cytes in humans. The first is a proinflammatory type represented by classically activated
M1 M
φ
macrophages. They secrete high levels of IL-1
β
, IL-6, IL-12, IL-23, TNF, and
reactive oxygen species, which have the main impact on the continuation of the RA in-
flammation. The second type is anti-inflammatory and refers to alternatively activated
M
φ
M2 macrophages [
80
]. They, in turn, provide high concentrations of TGF-
β
, IL-1Ra,
decoy IL-1RII, IL-10, and low IL-12 content. The M
φ
M2 macrophage functions are anti-
inflammatory activity, tissue remodeling, and wound healing. Both the proinflammatory
and anti-inflammatory macrophage phenotypes take part in inflammation regulation. Their
differentiation from monocytes with the use of the GM-CSF and M-CSF induction cytokines
leads to Mø M1 and Mø2 M2 formation, correspondingly [
68
]. Moreover, activated M
φ
M2 macrophages have three subsets according to their stimulating cytokines. There are
three M2 forms. The first one is an alternative M2 form called M2a and triggered by IL-4
and IL-13. The second one is M2b, which is initiated by the impact of the IgG-containing
immune complex and the TLR agonists or IL-1R. The third M2 form is M2c, which is
induced by IL-10 and glucocorticoid hormones [60].
Th1 cells, Th2 cells, and Th17 cells, along with their associated cytokines, play
a crucial role in the polarization, recruitment, activation, and differentiation of mono-
cytes/macrophages [
59
]. The activation and differentiation of M1 macrophages are driven
by the Th1 cytokine IFN-
γ
in the presence of LPS and TNF. On the other hand, M2
macrophages are driven by Th2 cytokines such as IL-4, IL-10, and IL-13. CXCL5, CXCL8,
CXCL9, CXCL10, and CXCL13 are typical chemokines expressed by M1 macrophages. No-
tably,
in vitro
studies have demonstrated that M-CSF polarized M2 macrophages can pro-
duce proinflammatory cytokines in response to an IgG immune complex containing ACPA.
This finding is also relevant in the context of RA since the synovium exhibits elevated levels
of M-CSF [
67
]. Furthermore, activin A, found in RA synovial fluid, induces increased ex-
pression of MMP12 (a proinflammatory polarization marker) on monocytes/macrophages,
polarizing them into proinflammatory M1 phenotypes. Given these findings, activin A
levels may serve as a potential biomarker for various therapies.
The overexpression of the Silent Information Regulator-2 homolog, SIRT1, on mono-
cytes has been shown to downregulate PU.1 phosphorylation, thereby inhibiting their
Diseases 2024,12, 81 14 of 20
differentiation into macrophages. SIRT1 overexpression can inhibit the production of proin-
flammatory cytokines by blocking the NF-kB pathway in RA patients. Consequently, SIRT1
has emerged as a significant therapeutic target of interest due to its role in regulating the
inflammatory process in RA [
81
]. It is worth mentioning that monocytes/macrophages
secrete the cytosolic phospholipase A2a enzyme, which plays a crucial role in generating
prostaglandin E2 from cell membrane phospholipids [
82
]. Prostaglandin E2, in turn, con-
tributes to the induction and maintenance of RA inflammation. In addition, under the
influence of LPS, TNF, and IL-1
β
, osteoblastic stromal cells produce cytosolic phospholi-
pase A2a, ultimately leading to the generation of prostaglandin E2. This process has been
associated with the promotion of inflammatory bone resorption in RA.
3.4. Monocytes Are Osteoclast Predecessors
Periarticular osteopenia and bone erosion adjacent to the pannus formation of synovial
joints are two of the most destructive events observed in RA with high disease activity. The
main culprits responsible for this erosion are osteoclasts, the primary bone resorbing cells.
Osteoclasts originate from circulating monocytes and resident macrophages, which contin-
uously migrate into the inflamed synovium and differentiate into osteoclasts, particularly
in active disease and bone erosion. Recent research has shown that monocytes expressing
CD14
+
and lacking CD16
−
are the main precursors of osteoclast progenitors, as opposed to
CD16
+
subsets [
12
]. These monocytes upregulate the RANK on their surface and interact
with RANKL, which is predominantly expressed by FLSs, osteoblasts, and activated T cells.
Th17 cells also play a significant role in promoting osteoclastogenesis as they stimulate the
expression of RANKL, which preferably interacts with the CD14
+
monocytes expressing
the surface marker CCR6, characteristic of Th17 cells [
83
]. Furthermore, the production of
IL-17, along with TNF, IL-1
β
, and IL-6, can amplify osteoclastogenesis. Additionally, the
mesenchymal stromal cells secreted by FLSs, when synergized with RANKL, also aid in
promoting osteoclastogenesis [56].
The mature activated osteoclasts release hydrochloric acid near their ruffled border
to dissolve the calcium from the bone matrix and also produce MMPs and cathepsin K,
which break down the remaining bone matrix, resulting in bone erosion adjacent to the
joints [
48
]. RA monocytes exhibit increased expression of the co-stimulatory osteoclast-
associated receptor, which, upon interaction with the collagen type II and collagen type
I ligands in the inflamed synovial articular cartilage, further promotes the formation of
osteoclasts. However, the osteoblasts’ soluble decoy receptor osteoprotegerin negatively
affects osteoclastogenesis and disrupts the osteoprotegerin/RANK ratio, consequently
favoring osteogenesis [
12
]. The activated T cells expressing the inducible costimulator,
when interacting with the inducible costimulator ligand (CD275) expressed by monocyte-
derived osteoclast-like cells, obstruct their differentiation into osteoclasts. This interaction
suppresses the expression of tartrate-resistant acid phosphatase, the nuclear factor of
activated T cells, and the osteoclast-associated receptors in monocyte-derived osteoclast-
like cells during their maturation into osteoclasts. Thus, the CD275-inducible costimulator
system directly interferes with osteoclastogenesis [84].
Clinical trials in RA focused on the monocytes that are of interest. The Chinese
government approved sinomenine as an anti-inflammation drug for RA treatment. Liu W.
et al. (2018) screened the various secretory cytokines in both LPS-induced and sinomenine-
treated RAW264.7 cells, followed by an estimation of the sinomenine ability to modulate
the cytokine secretion in a cell model, a collagen-induced arthritis mouse model, and RA
patients [
2
]. The results demonstrated that sinomenine regulated the IL-6, GM-CSF, IL-12
p40, IL-1
α
, TNF, IL-1
β
, CXCL1, Eotaxin-2, IL-10, M-CSF, RANTES, and CCL2 secretion
in vivo
and
in vitro
and reduced the RA activity and the 28-joint disease activity score in
a clinical setting. Moreover, sinomenine attenuated the CD11b
+
F4/80
+
CD64
+
resident
macrophages in the synovial tissue, the CD11b
+
Ly6C
+
CD43
+
macrophages in the spleen,
and draining the lymph nodes in collagen-induced arthritic mice. The percentage of
CD14
+
CD16
+
peripheral blood mononuclear cells was reduced by sinomenine in the
Diseases 2024,12, 81 15 of 20
RA patients. In conclusion, sinomenine controls the secretion of multiple inflammatory
cytokines and monocyte/macrophage subsets, thus decreasing RA development. Along
with methotrexate, sinomenine could be an alternative as a cost-effective anti-inflammatory
agent for treating RA [2].
4. Discussion
The innate and adaptive autoimmunity cells within the interconnected network drive
the continuous progression of RA pathogenesis. Numerous studies conducted to date, as
well as targeted therapies, have demonstrated that FLSs, macrophages, Th1 cells, and B
cells play a pivotal role in initiating and advancing synovial inflammation [
85
]. Recently,
Th17 cells have been implicated as the primary drivers of immune cell activation and
cytokine production [
60
]. Furthermore, they are primarily responsible for inducing RANKL
production by FLSs and osteoblasts, thereby promoting osteoclastogenesis [
78
]. Presently,
the data suggest that monocyte activation occurs within the circulation of individuals with
RA. These activated monocytes respond to chemotactic ligands such as CCL2 and CX3CL1,
migrating from the circulation into the RA synovium and releasing proinflammatory
cytokines [
83
]. Thus, their involvement in perpetuating synovial inflammation in RA
cannot be disregarded. Additionally, they function as APCs, activating other autoimmune
cells and stimulating the production of their respective cytokines. Evidently, they partake
in a positive feedback loop with the other key cells involved in RA pathogenesis. The
increased expression of various antigens on the monocyte membrane surface can be a
potential target for therapy in RA. Evaluating the levels of total monocytes and their
subsets in the peripheral blood of the RA patients, both before and after therapy, can
provide rheumatologists with guidance in determining the preference for treatment with
DMARDs or biologic agents [
26
]. Elevated levels of monocytes in the peripheral blood of
RA patients may serve as an additional biomarker for high disease activity, as evidenced by
the correlation with the disease activity score-28 scores and the serum levels of C-reactive
protein and the erythrocyte sedimentation rate.
The display of a heterogeneous character by the monocytes enables the differentiation
into subsets based on the microenvironment or inflammatory stages and, more impor-
tantly, the ability to transform into intermediate monocytes. This emphasizes the high
plasticity of the monocytes in the pathogenesis of RA. Inflammatory M1 macrophages,
which are crucial players in the development of RA, are primarily derived from circulating
monocytes, specifically intermediate monocytes [
2
]. This raises the question of which
chemical events, signals, or specific factors, apart from the arthritic joint environment,
trigger the conversion of CD14 monocytes to CD14
++
CD16
+
intermediate inflammatory
macrophages [
86
]. Focusing on these studies would not only benefit RA patients but also
shed light on other diseases where CD14
++
CD16
+
macrophages play a significant role,
such as atherosclerosis. Classical monocytes expressing CD14
++
and CD16
−
are responsible
for the bone erosion observed in erosive RA [
87
]. Osteoclastogenesis, the formation of
osteoclasts, is influenced by the presence of FLSs and Th17 cells. FLSs produce RANKL
and M-CSF to promote osteoclastogenesis [
67
]. Additionally, IL-17, produced by Th17 cells,
induces the production of RANKL by FLSs. Furthermore, IL-17, along with TNF, IL-1
β
,
and IL-6, amplifies osteoclastogenesis [
88
]. However, it is equally important to consider
the availability of classical monocytes in the RA synovium or microcirculation near RA
synovial joints in erosive rheumatoid arthritis as they have been shown to be precursor
cells capable of differentiating into osteoclasts.
5. Conclusions
Recent discussions have highlighted the significant advancements in our understand-
ing of how innate immune cells and signaling play a crucial role in driving the pathogenesis
of RA. This complexity of the disease underscores the importance of targeting the innate
immune system and its components for potential therapeutic interventions that can reduce
the incidence and enhance the quality of life for RA patients. To further explore this,
Diseases 2024,12, 81 16 of 20
additional research is needed to investigate how the heterogeneity and plasticity of mono-
cytes, the antigenic expressions on their surface, the differentiation into macrophages and
osteoclasts, the migration into the synovium, and the role of their cytokines in RA influence
their function in RA development. Moreover, studying how macrophage polarization in
inflamed joints changes during the course of RA can provide valuable insights into the
intricate and pivotal role of these cells in the disease process.
Author Contributions: Conceptualization, D.I.S. and N.G.N.; methodology, A.N.O.; software, D.I.S.;
validation, D.I.S., N.G.N. and A.N.O.; formal analysis, A.Y.P.; investigation, D.I.S.; resources, N.G.N.;
data curation, D.I.S.; writing—original draft preparation, D.I.S.; writing—review and editing, N.G.N.;
visualization, D.I.S.; supervision, N.G.N.; project administration, A.Y.P.; funding acquisition, A.Y.P.
All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Russian Scientific Foundation, grant number 20-15-00337.
The APC was funded by 20-15-00337.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data available from the authors.
Acknowledgments: The authors would like to thank Alina I. Salnikova, translator and interpreter at
the European Medical Center (Moscow, Russia), for her kind assistance in English editing. Figure
was created with Biorender.com, accessed on 1 January 2024.
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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