From murine to human nude/SCID: the thymus, T-cell development and the missing link.
ABSTRACT Primary immunodeficiencies (PIDs) are disorders of the immune system, which lead to increased susceptibility to infections. T-cell defects, which may affect T-cell development/function, are approximately 11% of reported PIDs. The pathogenic mechanisms are related to molecular alterations not only of genes selectively expressed in hematopoietic cells but also of the stromal component of the thymus that represents the primary lymphoid organ for T-cell differentiation. With this regard, the prototype of athymic disorders due to abnormal stroma is the Nude/SCID syndrome, first described in mice in 1966. In man, the DiGeorge Syndrome (DGS) has long been considered the human prototype of a severe T-cell differentiation defect. More recently, the human equivalent of the murine Nude/SCID has been described, contributing to unravel important issues of the T-cell ontogeny in humans. Both mice and human diseases are due to alterations of the FOXN1, a developmentally regulated transcription factor selectively expressed in skin and thymic epithelia.
- SourceAvailable from: ncbi.nlm.nih.govClinical & Experimental Immunology 05/2003; 132(1):9-15. · 3.41 Impact Factor
Article: T cell immunodeficiency.[show abstract] [hide abstract]
ABSTRACT: T cell immunodeficiency can occur as one of a group of primary disorders or develop secondary to chronic infection, illness or drug therapy. Primary T cell disorders are rare, accounting for approximately 11% of reported primary immunodeficiencies, and generally present in infancy or early childhood. Early recognition is very important as many of these patients will require bone marrow transplantation prior to the onset of severe infection or other complications. Because of their rarity, these infants usually present to clinicians who have little or no prior experience of these conditions, and therefore laboratory-based clinicians with knowledge of the key laboratory/pathological abnormalities and clinical features have a valuable role in identifying the possibility of immunodeficiency. Secondary T cell deficiency is a cardinal feature of HIV infection and the specific susceptibility to infectious micro-organisms is highlighted. The possibility of T cell immunodeficiency should be considered in any patient presenting with unusual or severe viral, fungal or protozoal infection.Journal of clinical pathology 10/2008; 61(9):988-93. · 2.43 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: CELL membrane structures controlled by genes in the major histocompatibility complex (H-2 in mice) are involved in most immune interactions between T lymphocytes and other cells1. Cytotoxic T lymphocytes (CTL) immunised against viruses2, haptens3, minor histocompatibility antigens4 or tumour antigens5, are specific for self H-2 antigens as well as for the foreign antigen. But CTL are not restricted to recognising antigens in combination with only self H-2. H-2d homozygous CTL which have matured in an irradiated H-2d/H-2k host can respond to antigen plus H-2k in addition to antigen plus H-2d (refs 6-8). It is not known whether the H-2 environment in which T cells mature influences their range of specificity, that is, whether CTL from a normal mouse can respond quantitatively as well to antigen plus foreign H-2 as they do to antigen plus self H-2. These experiments were designed to test this influence. The results suggest that host H-2 antigens do exert an effect on the specificity of T-cell responses.Nature 10/1977; 269(5627):417-8. · 38.60 Impact Factor
Hindawi Publishing Corporation
Clinical and Developmental Immunology
Volume 2012, Article ID 467101, 12 pages
FromMurineto HumanNude/SCID: TheThymus,T-Cell
Rosa Romano, LoredanaPalamaro,AnnaFusco, Leucio Iannace,Stefano Maio,
Department of Pediatrics, “Federico II” University, Via Pansini 5, 80131 Naples, Italy
Correspondence should be addressed to Claudio Pignata, firstname.lastname@example.org
Received 12 October 2011; Accepted 9 December 2011
Academic Editor: Ana Lepique
Copyright © 2012 Rosa Romano et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
defects, which may affect T-cell development/function, are approximately 11% of reported PIDs. The pathogenic mechanisms are
related to molecular alterations not only of genes selectively expressed in hematopoietic cells but also of the stromal component
of the thymus that represents the primary lymphoid organ for T-cell differentiation. With this regard, the prototype of athymic
disorders due to abnormal stroma is the Nude/SCID syndrome, first described in mice in 1966. In man, the DiGeorge Syndrome
of the murine Nude/SCID has been described, contributing to unravel important issues of the T-cell ontogeny in humans. Both
mice and human diseases are due to alterations of the FOXN1, a developmentally regulated transcription factor selectively ex-
pressed in skin and thymic epithelia.
Primary immunodeficiencies (PIDs) are severe disorders of
the immune system in which patients cannot produce a pro-
per protective immune response, leading to an increased
susceptibility to infections. Nowadays, more than 200 well-
characterized genetic immune deficiencies have been iden-
tified thanks to the advances in molecular genetics and
of the immune system that is primarily involved including
T, B, natural killer (NK) lymphocytes, phagocytic cells, and
complement proteins .
Primary T-cell defects are rare disorders, accounting for
approximately 11% of reported PIDs . These diseases may
be considered true experiments of the nature in that the
recognition of the molecular mechanisms underlying their
pathogenesis led to clarify the phases of the T-cell differenti-
ation process and the physiological mechanisms of the T-cell
responses. Studiesin this fieldled to unravelthe checkpoints,
the thymic microenvironment.
2.T-Cell Development andThymus
The thymus is the primary lymphoid organ that supports T-
cell differentiation and repertoire selection [3, 4]. The intra-
thymic development of T cells consists of several phases
that require a dynamic relocation of developing lymphocytes
within multiple architectural structures of this organ. As
shown in Figure 1, these steps are (1) the entry of lymphoid
progenitor cells into the thymus, (2) the generation of CD4+
CD8+double positive (DP) thymocytes in the cortex, (3) the
positive selection of DP thymocytes in the cortex, and (4) the
interaction of positively selected thymocytes with medullary
thymic epithelial cells (mTECs) to complete the thymocyte
the thymus .
Thymus anlagen arises as bilateral structures from the
third pharyngeal pouch in the embryonic foregut [6, 7]. The
interaction of the epithelial component with the lymphoid
progenitor takes place as early as embryonic day 11.5 in mice
and at the eighth week of gestation in humans [8, 9].
At an early stage, these precursors have both lymph-
oid and myeloid potential [10, 11] and are characterized by
2Clinical and Developmental Immunology
Figure 1: Steps of T-cell development. The lymphoid progenitor cell goes into the thymus through the cortico-medullary junction. DN
thymocytes (CD4−CD8−) migrate across the subcapsular region and then the outer cortex. Interaction between DN cells and cTECs
generates DP thymocytes (CD3+CD4+CD8+). Positively selected thymocytes interact with mTECs to complete the maturation process. In
the medulla, self-reactive thymocytes are deleted, SP (CD3+CD4+or CD3+CD8+) thymocytes are generated, and, eventually, the export of
mature T cells from the thymus takes place.
along with the CCR7, plays a central role in this precocious
stage of thymus colonization. At this stage of differentiation,
lymphoid cells also express the stem- and progenitor-cell
markers KIT (also known as CD117), the stem-cell antigen-1
3 (FLT3) [12–14].
Following the entry into the thymus through the cor-
ticomedullary junction, lymphoid progenitor cells begin
their commitment toward the T-cell lineage. The devel-
opmental pathway is traditionally divided into three sub-
sequent steps, as defined by peculiar immunophenotypic
patterns: the CD4−CD8−double negative (DN) stage, the
CD4+CD8+double positive (DP) stage, and the CD4−CD8+
or CD4+CD8−single positive (SP) stage. In mice, an immat-
ure single positive (ISP) CD8+CD4−cell may be detected
between the DN and DP stages. This population can be
easily distinguished from the mature SP cell by the high lev-
els of expression of T-cell receptor (TCR) β and CD3 and the
low level of CD24 (heat stable antigen, HSA). DN cells in
mice can be further subdivided based on the expression
of CD44 and CD25 in the following populations: CD44+
CD25−(DN1), CD44+CD25+(DN2), CD44−CD25+(DN3),
and CD44−CD25−(DN4) .
From the early T-cell lineage progenitor (ETP) stage
to the double-negative 3 (DN3) stage, T-cell differentiation
is independent from the TCR and is dependent on the
migration through the distinct thymic structures . These
phases are regulated by the expression levels of specific trans-
cription factors and by a fine tuned interplay between them
At the beginning, ETPs and DN2 cells exhibit a high pro-
liferative capability. Differently, at the DN3 stage, when a
fully rearranged TCR occurs, the proliferation stops. In the
initial thymocyte development till the DN3 stage, Notch-
mediated signals play a pivotal role [17, 18] also supported
by signals delivered through the interleukin-7 receptor (IL-
7R) [19, 20].
The immature thymocytes journey through the thymus
hasalsotheadditional effectof promoting thedifferentiation
of thymic stromal precursors into mature thymic epithelial
cells, thus playing an important role in the formation of
Clinical and Developmental Immunology3
Subcapsular zone Cortex Medulla
DN4ISP DP SP
Figure 2: Differential gene expression profile, which modulates the discrete stages of the T-cell development. The lymphoid progenitors,
entering into thymus and expressing the markers of HSCs, are primed to Notch and IL-7 signaling until DN1 stage. During the transition
DN1/DN2, immature thymocytes lose multilineage potential through the downregulation of genes involved in the differentiation towards
other cellular lineages, as PU.1, TAL1, GATA-2, and C/EBPα. At the DN2 stage, Myb, GATA-3, HEBalt, GLI-2, and Bcl-11b are upregulated.
At the DN3 stage, the genes required for a proper TCR assembly as Rag-1, Rag-2, and pTα are expressed, thus leading to the β-selection.
Following β-selection check-point, DN4 cells are fully committed to the TCRαβ+T-cell lineage.
cytes during the DN1-DN3 stages participate to the differ-
entiation process of TEC precursor cells into cortical TECs
The DN1 cell thymocytes keep the potential to differenti-
The transition to DN2 is characterized by the upregulation
of a number of genes involved in the process, including genes
needed for rearrangement and/or expression of the pre-TCR
signaling complex components (Figure 2) . At this stage,
the thymocytes lose the multilineage potential due to silenc-
ing of genes involved in the differentiation towards other
cellular lineages. Nevertheless, this potential is not com-
pletely lost, since cells with the DN2 phenotype can still
differentiate into NK cells, DCs, or macrophages under cer-
tain circumstances [29, 30].
DN2 stage T cells are fully responsive to IL-7 and SCF
due to the high expression of IL-7Rα and c-kit. The DN2
stage is characterized by the upregulation of CD25 molecule
(interleukin-2 receptor α, IL-2Rα) and CD90 (Thy-1) .
Moreover, the genes which favor the myeloid, NK, and
dendritic fate, so-called T-cell antagonists, as PU.1, stem-cell
leukemia (SCL also known as TAL1), GATA binding protein-
2 (GATA-2), and CCAAT-enhancer binding protein α
(C/EBPα) are silenced before that β or γδ selection takes
place (Figure 2) . During this phase only a few transcrip-
tion factors, including the zinc-finger transcription factor,
the tumor suppressor factor B-cell lymphoma/leukemia 11b
(BCL-11b) , basic helix-loop-helix (bHLH) transcrip-
tion factors alternative (HEBalt) , and, more transiently,
glioma-associated oncogene 2 (GLI-2), a transcription factor
involved in the sonic hedgehog signaling , are expressed
The following DN2 to DN3 stage transition requires the
expression of different arrays of genes, as Runt-related tran-
plexes, the transcription factor Myb, GATA-3, and Bcl-11b,
which allow full TCRβ gene rearrangement in thymocytes,
that become competent to undergo β-selection [35–37].
Several important events occur during the DN2/3 transition,
as the induction of recombinase activating gene-1 (Rag-1)
and Rag-2, the upregulation of pre-Tα (pTα), and the re-
arrangement of TCRδ and γ. CD3ε and IL-7Rα (CD127) are
also upregulated at this phase  along with the turn-on of
the lck tyrosine kinase implicated in the pre-TCR and TCR
signaling . At this point, T-cell precursors lose their
capability to follow a non-T-cell fate choice .
The cells overcoming β-selection express the pre-TCR
complex on their surface and reach the DN3 stage .
Thereafter, the E-proteins E2A and HEB play a crucial role
in several processes and are required for the progression of
the T-cell development. In fact, these proteins are involved
in the TCR gene rearrangement , in conferring the com-
petence to undergo β-selection, and in the arrest of thymo-
cyte proliferation at the DN3 stage .
At the DN3 stage, pre-TCR signaling results in the down-
regulation of CD25, pTα, Rag-1, and Rag-2, which leads to
the appearance of DN4 cells. These cells are fully committed
cytes, which have properly rearranged TCRβ chains, show a
burst of proliferation and a subsequent upregulation of CD8
(DP). Eventually, DP cells rearrange TCRα gene, leading to
TCRα assembly into a TCR complex.
The newly generated DP thymocytes are localized in the
cortex and express low levels of the TCRαβ complex. This
4Clinical and Developmental Immunology
DP population consists of T cells with an unselected reper-
take place. In the cortex, the DP thymocytes interact through
their TCR with peptide-MHC complexes expressed by stro-
mal cells, as cTECs and dendritic cells . When TCR
thymocytes receive survival signals. This process, referred to
react to foreign antigens, but not to self-antigens . Lately,
positively selected DP thymocytes are ready to differentiate
into SP cells, that is, CD4+CD8−or CD4−CD8+and relocate
into the medulla. At this site, newly generated SP thymocytes
are further selected by the medullary stromal cells, including
autoimmune regulator- (AIRE-) expressing mTECs. The
cells which are reactive to tissue-specific self antigens are
deleted, thus avoiding autoimmunity . SP thymocytes
egress from the thymus as recent thymic emigrants (RTEs),
na¨ ıve cells expressing the CD62 ligand (CD62L), also known
as lymphocyte- (L-) selectin, CD69, and the CD45RA iso-
form. These RTE cells are fully mature T cells that exert
proper functional capabilities of cell-mediated immunity
3.Pathogenetic Mechanismsof T-CellDefects
Most of the pathogenic mechanisms underlying primary T-
cell disorders are related to molecular alterations of genes
selectively expressed in hematopoietic cells. However, since
cytes and thymic microenvironment, a severe T-cell defect
may also be due to alteration of the stromal component of
T-cell defect varies a lot ranging from the syndrome of severe
combined immunodeficiency (SCID), characterized by a
complete absence of T-cell functions to combined immun-
odeficiency disorders, in which there are a low number of
T cells whose function is not adequate .
SCIDs comprise a heterogeneous group of monogenic
disorders characterized by a virtual lack of functional peri-
pheral T cells. To date, more than 20 different genetic defects
involved in the pathogenesis of SCID in humans have been
identified [52, 53]. Typically, patients with SCID show a
severe defect in T-cell differentiation and a direct or indirect
impairment of B-cell development and function. On the
basis of the involvement of different cell lines in the patho-
genesis of the disease and of the subsequent different clinical
phenotypes, SCIDs have been till now classified according
to the presence or absence of T, B, and NK cells (Table 1).
Impaired survival of lymphocyte precursors is observed
in reticular dysgenesis (RD) and in adenosine deaminase
(ADA) deficiency. In RD the mutations of the adenylate
kinase 2 gene (AK2) result in increased apoptosis of myeloid
and lymphoid precursors. As a consequence, patients with
RD show marked lymphopenia and neutropenia [54, 55].
ADA deficiency is characterized by the accumulation of
high intracellular levels of toxic phosphorylated metabolites
Table 1: SCIDs classification. SCIDs have been so far classified ac-
cording to the presence or absence of T, B, and NK cells, as a conse-
quence of different molecular defects.
Gene defect Form of SCID
Rag-1 or Rag-2 artemis
of adenosine and deoxyadenosine that cause apoptosis of
lymphoid precursors in the bone marrow and thymus [56,
The majority of SCIDs in human subjects derive from
alterations of the cytokine-mediated signaling apparatus.
SCID-X1 represents the most common form of SCID and is
caused by mutations of the IL-2 receptor γ gene (IL-2Rγ),
which encodes for the common γ-chain (γ-c) shared by
cytokine receptors, including those for IL-2, IL-4, IL-7, IL-9,
IL-15, and IL-21. Patients usually have few or no T and NK
cells but a normal or elevated number of B cells which fail to
produce immunoglobulins normally . γ-c also plays ef-
fects on cell cycle control and participates to the growth of
tumoral cells, as well [59, 60]. Defects of JAK3, an intra-
cellular tyrosine kinase physically and functionally coupled
to γ-c, result in a syndrome whose immunologic phenotype
is undistinguishable from that of SCID-X1 . Mutations
in the gene encoding for the α-chain of the IL-7R abrogate
T lymphocyte development but leave B and NK cell develop-
ment intact . Mutations in critical genes needed for the
expression of pre-T-cell receptor, as Rag-1 and Rag-2, result
in a functional inability to form antigen receptors through
genetic recombination, compromising the production of
functional T cells. These proteins recognize recombination
signal sequences and introduce a DNA double-stranded
break, permitting V, D, and J gene rearrangements [63, 64].
Lymphocyte phenotype differs from those of patients with
SCID caused by γ-c, Janus kinase-3 (Jak-3), IL-7Rα, or ADA
deficiencies in that they lack both B and T lymphocytes since
pre-TCR and pre-B-cell receptor (BCR) share similar molec-
ular mechanisms requiring Rag-1 and 2 expression .
Defects of pre-TCR and pre-BCR expression might also
reflect mutations in genes that encode proteins involved in
nonhomologous end-joining (NHEJ) and DNA repair and,
in particular, Artemis, DNA protein-kinase catalytic subunit
(DNA-PKcs), Cernunnos/XLF, and DNA ligase IV [65–
69]. In all these diseases, the generation of both T and
B lymphocytes is severely compromised. However, it should
be noted that a functional T-cell defect may also be due
to infections [70, 71] or during the reconstitution phase
following stem cell transplantation .
It is noteworthy that all the genes whose alterations lead
to the above mentioned forms of SCID selectively impair
Clinical and Developmental Immunology5
the lymphocyte functionality and the ability of these cells
to proceed in the developmental pathway. In some cases, as
in the case of TrkA mutation , the gene has pleiotropic
effects resulting in complex multisystemic disorders associ-
ated to immunodeficiency.
4.The MurineModel of Athymia:nu/nuMice
The first example of SCID not primarily related to a hemato-
poietic cell abnormality but rather to an intrinsic thymic
epithelial cell defect is the Nude/SCID phenotype, whose
identification contributed to unravel important issues of
The “nude” phenotype, identified for the first time in
mice, results from inactivating mutations in a single gene,
originally named winged-helix-nude (whn) and recently
known as forkhead box n1 (foxn1) . This murine model
was described by Flanagan in 1966, when spontaneously ap-
peared in the Virus Laboratory of Ruchill Hospital in Glas-
gow (UK) [75–77]. Mice homozygous for the mutation
“nude” are hairless, have retarded growth, decreased fertility,
and die by 5 months of life for infections. The hairlessness is
due to the coiling of the incomplete hair shafts in the dermis
caused by the absence of free sulfhydryl groups in the mid-
follicle region . The “nude” foxn1 gene does not affect
the growth of hair follicles, but the epidermal differentiation
ferentiation of keratinocytes in the hair follicle [79, 80]. The
“nude” mice are affected by severe infertility and show small
ovaries with low egg counts in the females and no motile
sperm in the males . This condition may be the result
of changes in hormonal status, as demonstrated by altered
serum levels of estradiol, progesterone, and thyroxine .
The thymus is absent at birth  and there are very few
lymph nodes .
Since the abnormal, or even absent, thymus is the hall-
mark of the “nude” phenotype, these animals develop a pro-
found T-cell deficiency and a severely impaired immune
response of either cell-mediated and, indirectly, humoral
thymic rudiment composed of vesicles or canaliculi delim-
ited by epithelial-like cells, with no trace of lymphoid cells.
By the day 14, the “nude” thymus is much smaller compared
to the normal .
Nu/nu mice show lymphopenia and also low immuno-
globulin levels. In the absence of normal T cells originated
from the thymus, the development of the antibody forming
cells is delayed, although “nude” mice do not lack precursors
ing cells may mature in the absence of the thymus, albeit at a
slower rate . In “nude” mice lymph nodes, the outer cor-
tex with primary nodules and the medullary cords are nor-
mal. In the spleen sections from the nu/nu mice, the pro-
portion of red to white pulp is greater than normal and, in
seen in the red pulp. In some spleens, Malpighian follicles,
although present, are fewer and smaller than in controls and
a depletion of lymphocytes is constant in the close proximity
of the central arteriole in the thymus-dependent area. The
depletion in the splenic thymus-dependent areas is not as
of an athymic disorder has long been considered the
DiGeorge’s Syndrome (DGS), even though main features of
nological signs, are not completely overlapping.
5.The Athymic DiGeorgeSyndrome
The DGS, along with velocardiofacial syndrome and con-
otruncal anomaly face syndrome, is frequently associated
to a common heterozygous intrachromosomal deletion in
22q11.2. However, a DGS-like phenotype can have alterna-
tive etiologies, including maternal diabetes, fetal alcohol syn-
drome, and teratogenesis, even though the molecular mech-
anisms underlying these forms are still unknown . DGS
has an estimated incidence of 1 in 4000 live births [87, 88]
and, thus, it is the most common microdeletion syndrome in
humans and the second most common chromosomal disor-
der after Down’s syndrome. The deletion is due to a meiotic
nonallelic homologous recombination between flanking 250
consisting in low-copy repeats/segmental duplications in the
de novo deletions, approximately 5% of cases are inherited as
a hemizygous 3 Mb deletion, containing about 30 genes [89,
90, 94, 95], is found, whereas approximately 8% of patients
carry a smaller deletion of 1.5 Mb, encompassing 24 genes
, even though no difference in the clinical presentation is
appreciable in the smaller deletion .
The main features of this syndrome are mild facial dys-
morphism, submucous cleft palate, velopharyngeal insuffi-
ciency, speech delay, recurrent infections, variable immun-
odeficiency secondary to thymic aplasia or hypoplasia, and
in some cases [99–102]. Children with the DGS, according to
plete or partial DGS. The “complete” form represents a small
percentage of patients, accounting to the 0.5% of all patients.
These patients show a severe combined immunodeficiency
phenotype with near absent T lymphocytes. The majority of
patients have a “partial” phenotype and an immune defect
usually manifesting as mild to moderate T lymphocytopenia.
The T-cell proliferation is usually normal or in very few cases
low normal. These patients have been reported to have a
on the basis of the immunological impairment, suggesting
that anatomical defects, gastroesophageal reflux, allergies,
cardiac disease, and poor nutrition may also contribute to
“partial” DGS patients have severe infections as reported in
SCID and, moreover, T-cell proliferation is usually normal.
A moderate CD4 lymphocytopenia with low to normal
CD8 T lymphocytes is usually found. An age-related de-
crease of T lymphocytes is also seen in DGS patients. TCR
6Clinical and Developmental Immunology
repertoire analysis in 22q11.2 deletion patients has shown
significant oligoclonal peaks and Vβ family dropouts when
compared to controls. In a study of nine patients with a
CD8+TCR repertoire, using both flow cytometric and third
complementarity determining region (CDR3 spectratyping)
fragment analysis, has been documented . In another
study, the spectratyping showed alterations in the repertoire,
which, however, improved over the time .
Immune deficiency in these patients seems to be associ-
ated to an increased incidence of autoimmune diseases [106–
108], in particular cytopenias [109, 110], arthritis , and
which belongs to the family of T-box transcription factors,
which share a common DNA binding domain is called “T-
box” . A specific role for Tbx1 in DGS and thymus
development came out from the peculiar expression pattern
in both the third pharyngeal pouch endoderm and the ad-
jacent mesenchyme and not in the neural crest cells .
Furthermore, the homozygous loss of Tbx1 causes thymic
hypoplasia, as well [96, 115–117]. Of note, mice heterozy-
gousfora null alleleofTbx1 demonstrate only a mild pheno-
type without thymus anomalies . Thus, evidence would
suggest, at least in mice, that gene dosage of Tbx1 is crucial
in the pathogenesis of DGS. However, in the same region
there are other genes potentially implicated in the pathogen-
esis of DGS, such as Crkl, which encodes an adaptor pro-
tein implicated in growth factor and adhesion molecule
defects in neural crest derivatives including aortic arch
arteries, thymus, and craniofacial structures  and in pre-
natal death. However, the deletion at the heterozygous state
does not cause any clinical sign, thus indicating that a com-
bination of gene alterations is needed for the full expressivity
of the phenotype .
The human equivalent of the “nude” murine phenotype was
first described in two sisters in 1996, after more than 30 years
from the initial mouse description and, subsequently, associ-
ated to FOXN1 gene alterations.
The human Nude/SCID is an autosomal recessive disor-
der , whose hallmark is the T-cell immunodeficiency
due to the complete absence of the thymus. This immunod-
eficiency presents in a quite similar fashion to the classical
SCID phenotype, thus being more severe than DGS. Along
with the severe infections, other features of the syndrome
are ectodermal abnormalities, as alopecia and nail dystrophy
. Of note, the nail dystrophy can be observed also in
subjects carrying the genetic alteration in heterozygosity.
The most frequent nail alteration is the koilonychia (spoon
nail), characterized by a concave surface and raised edges
of the nail plate, associated with significant thinning of the
plate itself; a canaliform dystrophy associated to a transverse
groove of the nail plate (Beau line) may also be found
(Figure 3). However, the most specific phenotypic alteration
is leukonychia, characterized by a typical arciform pattern
resembled to a half-moon and involving the proximal part
of the nail plate. These alterations of digits and nails have
also been reported in a few strains of “nude” mice. FOXN1
is known to be selectively expressed in the nail matrix where
the nail plate originates, thus confirming that this transcrip-
tion factor is involved in the maturation process of nails
and suggesting nail dystrophy as an indicative sign of hetero-
zygosity for this molecular alteration .
Interestingly, additional studies have also reported on
anomalies of brain structures, suggesting a potential role
of this transcription factor in brain embryogenesis, as also
suggested by its expression in epithelial cells of the develop-
ing choroids plexus, a structure filling the lateral, third, and
fourth ventricles. However, the severe neural tube defects,
including anencephaly and spina bifida, have been only in-
alteration represents a cofactor and is not sufficient per se to
alter brain embryogenesis. The anomalies of brain structure
high frequency of FOXN1 alteration .
Prenatal alteration of the FOXN1 gene in humans pre-
vents the development of the T-cell compartment as early as
at 16 weeks of gestation . By contrast, stem cells, B, and
NK lymphocytes are normal. CD4+cells are more affected
than CD8+cells, even though the latter are also profoundly
reduced. No CD4+CD45RA+naive cells can be usually found
. CD8 cells coexpressing CD3 are very scarce and a
few CD3+CD8+CD45RA+na¨ ıve cells can be detected .
not of lymphocytes expressing TCRγδ, is observed .
TCR gene rearrangement, although altered, occurs to some
extent, suggesting the possibility of an extrathymic and
FOXN1-independent site of differentiation. However, it
blockage, are unable to sustain a productive immune res-
ponse into the periphery.
Taken together, the data so far available underline the
crucial role of FOXN1 in the early prenatal stages of T-cell
ontogeny in humans .
7.Role of FOXN1 inImmune System
FOXN1 belongs to the forkhead-box gene family that com-
prises a diverse group of “winged helix” transcription factors
implicated in a variety of cellular processes: development,
metabolism, cancer, and aging . These transcription
regulated and of directing tissue specific transcription and
those of the liver, lung, intestine, kidney, and urinary tract,
later, its expression is confined to skin and thymus epithelia,
where FOXN1 is absolutely required for the normal differen-
tiation of hair follicles and TECs.
FOXN1 gene, spanning about 30kb [125, 126], is an
epithelial cell-autonomous gene and is highly conserved in
sequence and function in rodents and humans. Interestingly,
Clinical and Developmental Immunology7
Figure 3: Nail dystrophy patterns in subjects carrying heterozygous mutations in FOXN1 gene: (a) koilonychias, (b) canaliform dystrophy,
and (c) leukonychia.
an extensive screening of cDNA clones obtained from skin
cells revealed the presence of two different noncoding first
exons , the exons 1a and 1b, that undergo to alternative
splicing to either of two splice acceptor sites of the exon 2,
located upstream of the initiation codon. This suggests the
presence of two distinct promoters of exons 1a and 1b .
The alternative usage of the exon 1a or 1b seems to direct the
tissue specificity , in that promoter 1a is active in thy-
mus and skin, while promoter 1b is active only in skin.
The molecular mechanisms by which FOXN1 expression
and activity are regulated are only incompletely understood.
It is suggested that FOXN1 might, subsequently, upregulate
the expression of fibroblast growth factor (FGF) receptors,
which in turn modulate the thymic stroma differentiation
and thymopoiesis . In vitro exposure of thymic epithe-
lial cells to some Wnt proteins is sufficient to upregulate
FOXN1 protein expression in both an endocrine and par-
acrine fashion . Wnts belong to a large family of se-
creted glycoproteins that have important roles in cell-fate
The prenatal thymus development, the maintenance of
a proper thymic microenvironment, and the efficient T-cell
production require an appropriate crass-talk between thy-
mocytes and thymic stromal cells . Postnatally, the thy-
mic involution results in dramatically reduced T-cell genera-
tion in an age-dependent fashion .
Indeed, recent evidence has implicated both TEC- and
hematopoietic stem cell- (HSC-) intrinsic defects in involu-
tion of the organ [130–133]. Foxn1 is expressed in all TECs
during initial thymus organogenesis and is required for the
initial phase of their differentiation [75, 134, 135]. Foxn1
exerts an important role  in inducing both cortical and
medullary differentiation [137, 138]. Although foxn1 has
long been studied, most of the studies thus far available are
restricted to fetal differentiation process, while its postnatal
role in the mature thymus still remains to be fully elucidated.
However, it is largely unknown whether the role of foxn1
in the thymus and skin is identical. One important difference
is that foxn1 is involved in morphogenesis of the three-
dimensional thymic microstructure, which is important for
the functionality of the thymus . Moreover, the differ-
entiation of the immature epithelial cells into functional
cTECs and mTECs is foxn1-dependent. In particular, foxn1
mainly regulates TEC patterning in the fetal stage  and
TEC homeostasis in the postnatal thymus . TECs are
implicated in either thymus organogenesis or in most stages
of maturation of thymocytes [142, 143]. The inborn null
mutation in foxn1  causes a differentiation failure in
TECs thereby halting thymic development at a rudimentary
stage. The thymic lobar architecture is still present but the
topoietic precursor cells (HPCs) into the epithelial cluster
and thus preclude the generation of thymocytes . These
results argue strongly for a failure in thymocytes-epithelial
crosstalk, thus, explaining the blockage of thymic lympho-
poiesis [75, 136]. The organ is, therefore, an alymphoid two-
dimensional (2D) rudiment with a cystic structure [72, 82,
Because of the significant expression levels of FOXN1 in
skin elements, keratinocytes have been successfully used to
support a full process of human T-cell development in vitro,
finding would imply a role for skin as a primary lymphoid
Primary T-cell defects are rare disorders, accounting for ap-
proximately 11% of reported PIDs. These disorders include
a wide spectrum of diseases that affect T-cell development
and/or function. The pathogenic mechanisms are mostly
related to molecular alterations of genes selectively expressed
ations of the stromal component of the thymus, which is the
primary lymphoid organ that supports T-cell differentiation
and repertoire selection. In this organ, the dynamic reloca-
tion in multiple architectural structures requires the cross-
talk between thymocytes and thymic microenvironment.
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selectively expressed in skin and thymic epithelia. In mice
and humans its alteration leads to thymic agenesia and
severe T-cell deficiency. The Nude/SCID immunodeficiency
is much more severe than DGS, indicating that the FOXN1
expression is absolutely required for an efficient production
of mature T cells. The studies on the human Nude/SCID
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