Article

The dynamic co-evolution of memory and regulatory CD4+ T cells in the periphery

Department of Immunology and Molecular Pathology, Division of Infection and Immunity, Windeyer Institute of Medical Sciences, University College London, London W1T 4JF, UK.
Nature reviews. Immunology (Impact Factor: 34.99). 04/2007; 7(3):231-7. DOI: 10.1038/nri2037
Source: PubMed

ABSTRACT

Whereas memory T cells are required to maintain immunity, regulatory T cells have to keep the immune system in check to prevent excessive inflammation and/or autoimmunity. Both cell types must be present during the lifetime of the organism. However, it is not clear whether both subsets are regulated in tandem or independently of each other, especially because thymic involution severely restricts the production of T-cell populations during ageing. In this Opinion article, we discuss recent evidence in both mice and humans that supports the hypothesis that some CD4(+)CD25(+)FOXP3(+)regulatory T cells can differentiate from rapidly proliferating memory T cells in the periphery.

Full-text

Available from: Milica Vukmanovic-Stejic, Mar 16, 2015
There was initial scepticism when studies
by Sakaguchi et al. in 1995 hinted at a
re-emergence of the suppressor-T-cell
concept
1
, which fell out of favour in
the 1980s as, at the time, there were no
consistent ways to study the phenotype
or function of these cells
2
. However, these
cells have once again taken centre stage
in immunology research. They are now
identified as being
CD4 positive, with high
levels of cell-surface expression of
CD25;
they are known as T
Reg
cells (CD4
+
CD25
+
regulatory T cells)
3,4
and have been shown
to have a regulatory function in various
disease states in mice
3,4
. T
Reg
cells are also
found in humans and, as in mice, they only
constitute a small proportion (<10%) of
the CD4
+
T-cell pool
3,4
. T
Reg
cells can also
be identified by their expression of the
forkhead winged-helix transcription factor
FOXP3 (forkhead box P3)
5,6
.
The expression of FOXP3 has been
proposed to be the crucial switch factor in
the induction of the T
Reg
-cell population.
The immune dysregulation that is found in
Foxp3-knockout mice, which develop a fatal
autoimmune disease, and the association
of defective FOXP3 function with IPEX
(immunodysregulation, polyendocrinopathy
and enteropathy, X-linked) syndrome in
humans substantiates a role for this molecule
in immune regulation
5,6
. Patients with IPEX
develop autoimmune diseases, such as
type 1 diabetes and thyroditis, inflammatory
bowel disease and severe allergies
7
. These
observations indicate the exciting possibility
of therapeutic intervention, directed towards
the T
Reg
cells themselves or at key target
molecules, such as FOXP3, to modulate
disease activity that arises from immune
dysregulation
8
.
In addition to this naturally occurring
population of T
Reg
cells, which develop in
the thymus
1
, other regulatory T-cell popula-
tions are generated in the periphery. These
are often termed adaptive T
Reg
cells and
they are remarkably similar to the naturally
occurring T
Reg
-cell populations
9
. It is impor-
tant to understand the biology of regulatory
T cells and how they are generated in vivo
before any attempts are made to manipulate
them therapeutically. This note of caution is
particularly relevant given the severe prob-
lems that arose from the use of super agonistic
CD28-specific antibodies in a recent
clinical trial, in which the reagent, although
tolerated well in animals, had unexpected
effects in humans
10
. In this Opinion article,
we address the question of how FOXP3
+
T
Reg
cells are maintained throughout life and
question to what extent studies in mice
can be extrapolated to humans.
Heterogeneity of T
Reg
cells
Many regulatory T-cell populations have been
described
(TABLE 1). In this article, we focus
on FOXP3
+
T
Reg
cells, which are the best-
defined population at present. In addition
to CD25 and FOXP3, several other markers,
such as CTLA4 (cytotoxic T-lymphocyte
antigen 4), GITR (glucocorticoid-induced
tumour-necrosis-factor-receptor-related
protein), HLA-DR, OX40 and LAG3
(lymphocyte-activation gene 3) have been
associated with this natural T
Reg
-cell popula-
tion
3,4
. There are some important differences
between mouse and human T cells, as human
CD4
+
CD25
effector T cells can upregulate
FOXP3 expression transiently, following
activation
11–13
, whereas mouse effector T cells
do not
14
. So, in conditions of immune stimu-
lation, FOXP3 might not be a good marker
for human T
Reg
cells, as it can be expressed by
both regulatory and activated effector T-cell
populations.
At present, the best way to identify both
mouse and human T
Reg
cells, in situations
where there is no overt immune activation
in vivo, is by the co-expression of high levels
of CD25 and FOXP3
(REF. 15). In addi-
tion, low-level expression of
CD127 (the
interleukin-7 receptor α-chain) can be used
as a further discriminating marker for this
population
16,17
. However, it is important to
acknowledge the limitations of CD25 by itself
as a marker for regulatory populations, as it is
also upregulated by activated effector T cells,
and some regulatory CD4
+
T cells that are
FOXP3
+
CD127
low
express little or no CD25
(REFS 16,17). Although most FOXP3
+
natu-
rally occurring T
Reg
cells in adults express
markers of primed T cells, such as
CD45RO,
a small proportion of these cells express the
marker CD45RA, and this CD45RA popula-
tion decreases with age
18–20
. However, both
populations have identical mechanisms of
suppression.
Lifelong maintenance of T
Reg
cells
T
Reg
cells can develop in the thymus by the
high-affinity interaction of the T-cell recep-
tor (TCR) with self-peptides, as reviewed
extensively elsewhere
21
. However, whereas
OPINION
The dynamic co-evolution of
memory and regulatory CD4
+
T cells
in the periphery
Arne N. Akbar, Milica Vukmanovic-Stejic, Leonie S. Taams and Derek C. Macallan
Abstract | Whereas memory T cells are required to maintain immunity, regulatory
T cells have to keep the immune system in check to prevent excessive inflammation
and/or autoimmunity. Both cell types must be present during the lifetime of the
organism. However, it is not clear whether both subsets are regulated in tandem or
independently of each other, especially because thymic involution severely restricts
the production of T-cell populations during ageing. In this Opinion article, we
discuss recent evidence in both mice and humans that supports the hypothesis that
some CD4
+
CD25
+
FOXP3
+
regulatory T cells can differentiate from rapidly
proliferating memory T cells in the periphery.
NATURE REVIEWS
|
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|
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PERSPECTIVES
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Page 1
Thymus
Naturally occuring
T
Reg
cell
Naive T cell
Proliferation
Memory T-cell pool
Self-
regeneration
Telomere erosion
Telomere erosion
Memory-derived T
Reg
cell
Origin
Maintenance
ab
the output of an infant’s thymus is in the
order of 10
9
CD4
+
T cells per day, by 20
years of age it is reduced to 15% of this value
and continues to decrease thereafter
22
. Old
individuals (>70 years of age) have decreased
numbers of CD45RA
+
T
Reg
cells
18,20
, as well
as decreased numbers of conventional
naive T cells, which supports the assump-
tion that thymic involution equally affects
the generation of nascent regulatory and
non-regulatory T cells. However, compared
to young individuals (<35 years old), old
individuals have the same or increased
numbers of CD45RO
+
T
Reg
cells with the
same function as those from young indi-
viduals
23,24
, therefore, other mechanisms in
addition to thymic output must contribute to
the lifelong maintenance of these cells.
There are three potential mechanisms to
explain how T
Reg
cells are maintained in vivo.
First, there might be naturally occurring
T
Reg
cells of thymic origin that are resistant
to death. However, we and others have
shown that most T
Reg
cells in adults express
CD45RO and these cells are highly suscep-
tible to apoptosis
25–27
. Second, T
Reg
cells in
adults might originate from thymic-derived
T
Reg
cells that are continuously replenished
by proliferation in the periphery
(FIG. 1a).
Indeed, mouse T
Reg
cells have been shown
to proliferate extensively while maintain-
ing their regulatory function in vivo
28–30
,
and such proliferation also occurs for both
human and mouse T
Reg
cells in vitro
31–34
. This
might explain why freshly isolated T
Reg
cells
from both humans and mice express a highly
differentiated phenotype (CD45RB
low
),
indicating that they have proliferated exten-
sively
26,35–38
. Third, T
Reg
cells might also be
generated from activated effector or memory
CD4
+
CD25
T cells in the periphery, and
these cells are known as adaptive or induced
T
Reg
cells
39–41
(FIG. 1b). The phenotype and
function of adaptive T
Reg
cells is remarkably
similar to naturally occurring T
Reg
cells
(TABLE 1).
The small subset of the human naturally
occurring T
Reg
-cell population that expresses
CD45RA has robust proliferative activity
and might represent a self-regenerating
T
Reg
-cell population
18–20
. However, CD45RA
+
T
Reg
cells upregulate CD45RO expression after
activation and become highly susceptible
to apoptosis
18–20
, and are therefore likely to
be lost rapidly after proliferation. Although
it is possible that these CD45RA
+
T
Reg
cells
may re-populate the CD45RO
+
T
Reg
-cell
pool throughout life, the observations that
CD45RA
+
T
Reg
cells decrease considerably
with age, together with their susceptibility
to death, suggests that it is unlikely
18–20
. The
occurrence of thymic involution indicates
that T
Reg
cells in old humans have to be gener-
ated from either proliferative natural T
Reg
cells
or T
Reg
cells induced from effector memory
T cells, in both cases proliferation is central
to their lifelong maintenance. This raises a
problem because there is a limit to the extent
to which cells can proliferate continuously
in vivo that is set by the extent of telomere
erosion
42,43
. Therefore, the two crucial issues
Table 1 | Types of regulatory T cell: origin, phenotype and function
Cell type Phenotype* Mechanism* Origin References
Natural
T
Reg
cells
FOXP3
+
CTLA4
+
GITR
+
CD45RB
low
CD62L
+/–
CD127
low
Contact dependent,
IL-10, TGFβ, CTLA4
and GITR
Thymus 18,20,26,
35,37,38,
86–89
Natural
T
Reg
cells
(expanded)
FOXP3
+
HLA-DR
+
CD62L
+/–
CD69
+
Contact dependent Expansion of natural
T
Reg
cells
28,30–34,44
Induced T
Reg
cells
FOXP3
+
CTLA4
+
GITR
+
CD45RB
low
Contact dependent
and in some cases
TGFβ
Conversion and/or
expansion of non-
regulatory CD4
+
T cells
13,39–41,47,
48,51,52,
60,90,91
T
H
3 cells FOXP3
+/–
(not well
defined)
TGFβ and/or IL-10
Periphery 55,56
T
R
1 cells FOXP3
+
IL-10 Periphery 54,92,93
CD8
+
T cells CD28
+/–
Cell contact, ILT3
and ILT4
Periphery 94
*For simplicity, we have provided a consensus of the most widely accepted data on the characteristics of regulatory
T cells.
Most of the natural regulatory CD4
+
CD25
+
T (T
Reg
) cells express CD45RO, which is characteristic of a memory
phenotype. However, a small proportion of circulating T
Reg
cells express CD45RA and are sometimes referred to as
naive natural T
Reg
cells. These cells are abundant in cord blood, but decrease with age. In terms of phenotype and
function, they are very similar to the CD45RO
+
T
Reg
cells. CTLA4; cytotoxic T-lymphocyte antigen 4; FOXP3, forkhead
box P3; GITR, glucocorticoid-induced tumour-necrosis-factor-receptor-related protein; IL, interleukin;
ILT, immunoglobulin-like transcript; TGFβ, transforming growth factor-β; T
H
3, T helper 3; T
R
1, T regulatory 1.
Figure 1 | Possible mechanisms for the origin and maintenance of human CD4
+
CD25
+
regula-
tory T cells in vivo. Depicted are two possible models of CD4
+
CD25
+
regulatory T (T
Reg
)-cell mainte-
nance. a | In the first model, naturally occurring T
Reg
cells are generated in the thymus as a separate
lineage. However, as thymic output decreases, T
Reg
cells need to be maintained by continuous prolif-
eration in the periphery. Telomere erosion and lack of telomerase activity would limit their capacity
for self-regeneration. b | In the second model, T
Reg
cells are continuously generated from responding,
effector memory T-cell populations in the periphery.
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Thymic emigrants
Naive T-cell pool
CD4
+
CD45RA
+
Memory T-cell pool
CD4
+
CD25
CD45RO
+
Regulatory T-cell pool
CD4
+
CD25
hi
CD45RO
+
New cells: <0.5%
per day
New cells:
3%
per day
New cells:
8% per day
Division of cells within
the naive pool
Division of cells within
the memory pool
Activation and
proliferation of
naive cells
Production of
new T
Reg
cells
that have to be addressed to assess the resid-
ual replicative capacity of T
Reg
cells are: first,
the rate of turnover of these cells, and second,
whether they have sufficient telomeric reserve
to sustain the rate of division throughout life.
Turnover rate of human T
Reg
cells in vivo
The development of the deuterated glucose
method for labelling proliferating cells
in vivo has enabled the turnover rate of
different subsets of human T cells to be
determined
(FIG. 2; TABLE 2). However, there
are few data available on the turnover rate
of T
Reg
cells. In one study in mice, using a
combination of BrdU (bromodeoxyuridine)
and CFSE (5,6-carboxyfluorescein diacetate
succinimidyl ester) labelling, it was shown
that a subset of T
Reg
cells that was gener-
ated in vitro has a rapid turnover rate after
adoptive transfer in vivo
44
. In a second study,
in patients with acute myeloid leukaemia,
there was evidence for a relatively high
rate of T
Reg
-cell proliferation in vivo
45
.
More recently, a study of T
Reg
-cell turnover
in healthy individuals without any overt
immune activation, showed that CD45RO
+
T
Reg
cells had a rapid doubling time of 8 days,
compared with the CD4
+
CD25
CD45RO
+
memory T cells (doubling time of 28 days)
or CD4
+
CD25
CD45RA
+
naive T cells
(doubling time of 199 days)
24
. In this study,
CD45RO
+
T
Reg
cells expressed FOXP3
(REF. 24) and were CD127
low
(A. McQuaid,
M.V.-S. and A.N.A., unpublished observa-
tions). The rapid proliferation was matched
by the rapid death rate of CD45RO
+
T cells.
Furthermore, the kinetics for prolifera-
tion and death of these regulatory T cells
in young and old individuals were very
similar
24
. This suggests that human T
Reg
cells
comprise a dynamic population, which is
generated continuously from either pre-
existing CD45RO
+
T
Reg
cells
28,30,44
or from
CD4
+
CD25
memory T cells that proliferate
rapidly then enter the T
Reg
-cell pool during
the labelling period
(FIG. 1).
Can T
Reg
cells proliferate indefinitely?
Every time a cell divides, between 50–100
base pairs of telomeric DNA are lost from
the ends of chromosomes
42,43
. The induction
of expression of the enzyme telomerase can
retard telomere erosion and it is possible
that some T-cell populations might be main-
tained indefinitely because this enzyme is
constitutively active
42,43
. The telomere length
in human T cells is 4–12 kilobases (kb)
and growth arrest or replicative senescence
occurs when telomere length falls below
4 kb
42,43
. The telomere length of human
CD45RO
+
T
Reg
cells was found to be short
compared with either CD4
+
CD25
CD45RO
+
memory T cells or the total CD4
+
T-cell pool
in the same subjects
24,46
. In addition, the
T
Reg
cells from both young and old subjects
were unable to upregulate telomerase after
activation
24
. It is therefore unlikely that the
T
Reg
-cell pool in old humans is derived from
self-regenerating populations of naturally
occurring T
Reg
cells that continuously pro-
liferate throughout life
24
. Furthermore, the
propensity of replicative senescence together
with their susceptibility to apoptosis
26,27
Table 2 | Relative in vivo proliferation rates of CD4
+
T-cell subtypes
Method Cell type studied Main phenotypic definition Proliferation rate References
Persistence of chromosomal
abnormalities
Human PBMCs CD45RO
+
versus CD45RA
+
Memory T cells > naive T cells 95
BrdU incorporation Mouse spleen and
lymph-node T cells
CD44
+
and CD45RB
+
Memory T cells > naive T cells 96
Deuterium-labelled glucose Human PBMCs CD45RO
+
or CD45RA
+
and
CD62L
Memory T cells > naive T cells 97,98
Deuterium-labelled water Human PBMCs CD45RA
+
CD62L
+
versus
CD45RA
CD62L
Effector memory T cells > naive
T cells
99
Deuterium-labelled glucose Human PBMCs CCR7
+
versus CCR7
(CD45RO
+
and CD45RA
+
populations)
Effector memory T cells > central
memory T cells > naive T cells
100
Survival after adoptive transfer
using CFSE and BrdU labelling
Mouse spleen and
lymph-node T cells
CD25, CD62L, CD44 ‘Activated’ T
Reg
cells (defined as
CD62L
low
CD44
hi
) > ‘resting’ T
Reg
cells
44
Ki67 labelling Human PBMCs CD25
hi
, CD25
mid
and CD25
low
(CD4
+
T-cell populations)
CD25
hi
≈ CD25
mid
≈ CD25
low
45
Deuterium-labelled glucose Human PBMCs CD25
hi
versus CD25
(CD45RO
+
population) versus CD45RA
+
T
Reg
cells > memory T cells > naive
T cells
24
BrdU, bromodeoxyuridine; CFSE, carboxy-fluorescein diacetate succinimidyl ester; PBMCs, peripheral blood mononuclear cells.
Figure 2 | A model of regulatory T-cell homeostasis derived from deuterium-labelled glucose
measurements of in vivo proliferation. The size of the circle schematically represents the size of
the T-cell population; the size of the dark wedge represents the proportion of new cells per unit of time
within that population. Most circulating CD4
+
T cells are slowly dividing or non-dividing CD45RA
+
naive T cells. Cells within the smaller memory T-cell pool divide more often, whereas T
Reg
cells
(CD4
+
CD25
+
regulatory T cells) comprise the smallest population (about one-thirtieth of circulating
CD4
+
T cells) with the fastest rate of turnover
24
. Cell death is not shown for the sake of clarity but must
balance the appearance of new cells within each pool. This data is compatible with the available
reports in mice
44
and humans
45
showing that T
Reg
cells have a high rate of proliferation.
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Page 3
would explain why these cells do not accu-
mulate in vivo despite their rapid prolifera-
tion. This raises the question of whether
T
Reg
cells are derived from proliferating
memory CD4
+
CD25
CD45RO
+
T cells. It is
important to emphasize that as mice have a
considerably shorter lifespan than humans,
but have up to tenfold longer telomeres
25
,
the thymus-derived T
Reg
-cell population
might not be restricted to the same extent by
continuous proliferation. Self-regenerating
naturally occurring T
Reg
cells might therefore
constitute a greater proportion of the T
Reg
-cell
pool in mice compared with humans.
Are memory T cells and T
Reg
cells related?
Studies in mice have shown that T
Reg
cells
can be generated from CD4
+
CD25
effec-
tor T cells under certain conditions
40,41,47,48
(TABLE 3). To address the relationship
between human CD45RO
+
T
Reg
cells and
CD4
+
CD25
CD45RO
+
memory T cells, sev-
eral researchers have compared the TCR Vβ
repertoires in the two subsets to determine
to what extent both populations express
receptors that belong to the same TCR
family. These studies showed that these two
populations were related
46,49
. These studies
were extended to investigate the clonal rela-
tionship between T
Reg
-cell and memory T-cell
populations within an individual TCR Vβ
family from the same individual
24
. The key
observation was that there was an average
of 80% clonal homology between CD45RO
+
T
Reg
-cell and CD4
+
CD25
CD45RO
+
memory
T-cell populations, and this related identity
has been confirmed by other research-
ers using DNA spectratyping
49
or CDR3
(complementarity-determining region 3)
sequence analysis
50
. Furthermore, antigen-
related expansions of CD4
+
CD25
effector
T cells within certain Vβ families were
associated with parallel increases of FOXP3
+
T
Reg
cells with the same TCR Vβ usage
24
. As it
is unlikely that these human regulatory T cells
are self-regenerating, at least a proportion of
them must be derived from rapidly dividing
memory CD4
+
T cells. However, an alterna-
tive possibility that must be considered is
that the shared clonality in CD4
+
CD25
and
CD4
+
CD25
+
T-cell subpopulations might
actually represent shared sequences between
CD25
T
Reg
-cell-like and CD25
+
T
Reg
-cell
populations. This might be resolved in the
future by the use of MHC class II tetramers
to show that the same antigen-specific CD4
+
T cells are equally represented in regulatory
and non-regulatory pools.
From memory T cell to T
Reg
cell. It has been
shown in both mice and humans that trans-
forming growth factor-β (TGFβ)
48,51,52
and
interferon-γ (IFNγ)
53
can induce CD4
+
CD25
FOXP3
effector T cells to become FOXP3
+
T
Reg
cells (TABLE 3). As these cytokines signal
through different pathways, it will be impor-
tant to determine their point of convergence.
Other regulatory populations, such as those
that secrete interleukin-10 (IL-10; T regula-
tory 1 (T
R
1) cells)
54
or TGFβ (T helper 3
(T
H
3) cells)
55,56
, can also be generated from
naive and/or memory CD4
+
CD25
T cells
under specific conditions, but their relation-
ship to naturally occurring or adaptive
FOXP3
+
T
Reg
cells is unclear. One important
caveat that applies to humans is that not all
CD4
+
CD25
FOXP3
T cells that are induced
to express FOXP3 also acquire regulatory
function
12
. Although this has led to the sug-
gestion that FOXP3 is not an optimal marker
for human T
Reg
cells, we think this is too
narrow a conclusion. The fact that FOXP3
is upregulated by CD4
+
CD25
T cells after
activation is consistent with our hypothesis
that activated T cells can differentiate into
T
Reg
cells under appropriate conditions. The
crucial question to be addressed is what
signals induce the stable upregulation of
FOXP3 and committed regulatory function
in CD4
+
CD25
T cells.
Apart from cytokine induction, it has
been shown that the induction of anergy in
activated CD4
+
T cells by providing a TCR
signal in the absence of co-stimulation,
or by specific peptide being presented by
activated T cells to each other in humans or
Table 3 | Studies showing extra-thymic generation of CD4
+
CD25
+
regulatory T cells from CD4
+
CD25
non-regulatory T cells
T-cell source T
Reg
-cell-inducing stimulus Induction of
FOXP3 expression
References
Mouse in vitro studies
CD4
+
CD25
T cells
TGFβ
Ye s 4 8 , 5 1
CD4
+
CD25
T cells
IFNγ
Ye s 5 3
Mouse in vivo studies
CD4
+
CD25
T cells Homeostatic proliferation and/or conversion Yes 47,101
CD4
+
CD25
T cells (OVA-specific
TCR-transgenic Rag
–/–
mice)
Intravenous or oral administration of OVA ND 79
CD4
+
CD25
T cells (HA-specific
TCR-transgenic Rag
–/–
mice)
Prolonged exposure to a low dose of antigen Yes 39,41
CD4
+
(OVA-specific TCR-transgenic
Rag
–/–
mice)
Prolonged exposure to peripheral antigen Yes 40
Human in vitro studies
CD4
+
CD25
T cells CD3 and/or CD28 or antigen-specific stimulation Yes 13,90,102
CD4
+
CD25
T cells
IFNγ
Ye s 5 3
CD4
+
CD25
CD45RA
+
T cells
TGFβ
Yes 48,52,103
CD4
+
CD25
CD45RA
+
T cells Thrombospondin and/or CD47 interaction Yes 60
CD4
+
CD25
CD45RA
+
T cells IL-4 and/or IL-13 Yes 91
CD4
+
effector T-cell clones T-cell-to-T-cell antigen presentation; immobilized
CD3-specific antibody
Ye s A . N . A . et al., unpublished
observations
FOXP3, forkhead box P3; HA, haemagglutinin; IFNγ, interferon-γ; ND, not determined; OVA, ovalbumin; Rag, recombination-activating gene; TCR, T-cell receptor; TGFβ,
transforming growth factor-β; T
Reg
cell, CD4
+
CD25
+
regulatory T cell.
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a
b
CD4
+
T cells
Effector memory
T cells
Memory T cells
T
Reg
cells
Apoptosis
Apoptosis
Intermittent
exposure
Self-antigens,
dietary antigens
Persistent
antigen
Continuous
exposure
T
Reg
cellsT
Reg
cells
Antigen removedForeign antigen
Immunity
Tolerance
rats, induces the development of regulatory
T cells in vitro
57–59
. However, it is not clear
whether these populations are identical to
the FOXP3
+
T
Reg
cells that are found in vivo
25
.
It has also been shown that the interaction
between thrombospondin and CD47 on the
surface of human CD4
+
CD25
FOXP3
naive
or memory T cells induced the development
of a FOXP3
+
T
Reg
-cell population
60
. These
data collectively highlight that the CD4
+
T-cell repertoire in both mice and humans is
considerably more flexible than previously
appreciated and that exogenous mediators
and specific cell-surface triggers might
be able to regulate the differentiation of
responsive T-cell populations into regulatory
T-cell populations. The context and location
of antigenic encounter, for example in the
presence of IFNγ, IL-10 or TGFβ, could
influence which type of regulatory T cell
(FOXP3
+
T
Reg
cell, T
R
1 cell or T
H
3 cell) is
induced. In addition, as FOXP3
+
T
Reg
cells
have a highly differentiated phenotype
26,35
, it
is likely that the cues that induce this stable
transition affect only memory CD4
+
T cells
that have reached a particular stage of matu-
rity. This is a characteristic that has been
observed for other functional lineage-related
transcription factors, such as T-bet, which is
involved in inducing a T
H
1-type functional
profile in CD4
+
T cells
61
.
Co-evolution of memory T cells and T
Reg
cells.
Based on the earlier discussion, we postulate
different pathways for the maintenance
of T
Reg
cells in mice and humans. In mice,
FOXP3
+
T
Reg
cells could originate either
from naturally occurring, thymus derived
T
Reg
cells that are self-regenerating or from
activated and proliferating CD4
+
CD25
FOXP3
memory T cells (FIG. 1). However,
in humans, especially older individuals, we
postulate that FOXP3
+
T
Reg
cells are generated
continuously by activated and proliferating
CD4
+
CD25
FOXP3
T cells. The rapid gen-
eration of these cells is counterbalanced by
rapid loss, thereby maintaining homeostasis,
which allows a relatively constant number of
T
Reg
cells to be maintained during ageing
23,24
.
This hypothesis is supported by the observa-
tion that T
Reg
cells develop in parallel with
activated CD4
+
T cells within the memory
T-cell pool
24,50
. The inescapable conclusion
from this is that most human FOXP3
+
T
Reg
cells are transient cells that are generated,
lost and replaced continuously in vivo.
This proposes a simple model that links
the level of regulatory T-cell generation
to the extent of immune stimulation that
occurs
(FIG. 3). A robust antigenic challenge
that results in the extensive proliferation of
specific CD4
+
T cells will give rise to many
short-lived T
Reg
cells of the same specificity,
whereas a smaller immune response would
induce less T
Reg
-cell generation. When the
immune response subsides, owing to removal
of the antigen, the regulatory cells disappear
rapidly as they are susceptible to apoptosis
26
.
This prevents the accumulation of regulatory
cells that could paralyse specific immunity
and therefore allows subsequent responsive-
ness to a previously encountered antigen.
Conversely, in the case of self-antigens and
dietary antigens, there will be prolonged
antigenic encounter, leading to chronic
stimulation of antigen-specific CD4
+
T cells.
This in turn will lead to the continuous gen-
eration of T
Reg
cells. This would prevent the
development of autoimmunity, and might be
a crucial mechanism at the interface between
antigen-induced immunity and tolerance.
Implications for therapy
According to this model of T
Reg
-cell genera-
tion, strategies to deplete T
Reg
cells in vivo
to boost antitumour immune responses
62,63
might only have limited benefit, as these
cells may be continuously replenished
from the memory pool
64
. In fact, in a
recent clinical trial, using a recombinant
IL-2–diphtheria-toxin conjugate to deplete
T
Reg
cells and thereby boost the antitumour
response, T
Reg
cells were restored to 75% of
the original number within two months
63
. In
addition, infusion of these cells into humans
to reduce inflammation or autoimmunity
will only confer short-term benefit as these
cells are susceptible to apoptosis and might
be cleared rapidly in vivo. In this context,
factors such as IL-2 that have been shown to
be essential for the generation of T
Reg
cells,
might actually function by counteracting
T
Reg
cell susceptibility to cell death
65–67
.
However, the proximity of these cells to rep-
licative senescence because of short telom-
eres might be a useful characteristic from the
point of view of safety, as their capacity for
continuous proliferation would be limited in
situations of adoptive transfer.
If T
Reg
cells differentiate from memory
or activated CD4
+
T cells, then changes
in FOXP3
+
T
Reg
cells that have been found in
human diseases, such as multiple sclerosis
68
,
Figure 3 | A model that links the duration and intensity of immune stimulation to regulatory
T-cell generation.
a | Antigenic challenge resulting in extensive proliferation of specific CD4
+
T cells will give rise to many short-lived T
Reg
cells (CD4
+
CD25
+
regulatory T cells) of the same
specificity. The number of T
Reg
cells generated will be proportional to the size of the immune
response. Once the antigen is removed, the immune response subsides and the regulatory cells
disappear rapidly, as they are susceptible to apoptosis. This prevents the over-accumulation of
regulatory T cells that could paralyse subsequent responses to the same antigen.
b | Prolonged
antigenic encounter, as in the case of self-antigens and dietary antigens, will lead to chronic
stimulation of antigen-specific CD4
+
T cells. This in turn will lead to the continuous generation of
T
Reg
cells that might prevent the development of autoimmunity.
PERSPECTIVES
NATURE REVIEWS
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IMMUNOLOGY VOLUME 7
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MARCH 2007
|
235
© 2007 Nature Publishing Group
Page 5
rheumatoid arthritis
69–71
, type 1 diabetes
72
and
atopic eczema
73
, could be linked to defective
differentiation from effector or memory
CD4
+
T cells rather than to their dysfunction
per se. In addition, if T
Reg
cells are generated
continuously throughout life, then persistent
infection with viruses, such as cytomegalo-
virus (CMV), that induce the large-scale
differentiation of human memory T cells
74–76
would be expected to also induce a dispropor-
tionate representation of CMV-specific regu-
latory T cells in the total T
Reg
-cell pool, which
indeed has been shown
24
. Such an imbalance
might contribute to the CMV-related
immune defect found in old individuals
77
.
Finally, the concept that T
Reg
cells might
develop from CD4
+
CD25
memory or acti-
vated CD4
+
T cells by antigenic stimulation
might explain some historical data. Low-zone
tolerance, which is achieved by the repeated
administration of low doses of antigen to
an animal, is one of the fundamental tenets
of immunology but the mechanism for this
remains unclear
78
. More recent data in mice,
in which continuous infusion of antigen
induces T
Reg
cells, provide a modern counter-
part of these studies, suggesting that low-zone
tolerance might occur as a result of regulatory
T-cell induction from the responding popula-
tion
39–41,79
. The possibility of clinically appli-
cable induction of regulatory T cells that can
restore or maintain self-tolerance was clearly
shown by studies of autoimmune diabetes in
mice. In this study, treatment of non-obese
diabetic mice with CD3-specific antibodies
induced TGFβ-dependent regulatory T cells
from CD25
T-cell populations
80
. Treatment
with CD3-specific antibody was also effective
in dampening disease progression in patients,
presumably by a similar mechanism
81
. Peptide
immunotherapy in humans has been used
successfully to treat allergies by inducing
responsive cells to secrete IL-10, and this is
analogous to the studies of low-zone toler-
ance
82
. The IL-10-producing CD4
+
T
R
1-cell
population that is induced by peptide
immunotherapy overlaps with the T
Reg
-cell
population
83–85
. These studies indicate that the
way forward in the use of human T
Reg
cells for
therapy might be to identify and manipulate
mechanisms that regulate their induction
from responsive populations in vivo.
Future perspectives
The regulatory T-cell field has diversified
considerably and increased in complexity
in the last decade, as different researchers
use different systems and focus exclusively
on particular regulatory T-cell populations.
Searching for points of convergence between
regulatory T-cell types and experimental
systems used is essential to allow simplifica-
tion of the field in the future. The model
proposed here provides a simple explanation
of how the activation of memory T cells is
closely linked to the induction of regula-
tory activity. The current limitation to the
model proposed here is that the relationship
between different types of human regula-
tory T cell is unclear and it remains to be
determined whether they arise from a com-
mon precursor. Second, although the shared
clonotype between T
Reg
cells and memory
T cells provides compelling evidence that
both populations are related, naturally
occurring T
Reg
cells are considered to be
specific for self-antigens, whereas adaptive
T
Reg
cells have the same non-self-antigen
specificity as the effector memory T-cell
pool. The possible implication for auto-
immunity of the natural attrition of the natu-
rally occurring T
Reg
-cell population owing to
thymic involution, in parallel with the accu-
mulation of the adaptive T
Reg
-cell population
during ageing, requires further study. The
current challenge is to identify mechanisms
that induce the stable development of regula-
tory CD4
+
T cells from activated populations
to pinpoint therapeutic targets for the in vivo
manipulation of these cells in humans.
Arne N. Akbar and Milica Vukmanovic-Stejic are at the
Department of Immunology and Molecular Pathology,
Division of Infection and Immunity, Windeyer Institute
of Medical Sciences, University College London,
London W1T 4JF, UK.
Leonie S. Taams is at the Department of
Immunobiology, King’s College London, Guy’s Hospital,
London SE1 9RT, UK.
Derek C. Macallan
is at the Centre for Infection,
Division of Cellular and Molecular Medicine,
St George’s University of London, London
SW17 0RE, UK.
Correspondence to A.N.A.
e-mail: a.akbar@ucl.ac.uk
doi:10.1038/nri2037
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