Repeated antigenic encounter drives proliferation and differentiation of memory T cell pools. An important question is whether certain specific T cells may be driven eventually to exhaustion in elderly individuals since the human life expectancy is increasing. We found that CMV-specific CD4+ T cells were significantly expanded in healthy young and old carriers compared with purified protein derivative-, varicella zoster virus-, EBV-, and HSV-specific populations. These CMV-specific CD4+ T cells exhibited a late differentiated phenotype since they were largely CD27 and CD28 negative and had shorter telomeres. Interestingly, in elderly CMV-seropositive subjects, CD4+ T cells of different specificities were significantly more differentiated than the same cells in CMV-seronegative individuals. This suggested the involvement of bystander-secreted, differentiation-inducing factors during CMV infection. One candidate was IFN-α, which induced loss of costimulatory receptors and inhibited telomerase in activated CD4+ T cells and was secreted at high levels by CMV-stimulated plasmacytoid dendritic cells (PDC). The CMV-specific CD4+ T cells in elderly subjects had severely restricted replicative capacity. This is the first description of a human memory T cell population that is susceptible to being lost through end-stage differentiation due to the combined effects of lifelong virus reactivation in the presence of bystander differentiation-inducing factors.
Anergic/suppressive CD4+CD25+ T cells have been proposed to play an important role in the maintenance of peripheral tolerance. Here we demonstrate that in humans these cells suppress proliferation to self antigens, but also to dietary and foreign antigens. The suppressive CD4+CD25+ T cells display a broad usage of the T cell receptor Vβ repertoire,suggesting that they recognize a wide variety of antigens. They reside in the primed/memory CD4+CD45RO+CD45RBlow subset and have short telomeres, indicating that these cells have the phenotype of highly differentiated CD4+ T cells that have experienced repeated episodes of antigen‐specific stimulation in vivo. This suggests that anergic/suppressiveCD4+CD25+ T cells may be generated in the periphery as a consequence of repeated antigenic encounter. This is supported by the observation that highly differentiated CD4+T cells can be induced to become anergic/suppressive when stimulated by antigen presented by non‐professional antigen‐presenting cells. We suggest that besides being generated in the thymus, CD4+CD25+ regulatory T cells may also be generated in the periphery. This would provide a mechanism for the generation of regulatory cells that induce tolerance to a wide array of antigens that may not be encountered in the thymus.
Human γδ T cells expressing the Vδ3 TCR comprise a minor lymphocyte subset in blood but are enriched in liver and in patients with some chronic viral infections and leukemias. We analysed the frequencies, phenotypes, restriction elements and functions of fresh and expanded peripheral blood Vδ3 T cells. Vδ3 T cells accounted for ~0.2% of circulating T cells, included CD4+, CD8+ and CD4−CD8− subsets, and variably expressed CD56, CD161, HLA-DR and NKG2D, but not NKG2A nor NKG2C. Vδ3 T cells were sorted and expanded by mitogen stimulation in the presence of IL-2. Expanded Vδ3 T cells recognised CD1d, but not CD1a, CD1b nor CD1c. Upon activation, they killed CD1d+ target cells, released Th1, Th2 and Th17 cytokines and induced maturation of dendritic cells into APCs. Thus, Vδ3 T cells are glycolipid-reactive T cells with distinct antigen specificities but functional similarities to natural killer T cells.
Naturally occurring CD4 + CD25 hi Foxp3 + Tregs (nTregs) are highly proliferative in blood. However, the kinetics of their accumulation and proliferation during a localized antigen-specific T cell response is currently unknown. To explore this, we used a human experimental system whereby tuberculin purified protein derivative (PPD) was injected into the skin and the local T cell response analyzed over time. The numbers of both CD4 + Foxp3 -(memory) and CD4 + Foxp3 + (putative nTreg) T cells increased in parallel, with the 2 populations proliferating at the same relative rate. In contrast to CD4 + Foxp3 -T cell populations, skin CD4 + Foxp3 + T cells expressed typical Treg markers (i.e., they were CD25 hi , CD127 lo , CD27 + , and CD39 + ) and did not synthesize IL-2 or IFN-γ after restimulation in vitro, indicating that they were not recently activated effector cells. To determine whether CD4 + Foxp3 + T cells in skin could be induced from memory CD4 + T cells, we expanded skin-derived memory CD4 + T cells in vitro and anergized them. These cells expressed high levels of CD25 and Foxp3 and suppressed the proliferation of skin-derived responder T cells to PPD challenge. Our data therefore demonstrate that memory and CD4 + Treg populations are regulated in tandem during a secondary antigenic response. Furthermore, it is possible to isolate effector CD4 + T cell populations from inflamed tissues and manipulate them to generate Tregs with the potential to suppress inflammatory responses. IntroductionNaturally occurring CD4 + CD25 hi Foxp3 + Tregs (nTregs) can prevent reactivity to both self and non-self antigens (1-4). Although early studies suggested that these cells are generated as a distinct population in the thymus, CD4 + CD25 hi Foxp3 + Tregs, which are phenotypically and functionally identical to the thymus-derived population, can also be generated after antigen-induced proliferation of CD4 + T cells in peripheral tissues in mice (5, 6). The rapid division of CD4 + CD25 hi Foxp3 + Tregs that has been shown to occur in vivo in mice (7) and humans (8) may be a mechanism for maintaining nTregs. This has particularly important implications for the lifelong maintenance of human Tregs after thymic involution, since CD4 + CD25 hi Foxp3 + T cells in humans have limited capacity for extensive self-renewal, due to short telomeres, and lack telomerase activity (8). It is important to consider the possible difference in behavior and characteristics of nTregs in mice and humans, especially given the potential for species-specific differences that might lead to side effects during therapy (9).The regulation of immunity and pathology by intervention at the Treg axis has been very successful in animal models, where it has been shown that CD4 + CD25 hi Foxp3 + T cells can be harnessed to prevent autoimmunity (10, 11), inflammatory disease (12),
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