The in vivo mechanism of regulatory T cell (T(reg) cell) function in controlling autoimmunity remains controversial. Here we have used two-photon laser-scanning microscopy to analyze lymph node priming of diabetogenic T cells and to delineate the mechanisms of T(reg) cell control of autoimmunity in vivo. Islet antigen-specific CD4(+)CD25(-) T helper cells (T(H) cells) and T(reg) cells swarmed and arrested in the presence of autoantigens. These T(H) cell activities were progressively inhibited in the presence of increasing numbers of T(reg) cells. There were no detectable stable associations between T(reg) and T(H) cells during active suppression. In contrast, T(reg) cells directly interacted with dendritic cells bearing islet antigen. Such persistent T(reg) cell-dendritic cell contacts preceded the inhibition of T(H) cell activation by dendritic cells, supporting the idea that dendritic cells are central to T(reg) cell function in vivo.

Classically, the CD28/cytotoxic T-lymphocyte antigen-4 (CTLA-4) and B7 families of cell surface molecules regulate complex signaling pathways that profoundly affect T-cell responses. The recent identification and characterization of additional CD28 and B7 family members including programmed death-1 (PD-1), programmed death ligand-1 (PD-L1) (B7-H1), and PD-L2 (B7-DC) has added to the complexity and greater appreciation of how surface molecules control T-cell activation and peripheral tolerance. CD28/B7 interactions mediate co-stimulation and significantly enhance peripheral T-cell responses. CTLA-4, in contrast, interacting with the same B7 molecules, results in decreased T-lymphocyte activity and regulates the immune response. Similarly, PD-1 interactions with PD-L1 and PD-L2 downmodulate T-cell immune responses. Despite these similarities, the regulatory roles of the CTLA-4 and PD-1 pathways are distinct. This may be due, at least in part, to the differential expression patterns of the CTLA-4 and PD-1 ligands both temporally and spatially. This article examines the role of CTLA-4 and PD-1 in limiting autoreactivity and establishing peripheral self-tolerance with the hypothesis that CTLA-4 signals are required early in the lymph node during initiation of an immune response and PD-1 pathways act late at the tissue sites to limit T-cell activity.

Programmed death-1 (PD-1) is an inhibitory molecule expressed on activated T cells, however, the biological context in which PD-1 controls T cell tolerance remains unclear. Using two-photon laser-scanning microscopy, we showed that unlike naïve or activated islet antigen-specific T cells, tolerized islet antigen-specific T cells moved freely and did not swarm around antigen-bearing dendritic cells (DC) in pancreatic lymph nodes. Inhibition of T cell receptor (TCR)-driven stop signals depended on continued PD-1-PD-L1 interactions, as antibody blockade of PD-1 or PD-L1 decreased T cell motility, enhanced T cell-DC contacts, and caused autoimmune diabetes. CTLA-4 blockade did not alter T cell motility or abrogate tolerance. Thus, PD-1-PD-L1 interactions maintain peripheral tolerance by mechanisms fundamentally distinct from those of CTLA-4.

A new regulatory T (T reg) cell–specific, FoxP3-GFP-hCre bacterial artificial chromosome transgenic mouse was crossed to a conditional Dicer knockout (KO) mouse strain to analyze the role of microRNAs (miRNAs) in the development and function of T reg cells. Although thymic T reg cells developed normally in this setting, the cells showed evidence of altered differentiation and dysfunction in the periphery. Dicer-deficient T reg lineage cells failed to remain stable, as a subset of cells down-regulated the T reg cell–specific transcription factor FoxP3, whereas the majority expressed altered levels of multiple genes and proteins (including Neuropilin 1, glucocorticoid-induced tumor necrosis factor receptor, and cytotoxic T lymphocyte antigen 4) associated with the T reg cell fingerprint. In fact, a significant percentage of the T reg lineage cells took on a T helper cell memory phenotype including increased levels of CD127, interleukin 4, and interferon γ. Importantly, Dicer-deficient T reg cells lost suppression activity in vivo; the mice rapidly developed fatal systemic autoimmune disease resembling the FoxP3 KO phenotype. These results support a central role for miRNAs in maintaining the stability of differentiated T reg cell function in vivo and homeostasis of the adaptive immune system.

Experimental autoimmune encephalomyelitis (EAE) is a CD4+ T lymphocyte–mediated disease of the central nervous system (CNS) characterized by mononuclear cell infiltration, demyelination, and paralysis. We previously demonstrated a role for chemokines in acute and relapsing EAE pathogenesis. Presently, we investigated the role of CC chemokine receptor 2 (CCR2) in acute EAE. CCR2−/− mice did not develop clinical EAE or CNS histopathology, and showed a significant reduction in T cell– and CNS-infiltrating CD45highF4/80+ monocyte subpopulations. Peripheral lymphocytes from CCR2−/− mice produced comparable levels of interferon-gamma (IFN-γ) and interleukin (IL)-2 in response to antigen-specific restimulation when compared with control mice. Adoptively transferred myelin oligodendrocyte glycoprotein 35-55–specific T cells lacking expression of CCR2 were able to induce EAE, whereas CCR2−/− recipients of wild-type T cells failed to develop disease. These results suggest that CCR2 expression on host-derived mononuclear cells is critical for disease induction.

The past decade has seen a significant increase in the number of potentially tolerogenic therapies for treatment of new-onset diabetes. However, most treatments are antigen nonspecific, and the mechanism for the maintenance of long-term tolerance remains unclear. In this study, we developed an antigen-specific therapy, insulin-coupled antigen-presenting cells, to treat diabetes in nonobese diabetic mice after disease onset. Using this approach, we demonstrate disease remission, inhibition of pathogenic T cell proliferation, decreased cytokine production, and induction of anergy. Moreover, we show that robust long-term tolerance depends on the programmed death 1 (PD-1)–programmed death ligand (PD-L)1 pathway, not the distinct cytotoxic T lymphocyte–associated antigen 4 pathway. Anti–PD-1 and anti–PD-L1, but not anti–PD-L2, reversed tolerance weeks after tolerogenic therapy by promoting antigen-specific T cell proliferation and inflammatory cytokine production directly in infiltrated tissues. PD-1–PD-L1 blockade did not limit T regulatory cell activity, suggesting direct effects on pathogenic T cells. Finally, we describe a critical role for PD-1–PD-L1 in another powerful immunotherapy model using anti-CD3, suggesting that PD-1–PD-L1 interactions form part of a common pathway to selectively maintain tolerance within the target tissues.

CD8+ T cell immunosurveillance dynamics influence the outcome of intracellular infections and cancer. Here we used two-photon intravital microscopy to visualize the responses of CD8+ resident memory T cells (TRM cells) within the reproductive tracts of live female mice. We found that mucosal TRM cells were highly motile, but paused and underwent in situ division after local antigen challenge. TRM cell reactivation triggered the recruitment of recirculating memory T cells that underwent antigen-independent TRM cell differentiation in situ. However, the proliferation of pre-existing TRM cells dominated the local mucosal recall response and contributed most substantially to the boosted secondary TRM cell population. We observed similar results in skin. Thus, TRM cells can autonomously regulate the expansion of local immunosurveillance independently of central memory or proliferation in lymphoid tissue.

Immunosurveillance of secondary lymphoid organs (SLO) is performed by central memory T cells that recirculate through blood. Resident memory T (Trm) cells remain parked in nonlymphoid tissues and often stably express CD69. We recently identified Trm cells within SLO, but the origin and phenotype of these cells remains unclear. Using parabiosis of "dirty" mice, we found that CD69 expression is insufficient to infer stable residence of SLO Trm cells. Restimulation of nonlymphoid memory CD8 T cells within the skin or mucosa resulted in a substantial increase in bona fide Trm cells specifically within draining lymph nodes. SLO Trm cells derived from emigrants from nonlymphoid tissues and shared some transcriptional and phenotypic signatures associated with nonlymphoid Trm cells. These data indicate that nonlymphoid cells can give rise to SLO Trm cells and suggest vaccination strategies by which memory CD8 T cell immunosurveillance can be regionalized to specific lymph nodes.