Regulatory T cells (Treg), a subset of CD4+ T cells, dramatically accumulate with age in humans and mice and contribute to age-related immune suppression. Recently, we showed that a majority of accumulating Treg in aged mice expressed low levels of CD25 and their accrual is associated with declining levels of IL-2 in aged mice. Here, we further investigated the origin of CD25lo Treg in aged mice. First, aged Treg had high expression of neuropilin-1 and Helios and had a broad Vβ repertoire. Next, we analyzed the gene expression profile of Treg, naïve, and memory T cells in aged mice. We found that the gene expression profile of aged CD25lo Treg were more related to young CD25lo Treg than to either naïve or memory T cells. Further, the gene expression profile of aged Treg was consistent with recently described “effector” Treg. Additional analysis revealed that nearly all Treg in aged mice were of an effector phenotype (CD44hiCD62Llo) and could be further characterized by high levels of ICOS and CD69. ICOS contributed to Treg maintenance in aged mice, as in vivo antibody blockade of ICOSL led to a loss of effector Treg, and this loss was rescued in Bim-deficient mice. Further, serum levels of IL-6 increased with age and contributed to elevated expression of ICOS on aged Treg. Finally, Treg accrual was significantly blunted in aged IL-6-deficient mice. Together, our data show a role for IL-6 in promoting effector Treg accrual with age likely through maintenance of ICOS expression.

Summary T cells play a critical role in immune responses as they specifically recognize peptide/MHC complexes with their T cell receptors (TCRs) and initiate adaptive immune responses. While T cells are critical for performing appropriate effector functions and maintaining immune memory, they also can cause autoimmunity or neoplasia if misdirected or dysregulated. Thus, T cells must be tightly regulated from their development onward. Maintenance of appropriate T cell homeostasis is essential to promote protective immunity and limit autoimmunity and neoplasia. This review will focus on the role of cell death in maintenance of T cell homeostasis and outline novel therapeutic strategies tailored to manipulate cell death to limit T cell survival (eg autoimmunity and transplantation) or enhance T cell survival (eg vaccination, immune-deficiency).

Ovarian carcinoma microenvironmental T cells exert clinically relevant pressure against malignant progression; however current immunotherapies rarely induce ovarian cancer regression. Here we investigate CD277-containing butyrophilin 3A1 (BTN3A1), a poorly investigated immunoregulatory pathway driven by myeloid and tumor cells in ovarian tumor beds. We show that BTN3A1 is overexpressed in ovarian cancer and is associated with a significant survival disadvantage in these patients (n=200). Concomitantly, ectopic expression of BTN3A1 on APCs inhibits αβ T cell proliferation and Th1 cytokine production. Proteomic analyses and binding assays demonstrate that BTN3A1 interacts with the CD45 phosphatase and elements of the TCR. Consequently, TCR ligation in the presence of BTN3A1 inhibits the segregation of CD45 from the immune synapse and blunts downstream signaling by antagonizing the phosphorylation of CD3Zeta, Lck, and Zap70. We developed fully human αCD277 antibodies which rescue αβ T cell proliferation and Th1 cytokine responses, while driving the infiltration of T cells into tumor beds, delaying ovarian tumor progression in novel BTN3A1+ humanized mice and xenograft studies. Paradoxically, αCD277 antibodies promote the activation of γδ T cells by driving a conformational transformation of BTN3A1. Thus, co-transfer of γδ and Ag-specific αβ T cells in the presence of αCD277 antibodies synergize to further impair malignant progression in vivo. Overall, we show that BTN3A1 drives αβ T cell dysfunction in ovarian cancer, while αCD277 antibodies transform this molecule from immunosuppressive to immunostimulatory by rescuing αβ T cells and activating γδ T cells, thus dynamically unleashing T cell-driven antitumor immunity.

DN T cells (CD4−CD8α−TCRαβ+) and CD8αα T cells compose a major lymphocyte population in the intestine and promote gut homeostasis. Their thymic development, i.e. agonist selection, has been shown to be controlled by the pro-apoptotic factor Bim. In addition to its role in the thymus, Bim also controls peripheral T cell survival. However, the role of Bim in thymic and peripheral homeostasis of DN T and CD8αα T cells remain unclear. Here, we found that T cell-specific expression of Bim during early, but not late, thymic development limits the levels of DN T and CD8αα T cells in the spleen and iIEL compartments. The loss of Bim altered the TCR repertoire of DN T cells, but not conventional CD4 or CD8 T cells. As IL-15 controls CD8αα cell homeostasis, and antagonizes Bim in other T cells, we examined whether the additional loss of Bim would restore CD8αα T cells in IL-15−/− mice. Strikingly, the additional loss of Bim restored splenic, but not intestinal, DN T and CD8αα T cells in IL-15−/− mice. Instead, IL-15 signaling via Stat5 synergized with TCR signaling in peripheral DN T cells to increase expression of CD8α. Furthermore, adoptive transfer of splenic DN T cells gave rise to CD8αα cells in immune intact recipients. Combined, these data show that Bim functions temporally to limit the development and TCR repertoire of CD8αα precursors in the thymus. Moreover, Bim limits IL-15-driven homeostasis of CD8αα cells in the secondary lymphoid organs; but does not restrict their IL-15-driven maturation that is critical for intestinal homeostasis.