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IL-21 regulates Th17 cell homeostasis, enhances the differentiation of memory B cells and antibody-secreting plasma cells, and promotes the maintenance of CD8+ T-cell responses. In this study, we investigated the phenotype, function, and frequency of blood and intestinal IL-21–producing cells in nonhuman primates that are hosts of progressive (rhesus macaques [RMs]) and nonprogressive (sooty mangabeys [SMs]) SIV infection. We found that, in both species, memory CD4+CD95+CCR6− T cells are the main IL-21 producers, and that only a small fraction of CD4+IL-21+ T cells produce IL-17. During chronic SIV infection of RMs, CD4+IL-21+ T cells were significantly depleted in both blood and rectal mucosa, with the extent of this depletion correlating with the loss of Th17 cells. Furthermore, treatment with IL-21 increased the in vivo levels of Th17 cells in SIV-infected RMs. In contrast, normal levels of CD4+IL-21+ T cells were found in SIV-infected SMs. Collectively, these data indicate that depletion of IL-21–producing CD4+ T cells distinguishes progressive from nonprogressive SIV infection of RMs and SMs, and suggest that depletion of CD4+IL-21+ T cells is involved in the preferential loss of Th17 cells that is associated with SIV disease progression. Further preclinical studies of IL-21 as a potential immunotherapeutic agent for HIV infection may be warranted.

Abstract 3289 Functional T cell immunity depends on the diversity of T cell receptor (TCR) repertoire. Numerous cytolytic assaults can perturb effector cell fitness by inhibiting thymic de novo T cell production or predisposing peripheral CD8+ T cells to exhaustion. In fact, age-associated thymic atrophy and PD-1 upregulation remain the critical components of immune dysfunction. Normal thymopoiesis arises from marrow-derived CD3−CD4−CD8− triple-negative T cell progenitors (TN) which seed the thymic cortex. Thereafter, TN develop into mature single-positive (SP) CD4 or CD8 T cells after expressing both CD4 and CD8 (double-positive-DP) transiently, leading to de novo T cell production. Interleukin-7 (IL7) is a singularly important common γ-chain interleukin involved in this process. In order to pharmacologically enhance thymopoiesis, we report the development of a novel biopharmaceutical based on the fusion of GMCSF and IL7, hereafter GIFT7. Systemic administration of GIFT7 compared to IL7 in young immune competent mice leads to a transient increase in the number of DN1 (5.48±1.01 v.s. 2.74±1.07 × 106, p=0.046) by day 7. Total thymic cellularity significantly increases in GIFT7-treated group by day 14 (237±51.3 v.s. 123±25.6 × 106, p=0.026). GIFT7-mediated hypertrophic effect is significantly more pronounced in aged thymi: 111±21.8, 67.8±13.9, 60.3±4.6 × 106 for 7-dose of (5ug kg−1) GIFT7, IL7, or PBS treatment respectively. We observed a significant increase in DN1 and DN4 subsets with the greatest fold difference in the CD44int DN1 subset. Histologically, GIFT7 administration leads to significant cortical thymic hyperplasia (cortex: medulla ratio 2.67:1). Functionally, GIFT7 enhances viral-specific CTL responsiveness as modeled by murine cytomegalovirus (MCMV) peptide MHC+ tetramer enumeration (10.4±3.8 v.s. 2.9±1.4 × 106 for GIFT7 and untreated respectively, p In the periphery, GIFT7 selectively expand a CD8+ subset with a Central Memory (CM) phenotype defined as CD8+CD44+CD62L+CCR7+KLRG−CD27+PD1−. This unique expansive effect of GIFT7 is also demonstrated on peripheral blood mononuclear cells (PBMC) derived from non-human primates (NHP), in that GIFT7 stimulation of pre-activated NHP-PBMC leads to a two-fold increase in total cell number after 3 days and >80% of Ki67+ expression in both CD4+ and CD8+ compartments. Interestingly, GIFT7-mediated proliferation in CD8+ T cells is accompanied by a 40% decrease in the mean fluorescent intensity (MFI) of PD1 expression. Our work identifies a CD44intCD25− subset of DN thymocytes most responsive to IL7R-mediated STAT5 hypersignalling and their function as T cell progenitors in GIFT7-driven thymic reconstitution. This points to the translational potential of GIFT7 or GIFT7-primed T lineage cells as treatment of immune malfunction. Disclosures: No relevant conflicts of interest to declare.