HIV-Infected T Cells Induce Granzyme B-Secreting Regulatory B Cells in An Interleukin 21-Dependent Fashion,

Abstract 3222

We and others have recently provided evidence that a series of lymphocyte subsets including, plasmacytoid dendritic cells, B cells and regulatory T cells are able to secrete the cytotoxic serine protease granzyme B (GrB) into the extracellular compartment, where it contributes to the suppression of T cell proliferation by a so far undefined GrB-dependent mechanism. For B cells, we found that viral antigens in the context of the acute phase cytokine interleukin (IL-) 21 can potently induce GrB. Here, we demonstrate that infection of CD4⁺ T cells with HIV-1 (NL4-3), but not mock infection, induces strong expression of IL-21 on both mRNA and protein levels in CD4⁺ T cells. Moreover, we found that HIV-infected CD4⁺ T cells are able to induce high levels of GrB in co-cultured B cells and that inhibition of IL-21 with specific antibodies abrogates T cell-mediated GrB induction in B cells. In support of these data, serum levels of both IL-21 and GrB are significantly higher in patients acutely infected with HIV as compared to healthy control subjects. Surprisingly, co-culture of CD4⁺ T cells with B cells during HIV-1 infection strongly suppressed both, proliferation of T cells as well as virus replication as indicated by significantly reduced p24 levels in culture supernatants. Notably, this effect was enhanced by external stimulation of B cells with IL-21, and was reduced by inhibition of GrB using specific GrB inhibitors. To further explore the underlying mechanisms of our findings, we performed confocal microscopy of T cell-B cell co-cultures and demonstrated that GrB-secreting B cells directly interact with CD4⁺ T cells, thereby transferring active GrB to them. Moreover, we found that GrB⁺ B cells decreased CD4⁺ T cell expression of the T cell receptor-zeta chain, a known GrB target, which is required for T cell proliferation, and known to be suppressed in HIV patients. In summary, we provide evidence that HIV induces the acute phase cytokine IL-21 in infected CD4⁺ T cells, thereby indirectly triggering the expression of GrB by B cells. GrB-secreting B cells may play a so far unappreciated role in decelerating the expansion of HIV, particularly in the early phase of acutely infected patients. Our study reveals a novel pathogenetic mechanism in HIV infection with potential relevance for the development of novel immunotherapeutic approaches.