HIV Triggers Interleukin 21-Mediated Induction of Granzyme B-Secreting B Cells with Regulatory and Antiviral Potential

Abstract 1042

Certain regulatory lymphocyte subpopulations including plasmacytoid dendritic cells and regulatory T cells secrete granzyme B (GrB), thereby suppressing T cell expansion. Recently, we found that B cells can also produce GrB and acquire regulatory potential in response to interleukin (IL-)21. Since HIV has been shown to be associated with elevated serum IL-21 levels, we hypothesized that GrB-expressing B cells may be induced during HIV infection. Here, we show that infection of CD4⁺ T cells with HIV 1 (NL4-3), but not mock infection, induces strong expression of IL-21 without upregulation of CD40 ligand. Importantly, we further demonstrate that such IL-21⁺CD40L^low T cells induce GrB in cocultured B cells in an IL-21-dependent fashion, rather than supporting their differentiation into plasma cells. In support of these findings, serum levels of both IL-21 and GrB are significantly higher in HIV-infected patients before HAART as compared to healthy controls. Up to 60% of freshly isolated B cells (36.2% ± 12.9%) from patients infected with HIV, but not B cells isolated from healthy individuals, express GrB. Of note, coculture of HIV-infected CD4⁺ T cells with GrB⁺ B cells result in GrB transfer, and strongly suppresses both, proliferation of T cells and viral replication as indicated by significantly reduced p24 levels in coculture supernatants. The observed effects are enhanced by IL-21, and reduced by GrB inhibition. In summary, we demonstrate that HIV infection induces IL-21 in CD4⁺ T cells, thereby indirectly triggering the development of GrB-secreting B cells with antiretroviral properties. GrB-secreting B cells may play a so far unappreciated role in decelerating HIV expansion, particularly in the early phase of infection. On the other hand, induction of GrB in HIV-specific B cells may interfere with their terminal differentiation into plasma cells, which may explain the lack of an efficient anti-HIV humoral immune response in HIV-infected patients.