Background:  The basis for deficient antibody production in HIV infection is not well understood. Generation of antibody-secreting plasma cells critically depends upon CD4 T follicular helper (TFH) cells and their secreted cytokine interleukin-21 (IL-21) during the germinal center (GC) reaction. Peripheral TFH constitute approximately 10 to 15% of CD4 T cells, express CXCR5, and share functional properties with GC TFH. We reported that a successful H1N1/09 vaccine response in HIV-infected patients and healthy donors (HD) is associated with an expansion of memory B cells with up-regulated IL-21R and increase in plasma IL-21. Here we investigated frequency and function of TFH in HIV infection.

Methods:  Study population consisted of 25 HIV-infected, ART-treated individuals and 17 HD. From this group, 16 patients and 8 HD described above received a single dose of H1N1/09 influenza vaccine. TFH were determined in fresh peripheral blood mononuclear cells by multiparameter flow cytometry and IL-21 was determined by ELISA. Differences between groups were analyzed by Student t-test or the 2-sample Wilcoxon rank-sum (Mann-Whitney) test.

Results:  Frequencies of peripheral TFH were equivalent in 25 patients and 17 HD and displayed a central memory phenotype. CXCR5, inducible co-stimulator (ICOS), and induced IL-21 expression in CD4 T cells correlated with each other. In vaccinees, serum H1N1 antibody titers >1:40 were evident in only 50% of patients (n = 8) and in all HD at week 4 post-vaccination (T2). Vaccine responder and non-responder patients were equivalent in mean age, plasma HIV RNA, CD4 and CD8 T cell counts, and CD20⁺ B cells. In vaccine responders, circulating TFH cells underwent a significant expansion at T2 compared to baseline (T1) in patients (p = 0.003) and HD (p = 0.001) with increase in Ki67⁺ cells, concomitantly with increase in H1N1 antigen-induced IL-21 in culture supernatants and intracytoplasmic IL-21 in CD4⁺ T cells that persisted at week 24. These changes were not seen in vaccine non-responders. At T2, TFH correlated with frequency of memory B cells and H1N1 antibody titers.

Conclusions:  Failure of expansion of TFH in the context of H1N1/09 vaccine responses is a novel immune deficiency that persists in a subset of otherwise stable HIV-infected patients. The CD4 TFH cells regulate interaction of CD4-derived IL-21 and IL-21R on B cells and promote expansion of memory B cells. These findings have important implications for vaccine design.