IL-21 and IL-4 Collaborate To Shape T-Dependent Antibody Responses.

The selection of affinity-matured Ab-producing B cells is supported by interactions with T follicular helper (Tfh) cells. In addition to cell surface–expressed molecules, cytokines produced by Tfh cells, such as IL-21 and IL-4, provide B cell helper signals. In this study, we analyze how the fitness of Th cells can influence Ab responses. To do this, we used a model in which IL-21R–sufficient (wild-type [WT]) and –deficient ( Il21r −/− ) Ag-specific Tfh cells were used to help immunodeficient Il21r −/− B cells following T-dependent immunization. Il21r −/− B cells that had received help from WT Tfh cells, but not from Il21r −/− Tfh cells, generated affinity-matured Ab upon recall immunization. This effect was dependent on IL-4 produced in the primary response and associated with an increased fraction of memory B cells. Il21r −/− Tfh cells were distinguished from WT Tfh cells by a decreased frequency, reduced conjugate formation with B cells, increased expression of programmed cell death 1, and reduced production of IL-4. IL-21 also influenced responsiveness to IL-4 because expression of both membrane IL-4R and the IL-4–neutralizing soluble (s)IL-4R were reduced in Il21r −/− mice. Furthermore, the concentration of sIL-4R was found to correlate inversely with the amount of IgE in sera, such that the highest IgE levels were observed in Il21r −/− mice with the least sIL-4R. Taken together, these findings underscore the important collaboration between IL-4 and IL-21 in shaping T-dependent Ab responses.