Abstract
IL-4 is known to be the quintessential regulatory cytokine, playing a role in a vast number of immune and non-immune functions. This cytokine is commonly secreted by type 2 helper T (TH2) cells and follicular helper T (TFH) cells after antigenic sensitization. TH2 cells have been classically thought to be the major contributor to B-cell help as a source of IL-4 responsible for class-switch recombination to IgG1 in mice (IgG4 in humans) and to IgE in mice and humans. Recent in vivo observations have shown that IgE and IgG1 antibody responses are mainly controlled by IL-4-secreting TFH cells but not by classical TH2 cells. IL-4 is distinctively regulated in these two T-cell subsets by the GATA-3-mediated HS2 enhancer in TH2 cells and the Notch-mediated conserved non-coding sequence 2 (CNS-2) enhancer in TFH cells. Moreover, the IL-4 derived from TFH cells has an essential role in germinal center (GC) formation in the secondary lymphoid organs during humoral immune responses.
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