Cytokines, soluble factors secreted by T cells and other immunocytes, play major roles at different stages of the mucosal immune response. Cytokines are involved in communication between cells of the immune system and are critical in determining both the type and the magnitude of immune responses. Whether the immune response is driven toward humoral or cell-mediated immunity is determined, in part, by the profile of cytokines produced by CD4+ T helper cell subpopulations. Cytokines also influence the development of different T cell subpopulations. For example, the Th2-derived factors IL-4 and IL-10 inhibit the development of Th1 cells in vitro, while IFNγ produced by Th1 cells suppresses the Th2 response. The range of potential immune responses to an infectious agent may therefore be tightly regulated by cytokines produced by T cells. Cytokine production at mucosae is apparently biased toward Th2 responses, which may be an important factor in the predominance of IgA antibodies in these tissues. This chapter reviews evidence in support of this concept from in vitro and in vivo studies and describes how the recent work with mice rendered deficient for Th2-type cytokines sheds further light on the role of these factors in mucosal immunoregulation. Some implications of this work for improved mucosal vaccination strategies are also discussed.
