T Follicular Helper (TFH) Cells in Normal and Dysregulated Immune Responses

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Abstract
T cell help for antibody production is a fundamental aspect of immune responses. Only recently has a better understanding of the cellular and molecular mechanisms for T cell help emerged. A subset of T cells, termed T follicular helper cells (TFHcells), provides a helper function to B cells and represents one of the most numerous and important subsets of effector T cells in lymphoid tissues. TFHcells are distinguishable from Th1 and Th2 cells by several criteria, including chemokine receptor expression (CXCR5), location/migration (B cell follicles), and function (B cell help). Central to the function of CD4+T cells is IL-21, a “helper” cytokine produced by TFHcells that potently stimulates the differentiation of B cells into Ab-forming cells through IL-21R. Consequently, dysregulation of TFHcell function, and over- or under-expression of TFHcell–associated molecules such as ICOS or IL-21, most likely contributes to the pathogenesis of certain autoimmune diseases or immunodeficiencies.