Follicular associated T cells and their B-cell helper qualities.

The immune system utilizes sophisticated cellular surveillance mechanisms to maintain the integrity of the multicellular host. Adaptive immunosurveillance in particular constitutes a powerful branch of the immune system that houses the capacity to mount exquisitely specific responses against a diverse array of foreign antigens. Central to the development of adaptive immunity is the activation of T and B cells. Upon antigen engagement, T and B cells have been observed to undergo striking changes in their migratory status and distribution within secondary lymphoid organs, a phenomenon that is to a large extent controlled through their altered responsiveness to homeostatic T- and B-zone chemokines. Changes in their chemokine receptor expression and/or sensitivity to their respective ligands assist in bringing rare antigen-specific T and B lymphocytes, dendritic cells and CD4+CD3(-) accessory cells together. Cognate interaction between these cells at the T-B junction can support the generation of extrafollicular foci of antibody producing plasma cells and the formation of germinal centers. Such T-dependent antibody responses are highly dependent on the functional properties and activity of a specialized subset of CXCR5+ICOS+ CD4 T cells referred to as T follicular helper cells (T FH). This review presents an overview of some of the defining characteristics of this subset of T-helper cells and the chemokine receptors and their ligands that help dictate the migratory activity of T(FH) cells within secondary lymphoid organs.