Accessory molecule regulation of naive CD4 T cell activation.

Naive CD4 T cell activation is a complex process involving many steps. T cell receptor (TCR) signals, provided by interaction with peptide/MHC on antigen-presenting cells (APC), control many events associated with activation. The extent of TCR signaling and the magnitude of the T cell response is in turn controlled by accessory molecules on APC, which stabilize T-APC interactions. Full T cell activation additionally requires multiple costimulatory signals, generated upon ligation of T cell coreceptors by accessory molecules, and these lead to IL-2 production, proliferation and differentiation of the naive cell into an effector state. This review summarizes the role played by accessory molecules in naive CD4 activation and discusses how integration of signals from these molecules, with signals from the TCR, may determine the outcome of T-APC interaction. The available data provide explanations for why only APC which express high levels of multiple costimulatory/adhesion molecules, such as dendritic cells and activated B cells, induce efficient naive T cell responses, and suggest that ICAM-1/LFA-1 and B7/CD28 interactions are major pathways used to initiate naive T cell activation.