Cognate T cell help for B cell activation

The CD40 ligand (CD40L) expressed by activated T cells is a 33 kDa type II membrane glycoprotein with sequence homology to tumor necrosis factor. T cells activated through TCR ligation transiently express on their surface CD40L, which interacts with CD40 expressed on B cells to deliver signals for B cell activation and proliferation. Signaling through CD40 in combination with T cell-derived cytokines such as IL-4 and IL-5 in mice leads to B cell differentiation accompanied by antibody secretion and Ig isotype switching. The CD40-CD40L interaction is also important for other B cell functions including the formation of germinal centers and the induction of memory B cells. The physiological significance of the CD40-CD40L interaction in vivo has become evident from the observation of patients with X-linked hyper-IgM syndrome, who have mutations in the gene encoding CD40L and are therefore unable to express functional CD40L on their T cells. These patients fail to produce Ig of isotypes other than IgM in response to antigen challenge and also lack germinal centers in their secondary lymphoid organs.

The cognate interaction of B and T cells triggers the engagement of many other accessory molecules on both cells, which serve to stabilize the B-T conjugation and to coordinate the responses of the interacting cells. They include B7-1 (CD80), B7-2 (CD86), CTLA-4, LFA-1 (CD1 la/18), LFA-2 (CD2), LFA-3 (CD58), ICAM-1 (CD54), VCAM (CD106) and VLA-4 (CD49d/29). The B7-1/B7-2 expressed on B cells interacts with the CD28 expressed on T cells to deliver a costimulatory signal to TCR-stimu-lated T cells, which then become fully activated to produce cytokines. T cell-derived cytokines such as IL-2, IL-4, IL-5, IL-6, IL-10, IL-13 and interferon y (IFNy) have been shown to enhance B cell activation and proliferation and to be essential costimuli for subsequent B cell differentiation and Ig production.

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