Two separate functions of class II (Ia) molecules: T-cell stimulation and B-cell excitation.

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Abstract
The role of major histocompatibility complex-encoded class II (Ia) molecules as transmembrane signaling receptors was evaluated in the T helper cell-dependent activation of B lymphocytes. The murine B-cell lymphoma CH12, which expresses both I-A and I-E class II molecules was used. CH12 cells carry IgM of known antigen specificity and require both specific antigen and Ia-restricted T-cell help for the induction of antibody secretion. In this respect, they resemble normal resting B cells. The ability of antigen-specific or alloreactive T helper cells reactive with either the I-A or the I-E molecules on CH12 to be activated and their ability to stimulate antibody production by CH12 was studied. Although CH12 cells present antigen to T helper cells that interact with either the I-A or the I-E molecules, CH12 cells are stimulated to secrete antibody only by T helper cells reactive with their I-E molecules. Class II molecules are transducers of signals for B-cell excitation in addition to serving a restricting function for helper T-cell stimulation. These 2 functions, T-cell stimulation and B-cell excitation, are discrete and need not be expressed by the same Ia molecule.

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