The aim of this project is to define at both the cellular and molecular level, the signalling events involved in immunoglobulin Ig heavy chain (CH) class switching. In previous studies we showed that Con A-activated T cells clones derives from the murine Peyer's patch could induce sIgM+ B cells to differentiate into sIgA + B cells. During this period we extended this observation to humans by preparing PHA-activated T cell clones derived from human appendiceal cells. In these studies we showed that appendiceal T cell clones could induce IgA synthesis in sIgM+,sIgA- and sIgM-, sIgA + B cells isolated by various techniques. In contrast, peripheral T cell clones could induce sIgM-, A+B cells but not sIgM+,A- B cells. Thus, switch T cells were present in human mucosal follicles but not peripheral blood. In other studies we determined the capacity of various T cell- derived lymphokines, particularly IL-4 and IL-5, to act as switch or post-switch IgA B cell differentiation factors. In these studies we prepared populations of sIgM+, sIgA- and sIgM-, sIgA + populations from the murine Peyer's patch and found that IL-4 and IL-5 containing supernatant derived from cultures of the EL-4 and D-10 T cell lines supported IgA synthesis in LPS-stimulated sIgM- sIgA+ B cells but not in sIgM+sIgA- B cell cultures. Recombinant IL-4 alone had little or no effect on IgA synthesis in any cell populations, whereas purified IL-5 had the same effect as the unpurified supernatant. These studies indicate that IL-5 acts as a post-switch IgA B cell differentiation factor. In a final series of studies we studied the effects of T cell-derived lymphokines of a B cell clone termed CH12.LX which is capable of undergoing terminal differentiation into an IgA-producing cell. CH12.LX B cells secreted large amounts of IgM and small amounts of IgA spontaneously in culture; LPS increased the IgA synthesis to some degree whereas IL-5 (purified) greatly increased IgA synthesis a while causing a decrease in IgM synthesis. Thus IL-5 can act either to preferentially increase proliferation of spontaneously arising IgA B cells or else act on pre-switched IgM B cells to increase their conversion to IgA cells.