The requirement for surface Ig signaling as a prerequisite for T cell:B cell interactions. A possible role for desialylation

Models for T cell:B cell collaboration suggest that activated B cells process and present Ag to Th cells which subsequently induce B cell proliferation and differentiation. In contrast to activated B cells, resting B cells have generally been shown to be less efficient APC. If this model of T:B collaboration is physiologically correct, then resting B cells must undergo a phenotypic change that permits effective interaction with T cells. In this report, the requirement for rapid signaling through surface Ig on resting B cells for the induction of T:B interaction was investigated with an in vitro clustering assay. Resting splenic B cells were unable to form specific conjugates with T cell clones, unless the B cells were first treated with neuraminidase to remove sialic acid. In contrast, LPS-activated B cells were able to form conjugates without prior treatment. The ability of antibody against LFA-1 or L3T4 to inhibit cluster formation depended on the state of B cell activation in that anti-LFA-1 and anti-L3T4 mAb inhibited cluster formation by neuraminidase-treated resting B cells, but not by LPS-activated B cells. In addition, Ag-specific B cells which were isolated by their capacity to bind specific Ag were able to form clusters without any additional treatment. Moreover, treatment of resting splenic B cells with anti-mu-antibody induced clustering potential in B cells in as little as 10 min, suggesting that signaling through surface Ig was sufficient to induce this phenotypic change in B cells. Furthermore, activation of protein kinase C and Ca2+ mobilization were shown to be involved in that PMA and ionomycin treatment were also able to induce clustering potential in resting B cells. The rapid induction of clustering potential in resting B cells after signaling through surface Ig may represent a fundamental change in B cell physiology which occurs after recognition of specific Ag and may be required for effective cognate recognition between resting hapten-specific B cells and carrier-specific T cells. The potential role of desialylation for the induction of T:B interaction is discussed.