EARLY SIGNALS IN ALLOANTIGEN INDUCED B-CELL PROLIFERATION - COMPARISON BETWEEN B-CELL TRIGGERING BY INTACT ALLOGENEIC CELLS AND SOLUBILIZEDALLOANTIGEN

Stimulation of B cells from BALB/c with allogeneic lymphocytes from C5 7BL/6 mice resulted in a slight increase in cytosolic Ca2+ but in the absence of proliferative response. Immunization of BALB/c mice with C5 7BL/6 total lymphocytes resulted in an enhancement of cytosolic Ca2+ a nd of B cell proliferation. Phosphatidylinositol specific phospholipas e C was activated immediately after allogeneic stimulation as deduced by the concomitant rise in inositol 1,4,5-trisphosphate and 1,2-diacyl glycerol. Translocation of protein kinase C from the cytosol toward th e membranes paralleled the elevation in cytosolic free Ca2+. Activatio n of BALB/c B cells with solubilized alloantigen from the plasma membr ane of C57BL/6 lymphocytes produced qualitatively the same early respo nses as the treatment with allogeneic cells, although quantitatively m ore intense. Concerning protein kinase C, an important degradation was observed in these conditions. Soluble alloantigen failed to promote B cell proliferation, being observed when cells were costimulated with a low concentrations (2 ng/ml) of phorbol 12,13-dibutyrate before allo antigen addition. Analysis of the molecular weight of the active fract ion of the solubilized alloantigen revealed the presence of a 51 kDa p rotein that mimicked all properties of the alloantigen preparation. Th is molecule was also recognized in Western blot by an anticlass I mAb and by the sera of immunized animals. A putative MHC class I antigen i s proposed as the nature of the active molecule, and its interaction w ith specific membrane Ig on the B cell is analyzed. Although the resul ts fit with a cellular response mediated through membrane Ig, the invo lvement of other B cell surface molecules interacting with the alloant igens cannot be disregarded.