CHARACTERIZATION OF A NOVEL COSTIMULATORY MOLECULE - A 155-160 KD B-CELL SURFACE PROTEIN PROVIDES ACCESSORY HELP TO CD4(-CELLS TO PROLIFERATE AND DIFFERENTIATE() T)

Optimal activation of T cells to clonally expand requires at least two distinct biological signals; one is generated by the interaction of t he T cell receptor (TcR) with peptides bound to MHC molecules. The oth er signal(s) is (are) generated by a functionally defined event called the costimulatory pathway. We have characterized the co-stimulatory p roperty of a murine B lymphocyte membrane protein (155-160 kD) on rest ing CD4(+) T cells. The study involved the isolation of a 155-160 kD p rotein (B1) from the membranes of LPS-stimulated B cells. When reconst ituted into lipid vesicles, B1 exerted a dose-dependent proliferative response to CD4(+) T cells, resulting in the predominant secretion of IL-4 and IL-5 after cross-linking receptors with anti-CD3 mAb. This pr otein is a phosphoglycoprotein which gives a single spot on two-dimens ional gel electrophoresis under reducing conditions and as a distinct peak on reverse phase-HPL. The B1 binds to the T cell surface as is de monstrated by electron microscopic autoradiography and scanning electr on microscopy, as well as competitive binding assays. It does not cros s-react with antibodies directed against ICAM-1, LFA-1 alpha, B7, HSA and VCAM-1, suggesting the novelty of the protein. Activation of CD4() T cells with B1 in the presence of anti-CD3 resulted in the transloc ation of protein kinase C (PKC). The B1 is barely detectable on the su rface of resting B cells and digestion of this protein with V8 proteas e and peptide N-glycosidase F resulted in distinct protein bands on an autoradiogram.