Characterization of novel costimulatory molecules. A protien of 38-42 kDa from B cell surface is concerned with T cell activation and differentiation

Optimal activation of T cells often requires signals delivered by the ligation of T cell receptor (TcR) and those resulting from costimulatory interaction between certain T cell surface accessory molecules and their respective counter receptors on antigen presenting cells. The molecular events underlying the co-stimulatory activity are still not understood fully. Here we describe a 38-42-kDa (B3) protein, present on the surface of lipopolysaccharide-activated B cells, which can provide costimulation to resting T cells leading to a predominant release of interleukin (IL)-4 and IL-5 and negligible amounts of IL-2 and interferon-γ . Binding assay and electron microscopic autoradiography data suggest that this molecule binds T cells, and the same can be competed by unlabeled B3. Characterization experiments point out that B3 shows up as a single prominent peak on reverse phase-high performance liquid chromatography, runs as a single spot in reducing two-dimensional gel electrophoresis, and is a phosphoglycoprotein. The Western analysis indicate that it does not cross-react with antibodies directed against murine ICAM-1, LFA-1α , VCAM-1, HSA, and B7 suggesting the novelty of the protein. The internal amino acid sequence of this molecule suggests that it does not belong to a known category of murine B cell surface molecules