THE ROLE OF THE B7-1A MOLECULE, AN ALTERNATIVELY SPLICED FORM OF MURINE B7-1 (CD80), ON T-CELL ACTIVATION

B7 molecules (CD80 (B7-1) and CD86 (B7-2/B70)) on APCs provide costimu latory signals for T cell proliferation. We previously described the p resence of an alternatively spliced form of murine CD80 (previously te rmed MB7-2 and renamed as B7-1a) that completely lacks the second Ig-l ike domain coded by exon 3 in activated murine B cells. In this study, we first examined whether B7-1a mRNA can be detected in vivo by RNase protection assay. The expression of B7-1a mRNA was only detected in l ymphoid organs although the level of expression was lesser than that o f CD80 mRNA. However, we demonstrated that the expression of B7-1a mRN A like CD80 mRNA was considerably augmented in spleen cells treated wi th either LPS in vitro or OVA/CFA conjugate in vivo. We next determine d the functional activity of B7-1a using Chinese hamster ovary (CHO) c ells transfected by B7 genes. When resting T cells were cocultured wit h CHO cells expressing B7-1a molecules in the presence of PMA/ionomyci n, T cell proliferation was not detected, while CHO cells either expre ssing CD80 or CD86 could promote the proliferation of resting T cells. In contrast to resting T cells, CHO cells expressing B7-1a could supp ort the proliferation of activated T cells. Thus, costimulatory activi ty of B7-1a molecules was dependent upon the activation stage of T cel ls. Therefore the IgV-like region of CD80 contains a critical region f or functional interaction with its ligands and can transduce a costimu latory signal for T cell proliferation.