We have explored the consequences for the B cell of cognate interaction with T cells. Early expression of the B cell-restricted cell surface activation antigen, BLAST-2, has been used as an assay system to measure direct T-B cell collaboration. BLAST-2 is preferentially expressed by allogenic B cells cultured with MHC class II antigen-restricted Th clone cells matched to the DR specificity of the target B cells. B cells cultured with DR-mismatched allospecific Th cells express minimal BLAST-2. Th cell-induced BLAST-2 expression appears to be accessory cell independent and occurs as early as 8 h after initiation of culture, with peak expression at 18 h. Direct T-B cell contact, rather than Th-derived lymphokines, provides the most efficient stimulus for BLAST-2 expression. Crosslinking of sIg on B cells is a poor stimulus for BLAST-2 expression. The BLAST-2 assay permits the evaluation of early events associated with B cell activation through cognate interactions, and may facilitate subsequent studies of the mechanism of B cell differentiation.
We used a cloned, TNP-specific, MHC-restricted, human Th cell line, E-11, and an assay of cognate Th-B cell interaction, BLAST-2 antigen expression on the B cell surface, to investigate the functional nature of the Th cell antigen receptor. We observed that E-11 induces BLAST-2 expression by resting B cells in a hapten-dependent, hapten-specific, but MHC nonrestricted manner. The implication of these results for the Th cell receptor are discussed.