The murine B7-1 (mB7-1) molecule expressed on APCs delivers a costimulatory signal for T cell activation through its T cell counter-receptor CD28, resulting in T cell proliferation and IL-2 production. Signaling through the TCR in the absence of CD28 signaling results in T cell anergy. We have analyzed the genomic structure of mB7-1 and here describe the identification of a previously unrecognized sixth exon at the far 3' end of the locus, which encodes an alternative cytoplasmic domain. Reverse transcriptase-PCR amplification of mB7-1 transcripts demonstrates that exon 6 is functionally spliced to the transmembrane-encoding exon 4. Furthermore, using 5' rapid amplification of cDNA ends, we determined that the 5'-untranslated region extends over 1505 bp beyond the previously reported transcriptional start site. In addition, we report the chromosomal location of mB7-1 to chromosome 16, band B5.

To analyze the functions of T cell costimulators in vivo, we have constructed a transgenic mouse strain that constitutively expresses murine B7-1 on mature B cells. Antibody responses to T-dependent hapten-protein conjugates and serum immunoglobulin levels are markedly depressed in B7-1 transgenic mice. This immune deficiency is not due to an intrinsic B cell defect, as antibody responses to T-independent hapten conjugates are normal. Furthermore, treatment with anti-B7-1 restores the capacity of transgenic mice to respond to hapten-protein conjugates, demonstrating that the deficient antibody responses are directly attributable to the expression of B7-1. These results suggest that the temporally regulated expression of costimulators such as B7-1 may contribute to either initiation or down-regulation (feedback inhibition) of T-dependent immune responses in vivo, and that the inhibitory function is dominant in transgenic mice that constitutively express high levels of this costimulator.