A novel paired immunoglobulin-like receptor (PIR) gene family in mice is the focus of these studies. Sequences of the prototypic full length cDNA clones, PIR-A and PIR-B, predict type I transmembrane proteins of 75 and 87 kD with similar ectodomains, but distinctive transmembrane and cytoplasmic regions. The predicted PIR-A protein has a relatively short cytoplasmic tail and a charged Arg residue in the transmembrane region, suggesting association with an additional transmembrane protein(s) to form a signal transducing unit. In contrast, the PIR-B protein has a nonpolar transmembrane region and a relatively long cytoplasmic tail with two consensus immunoreceptor tyrosine-based inhibitory motifs (ITIM). The predicted peptide sequences are invariant in five consecutive PIR-B cDNA clones, but differ for all seven PIR-A type cDNA clones. PIR-A and PIR-B expression appears to be restricted to B lymphocytes and myeloid cells, wherein both genes are expressed simultaneously. The present studies are designed to test the hypothesis that the PIR-A and PIR-B cDNAs encode receptor proteins with related ligand-binding specificity, but different intracellular signaling properties so that the PIR-A molecular complex may have activating potential, while PIR-B may have inhibitory potential via its ITIM-like motifs. The first experiments are designed to produce monoclonal antibodies against common and distinctive epitopes on the PIR-A and PIR- B molecules and to use these antibodies to determine their cellular distribution and relative expression levels, their structure and unit composition, and the effects of PIR-crosslinkage on B cell and macrophage responses. The second experiments will examine the functional potential of the ITIM-like motifs in the PIR-B molecules. The third set of experiments is designed to isolate and sequence genomic PIR-A and PIR-B clones (i) to determine the genetic basis for the sequence diversity of the PIR-A genes and (ii) to disrupt the single copy PIR-B gene in embryonic stem cells to generate PIR-B knockout mice with unbalanced PIR-A activity. The fourth set of experiments will examine PIR-A transcripts in representative macrophage and B cell lines to determine whether they express single or multiple receptor members. The final experiments are designed to identify the ligand(s) for these PIR molecules. These studies may reveal an important regulatory role for the PIR-A and PIR-B receptors in humoral, inflammatory and allergic responses.