CLONING OF 2 MEMBERS OF THE SIRP-ALPHA FAMILY OF PROTEIN-TYROSINE-PHOSPHATASE BINDING-PROTEINS IN CATTLE THAT ARE EXPRESSED ON MONOCYTES AND A SUBPOPULATION OF DENDRITIC CELLS AND WHICH MEDIATE BINDING TO CD4 T-CELLS

Recent experimental studies have greatly clarified the function of cel l surface molecules in the induction and modulation of T cell response s by antigen-presenting cells (APC). However, the differences in abili ty to stimulate T cells evident for different types and subpopulations of the same APC, such as dendritic cell subsets, is less well underst ood. This report details an investigation of an antigen expressed on m onocytes that is also expressed on a subset of cattle afferent lymph v eiled cells (ALVC). A cDNA library derived from cattle monocytes was s creened with monoclonal antibodies (mAb) for expression in COS-7 cells . Using separate mAb for screening, two cDNA were cloned, the sequence s of which showed a single long open reading frame encoding a predicte d type I glycoprotein of 506 amino acids that contained three immunogl obulin superfamily domains and a long 112-amino acid cytoplasmic tail. We have termed this antigen MyD-1, reflecting its myeloid and dendrit ic cell distribution. Analysis of the EMBL database revealed that the molecule is a member of the recently described family of signal regula tory proteins (SIRP). The outeremost ig domain was of the adhesion/rec eptor I-type, suggesting that MyD-1 might bind to a ligand on another cell. Evidence for this was subsequently obtained by demonstrating tha t COS-7 cells transfected with MyD-1 cDNA bound CD4 T cells and this b inding was blocked by specific mAb. The potential importance of this i nteraction was supported by the finding that the proliferation of rest ing memory CD4 T cells to ovalbumin-pulsed monocytes was significantly reduced in the presence of mAb to MyD-1. A role for the molecule in t he modulation of the monocyte/dendritic APC response is also predicted from the existence of multiple potential tyrosine phosphorylation sit es in the cytoplasmic domain, including the presence of an immunorecep tor tyrosine-based inhibitory motif (ITIM) and the observation that th e SIRP alpha family members have been shown to bind to SHP-1 and SHP-2 . Together these data indicate a possible functional importance for My D-1 in the regulation of monocyte and dendritic cell function.