MOLECULAR-CLONING, MAPPING TO HUMAN-CHROMOSOME-1 Q21-Q23, AND CELL-BINDING CHARACTERISTICS OF SP-ALPHA, A NEW MEMBER OF THE SCAVENGER-RECEPTOR-CYSTEINE-RICH (SRCR) FAMILY OF PROTEINS

CD5 and CD6, two type I cell surface antigens predominantly expressed by T cells and a subset of B cells, have been shown to function as acc essory molecules capable of modulating T cell activation, Here we repo rt the cloning of a cDNA encoding Sp alpha, a secreted protein that is highly homologous to CD5 and CD6, Sp alpha has the same domain organi zation as the extracellular region of CD5 and CD6 and is composed of t hree SRCR (scavenger receptor cysteine rich) domains, Chromosomal mapp ing by fluorescence in situ hybridization and radiation hybrid panel a nalysis indicated that the gene encoding Sp alpha is located on the lo ng arm of human chromosome 1 at q21-q23 within contig WC1.17. RNA tran scripts encoding Sp alpha were found in human bone marrow, spleen, lym ph node, thymus, and fetal liver but not in non-lymphoid tissues, Cell binding studies with an Sp alpha immunoglobulin (Sp alpha-mIg) fusion protein indicated that Sp alpha is capable of binding to peripheral m onocytes but not to T or B cells. Sp alpha-mig was also found to bind to the monocyte precursor cell lines K-562 and weakly to THP-1 but not to U937, Sp alpha-mig also bound to the B cell line Raji and weakly t o the T cell line HUT-78. These findings indicate that Sp alpha, a nov el secreted protein produced in lymphoid tissues, may regulate monocyt e activation, function, and/or survival.