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 accessory molecules capable of modulating T cell activation, Here we report 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 organization as the extracellular region of CD5 and CD6 and is composed of three SRCR (scavenger receptor cysteine rich) domains, Chromosomal mapping by fluorescence in situ hybridization and radiation hybrid panel analysis indicated that the gene encoding Sp alpha is located on the long arm of human chromosome 1 at q21-q23 within contig WC1.17. RNA transcripts encoding Sp alpha were found in human bone marrow, spleen, lymph 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 monocytes 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 to the T cell line HUT-78. These findings indicate that Sp alpha, a novel secreted protein produced in lymphoid tissues, may regulate monocyte activation, function, and/or survival.