Molecular cloning, genomic organization and cell-binding characteristics of mouse Spα

Several group B scavenger receptor cysteine-rich (SRCR) proteins have been shown to function as modulators in the immune response. Recently, we reported the cloning of a new member of this family, human Sp alpha (hSp alpha). Herein we report the cloning and characterization of the mouse homologue of hSpa. Like its human counterpart, mouse Sp alpha (mSp alpha), is a secreted protein containing three SRCR domains. Most lymphoid tissues express RNA transcripts encoding mSp alpha. Characterization of a genomic clone encoding the mature mSpa protein showed that each of the SRCR domains of mSpa is encoded by a single exon. Comparison of the sequence of mSP alpha with those of other published proteins indicates that it is the same as the recently reported protein named AIM (apoptosis inhibitor expressed by macrophages). Cell-binding studies with a mSp alpha immunoglobulin (mSp alpha-R gamma) fusion protein indicated that mSp alpha is capable of binding to spleen-derived CD19(+) B cells and minimally to peritoneal cavity-derived CD19(+) B cells but not to peripheral blood-derived B cells. Spleen-derived CD3(+) T cells also bound mSp alpha-R gamma; however, no binding was observed to either peripheral blood mononuclear cells or peritoneal cavity-derived CD3+ T cells. The mSp alpha-R gamma fusion protein was also shown to bind to the mouse cell lines WEHI3 (monocytic) and EL-4 (thymoma, T cell). The cloning of cDNA and genomic clones encoding mSp alpha and the identification of cells expressing a putative mSp alpha receptor(s) should facilitate in vivo studies designed to investigate the function of Sp alpha in the immune compartment.