Molecular cloning, genomic organization and cell‐binding characteristicsof mouse Spα

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Abstract
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 Spα (hSpα). Herein we report the cloning and characterization of the mouse homologue of hSpα. Like its human counterpart, mouse Spα (mSpα), is a secreted protein containing three SRCR domains. Most lymphoid tissues express RNA transcripts encoding mSpα. Characterization of a genomic clone encoding the mature mSpα protein showed that each of the SRCR domains of mSpα is encoded by a single exon. Comparison of the sequence of mSPα 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 mSpα immunoglobulin (mSpα-Rγ) fusion protein indicated that mSpα 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 mSpα-Rγ; however, no binding was observed to either peripheral blood mononuclear cells or peritoneal cavity-derived CD3+ T cells. The mSpα-Rγ 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 mSpα and the identification of cells expressing a putative mSpα receptor(s) should facilitate in vivo studies designed to investigate the function of Spα in the immune compartment.