Recombinant Human CD5L Protein, CF

CD5L (CD5 antigen-like), also known as Sp alpha and AIM, is a 50 kDa secreted glycoprotein that belongs to the SRCR (scavenger receptor cysteine rich) group B family of proteins. Group B proteins are distinguished by SRCR domains that are encoded by a single exon (1 - 3). The human CD5L cDNA encodes a 347 amino acid (aa) precursor that includes a 19 aa signal sequence and three SRCR domains (4, 5). Among group B proteins, CD5L is most closely related to CD5 and CD6, with which it shares 18% and 31% aa sequence identity, respectively. CD5L is up-regulated in macrophages at inflammatory sites. It sustains inflammatory reactions by both increasing the phagocytic capacity of macrophages and impeding the apoptosis of local macrophages, NK cells, and T cells (6, 7). Agonists of the LXR and RXR nuclear hormone receptors induce CD5L upregulation in macrophages and reduce macrophage apoptosis (8, 9). Oxidized LDL (which acts through LXR/RXR) is taken up by macrophages, promoting their development into foam cells. The increased level of CD5L protects foam cells from apoptosis but permits more rapid cellular accumulation and atherosclerotic plaque formation (9). In activated B cells, however, the combination of CD5L and TGF-beta inhibits proliferation. The binding of CD5L to splenic B cells is increased following TGF-beta exposure, suggesting that TGF-beta increases the expression or availability of an unidentified CD5L receptor (5, 10). CD5L also functions as a pattern recognition molecule by binding both lipoteichoic acid on Gram positive and lipopolysaccharide on Gram negative bacteria (11). In the thymic cortex, CD5L protects cortical CD4⁺CD8⁺ thymocytes from apoptosis (12). CD5L circulates in the serum in complex with IgM (13).