Coxsackievirus and adenovirus receptor (CAR) binds immunoglobulins

The coxsackievirus and adenovirus receptor protein (CAR) serves as the cell surface receptor for group B coxsackieviruses and most adenoviruses, but t he physiological function and ligand for this protein remain to be describe d. An affinity column was constructed with the recombinant extracellular do main of the CAR (rECAR) to isolate potential ligands by affinity chromatogr aphy. Immunoglobulins G and M were consistently isolated from human sera pa ssed through the column, suggesting that the CAR may be an immunoglobulin-b inding protein. Further investigation revealed that the affinity-purified i mmunoglobulins bound to rECAR-coated immunoassay plates, and the peroxidase -labeled rECAR bound the immunoglobulins on ligand-overlay blots. The perox idase-labeled rECAR was incorporated into immunoprecipitates formed between the affinity-purified immunoglobulins and rabbit antibodies against human immunoglobulins, but not into immunoprecipitates formed between mouse IgG a nd rabbit antibodies against mouse IgG. The CAR present in HeLa cell lysate s also bound to the affinity-purified immunoglobulins on Immobilon membrane s, showing that the association is not limited to the recombinant protein. These results demonstrate that the CAR binds IgG and IgM present in serum, and reveal a direct interaction between the coxsackievirus and adenovirus r eceptor and the immune system.