Sulphatide-binding properties are shared by serum amyloid P component and a polyreactive germ-line IgM autoantibody, the TH3 idiotype.

Serum amyloid P component (SAP) concentration was elevated in sera from leprosy patients, significantly so above endemic controls in lepromatous cases. In the sera of lepromatous leprosy (LL) patients who experienced an erythema nodosum leprosum (ENL) episode the SAP fell at the onset of ENL and remained low throughout, in two of three cases. Changes in SAP concentration parallel anti-sulphatide IgM concentrations. TH3, a monoclonal IgM germ-line antibody derived from a LL patient, and SAP share similar binding patterns. In this study we demonstrate binding to heparin and sulphatide. Moreover, SAP inhibited the binding of TH3 to sulphatide, as well as anti-sulphatide IgM found in a range of sera, and anti-sulphatide IgG in the only sera sample in which it was found. The observation that anti-TH3 idiotype monoclonal and polyclonal anti-SAP antibodies both inhibited the binding of TH3 and IgM in sera (but not IgG) to sulphatide without binding to sulphatide themselves further demonstrated similar binding specificities. The observations of similarity in binding reinforce ideas that SAP may function as a primitive opsonin, but the clear ability to inhibit binding of autoantibodies suggests that SAP may play a role in ameliorating tissue and particularly nerve damage in leprosy patients.