A new C1q solid-phase assay for the detection of IgM immune complexes: application to the follow-up of kidney graft recipients with recurrent cytomegalovirus infection.

A C1q solid-phase enzyme assay for detecting circulating immune complexes (CIC) containing immunoglobulins M was developed. IgM immune complexes (IgM-CIC) bind to purified C1q-coated microplates. The amount of bound IgM-CIC is determined by adding anti-mu-alkaline phosphatase conjugate. This assay proved IgM-CIC specific; it does not detect IgG-CIC nor free anti-CMV IgM. Furthermore Rheumatoid Factor (RF) does not interfere with this test. Then, 119 renal allograft recipients were followed-up for the presence of seric IgM-CIC. Among them, 86 patients developed a recurrent Cytomegalovirus (CMV) infection and 33 did not. In the non-infected population, IgM-CIC were detected in only 15.2% of the patients. In contrast, patients with recurrent CMV infection showed a significantly higher frequency of detectable IgM-CIC (62.8%) (p less than 0.01). These IgM-CIC were detected: (a) during the first two or three weeks after graft; (b) in the course of the second month post graft. This second peak of IgM-CIC was never observed in subjects non-infected with CMV. IgM-CIC occurred before or at the same time as the detectable anti-CMV IgM and virus excretion in urines. Presence of IgM-CIC was not influenced by graft rejection episodes. Such a marker might help in discriminating immune response to viral infection from immune graft rejection.