Antibody-independent activation of the classical complement pathway by cytomegalovirus-infected fibroblasts

Human fibroblasts weakly activated the alternative complement pathway, as a ssessed by C3b deposition, while 4- to 5-fold more C3b was observed 4 days after infection on cytomegalovirus (CMV)-infected fibroblasts when incubate d with human serum. CMV-infected fibroblasts activated via the classical co mplement pathway independent of specific anti-CMV antibody and incubation o f CMV-infected fibroblasts with serum deficient in complement components re vealed that Clq, but not mannan-binding lectin, was required for complement activation. The enhanced complement activation by CMV-infected cells was o bserved as early as 4 h after infection and required the active transcripti on of CMV genes. No difference in the complement activation by CMV-infected cells was observed with the use of CMV-seropositive or -seronegative serum as a complement source, suggesting that CMV infection induces or up-regula tes a protein that binds directly to Clq in a complement-activating conform ation.