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Retroviruses can activate the complement system in the absence of antibodies, and the purpose of this study was to examine whether the serum collectin, mannan-binding protein (MBP), could mediate such complement activation.Virus envelope proteins gp120 and gp110 from HIV-1 and HIV-2 were incubated in microtitre wells coated with anti-gp120 or anti-gp110 antibodies. After further incubation with serum, complement activation was measured as deposition of complement factor C4 and C3 onto the wells. Deposited C4 and C3 were detected with enzyme-labelled antibodies. Normal human serum depleted of endogenous lectins by affinity chromatography was used as the complement source. Serum from C1q-deficient patients was used in some experiments. Complement activation was then assessed with and without prior addition of MBP to the wells. Complement activation was also correlated with the quantity of endogenous MBP in a number of normal sera.Complement activation by HIV envelope glycoproteins was found to be mediated by the binding of MBP to carbohydrates on natural envelope protein produced in virus-infected cells, as well as on glycosylated recombinant envelope proteins produced in insect cells. Non-glycosylated recombinant envelope proteins produced in Escherichia coli did not induce this type of complement activation.Activation of the classical complement pathway by retrovirus envelope proteins can be initiated by the binding of MBP to carbohydrate side chains of envelope glycoproteins.