Different stimulating effects of complement C3b and complete Freund's adjuvant on antibody response

Upon activation, complement C3 undergoes a conformational change and acquir es the capacity to covalently bind to other proteins such as antigen and to interact with specific receptors; therefore, C3 is involved in cell mediat ed immune response. The adjuvant effect produced by linking C3-fragments to antigen has recently been described. We injected C3b-Ag complexes consisti ng of one molecule of C3b ester linked to one molecule of HEL to immunised mice, and we compared the C3b adjuvant activity with that of complete Freun d's adjuvant. IgG titers elicited by HEL emulsified in CFA (HEL + CFA) were higher than those elicited by HEL-C3b, but decreased rapidly after a peak response around day 45 whereas HEL-C3b resulted in a continuous increase of anti-MEL response. Mice immunised with HEL + CFA then boosted with HEL-C3b gave significantly higher response than those boosted with HEL + CFA, indi cating more efficient memory cell restimulation by C3b. HEL + CFA leads to better priming than HEL-C3b when mice are boosted with HEL-C3b. Thus, adjuv ant effect of C3b is different from that of CFA, leading to more stable IgG production and better memory stimulation. (C) 1999 Elsevier Science B.V. A ll rights reserved.