Role of Complement Receptor Type 2 and Endogenous Complement in the Humoral Immune Response to Conjugates of Complement C3d and Pneumococcal Serotype 14 Capsular Polysaccharide

Abstract

Conjugation of the complement fragment C3d to both T-cell-dependent (TD) protein and T-cell-independent type 2 (TI-2) polysaccharide antigens enhances the humoral immune response in mice immunized with either type of antigen. However, the ability of C3d-protein conjugates to enhance the antibody response in mice deficient in complement receptor types 1 and 2 (CR1 and CR2) has raised questions about the role of C3d-CR2 interactions in the adjuvant effect of C3d. In this study, we examined the role of CR2 binding and endogenous complement activation in the antibody response to conjugates of C3d and serotype 14 pneumococcal capsular polysaccharide (PPS14). To block binding of PPS14-C3d conjugates to CR2, mice were immunized with a mixture of vaccine and (CR2) 2 -immunoglobulin G1 (IgG1). Mice receiving (CR2) 2 -IgG1 at the time of primary immunization had a marked reduction in the primary anti-PPS14 antibody response but an enhanced secondary anti-PPS14 response, suggesting that C3d-CR2 interactions are required for the primary response but can have negative effects on the memory response. Further, compared with mice receiving PPS14-C3d having a high C3d/PPS14 ratio, mice immunized with PPS14-C3d with low C3d/PPS14 ratios had an enhanced secondary antibody response. Treatment of mice with cobra venom factor to deplete complement had insignificant effects on the antibody response to PPS14-C3d. Experiments with CBA/N xid mice confirmed that PPS14-C3d conjugates retain the characteristics of TI-2 rather than TD antigens. Thus, the adjuvant effect of C3d conjugated to PPS14 requires C3d-CR2 interactions, does not require activation of endogenous complement, and is not mediated by TD carrier effects.

C3d adjuvant effects are mediated through the activation of C3d‐specific autoreactive T cells

“…Adjuvants consisting of a polysaccharide antigen covalently linked to either a foreign carrier protein or a complement component C3d often enhance the generation of a primary humoral response 1 , 2 , 3 . The prevailing model suggests the adjuvant effect of C3d is due to B‐cell activation upon cross‐linking of the antigen‐recognition molecules (Ig) and complement‐binding molecules (CD21), resulting in a lowered activation threshold and more robust immune response 4 , 15 .…”

“…Two methods of adjuvant conjugation to antigen have been shown to greatly improve the efficacy of polysaccharide vaccines: use of an immunogenic foreign T‐cell epitope‐rich protein, and use of complement fragment C3d 1 , 2 , 3 . The adjuvant effect of C3d is said to result from the synergistic effect of two events: (1) polysaccharide antigen binding to the IgM/D B‐cell receptor, and (2) C3d binding to the B‐cell complement receptor 2 (CR2 or CD21).…”

C3d adjuvant activity is reduced by altering residues involved in the electronegative binding of C3d to CR2

“…C3d is an effective molecular adjuvant [11,13–18,21,22,30–35]. To determine if C3d residues 162, 163 and, 170 affect the adjuvant properties of C3d, mice were vaccinated with DNA plasmids expressing Env gp120 fused to either the wild-type C3d or the mutant C3d proteins.…”

C3d Binding to the Myelin Oligodendrocyte Glycoprotein Results in an Exacerbated Experimental Autoimmune Encephalomyelitis

“…CD21 is also involved in C3d-opsonized Ag uptake and retention by FDC in splenic follicles for B cell selection and propagation (21,22). Mice deficient for Cr2 (encoding both CD21 and CD35) or C3 have defects in mounting immune responses to T cell-dependent Ags and some T-independent Ags (23)(24)(25)(26)(27)(28). This attractive adjuvant property was further exploited in numerous DNA-based vaccine approaches using multimeric C3d.…”