Cutting Edge: C3d Functions as a Molecular Adjuvant in the Absence of CD21/35 Expression

Abstract

Complement component C3 covalently attaches to Ags following activation, where the C3d cleavage fragment can function as a molecular adjuvant to augment humoral immune responses. C3d is proposed to exert its adjuvant-like activities by targeting Ags to the C3d receptor (CD21/35) expressed by B cells and follicular dendritic cells. To directly assess the importance of CD21/35 in mediating the immunostimulatory effects of C3d, CD21/35-deficient (CD21/35−/−) mice were immunized with streptavidin (SA), SA-C3dg tetramers, recombinant HIV gp120 (gp120), or gp120 fused with linear multimers of C3d. Remarkably, SA- and gp120-specific Ab responses were significantly augmented in CD21/35−/− mice when these Ags were complexed with C3d in comparison to Ag alone. In fact, primary and secondary Ab responses and Ab-forming cell responses of CD21/35−/− mice approached those of wild-type mice immunized with SA-C3dg and gp120-C3d. Thus, C3d can function as a molecular adjuvant in the absence of CD21/35 expression.

Identification and Characterization of the C3 Binding Domain of the Staphylococcus aureus Extracellular Fibrinogen-binding Protein (Efb)

“…CR2 is expressed on B cells, T cells, and follicular dendritic cells and is implicated in the regulation of B and T cell responses (74,75). If rEfb-bound C3d can block binding to CR1/CR2; it may also serve to negatively affect B cell, T cell, and/or antigen processing functions by diminishing the potent adjuvant properties of antigenlinked C3d (76). Although no similarity between the predicted rEfb structure and the ␤-sheet-rich Factor H was observed, we examined the possibility that rEfb may bind to the Factor H-binding site in C3d since Factor H also inhibits complement activity by binding to various domains in C3.…”

C3d Enhances Immune Responses Using Low Doses of DNA Expressing the HIV-1 Envelope from Codon-Optimized Gene Sequences

“…In order to enhance the immune responses elicited by a DNA vaccine, we used two effective approaches which, independently, enhance antibody titer and cellular responses against HIV-1 envelope glycoproteins; the use of C3d as a molecular adjuvant [5,16,19,21,24,31,32] and enhanced protein expression from codon-optimized gene sequences [2,3,10,37,41]. Therefore, in this study, DNA vaccines expressing one of four gp120 molecules from wild type HIV-1 gene sequences or sequences that were optimized for expression in mammalian cells (codonoptimized) were used to immunize mice.…”

“…In addition, higher titers of neutralizing antibodies were elicited in rodents vaccinated with gp120-mC3d 3 compared to gp120 alone [16]. The precise mechanism of C3d enhancement is unclear; however both CR2-dependent and CR2-independent pathways play a role in C3d immune enhancement [19].…”

“…Independently, each approach enhances antibody titer and cellular responses against Env compared to DNA plasmidsexpressing wild-type (wt) env gene inserts only. The fusion of C3d to an antigen results in enhanced immunogenicity to the fused antigen [5,16,17,19,21,24,31,32,35,36]. Rodents inoculated with DNA plasmids expressing HIV-1 Env fused to multiple copies of human or murine C3d (mC3d) had enhanced anti-Env specific IgG titers and accelerated affinity maturation of antibody [16].…”

Expression of complement receptors 1 (CR1/CD35) and 2 (CR2/CD21), and co-signaling molecule CD19 in cattle

“…There is also evidence that C3d can have adjuvant effects in the absence of CR2. C3d has been shown to act as an adjuvant in CR1--/CR2-- knockout mice in which HIV gp120/C3d streptavidin tetramers induced a greater IgM and IgG antibody response than gp120 alone (Haas et al, 2004). One proposed mechanism involves helper T cells.…”