A Minimum CR2 Binding Domain of C3d Enhances Immunity Following Vaccination

Abstract

The degradation product of the third (C3) complement component, C3d, links innate and adaptive immunity, and the covalent attachment of C3d to an antigen enhances antigen-specific immune responses. C3d has been hypothesized to enhance immunity by direct interaction with complement receptor 2 (CR2/CD21) on immune cells. However, the domains on C3d important for CR2 binding have been controversial, with various studies reaching contradictory conclusions. In addition, the concept of B-cell activation via CR2 by C3d has been questioned, since mice lacking CR2 still elicit C3d-enhanced immunity following vaccination. Therefore, the goal of this study was to determine if a peptide representing one of the proposed CR2 binding domains of C3d could substitute for the entire protein and enhance antigen-specific immunity. Mice (BALB/c) were vaccinated with the HIV-1 gp120 envelope glycoprotein (Env(gp120)) alone or fused to multiple copies of the murine C3d or a twenty-eight amino-acid peptide (P28) containing a minimum CR2 binding domain. Each immunogen was expressed from DNA plasmid in vivo or injected as purified recombinant protein. The fusion of the P28 peptide to Env(gp120) enhanced both humoral and cell-mediated immune responses with similar efficiency as Env(gp120) conjugated to C3d. The fusion of C3d or P28 to Env(gp120) elicited higher-titer anti-Env specific antibody, enhanced avidity maturation of the elicited antibody, and elicited higher numbers of IFN-gamma and IL-4 secreting cells compared to Env(gp120) immunizations. This CR2-binding domain specific 28 amino acid peptide can substitute for the entire C3d molecule and enhance immunity. These results indicate that the adjuvant properties of C3d are associated with CR2 interaction.

Cancer vaccines and carbohydrate epitopes

“…CD21 is also known as CR2 and is the C3d receptor; interaction of this receptor with C3d activates B-cells [14–17]. CR2 is expressed on B cells, follicular dendritic cells, epithelial cells, and a sub-group of T cells [16]. Interaction of CD21 with C3d activates B-cells creates immunologic memory, and causes immunoglobulin class switching but conversely is involved in B cell tolerance [17].…”

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

“…The co‐activation of these two signals has been shown to lower the threshold of activation and to promote a more robust immune response by inducing isotype switching, somatic hypermutation and B‐cell memory generation 4 . Yet in mice lacking complement receptors CR1 (CD35) and CR2 (CD21), immune responses to protein antigen‐C3d conjugates were equally enhanced to the levels seen in wild‐type mice 5 , 6 , 7 . Further, C3 −/− mice, but not CD21 −/− , have been shown to be deficient in T‐dependent (Td) responses 8 .…”

Employing XIAP to Enhance the Duration of Antigen Expression and Immunity Against an Avian Influenza H5 DNA Vaccine

“…It was previously reported that fusion of multimers of the P28 peptide to an antigen may enhance the level of immunogenicity (Bergmann-Leitner et al, 2007;Bower & Ross, 2006). Thus, hemagglutinin sequence (GenBank ABQ45850) from the influenza A virus (A/chicken/India/NIV33487/2006, H5N1, clade 2.2) was fused to 4 copies of P28 sequence to make H5-P28 4 (Figures 1a-I).…”