Was it there all the time?

Complement 3d: from molecular adjuvant to target of immune escape mechanisms

C3d is a fragment of the complement factor C3 and is generated in the course of complement activation. When bound to antigen in single or multiple copies, the B cell receptor and complement receptor 2 become co-crosslinked resulting in decreased or increased B cell responses depending on the valence of the antigen-C3d construct. When antigen-C3d constructs are used for the purpose of generating a protective immune response (vaccines), they may either enhance the expected response or suppress it depending on the nature of the antigen. Various pathogens use C3d to evade the immune system by inhibiting complement activation, invading and homing in host cells or masking immunogenic areas of pathogen proteins. Therefore, future vaccination strategies for infectious diseases and cancer employing C3d as a molecular adjuvant need to be carefully evaluated before choosing a target antigen in order to take advantage of the adjuvant effect of the complement component while avoiding potential vaccine complications associated with immune escape mechanisms.

Complement Component C3d-Antigen Complexes Can Either Augment or Inhibit B Lymphocyte Activation and Humoral Immunity in Mice Depending on the Degree of CD21/CD19 Complex Engagement

C3d can function as a molecular adjuvant by binding CD21 and thereby enhancing B cell activation and humoral immune responses. However, recent studies suggest both positive and negative roles for C3d and the CD19/CD21 signaling complex in regulating humoral immunity. To address whether signaling through the CD19/CD21 complex can negatively regulate B cell function when engaged by physiological ligands, diphtheria toxin (DT)-C3d fusion protein and C3dg-streptavidin (SA) complexes were used to assess the role of CD21 during BCR-induced activation and in vivo immune responses. Immunization of mice with DT-C3d3 significantly reduced DT-specific Ab responses independently of CD21 expression or signaling. By contrast, SA-C3dg tetramers dramatically enhanced anti-SA responses when used at low doses, whereas 10-fold higher doses did not augment immune responses, except in CD21/35-deficient mice. Likewise, SA-C3dg (1 microg/ml) dramatically enhanced BCR-induced intracellular calcium concentration ([Ca2+]i) responses in vitro, but had no effect or inhibited [Ca2+]i responses when used at 10- to 50-fold higher concentrations. SA-C3dg enhancement of BCR-induced [Ca2+]i responses required CD21 and CD19 expression and resulted in significantly enhanced CD19 and Lyn phosphorylation, with enhanced Lyn/CD19 associations. BCR-induced CD22 phosphorylation and Src homology 2 domain-containing protein tyrosine phosphatase-1/CD22 associations were also reduced, suggesting abrogation of negative regulatory signaling. By contrast, CD19/CD21 ligation using higher concentrations of SA-C3dg significantly inhibited BCR-induced [Ca2+]i responses and inhibited CD19, Lyn, CD22, and Syk phosphorylation. Therefore, C3d may enhance or inhibit Ag-specific humoral immune responses through both CD21-dependent and -independent mechanisms depending on the concentration and nature of the Ag-C3d complexes.

The predictive value of peritubular capillaries C3d deposition in IgA glomerulonephritis

The IgA glomerulonephritis (IgAGN) is one of the most common primary glomerulonephritis and has a variable and difficult to predict evolution toward the end-stage nephrosclerosis. The deposition of C3d complement component in peritubular capillaries (PTCs) indicates a variant type of acute rejection while C3d deposition in primary glomerulonephritis (GN) is poorly documented. The aim of this study is to examine C3d expression in peritubular capillaries (PTCs) as a predictive marker and its correlation with the severity of renal injury in IgA glomerulonephritis. Polyclonal FITC conjugated rabbit anti-human C3c and C3d antibodies were used for direct immunofluorescent evaluation of the C3c and C3d deposits in 24 kidney biopsies with IgA glomerulonephritis. The study revealed that the C3d deposits in peritubular capillaries were associated with known predictors for rapid progression of IgAGN: glomerular sclerosis (63.6%), atrophic tubules (90.9%) and interstitial sclerosis (81.8%). The intensity of the C3c glomerular immunofluorescent deposits was related with active lesions. Thus, the predictive value of C3d deposition on PTCs in IgAGN is worth to be taken into consideration as an unfavorable outcome of the disease and request further long run investigations.

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

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.