Depletion of Complement Has Distinct Effects on the Primary and Secondary Antibody Responses to a Conjugate of Pneumococcal Serotype 14 Capsular Polysaccharide and a T-Cell-Dependent Protein Carrier

Abstract

Complement activation plays a critical role in the immune response to T-cell-dependent and T-cell-independent antigens. However, the effect of conjugation of T-cell-dependent protein carriers to T-cell-independent type 2 antigens on the requirement for complement in the humoral immune response to such antigens remains unknown. We studied the role of complement activation on the antibody response of BALB/c mice immunized with the T-cell-independent type 2 antigen serotype 14 pneumococcal capsular polysaccharide (PPS14), either in unmodified form or conjugated to ovalbumin (OVA). In mice immunized with either PPS14 or PPS14-OVA, depletion of endogenous complement at the time of primary immunization by treatment with cobra venom factor (CVF) diminished serum anti-PPS14 concentrations after primary immunization but enhanced antibody responses after secondary immunization. The secondary immunoglobulin G (IgG) anti-PPS14 antibody response after immunization with PPS14-OVA was especially enhanced by complement depletion, was observed at doses as low as 0.2 g of antigen, and was maximal when CVF was administered within 2 days of immunization. The avidity and opsonophagocytic functions of IgG anti-PPS14 antibodies were comparable in mice immunized with PPS14-OVA with or without complement depletion. Serum anti-PPS14 antibody concentrations were near normal, and the enhancing effects of CVF treatment on the secondary anti-PPS14 antibody response were also apparent in splenectomized mice immunized with PPS14-OVA. These results demonstrate that complement activation can have distinct effects on the primary and secondary antibody responses to a T-cell-independent type 2 antigen, either unmodified or conjugated to a T-cell-dependent protein carrier. These differences should be taken into consideration when using complement to modulate the immune response to vaccines.

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

“…However, complement components may also exhibit immunosuppressive activities (18). For example, C3 depletion by cobra venom factor can inhibit primary, yet enhance secondary, humoral immune responses to both unmodified and protein-conjugated pneumococcal polysaccharide (19). Furthermore, C3d attachment to certain Ags can inhibit humoral responses to the nontoxic diphtheria toxin fragment B (DT), human chorionic gonadotropin, bovine rotavirus VP7, bovine herpesvirus type 1 glycoprotein D, and malarial circumsporozoite protein (20 -23).…”

Immune Complex-Mediated Enhancement of Secondary Antibody Responses

“…For depletion of complement, mice received 5 mg Cobra Venom Factor (Sigma-Aldrich, St. Louis, MO) i.p. at 28, 24, and 4 h prior to secondary immunization as described (17). Emory University Institutional Animal Care and Use Committee approved all studies.…”

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

“…Vaccines were administered by subcutaneous injection at days 0 and 42 (and in some cases at day 84), with the total dose divided equally between two sites. Mice were treated with CVF at the time of primary immunization as previously described (21). For experiments in which mice were treated with (CR2) 2 -IgG1, PPS14-C3d was mixed with either PBS, 10F7MN control mouse IgG1, or (CR2) 2 -IgG1 at 10:1 or 20:1 molar ratios to C3d and incubated overnight at room temperature prior to immunization.…”

“…PPS14-C3d and PPS14-OVA were prepared by conjugating purified mouse C3d or OVA monomers to PPS14 as previously described (20,21). For the experiments portrayed in Fig.…”

“…The goals of the studies reported here were to determine (i) whether binding to CR2 is a necessary component of the antibody response to PPS14-C3d, (ii) whether conjugation of C3d to PPS14 bypasses the requirement for endogenous complement, and (iii) whether C3d behaves as a TD protein carrier. We also wished to see whether restricting inhibition of CR2 binding or complement activation to the time of primary immunization had different effects on the primary and secondary anti-PPS14 antibody responses to PPS14-C3d, similar to our previous findings for unmodified PPS14 and PPS14-ovalbumin (OVA) (21). Because this is difficult or impossible using C3 or complement receptor knockout mice, we treated mice with a soluble form of CR2 (7) to prevent binding of C3d to cell surface CR2 and used cobra venom factor (CVF) to deplete serum complement prior to immunization.…”