C3d enhancement of antibodies to hemagglutinin accelerates protection against influenza virus challenge

CD19, CD21, and CD22: multifaceted response regulators of B lymphocyte signal transduction

B lymphocyte development and function depend upon the activity of intrinsic and B cell antigen receptor (BCR)-induced signals. These signals are interpreted, amplified, fine-tuned, or suppressed through the precise actions of specialized cell surface coreceptors, or "response regulators," that inform B cells of their extracellular environment. Important cell surface response regulators include the CD19/CD21 complex, CD22, and CD72. CD19 establishes a novel Src-family protein tyrosine kinase (PTK) amplification loop that regulates basal signaling thresholds and intensifies Src-family PTK activation following BCR ligation. In turn, CD22 limits the intensity of CD19-dependent, BCR-generated signals through the recruitment of potent phosphotyrosine and phosphoinositide phosphatases. Herein we discuss our current understanding of how CD19/CD21 and CD22 govern the emergence and intensity of BCR-mediated signals, and how alterations in these tightly controlled regulatory activities contribute to autoimmunity in mice and humans.

Regulation of DNA-raised immune responses by cotransfected interferon regulatory factors

Interferon regulatory factor 1 (IRF-1), IRF-3, and IRF-7 have been tested as genetic adjuvants for influenza virus hemagglutinin (HA) and nucleoprotein vaccine DNAs. Cotransfection of HA with IRF-3 and IRF-7 increased CD4 T-cell responses by 2- to 4-fold and CD8 T-cell responses by more than 10-fold. Following intramuscular deliveries of DNA, both CD4 and CD8 T cells were biased towards type 1 immune responses and the production of gamma interferon. Following gene gun bombardments of DNA, both were biased towards type 2 immune responses and the production of interleukin-4. The biases of the T-cell responses towards type 1 or type 2 were stronger for immunizations with IRF-3 as an adjuvant than for immunizations with IRF-7 as an adjuvant. Moderate adjuvant effects for antibody were observed. The isotypes of the antibody responses reflected the method of DNA delivery; intramuscular deliveries of DNA predominantly raised immunoglobulin G2a (IgG2a), whereas gene gun deliveries of DNA predominantly raised IgG1. These biases were enhanced by the codelivered IRFs. Overall, under the conditions of our experiments, IRF-3 had good activity for T cells, IRF-7 had good activity for both antibody and T cells, and IRF-1 had good activity for antibody.

Immunomodulation by apoptosis-inducing caspases for an influenza DNA vaccine delivered by gene gun

Apoptosis-inducing caspases have been tested for immunomodulatory effect on a gene gun-delivered DNA vaccine which expresses influenza hemagglutinin. Attenuated murine caspase 2 and a chimera of murine caspase 2 prodomain and human caspase 3 strongly enhanced humoral and cell-mediated immune response to hemagglutinin when they were co-administered with an immunogen DNA. In contrast, wild-type caspases did not enhance the DNA-raised immune response. Caspase dose-dependent antibody response curve revealed that the antibody level was in inverse relation to the amount of administered caspase. These findings indicate that bland apoptosis of antigen-harboring cells can elicit enhanced immune responses. Extensive apoptosis interferes with the generation of immune response. Gene gun delivery involving caspases elicited type-2 immune responses that characterized with dominant IL-4 and IgG1 production. ELISPOT assays showed that CD4 T cells were preferentially activated, while CD8 T cell response remained at marginal level. Using attenuated caspases for gene gun DNA vaccination is a useful approach to amplify type-2 immune responses.

Genetic immunization of neonates

The vaccination of neonates is generally difficult due to immaturity of the immune system, higher susceptibility to tolerance and potential negative interference of maternal antibodies. Studies carried out in rodents and non-human primates showed that plasmid vaccines expressing microbial antigens, rather than inducing tolerance, triggered significant humoral and cellular immunity with a Th1 component. The ability of bacterial CpG motifs to activate immature antigen-presenting cells is critical for the neonatal immunogenicity of DNA vaccines. In addition, the endogenous production of antigen subsequent to transfection of antigen-presenting cells may explain the lack of inhibition by maternal antibodies of cellular responses. Together, these features make the plasmid vaccines an appealing strategy to prime immune responses against foreign pathogens, during early life. In combination with subsequent boosting using conventional vaccines, DNA vaccine-based regimens may provide a qualitatively superior immunity against microbes. Thorough understanding of immunomodulatory properties of plasmid-vectors may extend their use for early prophylaxis of inflammatory disorders.

DNA fusion vaccines against B-cell tumors

DNA vaccination is currently being explored as a potential strategy for combatting cancer. However, tumor antigens are often weak and the immune system of patients may be compromised. For B-cell tumors, immunoglobulin idiotypic antigens provide defined targets but are poorly immunogenic. Fusion of a sequence derived from tetanus toxin to the genes encoding idiotypic determinants has proved highly effective in activating protective anti-tumor immunity. DNA fusion vaccines containing immuno-enhancing sequences can augment and direct immune attack on a range of target antigens. Gene-based fusion vaccines offer ease of manipulation and flexible design to activate effective attack on cancer.

Generation of recombinant human C3dg tetramers for the analysis of CD21 binding and function

CD21 (complement receptor 2, CR2) binds the terminal proteolytic fragments of the third component of complement (C3) that have been covalently attached to immune complexes or other targets during the activation of complement. We used the technique of in vivo biotinylation to create a recombinant multivalent ligand for CD21. A sequence coding for a biotinylation signal peptide was added to the 3' end of the human C3dg cDNA. The modified C3dg was expressed in Escherichia coli and biotinylated intracellularly by the bacterial biotin holoenzyme synthetase (BirA) enzyme. Monomeric C3dg was unable to bind to CD21 as determined by flow cytometry, while biotinylated recombinant C3dg (rC3dg) complexed with fluorochrome-conjugated streptavidin bound tightly. Binding was observed using CD21 positive B cells but not seen on pre-B cells that do not express this complement receptor. Two assays were used to assess the functional capacity of the recombinant C3dg. First, multimeric C3dg caused the phosphorylation of the mitogen-activated kinase, p38, in mature B lymphoma cells. Second, C3dg greatly enhanced the activation of primary B cells in combination with a sub-stimulatory concentration of anti-IgM monoclonal antibody. These results illustrate the utility of the technique of in vivo biotinylation to generate ligands for cell surface receptors that require multimerization for high avidity binding and function.

Fusion of C3d molecule with bovine rotavirus VP7 or bovine herpesvirus type 1 glycoprotein D inhibits immune responses following DNA immunization

The binding of the complement C3d molecule with receptors on B cells and/or follicular dendritic cells (FDCs) influences the induction of humoral immune responses. For example, C3d fused to an antigen has been shown to have a strong adjuvant effect on antibody production. We investigated the possibility that co-expression of antigen and C3d as a fusion protein could enhance antigen-specific immune responses, following plasmid immunization. One or two copies of murine C3d-cDNA, C3d or (C3d)(2), respectively, were cloned together with bovine rotavirus (BRV) VP7 or bovine herpesvirus type 1 (BHV-1) glycoprotein D (gD) genes. All constructs contained a signal peptide that resulted in the secretion of the expressed proteins. In vitro, the characterization of the chimeric proteins indicated that both VP7 and gD retained their antigenicity and the C3d remained biologically active. However, immunization with plasmids encoding VP7-C3d chimeras did not enhance rotavirus-specific antibody responses and the frequency of BRV-specific IFN-gamma secreting cells in the spleens were significantly lower in mice immunized with pVP7-(C3d)(2) when compared with mice immunized with plasmid encoding VP7. The same pattern of immune responses was observed for plasmids encoding gD-C3d. Both gD-specific antibody responses and the frequency of gD-specific IFN-gamma secreting cells were significantly lower in mice immunized with plasmid expressing gD-C3d chimeras when compared with mice immunized with plasmid encoding gD alone. These results indicate that co-expression of C3d with an antigen actually inhibit both humoral and cell-mediated antigen-specific immune responses.

C3d enhancement of neutralizing antibodies to measles hemagglutinin

Measles remains a major cause of worldwide infant mortality despite the use of current live attenuated vaccines. New approaches to measles virus (MV) vaccine development are critical to interrupt the spread of MV. In this study, we report the results using a DNA vaccine expressing a fusion of the measles hemagglutinin (H) protein and the complement component, C3d, to enhance the titers of neutralizing antibody. Plasmids were generated that expressed a secreted (s) form of H and the same form fused to three tandem copies of the murine homologue of C3d (sH-3C3d). Analysis of titers of the antibody raised in vaccinated mice indicated that immunizations with the DNA expressing sH-3C3d had higher titers of anti-H antibodies compared to serum from mice vaccinated with DNA expressing sH only. In addition, sH-3C3d elicited higher neutralizing antibody titers that inhibited MV induced plaque formation.

Cutting edge: myeloid complement C3 enhances the humoral response to peripheral viral infection

HSV-1 is the causative agent of cutaneous lesions, commonly referred to as cold sores. Primary exposure to the virus ordinarily occurs through the periphery, in particular through abraded skin or mucosal membranes. Under certain circumstances (e.g., in neonatals or AIDS patients), the infection becomes disseminated, often with severe consequences. Spread of HSV-1 is limited by virus-specific Ab. The development of an efficient humoral response to the virus is dependent on innate immunity component complement C3. The liver is the major source of C3, but there are also extrahepatic origins of C3 such as lymphoid macrophages. In the present study, the significance of C3 synthesis by bone marrow-derived cells was assessed by the transfer of wild-type bone marrow into irradiated C3-deficient mice. Using these chimeric mice, extrahepatic C3 was determined sufficient to initiate specific Ab and memory responses to a peripheral HSV-1 infection.

Enhanced avidity maturation of antibody to human immunodeficiency virus envelope: DNA vaccination with gp120-C3d fusion proteins

DNA vaccination can elicit both humoral and cellular immune responses and can confer protection against several pathogens. However, DNA vaccines expressing the envelope (Env) protein of human immunodeficiency virus (HIV) have been relatively ineffective at generating high titer, long-lasting, neutralizing antibodies in a variety of animal models. In this study, we report that fusion of Env and the complement component, C3d, in a DNA vaccine, enhances the titers of antibody to Env. Plasmids were generated that expressed a secreted form of Env (sgp120) from three isolates of HIV and these same forms fused to three tandem copies of the murine homologue of C3d (sgp120-3C3d). Analyses of titers and avidity maturation of the raised antibody indicated that immunizations with each of the sgp120-3C3d-expressing DNAs accelerated both the onset and the avidity maturation of antibody to Env.

Apoptosis-mediated enhancement of DNA-raised immune responses by mutant caspases

Apoptotic bodies can be used to target delivery of DNA-expressed immunogens into professional antigen-presenting cells (APCs). Here we show that antigen-laden apoptotic bodies created by vectors co-expressing influenza virus hemagglutinin (HA) or nucleoprotein (NP) genes and mutant caspase genes markedly increased T-cell responses. Both CD8 and CD4 T-cell responses were affected. The adjuvant activity was restricted to partially inactivated caspases that allowed immunogen expression before the generation of apoptotic bodies. Active-site mutants of murine caspase 2 and an autocatalytic chimera of murine caspase 2 prodomain and human caspase 3 induced apoptosis that did not interfere with immunogen expression. The adjuvant activity also enhanced B-cell responses, but to a lesser extent than T-cell responses. The large increases in T-cell responses represent one of the strongest effects to date of a DNA adjuvant on cellular immunity.

Increased immunogenicity and induction of class switching by conjugation of complement C3d to pneumococcal serotype 14 capsular polysaccharide

Previous studies have demonstrated an adjuvant effect for the C3d fragment of complement C3 when coupled to T-dependent protein antigens. In this study, we examined the antibody response to covalent conjugates of C3d and a T-independent antigen, the capsular polysaccharide of serotype 14 Streptococcus pneumoniae (PPS14). We prepared a conjugate of mouse C3d and PPS14 and compared its immunogenicity with that of a conjugate of PPS14 and ovalbumin (OVA). When BALB/c mice were immunized with PPS14-C3d, there was a significant increase in serum anti-PPS14 concentrations compared with either native PPS14 or control PPS14-glycine conjugates. This was accompanied by a switch in anti-PPS14 from predominantly immunoglobulin M (IgM) to IgG1 by day 25 following primary immunization. Following secondary immunization with PPS14-C3d, there was a marked booster response and a further increase in the ratio of IgG1 to IgM anti-PPS14. Although the primary antibody response to the PPS14-OVA conjugate exceeded that induced by immunization with PPS14-C3d, serum anti-PPS14 concentrations after a second injection of PPS14-C3d were nearly identical to those induced by secondary immunization with PPS14-OVA. Experiments with athymic nude mice suggested that T cells were not required for the adjuvant effect of C3d on the primary immune response to PPS14 but were necessary for enhancement of the memory response after a second injection of PPS14-C3d. These studies show that the adjuvant effects of C3d extend to T-independent antigens as well as T-dependent antigens. As a means of harnessing the adjuvant potential of the innate immune system, C3d conjugates may prove useful as a component of vaccines against encapsulated bacteria.