C3d of complement as a molecular adjuvant: bridging innate and acquired immunity

An optimal immune response should differentiate between harmful and innocuous antigens. Primitive systems of innate immunity, such as the complement system, may play a role in this distinction. When activated, the C3 component of complement attaches to potential antigens on microorganisms. To determine whether this alters acquired immune recognition, mice were immunized with a recombinant model antigen, hen egg lysozyme (HEL), fused to murine C3d. HEL bearing two and three copies of C3d was 1000- and 10,000-fold more immunogenic, respectively, than HEL alone. Thus, C3d is a molecular adjuvant of innate immunity that profoundly influences an acquired immune response.

Complement control protein factor H: the good, the bad, and the inadequate

The complement system is an essential component of the innate immune system that participates in elimination of pathogens and altered host cells and comprises an essential link between the innate and adaptive immune system. Soluble and membrane-bound complement regulators protect cells and tissues from unintended complement-mediated injury. Complement factor H is a soluble complement regulator essential for controlling the alternative pathway in blood and on cell surfaces. Normal recognition of self-cell markers (i.e. polyanions) and C3b/C3d fragments is necessary for factor H function. Inadequate recognition of host cell surfaces by factor H due to mutations and polymorphisms have been associated with complement-mediated tissue damage and disease. On the other hand, unwanted recognition of pathogens and altered self-cells (i.e. cancer) by factor H is used as an immune evasion strategy. This review will focus on the current knowledge related to these versatile recognition properties of factor H.

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

The ability of the C3d component of complement to enhance antibody responses and protective immunity to influenza virus challenges was evaluated using a DNA vaccine encoding a C3d fusion of the hemagglutinin (HA) from influenza virus. Plasmids were generated that encoded a transmembrane HA (tmHA), a secreted form of HA (sHA), or a sHA fused to three tandem copies of the murine homologue of the C3d (sHA-3C3d). Analysis of the titers, avidity maturation, and hemagglutinin-inhibition activity of raised antibody revealed that immunizations with sHA-3C3d DNA accelerated both the avidity maturation of antibody to HA and the appearance of hemagglutinin-inhibition activity. These accelerated antibody responses correlated to a more rapid appearance of protective immunity. They also correlated to complete protection from live virus challenge by a single vaccination at a dose ten times lower than the protective dose for non-C3d forms of HA.

Complement activation in experimental and human temporal lobe epilepsy

We investigated the involvement of the complement cascade during epileptogenesis in a rat model of temporal lobe epilepsy (TLE), and in the chronic epileptic phase in both experimental as well as human TLE. Previous rat gene expression analysis using microarrays indicated prominent activation of the classical complement pathway which peaked at 1 week after SE in CA3 and entorhinal cortex. Increased expression of C1q, C3 and C4 was confirmed in CA3 tissue using quantitative PCR at 1 day, 1 week and 3-4 months after status epilepticus (SE). Upregulation of C1q and C3d protein expression was confirmed mainly to be present in microglia and in a few hippocampal neurons. In human TLE with hippocampal sclerosis, astroglial, microglial and neuronal (5/8 cases) expression of C1q, C3c and C3d was observed particularly within regions where neuronal cell loss occurs. The membrane attack protein complex (C5b-C9) was predominantly detected in activated microglial cells. The persistence of complement activation could contribute to a sustained inflammatory response and could destabilize neuronal networks involved.

The dual function of the splenic marginal zone: essential for initiation of anti-TI-2 responses but also vital in the general first-line defense against blood-borne antigens

The splenic marginal zone (S-MZ) is especially well equipped for rapid humoral responses and is unique in its ability to initiate an immune response to encapsulated bacteria (T-cell independent type 2 (TI-2) antigens). Because of the rapid spreading through the blood, infections with blood-borne bacteria form a major health risk. To cope with blood-borne antigens, a system is needed that can respond rapidly to a great diversity of organisms. Because of a number of unique features, S-MZ B cells can respond rapid and efficient to all sorts of blood-borne antigens. These unique features include a low blood flow microenvironment, low threshold for activation, high expression of complement receptor 2 (CR2, CD21) and multireactivity. Because of the unique high expression of CD21 in a low flow compartment, S-MZ B cells can bind and respond to TI-2 antigens even with relatively low-avid B cell receptors. Although TI-2 antigens are in general poorly opsonized by classic opsonins, a particular characteristic of these antigens is their ability to bind very rapidly to complement fragment C3d without the necessity of previous immunoglobulin binding. TI-2 primed S-MZ B cells, already by first passage through the germinal centre, will meet antigen-C3d complexes bound to follicular dendritic cells, allowing unique immediate isotype switching. This explains that the primary humoral response to TI-2 antigens is unique in its characterization by a rapid increase in IgM concurrent with IgG antibody levels.

Targeted complement inhibition by C3d recognition ameliorates tissue injury without apparent increase in susceptibility to infection

Previous studies indicate a pivotal role for complement in mediating both local and remote injury following ischemia and reperfusion of the intestine. Here, we report on the use of a mouse model of intestinal ischemia/reperfusion injury to investigate the strategy of targeting complement inhibition to sites of complement activation by linking an iC3b/C3dg-binding fragment of mouse complement receptor 2 (CR2) to a mouse complement-inhibitory protein, Crry. We show that the novel CR2-Crry fusion protein targets sites of local and remote (lung) complement activation following intestinal ischemia and reperfusion injury and that CR2-Crry requires a 10-fold lower dose than its systemic counterpart, Crry-Ig, to provide equivalent protection from both local and remote injury. CR2-Crry has a significantly shorter serum half-life than Crry-Ig and, unlike Crry-Ig, had no significant effect on serum complement activity at minimum effective therapeutic doses. Furthermore, the minimum effective dose of Crry-Ig significantly enhanced susceptibility to infection in a mouse model of acute septic peritonitis, whereas the effect of CR2-Crry on susceptibility to infection was indistinguishable from that of PBS control. Thus, compared with systemic inhibition, CR2-mediated targeting of a complement inhibitor of activation improved bioavailability, significantly enhanced efficacy, and maintained host resistance to infection.

CD19-regulated signaling thresholds control peripheral tolerance and autoantibody production in B lymphocytes

The CD19 cell surface molecule regulates signal transduction events critical for B lymphocyte development and humoral immunity. Increasing the density of CD19 expression renders B lymphocytes hyper-responsive to transmembrane signals, and transgenic mice that overexpress CD19 have increased levels of autoantibodies. The role of CD19 in tolerance regulation and autoantibody generation was therefore examined by crossing mice that overexpress a human CD19 transgene with transgenic mice expressing a model autoantigen (soluble hen egg lysozyme, sHEL) and high-affinity HEL-specific IgMa and IgDa (IgHEL) antigen receptors. In this model of peripheral tolerance, B cells in sHEL/IgHEL double-transgenic mice are functionally anergic and do not produce autoantibodies. However, it was found that overexpression of CD19 in sHEL/IgHEL double-transgenic mice resulted in a breakdown of peripheral tolerance and the production of anti-HEL antibodies at levels similar to those observed in IgHEL mice lacking the sHEL autoantigen. Therefore, altered signaling thresholds due to CD19 overexpression resulted in the breakdown of peripheral tolerance. Thus, CD19 overexpression shifts the balance between tolerance and immunity to autoimmunity by augmenting antigen receptor signaling.

Stable transfection of Epstein-Barr virus (EBV) nuclear antigen 2 in lymphoma cells containing the EBV P3HR1 genome induces expression of B-cell activation molecules CD21 and CD23

A set of B-cell activation molecules, including the Epstein-Barr virus (EBV) receptor CR2 (CD21) and the B-cell activation antigen CD23 (Blast2/Fc epsilon RII), is turned on by infecting EBV-negative B-lymphoma cell lines with immortalizing strains of the viruslike B95-8 (BL/B95 cells). This up regulation may represent one of the mechanisms involved in EBV-mediated B-cell immortalization. The P3HR1 nonimmortalizing strain of the virus, which is deleted for the entire Epstein-Barr nuclear antigen 2 (EBNA2) protein open reading frame, is incapable of inducing the expression of CR2 and CD23, suggesting a crucial role for EBNA2 in the activation of these molecules. In addition, lymphoma cells containing the P3HR1 genome (BL/P3HR1 cells) do not express the viral latent membrane protein (LMP), which is regularly expressed in cells infected with immortalizing viral strains. Using electroporation, we have transfected the EBNA2 gene cloned in an episomal vector into BL/P3HR1 cells and have obtained cell clones that stably express the EBNA2 protein. In these clones, EBNA2 expression was associated with an increased amount of CR2 and CD23 steady-state RNAs. Of the three species of CD23 mRNAs described, the Fc epsilon RIIa species was preferentially expressed in these EBNA2-expressing clones. An increased cell surface expression of CR2 but not of CD23 was observed, and the soluble form of CD23 molecule (SCD23) was released. We were, however, not able to detect any expression of LMP in these cell clones. These data demonstrate that EBNA2 gene is able to complement P3HR1 virus latent functions to induce the activation of CR2 and CD23 expression, and they emphasize the role of EBNA2 protein in the modulation of cellular gene implicated in B-cell proliferation and hence in EBV-mediated B-cell immortalization. Nevertheless, EBNA2 expression in BL/P3HR1 cells is not able to restore the level of CR2 and CD23 expression observed in BL/B95 cells, suggesting that other cellular or viral proteins may also have an important role in the activation of these molecules: the viral LMP seems to be a good candidate.

The C-terminus of complement regulator Factor H mediates target recognition: evidence for a compact conformation of the native protein

The complement inhibitor Factor H has three distinct binding sites for C3b and for heparin, but in solution uses specifically the most C-terminal domain, i.e. short consensus repeats (SCR) 20 for ligand interaction. Two novel monoclonal antibodies (mABs C14 and C18) that bind to the most C-terminal domain SCR 20 completely blocked interaction of Factor H with the ligands C3b, C3d, heparin and binding to endothelial cells. In contrast, several mAbs that bind to the N-terminus and to the middle regions of the molecule showed no or minor inhibitory effects when assayed by enzyme-linked immunosorbent assay (ELISA) and ligand interaction assays. This paradox between a single functional binding site identified for native Factor H versus multiple interaction sites reported for deletion constructs is explained by a compact conformation of the fluid phase protein with one accessible binding site. On zymosan particles mAbs C14 and C18 blocked alternative pathway activation completely. Thus demonstrating that native Factor H makes the first and initial contact with the C terminus, which is followed by N terminally mediated complement regulation. These results are explained by a conformational hypothetical model: the native Factor H protein has a compact structure and only one binding site accessible. Upon the first contact the protein unfolds and exposes the additional binding sites. This model does explain how Factor H mediates recognition functions during complement control and the clustering of disease associated mutations in patients with haemolytic uraemic syndrome that have been reported in the C-terminal recognition domain of Factor H.

Antibody-mediated rejection in human cardiac allografts: evaluation of immunoglobulins and complement activation products C4d and C3d as markers

Antibody-mediated rejection (AMR) in human heart transplantation is an immunopathologic process in which injury to the graft is in part the result of activation of complement and it is poorly responsive to conventional therapy. We evaluated by immunofluorescence (IF), 665 consecutive endomyocardial biopsies from 165 patients for deposits of immunoglobulins and complement. Diffuse IF deposits in a linear capillary pattern greater than 2+ were considered significant. Clinical evidence of graft dysfunction was correlated with complement deposits. IF 2+ or higher was positive for IgG, 66%; IgM, 12%; IgA, 0.6%; C1q, 1.8%; C4d, 9% and C3d, 10%. In 3% of patients, concomitant C4d and C3d correlated with graft dysfunction or heart failure. In these 5 patients AMR occurred 56-163 months after transplantation, and they responded well to therapy for AMR but not to treatment with steroids. Systematic evaluation of endomyocardial biopsies is not improved by the use of antibodies for immunoglobulins or C1q. Concomitant use of C4d and C3d is very useful to diagnose AMR, when correlated with clinical parameters of graft function. AMR in heart transplant patients can occur many months or years after transplant.

Complement-Mediated Activation of the Adaptive Immune Responses: Role of C3d in Linking the Innate and Adaptive Immunity

C3d is the final degradation product of the third component of complement (C3). When conjugated to an antigen, C3d enhances immune responses to the fused antigen. Therefore, this molecule has been used as an adjuvant to enhance the immune responses to various foreign and self-proteins. C3d binds to the complement receptor 2 (CR2) that is located on the surface of follicular dendritic cells (FDC), B cells, and T cells in many species. C3d stimulates antigen presentation by FCD and helps to maintain immunological B cell memory. On B cells, C3d interaction with CR2 will collect molecules such as CD19, TAPA (CD81), and Lew 13. CD19 has a long intracellular tail that triggers a signaling cascade that results in cell activation and proliferation. Furthermore, simultaneous C3d-CR2 ligation and surface immunoglobulin (sIg) by antigen, activates two signaling pathways that cross-talk and synergize to activate B cells. However, C3d can enhance antibody titers in the absence of CR2 binding, indicating CR2-independent mechanism(s) of enhancement of the immune response. This review focuses on the complexity of the C3d-CR2 interaction, the importance of this interaction for the enhancement of the immune response by C3d and its derivatives, as well as the paradoxical enhancement of the immune response in the absence of CR2.

Insights into the regulatory mechanism controlling the inhibition of vaccine-induced seroconversion by maternal antibodies

The inhibition of vaccination by maternal antibodies is a widely observed phenomenon in human and veterinary medicine. Maternal antibodies are known to suppress the B-cell response. This is similar to antibody feedback mechanism studies where passively transferred antibody inhibits the B-cell response against particulate antigens because of epitope masking. In the absence of experimental data addressing the mechanism underlying inhibition by maternal antibodies, it has been suggested that epitope masking explains the inhibition by maternal antibodies, too. Here we report that in the cotton rat model of measles virus (MV) vaccination passively transferred MV-specific immunoglobulin G inhibit B-cell responses through cross-linking of the B-cell receptor with FcγRIIB. The extent of inhibition increases with the number of antibodies engaging FcγRIIB and depends on the Fc region of antibody and its isotype. This inhibition can be partially overcome by injection of MV-specific monoclonal IgM antibody. IgM stimulates the B-cell directly through cross-linking the B-cell receptor via complement protein 3d and antigen to the complement receptor 2 signaling complex. These data demonstrate that maternal antibodies inhibit B-cell responses by interaction with the inhibitory/regulatory FcγRIIB receptor and not through epitope masking.

Mechanisms of resistance of Porphyromonas gingivalis to killing by serum complement

The complement system plays an important role in the host defense against infection, and the formation of the terminal complement complex on the bacterial surface has been shown to be particularly important in killing of gram-negative bacteria. The gram-negative periodontal pathogen Porphyromonas gingivalis is resistant to complement killing, and possible mechanisms suggested for this resistance include protease production and capsule formation. In this study, P. gingivalis Arg- and Lys-gingipain deletion mutants and polysaccharide synthesis deletion mutants have been used to investigate these hypotheses. When Arg- and Lys-gingipain protease mutants were incubated in 20% normal human serum, deposition of complement components on the cell surface was significantly increased compared to that for the wild-type organism. However, despite the increased deposition, the protease mutants maintained resistance to killing and their viability was equal to that seen with heat-inactivated serum. Similar data were obtained when the wild-type organism was treated with gingipain protease inhibitors. K-antigen expression mutants were also resistant to killing. However, mutants which no longer synthesized a surface anionic polysaccharide (APS) (a phosphorylated branched mannan) were extremely sensitive to serum killing. These mutants lack the organized dense glycan surface layer present on the parent strain on the basis of electron microscopy. We conclude that the production of APS at the surface of P. gingivalis rather than Arg- and Lys-gingipain synthesis is the principal mechanism of serum resistance in P. gingivalis.

Elicitation of neutralizing antibodies with DNA vaccines expressing soluble stabilized human immunodeficiency virus type 1 envelope glycoprotein trimers conjugated to C3d

DNA vaccines expressing the envelope (Env) of human immunodeficiency virus type 1 (HIV-1) have been relatively ineffective at generating high-titer, long-lasting immune responses. Oligomeric or trimeric (gp140) forms of Env that more closely mimic the native proteins on the virion are often more effective immunogens than monomeric (gp120) envelopes. In this study, several forms of Env constructed from the HIV-1 isolate YU-2 (HIV-1(YU-2)) were tested for their immunogenic potential: a trimeric form of uncleaved (-) Env stabilized with a synthetic trimer motif isolated from the fibritin (FT) protein of the T4 bacteriophage, sgp140(YU-2)(-/FT), was compared to sgp140(YU-2)(-) without a synthetic trimerization domain, as well as to monomeric gp120(YU-2). DNA plasmids were constructed to express Env alone or fused to various copies of murine C3d (mC3d). BALB/c mice were vaccinated (day 1 and week 4) with DNA expressing a codon-optimized envelope gene insert, alone or fused to mC3d. Mice were subsequently boosted (week 8) with the DNA or recombinant Env protein. All mice had high anti-Env antibody titers regardless of the use of mC3d. Sera from mice vaccinated with DNA expressing non-C3d-fused trimers elicited neutralizing antibodies against homologous HIV-1(YU-2) virus infection in vitro. In contrast, sera from mice inoculated with DNA expressing Env-C3d protein trimers elicited antibody that neutralized both homologous HIV-1(YU-2) and heterologous HIV-1(ADA), albeit at low titers. Therefore, DNA vaccines expressing trimeric envelopes coupled to mC3d, expressed in vivo from codon-optimized sequences, elicit low titers of neutralizing antibodies against primary isolates of HIV-1.

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.

The crystal structure of human CD21: Implications for Epstein-Barr virus and C3d binding

Human complement receptor type 2 (CD21) is the cellular receptor for Epstein-Barr virus (EBV), a human tumor virus. The N-terminal two short consensus repeats (SCR1-SCR2) of the receptor interact with the EBV glycoprotein gp350/220 and also with the natural CD21 ligand C3d. Here we present the crystal structure of the CD21 SCR1-SCR2 fragment in the absence of ligand and demonstrate that it is able to bind EBV. Based on a functional analysis of wild-type and mutant CD21 and molecular modeling, we identify a likely region for EBV attachment and demonstrate that this region is not involved in the interaction with C3d. A comparison with the previously determined structure of CD21 SCR1-SCR2 in complex with C3d shows that, in both cases, CD21 assumes compact V-shaped conformations. However, our analysis reveals a surprising degree of flexibility at the SCR1-SCR2 interface, suggesting interactions between the two domains are not specific. We present evidence that the V-shaped conformation is induced by deglycosylation of the protein, and that physiologic glycosylation of CD21 would result in a more extended conformation, perhaps with additional epitopes for C3d binding.

Antibody-dependent enhancement of simian immunodeficiency virus (SIV) infection in vitro by plasma from SIV-infected rhesus macaques

Plasma from two rhesus macaques (Macaca mulatta) experimentally infected with the simian immunodeficiency virus (SIV; isolate SIVmac251) enhanced SIVmac infection of a human CD4+ lymphoblastoid cell line, MT-2. Prechallenge plasma samples from these animals and serum from SIV-negative macaques did not enhance infection. Compared with controls, infection enhancement was characterized by the rapid appearance of syncytium formation (3 to 4 days sooner), reverse transcriptase release (10-fold increase), and cytopathic effect (60% cell killing). Enhancement of activity was dependent on the presence of diluted, fresh SIV-negative macaque serum as a source of complement. A requirement for complement was shown by the absence of enhancement in heat-inactivated serum and by dose-dependent inhibition of enhancement in the presence of polyclonal antibody to monkey complement component C3. Monoclonal antibody to CD4 (OKT4a) blocked enhancement completely, while monoclonal antibody to the human complement component C3d receptor CR2 (OKB7) reduced enhancement by greater than 50%, indicating a requirement for CD4 and CR2 in mediating this phenomenon. SIV infection-enhancing activity appeared in macaques soon after experimental inoculation (28 days). The titer increased over time and peaked just prior to the death of both macaques from opportunistic infections and lymphoma. In vitro SIV infection enhancement is nearly identical to the in vitro complement-mediated, antibody-dependent enhancing (C'-ADE) activity observed in human immunodeficiency virus-positive human sera (Robinson et al., Lancet i:790-794, 1988; Robinson et al., J. Acq. Immun. Def. Synd. 2:33-42, 1989). These observations validate the macaque-SIV model for studies of C'-ADE.

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.

Function of C3 in a humoral response: iC3b/C3dg bound to an immune complex generated with natural antibody and a primary antigen promotes antigen uptake and the expression of co-stimulatory molecules by all B cells, but only stimulates immunoglobulin synthesis by antigen-specific B cells

Previous studies have shown that an optimal humoral response to a primary protein antigen requires C3 and CR2 (CD21). Sera from non-immunized donors contain natural IgM and IgG antibodies to the primary antigen keyhole limpet haemocyanin (KLH), and these have been previously shown to form immune complexes (IC) that activate the classical pathway of C, fixing iC3b/C3dg onto the KLH antigen. Such KLH IC bind to CR2 on KLH-non-specific B lymphocytes, resulting in antigen processing and MHC class II-dependent presentation to KLH-specific helper T cells. KLH IC also induce B lymphocytes to express the CD80 costimulatory molecule via simultaneous CR2 ligation with C3 and Fc gammaRII (CD32) stimulation by IgG natural antibody. The current study demonstrated that KLH IC ligation to either CR2 or Fc gammaRII resulted in activation of a second co-stimulatory molecule, LFA-1 (CD11a, CD18). The possibility of polyclonal B cell stimulation by the presentation of KLH-iC3b/C3dg by antigen-non-specific B cells was excluded by demonstration that in vitro cultivation of peripheral blood mononuclear cells (PBMC) with KLH-iC3b/C3dg elicited only anti-KLH, and did not stimulate synthesis of antibodies to hepatitis C virus (HCV) or tetanus toxoid (TT). Of greatest significance, a specific anti-KLH response was only detectable in cultures stimulated with KLH-iC3b/C3dg and not in cultures stimulated with KLH alone or KLH-IgG. Thus, iC3b/C3dg that was bound to a primary protein antigen enhanced recognition and specific immunoglobulin synthesis by antigen-specific B cells, even though the antigen was taken up and processed via CR2 by both antigen-specific and non-specific B cells.

Detachment of human immunodeficiency virus type 1 from germinal centers by blocking complement receptor type 2

After the transition from the acute to the chronic phase of human immunodeficiency virus (HIV) infection, complement mediates long-term storage of virions in germinal centers (GC) of lymphoid tissue. The contribution of particular complement receptors (CRs) to virus trapping in GC was studied on tonsillar specimens from HIV-infected individuals. CR2 (CD21) was identified as the main binding site for HIV in GC. Monoclonal antibodies (MAb) blocking the CR2-C3d interaction were shown to detach 62 to 77% of HIV type 1 from tonsillar cells of an individual in the presymptomatic stage. Although they did so at a lower efficiency, these antibodies were able to remove HIV from tonsillar cells of patients under highly active antiretroviral therapy, suggesting that the C3d-CR2 interaction remains a primary entrapment mechanism in treated patients as well. In contrast, removal of HIV was not observed with MAb blocking CR1 or CR3. Thus, targeting CR2 may facilitate new approaches toward a reduction of residual virus in GC.

Pneumococcal polysaccharides complexed with C3d bind to human B lymphocytes via complement receptor type 2

The immunoregulatory function of the complement system has been the focus of many investigations. In particular, fragments of complement factor C3 have been shown to play a role in B-lymphocyte activation and proliferation, lymphokine production, and the generation of in vitro antibody production. Purified pneumococcal polysaccharides (PS) can induce direct activation of C3 via the alternative pathway. Using sera of C1q-deficient patients and healthy subjects, we demonstrated that C3d, a split product of C3 that is generated after degradation of iC3b, can be bound to PS antigens. The binding of C3d to PS can occur in the absence of specific antibodies. Subsequently, we showed that PS complexed with C3d can be recognized by complement receptor type 2 that is expressed on B cells. Treatment of B cells with a monoclonal antibody recognizing the C3d-binding site of complement receptor type 2 reduces the binding of PS-C3d to the cells. In addition, we showed that PS4 complexed with C3d exerted an increased immunogenicity compared with free PS4. Our results show that the complement system plays a role in the activation of PS-specific B cells, carrying membrane receptors for C3d. Consequently, the complement system plays a regulatory role in the antibody response to T-cell-independent type 2 antigens such as PS.

Cerebral amyloid angiopathy in a 95+ cohort: complement activation and apolipoprotein E (ApoE) genotype

There is growing evidence that in Alzheimer's disease (AD) amyloid beta-protein (Abeta) triggers a chronic inflammatory reaction in cerebral amyloid plaques, including complement proteins. Abeta also accumulates cerebrovascularly in age- and AD-associated cerebral amyloid angiopathy (CAA). We investigated complement proteins in CAA in a population-based series using histological and immunohistochemical staining methods. The 74 subjects, aged 95 years or more, had undergone clinical neurological examination and apolipoprotein E (ApoE) genotyping. The brains had been studied for AD post-mortem, allowing us to relate the histopathological findings to clinical and genetic conditions. CAA with congophilic amyloid was found in 36/74 individuals (48.6%). The vascular amyloid deposits immunoreacted with antibodies to Abeta and complements 3d (C3d) and 9 (C9). The positivity in complement stains increased with growing severity of CAA (P = 0.001). The presence of CAA associated with ApoE epsilon4 (P = 0.0005) and overrepresentation of epsilon4 among those with moderate or severe vs. mild CAA (P = 0.03) was demonstrated. The presence of CAA associated with dementia (P = 0.01), which was contributed by both epsilon4+ (P = 0.02) and epsilon4- (P = 0.06) subjects. Our study shows that complement proteins are deposited in the affected vessels in Abeta-associated CAA. They may solely represent the cerebral Abeta- burden associated to inflammatory stimuli, or signal a contribution in the clearance of cerebral Abeta, thereby contributing to the events associated with evolution of clinical dementia. Our results demonstrate a strong association between CAA and ApoE epsilon4 as well as dementia and suggest that the contribution of CAA to dementia is largely independent of ApoE epsilon4.

Cutting edge: members of the Staphylococcus aureus extracellular fibrinogen-binding protein family inhibit the interaction of C3d with complement receptor 2

Staphylococcus aureus expresses a highly diversified arsenal of immune evasion proteins, many of which target the complement system. The extracellular fibrinogen-binding protein (Efb) and the Efb homologous protein (Ehp) have previously been demonstrated to bind to C3 and inhibit complement activation and amplification. In this study we present the first evidence that Efb and Ehp are also capable of inhibiting the interaction of C3d with complement receptor 2 (CR2), which plays an important role in B cell activation and maturation. The C-terminal domain of Efb efficiently blocked this interaction both in surface plasmon resonance-based competition studies and cellular assays and prevented the CR2-mediated stimulation of B cells. Furthermore, analyses of the available structural data were consistent with a molecular mechanism that reflects both steric and electrostatic effects on the C3d-CR2 interaction. Our study therefore suggests that S. aureus may disrupt both the innate and adaptive immune responses with a single protein module.

Vaccination with DNA plasmids expressing Gn coupled to C3d or alphavirus replicons expressing gn protects mice against Rift Valley fever virus

Background

Rift Valley fever (RVF) is an arthropod-borne viral zoonosis. Rift Valley fever virus (RVFV) is an important biological threat with the potential to spread to new susceptible areas. In addition, it is a potential biowarfare agent.

Methodology/Principal Findings

We developed two potential vaccines, DNA plasmids and alphavirus replicons, expressing the Gn glycoprotein of RVFV alone or fused to three copies of complement protein, C3d. Each vaccine was administered to mice in an all DNA, all replicon, or a DNA prime/replicon boost strategy and both the humoral and cellular responses were assessed. DNA plasmids expressing Gn-C3d and alphavirus replicons expressing Gn elicited high titer neutralizing antibodies that were similar to titers elicited by the live-attenuated MP12 virus. Mice vaccinated with an inactivated form of MP12 did elicit high titer antibodies, but these antibodies were unable to neutralize RVFV infection. However, only vaccine strategies incorporating alphavirus replicons elicited cellular responses to Gn. Both vaccines strategies completely prevented weight loss and morbidity and protected against lethal RVFV challenge. Passive transfer of antisera from vaccinated mice into naïve mice showed that both DNA plasmids expressing Gn-C3d and alphavirus replicons expressing Gn elicited antibodies that protected mice as well as sera from mice immunized with MP12.

Conclusion/Significance

These results show that both DNA plasmids expressing Gn-C3d and alphavirus replicons expressing Gn administered alone or in a DNA prime/replicon boost strategy are effective RVFV vaccines. These vaccine strategies provide safer alternatives to using live-attenuated RVFV vaccines for human use.

Rift Valley fever virus (RVFV) is an arthropod-borne phlebovirus associated with abortion storms, neonatal mortality in livestock and hemorrhagic fever or fatal encephalitis in a proportion of infected humans. Requirement of multiple booster immunizations to maintain the level of protective immunity with the inactivated vaccines and the ability of live-attenuated vaccines to cause detrimental side-effects are major limitations preventing the widespread use of current vaccines. In this paper, we describe the use of DNA and alphavirus replicon based vaccination approaches to elicit a protective immune response against RVFV. While both vaccines elicited high titer antibodies, DNA vaccination elicited high titer neutralizing antibodies, whereas the replicon vaccine elicited cellular immune responses. Both strategies alone or in combination elicited immune response that completely protected against not only mortality, but also illness. Even though the delivery vectors elicited some protection on their own, they did not prevent severe morbidity. These promising vaccines provide an alternative RVFV vaccine for livestock and humans.

Epitope mapping using the X-ray crystallographic structure of complement receptor type 2 (CR2)/CD21: identification of a highly inhibitory monoclonal antibody that directly recognizes the CR2-C3d interface

Complement receptor type 2 (CR2)/CD21 is a B lymphocyte cell membrane C3d/iC3b receptor that plays a central role in the immune response. Human CR2 is also the receptor for the EBV viral membrane glycoprotein gp350/220. Both C3d and gp350/220 bind CR2 within the first two of 15-16 repetitive domains that have been designated short consensus/complement repeats. Many mAbs react with human CR2; however, only one currently available mAb is known to block both C3d/iC3b and gp350/220 binding. We have used a recombinant form of human CR2 containing the short consensus/complement repeat 1-2 ligand-binding fragment to immunize Cr2(-/-) mice. Following fusion, we identified and further characterized four new anti-CR2 mAbs that recognize this fragment. Three of these inhibited binding of CR2 to C3d and gp350/220 in different forms. We have determined the relative inhibitory ability of the four mAbs to block ligand binding, and we have used overlapping peptide-based approaches to identify linear epitopes recognized by the inhibitory mAbs. Placement of these epitopes on the recently solved crystal structure of the CR2-C3d complex reveals that each inhibitory mAb recognizes a site either within or adjacent to the CR2-C3d contact site. One new mAb, designated 171, blocks CR2 receptor-ligand interactions with the greatest efficiency and recognizes a portion of the C3d contact site on CR2. Thus, we have created an anti-human CR2 mAb that blocks the C3d ligand by direct contact with its interaction site, and we have provided confirmatory evidence that the C3d binding site seen in its crystal structure exists in solution.