Disruption of the Cr2 locus results in a reduction in B-1a cells and in an impaired B cell response to T-dependent antigen

Synergy between the classical and alternative pathways of complement is essential for conferring effective protection against the pandemic influenza A(H1N1) 2009 virus infection

The pandemic influenza A(H1N1) 2009 virus caused significant morbidity and mortality worldwide thus necessitating the need to understand the host factors that influence its control. Previously, the complement system has been shown to provide protection during the seasonal influenza virus infection, however, the role of individual complement pathways is not yet clear. Here, we have dissected the role of intact complement as well as of its individual activation pathways during the pandemic influenza virus infection using mouse strains deficient in various complement components. We show that the virus infection in C3^-/- mice results in increased viral load and 100% mortality, which can be reversed by adoptive transfer of naïve wild-type (WT) splenocytes, purified splenic B cells, or passive transfer of immune sera from WT, but not C3^-/- mice. Blocking of C3a and/or C5a receptor signaling in WT mice using receptor antagonists and use of C3aR^-/- and C5aR^-/- mice showed significant mortality after blocking/ablation of C3aR, with little or no effect after blocking/ablation of C5aR. Intriguingly, deficiency of C4 and FB in mice resulted in only partial mortality (24%-32%) suggesting a necessary cross-talk between the classical/lectin and alternative pathways for providing effective protection. In vitro virus neutralization experiments performed to probe the cross-talk between the various pathways indicated that activation of the classical and alternative pathways in concert, owing to coating of viral surface by antibodies, is needed for its efficient neutralization. Examination of the virus-specific complement-binding antibodies in virus positive subjects showed that their levels vary among individuals. Together these results indicate that cooperation between the classical and alternative pathways not only result in efficient direct neutralization of the pandemic influenza virus, but also lead to the optimum generation of C3a, which when sensed by the immune cells along with the antigen culminates in generation of effective protective immune responses.

The pandemic influenza A(H1N1) 2009 virus is now circulating seasonally and causing a significant disease burden worldwide. Hence, it is important to delineate the immune components required for protection against its infection. Here we demonstrate that presence of intact complement is essential for clearing the pandemic influenza virus infection, wherein complement synthesized by B cells plays a major role. Further, we show that activation of the classical as well as alternative pathways is a requisite for efficient neutralization of the virus as well as the optimum generation of C3a, which is necessary for boosting the protective immune responses. Our results thus reveal that deficiencies of components of the classical and alternative pathways enhance the susceptibility to and severity of the pandemic influenza virus infection.

How Follicular Dendritic Cells Shape the B-Cell Antigenome

Follicular dendritic cells (FDCs) are stromal cells residing in primary follicles and in germinal centers of secondary and tertiary lymphoid organs (SLOs and TLOs). There, they play a crucial role in B-cell activation and affinity maturation of antibodies. FDCs have the unique capacity to bind and retain native antigen in B-cell follicles for long periods of time. Therefore, FDCs shape the B-cell antigenome (the sum of all B-cell antigens) in SLOs and TLOs. In this review, we discuss recent findings that explain how this stromal cell type can arise in almost any tissue during TLO formation and, furthermore, focus on the mechanisms of antigen capture and retention involved in the generation of long-lasting antigen depots displayed on FDCs.

FHR3 Blocks C3d-Mediated Coactivation of Human B Cells

The autoimmune renal disease deficient for complement factor H-related (CFHR) genes and autoantibody-positive form of hemolytic uremic syndrome is characterized by the presence of autoantibodies specific for the central complement regulator, factor H, combined with a homozygous deficiency, mostly in CFHR3 and CFHR1 Because FHR3 and FHR1 bind to C3d and inactivated C3b, which are ligands for complement receptor type 2 (CR2/CD21), the aim of the current study was to examine whether FHR3-C3d or FHR1-C3d complexes modulate B cell activation. Laser-scanning microscopy and automated image-based analysis showed that FHR3, but not FHR1 or factor H, blocked B cell activation by the BCR coreceptor complex (CD19/CD21/CD81). FHR3 bound to C3d, thereby inhibiting the interaction between C3d and CD21 and preventing colocalization of the coreceptor complex with the BCR. FHR3 neutralized the adjuvant effect of C3d on B cells, as shown by inhibited intracellular CD19 and Akt phosphorylation in Raji cells, as well as Ca(2+) release in peripheral B cells. In cases of CFHR3/CFHR1 deficiency, the FHR3 binding sites on C3d are occupied by factor H, which lacks B cell-inhibitory functions. These data provide evidence that FHR3, which is absent in patients with the autoimmune form of hemolytic uremic syndrome, is involved in B cell regulation.

CD21 and CD19 deficiency: Two defects in the same complex leading to different disease modalities

PURPOSE

Deficiencies in CD19 and CD81 (forming the CD19-complex with CD21 and CD225) cause a severe clinical phenotype. One CD21 deficient patient has been described. We present a second CD21 deficient patient, with a mild clinical phenotype and compared the immunobiological characteristics of CD21 and CD19 deficiency.

METHODS

CD21 deficiency was characterized by flowcytometric immunophenotyping and sequencing. Real-time PCR, in vitro stimulation and next generation sequencing were used to characterize B-cell responses and affinity maturation in CD21(-/-) and CD19(-/-) B cells.

RESULTS

A compound heterozygous mutation in CD21 caused CD21 deficiency. CD21(-/-) B cells responded normally to in vitro stimulation and AID was transcribed. Affinity maturation was less affected by CD21 than by CD19 deficiency.

CONCLUSIONS

Both CD21 and CD19 deficiencies cause hypogammaglobulinemia and reduced memory B cells. CD19 deficiency causes a more severe clinical phenotype. B-cell characteristics reflect this, both after in vitro stimulation as in affinity maturation.

Biallelic loss-of-function mutation in NIK causes a primary immunodeficiency with multifaceted aberrant lymphoid immunity

Primary immunodeficiency disorders enable identification of genes with crucial roles in the human immune system. Here we study patients suffering from recurrent bacterial, viral and Cryptosporidium infections, and identify a biallelic mutation in the MAP3K14 gene encoding NIK (NF-κB-inducing kinase). Loss of kinase activity of mutant NIK, predicted by in silico analysis and confirmed by functional assays, leads to defective activation of both canonical and non-canonical NF-κB signalling. Patients with mutated NIK exhibit B-cell lymphopenia, decreased frequencies of class-switched memory B cells and hypogammaglobulinemia due to impaired B-cell survival, and impaired ICOSL expression. Although overall T-cell numbers are normal, both follicular helper and memory T cells are perturbed. Natural killer (NK) cells are decreased and exhibit defective activation, leading to impaired formation of NK-cell immunological synapses. Collectively, our data illustrate the non-redundant role for NIK in human immune responses, demonstrating that loss-of-function mutations in NIK can cause multiple aberrations of lymphoid immunity.

How antibodies use complement to regulate antibody responses

Antibodies, forming immune complexes with their specific antigen, can cause complete suppression or several 100-fold enhancement of the antibody response. Immune complexes containing IgG and IgM may activate complement and in such situations also complement components will be part of the immune complex. Here, we review experimental data on how antibodies via the complement system upregulate specific antibody responses. Current data suggest that murine IgG1, IgG2a, and IgG2b upregulate antibody responses primarily via Fc-receptors and not via complement. In contrast, IgM and IgG3 act via complement and require the presence of complement receptors 1 and 2 (CR1/2) expressed on both B cells and follicular dendritic cells. Complement plays a crucial role for antibody responses not only to antigen complexed to antibodies, but also to antigen administered alone. Lack of C1q, but not of Factor B or MBL, severely impairs antibody responses suggesting involvement of the classical pathway. In spite of this, normal antibody responses are found in mice lacking several activators of the classical pathway (complement activating natural IgM, serum amyloid P component (SAP), specific intracellular adhesion molecule-grabbing non-integrin R1 (SIGN-R1) or C-reactive protein. Possible explanations to these observations will be discussed.

C3d plasma levels and CD21 expressing B-cells in children after ABO-incompatible heart transplantation: Alterations associated with blood group tolerance

BACKGROUND

Most children transplanted with ABO-incompatible (ABOi) hearts develop selective tolerance to donor A/B antigens, whereas anti-A/B antibodies typically re-accumulate in adults after ABOi kidney transplantation. Deficiency of essential factors linking innate and adaptive immunity in early childhood may promote development of tolerance, specifically interactions between complement split product C3d and its ligand CD21 on B cells, considering their role in augmenting "T-independent" B-cell activation.

METHODS

Blood and clinical data were analyzed from children after ABOi or ABO-compatible (ABOc) heart transplantation (HTx). Plasma C3d levels were quantified by enzyme-linked immunoassay. Peripheral blood mononuclear cells (PBMC) were phenotyped by flow cytometry; expression of B-cell co-receptor components CD21 and CD81 was quantified.

RESULTS

Fifty-five samples from pediatric HTx recipients (median age at transplant: 4.2 [range 0.03 to 20.4] months; age at sample collection: 14.6 [0.04 to 51.3] months; 53% ABOi) and 21 controls were studied. CD21-expressing B cells increased in trend with age (p = 0.079); longitudinal measures in individual patients showed a strong correlation with age. CD21 expression intensity in B-cells was not age-dependent. Plasma C3d levels did not correlate with age. Comparing ABOc vs ABOi HTx, CD21-expressing cell proportions were similar; however, serum C3d levels were significantly lower after ABOi HTx (p < 0.05).

CONCLUSIONS

In children, including HTx patients, CD21-expressing B-cells show a trend to increase with age, corresponding with improved responsiveness to polysaccharide antigens. This does not differ in patients with ABOi grafts developing tolerance to donor ABO antigens. C3d levels are not age-dependent, but reduced C3d levels after ABOi HTx suggest altered complement metabolism contributing to ABO tolerance.

Generation of a novel Cr2 gene allele by homologous recombination that abrogates production of Cr2 but is sufficient for expression of Cr1

The enhancing effects of the complement system for humoral immunity have primarily focused upon the recognition of complement-bound foreign antigens by a co-receptor complex of the antigen-specific B cell receptor (BCR) and complement receptor 2 (Cr2). In vivo experiments using Cr2 gene deficient mice (which lack the expression of both the Cr1 and Cr2 proteins) do demonstrate depressed humoral responses to immunization but cannot be used to define specific contributions of the singular Cr1 or Cr2 proteins on B cell functions. To study the effect of a Cr2 deficiency in a Cr1 sufficient environment we created a mouse line in which the alternative splice site required for the expression of the Cr2 isoform was removed. This mouse line, Cr2KO, still expressed Cr1 on B cells but was deficient for the full length Cr2 protein. Surprisingly a new alternative splice within the Cr2 gene created a truncated product that encoded a novel protein termed iCr2 that was expressed on the surface of the cells. The Cr2KO mouse thus provides a new model system for the analysis of Cr1 and Cr2 functions in the immune response of the mouse.

Carotid calcification in mice: a new model to study the effects of arterial stiffness on the brain

BACKGROUND

Arterial stiffness has been identified as an important risk factor for cognitive decline. However, its effects on the brain's health are unknown, and there is no animal model available to study the precise impact of arterial stiffness on the brain. Therefore, the objective of the study was to develop and characterize a new model specific to arterial stiffness in order to study its effects on the brain.

METHODS AND RESULTS

Calcium chloride (CaCl2) was applied to carotid arteries of mice, inducing an increase in collagen distribution and intima-media thickness, a fragmentation of elastin, a decrease in arterial compliance and distensibility, and an increase in cerebral blood flow pulsatility (n=3 to 11). Calcium deposits were only present at the site of CaCl2 application, and there was no increase in systemic blood pressure or change in vessel radius making this model specific for arterial stiffness. The effects of carotid stiffness were then assessed in the brain. Carotid calcification induced an increase in the production of cerebral superoxide anion and neurodegeneration, detected with Fluoro-Jade B staining, in the hippocampus (n=3 to 5), a key region for memory and cognition.

CONCLUSIONS

A new model of arterial stiffness based on carotid calcification was developed and characterized. This new model meets all the characteristics of arterial stiffness, and its specificity allows the study of the effects of arterial stiffness on the brain.

Optimal germinal center B cell activation and T-dependent antibody responses require expression of the mouse complement receptor Cr1

Follicular dendritic cells (FDCs) and complement receptor (Cr)1 and complement receptor (Cr)2 are important for the generation of humoral immunity. Cr1/2 expression on B cells and FDCs was shown to provide a secondary signal for B cell activation, to facilitate transport of Ag in immune follicles, and to enhance retention of immune complexes by FDCs. We show in this study that murine B cells predominantly express the Cr2 product from the Cr2 gene, whereas FDCs almost exclusively express the Cr1 isoform generated from the Cr2 gene. To define the specific role of Cr1, we created an animal that maintains normal cell-restricted expression of Cr2 but does not express Cr1. Cr1-deficient (Cr1KO) mice develop normal B1 and B2 immature and mature B cell subsets and have normal levels of naive serum Abs but altered levels of natural Abs. Immunization of the Cr1KO animal demonstrates deficient Ab responses to T-dependent, but not T-independent, Ags. Germinal centers from the immunized Cr1KO animal possess a deficiency in activated B cells, similar to that seen for animals lacking both Cr1 and Cr2 or C3. Finally, animals lacking only Cr1 respond similarly to wild-type animals to infections with Streptococcus pneumoniae, a pathogen to which animals lacking C3 or both Cr1 and Cr2 are particularly sensitive. Altogether, these data suggest that the production of Cr1, primarily by FDCs, is critical in the generation of appropriately activated B cells of the germinal center and the generation of mature Ab responses.

Synergistic induction of interferon α through TLR-3 and TLR-9 agonists identifies CD21 as interferon α receptor for the B cell response

Maternal antibodies inhibit seroconversion and the generation of measles virus (MeV)-specific antibodies (both neutralizing and non-neutralizing antibodies) after vaccination whereas T cell responses are usually unaffected. The lack of seroconversion leaves individuals susceptible to vaccine-preventable infections. Inhibition of antibody secretion is due to the inhibition of B cells through a cross-link of the B cell receptor with the inhibitory FcγIIB receptor (CD32) by maternal antibody/vaccine complexes. Here, we demonstrate that a combination of TLR-3 and TLR-9 agonists induces synergistically higher levels of type I interferon in vitro and in vivo than either agonist alone. The synergistic action of TLR-3 and TLR-9 agonists is based on a feedback loop through the interferon receptor. Finally, we have identified CD21 as a potential receptor for interferon α on B cells which contributes to interferon α-mediated activation of B cells in the presence of maternal antibodies. The combination leads to complete restoration of B cell and antibody responses after immunization in the presence of inhibitory MeV-specific IgG. The strong stimulatory action of type I interferon is due to the fact that type I interferon uses not only the interferon receptor but also CD21 as a functional receptor for B cell activation.

Maternal antibodies provide protection against infection with pathogens early in life but also interfere with vaccination. This interference is caused by a vaccine/maternal antibody complex which links the B cell receptor to the inhibitory CD32 molecule. Here, we show that this cross-link results in impaired B cell activation and proliferation which is correlated with diminished antibody responses. We also found that induction of large amounts of type I interferon restores the neutralizing antibody response in the presence of maternal antibodies. The best induction of type I interferon was accomplished by a combination of known activators of interferon secretion (a combination of TLR-3 and TLR-9 agonists). The strong stimulation by interferon is due to the previously unappreciated role of CD21 as functional receptor for interferon alpha. Our findings demonstrate that the dual receptor usage of type I interferon receptor and CD21 is crucial for B cell activation in the presence of maternal antibodies. This study suggests that measles vaccine, and potentially other vaccines, may induce optimal antibody responses when they are reconstituted with TLR-3 and TLR-9 agonists and thus these agonists may have great potential for clinical use.

Genetic control of antibody production during collagen-induced arthritis development in heterogeneous stock mice

OBJECTIVE

To identify genetic factors driving pathogenic autoantibody formation in collagen-induced arthritis (CIA), a mouse model of rheumatoid arthritis (RA), in order to better understand the etiology of RA and identify possible new avenues for therapeutic intervention.

METHODS

We performed a genome-wide analysis of quantitative trait loci controlling autoantibody to type II collagen (anti-CII), anti-citrullinated protein antibody (ACPA), and rheumatoid factor (RF). To identify loci controlling autoantibody production, we induced CIA in a heterogeneous stock-derived mouse cohort, with contribution of 8 inbred mouse strains backcrossed to C57BL/10.Q. Serum samples were collected from 1,640 mice before arthritis onset and at the peak of the disease. Antibody concentrations were measured by standard enzyme-linked immunosorbent assay, and linkage analysis was performed using a linear regression-based method.

RESULTS

We identified loci controlling formation of anti-CII of different IgG isotypes (IgG1, IgG3), antibodies to major CII epitopes (C1, J1, U1), antibodies to a citrullinated CII peptide (citC1), and RF. The anti-CII, ACPA, and RF responses were all found to be controlled by distinct genes, one of the most important loci being the immunoglobulin heavy chain locus.

CONCLUSION

This comprehensive genetic analysis of autoantibody formation in CIA demonstrates an association not only of anti-CII, but interestingly also of ACPA and RF, with arthritis development in mice. These results underscore the importance of non-major histocompatibility complex genes in controlling the formation of clinically relevant autoantibodies.

Human complement receptor 2 (CR2/CD21) as a receptor for DNA: implications for its roles in the immune response and the pathogenesis of systemic lupus erythematosus (SLE)

Human CR2 is a B cell membrane glycoprotein that plays a central role in autoimmunity. Systemic lupus erythematosus (SLE) patients show reduced CR2 levels, and complete deficiency of CR2 and CR1 promotes the development of anti-DNA antibodies in mouse models of SLE. Here we show that multiple forms of DNA, including bacterial, viral and mammalian DNA, bind to human CR2 with moderately high affinity. Surface plasmon resonance studies showed that methylated DNA bound with high affinity with CR2 at a maximal K(D) of 6nM. DNA was bound to the first two domains of CR2 and this binding was blocked by using a specific inhibitory anti-CR2 mAb. DNA immunization in Cr2(-/-) mice revealed a specific defect in immune responses to bacterial DNA. CR2 can act as a receptor for DNA in the absence of complement C3 fixation to this ligand. These results suggest that CR2 plays a role in the recognition of foreign DNA during host-immune responses. This recognition function of CR2 may be a mechanism that influences the development of autoimmunity to DNA in SLE.

Complement receptors 1 and 2 in murine antibody responses to IgM-complexed and uncomplexed sheep erythrocytes

Early complement components are important for normal antibody responses. In this process, complement receptors 1 and 2 (CR1/2), expressed on B cells and follicular dendritic cells (FDCs) in mice, play a central role. Complement-activating IgM administered with the antigen it is specific for, enhances the antibody response to this antigen. Here, bone marrow chimeras between Cr2^−/− and wildtype mice were used to analyze whether FDCs or B cells must express CR1/2 for antibody responses to sheep erythrocytes (SRBC), either administered alone or together with specific IgM. For robust IgG anti-SRBC responses, CR1/2 must be expressed on FDCs. Occasionally, weak antibody responses were seen when only B cells expressed CR1/2, probably reflecting extrafollicular antibody production enabled by co-crosslinking of CR2/CD19/CD81 and the BCR. When SRBC alone was administered to mice with CR1/2⁺ FDCs, B cells from wildtype and Cr2^−/− mice produced equal amounts of antibodies. Most likely antigen is then deposited on FDCs in a way that optimizes engagement of the B cell receptor, making CR2-facilitated signaling to the B cell superfluous. SRBC bound to IgM will have more C3 fragments, the ligands for CR1/2, on their surface than SRBC administered alone. Specific IgM, forming a complex with SRBC, enhances antibody responses in two ways when FDCs express CR1/2. One is dependent on CR1/2⁺ B cells and probably acts via increased transport of IgM-SRBC-complement complexes bound to CR1/2 on marginal zone B cells. The other is independent on CR1/2⁺ B cells and the likely mechanism is that IgM-SRBC-complement complexes bind better to FDCs than SRBC administered alone. These observations suggest that the immune system uses three different CR1/2-mediated effector functions to generate optimal antibody responses: capture by FDCs (playing a dominant role), transport by marginal zone B cells and enhanced B cell signaling.

Transcriptional effects of a lupus-associated polymorphism in the 5' untranslated region (UTR) of human complement receptor 2 (CR2/CD21)

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with a strong genetic component that determines risk. A common three single-nucleotide polymorphism (SNP) haplotype of the complement receptor 2 (CR2) gene has been associated with increased risk of SLE (Wu et al., 2007; Douglas et al., 2009), and a less common haplotype consisting of the major allele at SNP1 and minor alleles at SNP2 and 3 confers protection (Douglas et al., 2009). SNP1 (rs3813946), which is located in the 5' untranslated region (UTR) of the CR2 gene, altered transcriptional activity of a CR2 promoter-luciferase reporter gene construct transiently transfected into a B cell line (Wu et al., 2007) and had an independent effect in the protective haplotype (Douglas et al., 2009). In this study, we show that this SNP alters transcriptional activity in a transiently transfected non B-cell line as well as in stably transfected cell lines, supporting its relevance in vivo. Furthermore, the allele at this SNP affects chromatin accessibility of the surrounding sequence and transcription factor binding. These data confirm the effects of rs3813946 on CR2 transcription, identifying the 5' UTR to be a novel regulatory element for the CR2 gene in which variation may alter gene function and modify the development of lupus.

Essential role of EBF1 in the generation and function of distinct mature B cell types

The transcription factor EBF1 is essential for lineage specification in early B cell development. In this study, we demonstrate by conditional mutagenesis that EBF1 is required for B cell commitment, pro-B cell development, and subsequent transition to the pre-B cell stage. Later in B cell development, EBF1 was essential for the generation and maintenance of several mature B cell types. Marginal zone and B-1 B cells were lost, whereas follicular (FO) and germinal center (GC) B cells were reduced in the absence of EBF1. Activation of the B cell receptor resulted in impaired intracellular signaling, proliferation and survival of EBF1-deficient FO B cells. Immune responses were severely reduced upon Ebf1 inactivation, as GCs were formed but not maintained. ChIP- and RNA-sequencing of FO B cells identified EBF1-activated genes that encode receptors, signal transducers, and transcriptional regulators implicated in B cell signaling. Notably, ectopic expression of EBF1 efficiently induced the development of B-1 cells at the expense of conventional B cells. These gain- and loss-of-function analyses uncovered novel important functions of EBF1 in controlling B cell immunity.

Innate immunity mediates follicular transport of particulate but not soluble protein antigen

Ag retention on follicular dendritic cells (FDCs) is essential for B cell activation and clonal selection within germinal centers. Protein Ag is deposited on FDCs after formation of immune complexes with specific Abs. In this study, by comparing the same antigenic determinant either as soluble protein or virus-like particle (VLP), we demonstrate that VLPs are transported efficiently to murine splenic FDCs in vivo in the absence of prior immunity. Natural IgM Abs and complement were required and sufficient to mediate capture and transport of VLPs by noncognate B cells. In contrast, soluble protein was only deposited on FDCs in the presence of specifically induced IgM or IgG Abs. Unexpectedly, IgG Abs had the opposite effect on viral particles and inhibited FDC deposition. These findings identify size and repetitive structure as critical factors for efficient Ag presentation to B cells and highlight important differences between soluble proteins and viral particles.

Adenovirus-based vaccination against Clostridium difficile toxin A allows for rapid humoral immunity and complete protection from toxin A lethal challenge in mice

Clostridium difficile associated diarrhea (CDAD) is a critical public health problem worldwide with over 300,000 cases every year in the United States alone. Clearly, a potent vaccine preventing the morbidity and mortality caused by this detrimental pathogen is urgently required. However, vaccine efforts to combat C. difficile infections have been limited both in scope as well as to efficacy, as such there is not a vaccine approved for use against C. difficile to date. In this study, we have used a highly potent Adenovirus (Ad) based platform to create a vaccine against C. difficile. The Ad-based vaccine was able to generate rapid and robust humoral as well as cellular (T-cell) immune responses in mice that correlated with provision of 100% protection from lethal challenge with C. difficile toxin A. Most relevant to the clinical utility of this vaccine formulation was our result that toxin A specific IgGs were readily detected in plasma of Ad immunized mice as early as 3 days post vaccination. In addition, we found that several major immuno-dominant T cell epitopes were identified in toxin A, suggesting that the role of the cellular arm in protection from C. difficile infections may be more significant than previously appreciated. Therefore, our studies confirm that an Adenovirus based-C. difficile vaccine could be a promising candidate for prophylactic vaccination both for use in high risk patients and in high-risk environments.

Genetic CD21 deficiency is associated with hypogammaglobulinemia

BACKGROUND

Complement receptor 2 (CR2/CD21) is part of the B-cell coreceptor and expressed by mature B cells and follicular dendritic cells. CD21 is a receptor for C3d-opsonized immune complexes and enhances antigen-specific B-cell responses.

OBJECTIVE

Genetic inactivation of the murine CR2 locus results in impaired humoral immune responses. Here we report the first case of a genetic CD21 deficiency in human subjects.

METHODS

CD21 protein expression was analyzed by means of flow cytometry and Western blotting. CD21 transcripts were quantified by using real-time PCR. The CD21 gene was sequenced. Wild-type and mutant CD21 cDNA expression was studied after transfection of 293T cells. Binding of EBV-gp350 or C3d-containing immune complexes and induction of calcium flux in CD21-deficient B cells were analyzed by means of flow cytometry. Antibody responses to protein and polysaccharide vaccines were measured.

RESULTS

A 28-year-old man presented with recurrent infections, reduced class-switched memory B cells, and hypogammaglobulinemia. CD21 receptor expression was undetectable. Binding of C3d-containing immune complexes and EBV-gp350 to B cells was severely reduced. Sequence analysis revealed a compound heterozygous deleterious mutation in the CD21 gene. Functional studies with anti-immunoglobulin- and C3d-containing immune complexes showed a complete loss of costimulatory activity of C3d in enhancing suboptimal B-cell receptor stimulation. Vaccination responses to protein antigens were normal, but the response to pneumococcal polysaccharide vaccination was moderately impaired.

CONCLUSIONS

Genetic CD21 deficiency adds to the molecular defects observed in human subjects with hypogammaglobulinemia.

Requirement for complement in antibody responses is not explained by the classic pathway activator IgM

Animals lacking complement factors C1q, C2, C3, or C4 have severely impaired Ab responses, suggesting a major role for the classic pathway. The classic pathway is primarily initiated by antigen-Ab complexes. Therefore, its role for primary Ab responses seems paradoxical because only low amounts of specific Abs are present in naive animals. A possible explanation could be that the classic pathway is initiated by IgM from naive mice, binding with sufficient avidity to the antigen. To test this hypothesis, a knock-in mouse strain, Cμ13, with a point mutation in the gene encoding the third constant domain of the μ-heavy chain was constructed. These mice produce IgM in which proline in position 436 is substituted with serine, a mutation previously shown to abrogate the ability of mouse IgM to activate complement. Unexpectedly, the Ab response to sheep erythrocytes and keyhole limpet hemocyanin in Cμ13 mice was similar to that in WT mice. Thus, although secreted IgM and the classic pathway activation are both required for the normal primary Ab response, this does not require that IgM activate C. This led us to test Ab responses in animals lacking one of three other endogenous activators of the classic pathway: specific intracellular adhesion molecule-grabbing nonintegrin R1, serum amyloid P component, and C-reactive protein. Ab responses were also normal in these animals.

Complement activation and complement receptors on follicular dendritic cells are critical for the function of a targeted adjuvant

A detailed understanding of how activation of innate immunity can be exploited to generate more effective vaccines is critically required. However, little is known about how to target adjuvants to generate safer and better vaccines. In this study, we describe an adjuvant that, through complement activation and binding to follicular dendritic cells (FDC), dramatically enhances germinal center (GC) formation, which results in greatly augmented Ab responses. The nontoxic CTA1-DD adjuvant hosts the ADP-ribosylating CTA1 subunit from cholera toxin and a dimer of the D fragment from Staphylococcus aureus protein A. We found that T cell-dependent, but not -independent, responses were augmented by CTA1-DD. GC reactions and serum Ab titers were both enhanced in a dose-dependent manner. This effect required complement activation, a property of the DD moiety. Deposition of CTA1-DD to the FDC network appeared to occur via the conduit system and was dependent on complement receptors on the FDC. Hence, Cr2(-/-) mice failed to augment GC reactions and exhibited dramatically reduced Ab responses, whereas Ribi adjuvant demonstrated unperturbed adjuvant function in these mice. Noteworthy, the adjuvant effect on priming of specific CD4 T cells was found to be intact in Cr2(-/-) mice, demonstrating that the CTA1-DD host both complement-dependent and -independent adjuvant properties. This is the first demonstration, to our knowledge, of an adjuvant that directly activates complement, enabling binding of the adjuvant to the FDC, which subsequently strongly promoted the GC reaction, leading to augmented serum Ab titers and long-term memory development.

T cell hybridomas to study MHC-II restricted B-cell receptor-mediated antigen presentation by human B cells

MHC-II antigen presentation by B cells is essential in order for B cells to receive optimal costimulation from helper CD4+ T cells. This process is facilitated and focused through the extremely efficient uptake, processing, and presentation of antigen recognized by an individual B cell's unique B-cell receptor (BCR). The investigation of human B-cell antigen presentation has been limited by the varied specificity of BCR found in the mixed populations of B cells in vivo. As a result, there is no readily available method to measure BCR-mediated antigen presentation in this heterogeneous population of B cells. We have overcome this limitation by developing HLA-DR-restricted T-cell lines capable of recognizing a specific antigen taken up via the BCR and presented by the mixed B-cell population through this physiologically relevant mechanism. BCR-mediated presentation was enhanced >4 logs compared to presentation by B cells taking up the antigen through nonspecific mechanisms. The studies presented here characterize T-cell hybridoma lines developed for HLA-DRB1*0101+ and HLA-DRB1*1501+ B cells, but clones could be generated for other HLA-DR types using the methods described. These hybridomas have potential applications including study of the mechanisms of BCR-mediated enhancement of presentation, determination of adjuvant effects on presentation, and optimization of vaccine antigen preparations. Therefore, these T-cell lines could significantly facilitate the study of BCR-mediated antigen presentation required by T helper cell-dependent vaccines in humans.

Mice expressing human CR1/CD35 have an enhanced humoral immune response to T-dependent antigens but fail to correct the effect of premature human CR2 expression

We have previously demonstrated that mice expressing human complement receptor type 2 (CR2/CD21) during the CD43(+)/CD25(-) late pro-B cell stage of B cell development have marked changes in their subsequent B cell ontogeny. Here, we show that the humoral immune response to the T cell dependent antigen, sheep red blood cells (SRBCs) can be moderately enhanced with the addition of human CR1 (driven by the lambda promoter/enhancer transgene) to endogenous mCR1/CR2 expression on the B cell surface but that hCR1 expression alone (on the mouse CR1/2 deficient background) has no effect on the humoral immune response or general B cell development. Furthermore, expression of hCR1 had no recuperative effect on the markedly altered B cell phenotype noted with premature expression of hCR2 (either in the presence or absence of endogenous mCR1/2). We conclude that hCR1 alone cannot replace the role of CR2 in mice and that the effects of premature hCR2 expression during BCR development are not significantly altered by the addition of hCR1 at that developmental stage or beyond; thus hCR2 signaling in the mouse remains dominant over subsequent input from either hCR1 or endogenous receptors.

The lectin pathway of complement activation contributes to protection from West Nile virus infection

The function of the lectin pathway of complement activation in vivo against West Nile virus (WNV) or many other pathogenic viruses has not been defined. Mice deficient in lectin pathway recognition molecules (mannose binding lectin-A (MBL-A) and mannose binding lectin-C (MBL-C)) or the effector enzyme mannan-binding lectin-associated serine protease-2 (MASP-2), were more vulnerable to WNV infection than wild type mice. Compared with studies of mice deficient in factors of the classical or alternative pathway, MBL-A(-/-) × MBL-C(-/-) or MASP-2(-/-) mice showed a less severe course of WNV infection. Indeed, a deficiency in lectin pathway activation did not significantly affect the kinetics of viral spread to the central nervous system (CNS) nor did it profoundly alter generation of adaptive B and T cell immune responses. We conclude that MBL-mediated recognition and lectin pathway activation have important yet subordinate functions in protecting against WNV infection and disease.

The cytoskeleton coordinates the early events of B-cell activation

B cells contribute to protective adaptive immune responses through generation of antibodies and long-lived memory cells, following engagement of the B-cell receptor (BCR) with specific antigen. Recent imaging investigations have offered novel insights into the ensuing molecular and cellular events underlying B-cell activation. Following engagement with antigen, BCR microclusters form and act as sites of active signaling through the recruitment of intracellular signaling molecules and adaptors. Signaling through these "microsignalosomes" is propagated and enhanced through B-cell spreading in a CD19-dependent manner. Subsequently, the mature immunological synapse is formed, and functions as a platform for antigen internalization, enabling the antigen presentation to helper T cells required for maximal B-cell activation. In this review, we discuss the emerging and critical role for the cytoskeleton in the coordination and regulation of these molecular events during B-cell activation.

CR2+ marginal zone B cell production of pathogenic natural antibodies is C3 independent

Intestinal ischemia-reperfusion (IR)-induced damage requires complement receptor 2 (CR2) for generation of the appropriate natural Ab repertoire. Pathogenic Abs recognize neoantigens on the ischemic tissue, activate complement, and induce intestinal damage. Because C3 cleavage products act as ligands for CR2, we hypothesized that CR2(hi) marginal zone B cells (MZBs) require C3 for generation of the pathogenic Abs. To explore the ability of splenic CR2(+) B cells to generate the damaging Ab repertoire, we adoptively transferred either MZBs or follicular B cells (FOBs) from C57BL/6 or Cr2(-/-) mice into Rag-1(-/-) mice. Adoptive transfer of wild type CR2(hi) MZBs but not CR2(lo) FOBs induced significant damage, C3 deposition, and inflammation in response to IR. In contrast, similarly treated Rag-1(-/-) mice reconstituted with either Cr2(-/-) MZB/B1 B cells (B1Bs) or FOBs lacked significant intestinal damage and displayed limited complement activation. To determine whether C3 cleavage products are critical in CR2-dependent Ab production, we evaluated the ability of the natural Ab repertoire of C3(-/-) mice to induce damage in response to IR. Infusion of C3(-/-) serum into Cr2(-/-) mice restored IR-induced tissue damage. Furthermore, Rag-1(-/-) mice sustained significant damage after infusion of Abs from C3(-/-) but not Cr2(-/-) mice. Finally, adoptive transfer of MZBs from C3(-/-) mice into Rag-1(-/-) mice resulted in significant tissue damage and inflammation. These data indicate that CR2 expression on MZBs is sufficient to induce the appropriate Abs required for IR-induced tissue damage and that C3 is not critical for generation of the pathogenic Abs.

Trafficking of B cell antigen in lymph nodes

The clonal selection theory first proposed by Macfarlane Burnet is a cornerstone of immunology (1). At the time, it revolutionized the thinking of immunologists because it provided a simple explanation for lymphocyte specificity, immunological memory, and elimination of self-reactive clones (2). The experimental demonstration by Nossal & Lederberg (3) that B lymphocytes bear receptors for a single antigen raised the central question of where B lymphocytes encounter antigen. This question has remained mostly unanswered until recently. Advances in techniques such as multiphoton intravital microscopy (4, 5) have provided new insights into the trafficking of B cells and their antigen. In this review, we summarize these advances in the context of our current view of B cell circulation and activation.

Engineering complement activation on polypropylene sulfide vaccine nanoparticles

The complement system is an important regulator of both adaptive and innate immunity, implicating complement as a potential target for immunotherapeutics. We have recently presented lymph node-targeting, complement-activating nanoparticles (NPs) as a vaccine platform. Here we explore modulation of surface chemistry as a means to control complement deposition, in active or inactive forms, on polypropylene sulfide core, block copolymer Pluronic corona NPs. We found that nucleophile-containing NP surfaces activated complement and became functionalized in situ with C3 upon serum exposure via the alternative pathway. Carboxylated NPs displayed a higher degree of C3b deposition and retention relative to hydroxylated NPs, upon which deposited C3b was more substantially inactivated to iC3b. This in situ functionalization correlated with in vivo antigen-specific immune responses, including antibody production as well as T cell proliferation and IFN-γ cytokine production upon antigen restimulation. Interestingly, inactivation of C3b to iC3b on the NP surface did not correlate with NP affinity to factor H, a cofactor for protease factor I that degrades C3b into iC3b, indicating that control of complement protein C3 stability depends on architectural details in addition to factor H affinity. These data show that design of NP surface chemistry can be used to control biomaterials-associated complement activation for immunotherapeutic materials.

Acquisition and presentation of follicular dendritic cell-bound antigen by lymph node-resident dendritic cells

Follicular dendritic cells (DCs [FDCs]) are prominent stromal cell constituents of B cell follicles with the remarkable ability to retain complement-fixed antigens on their cell surface for extended periods of time. These retained immune complexes have long been known to provide the antigenic stimulus that drives antibody affinity maturation, but their role in cellular immunity has remained unclear. In this study, we show that FDC-retained antigens are continually sampled by lymph node-resident DCs for presentation to CD8 T cells. This novel pathway of antigen acquisition was detectable when FDCs were loaded with purified antigens bound into classical antigen-antibody immune complexes, as well as after pregnancy, when they are loaded physiologically with antigens associated with the complement-fixed microparticles released from the placenta into maternal blood. In both cases, ensuing antigen presentation was profoundly tolerogenic, as it induced T cell deletion even under inflammatory conditions. These results significantly broaden the scope of FDC function and suggest new ways that the complement system and persistent antigen presentation might influence T cell activation and the maintenance of peripheral immune tolerance.

Low concentrations of anti-Aβ antibodies generated in Tg2576 mice by DNA epitope vaccine fused with 3C3d molecular adjuvant do not affect AD pathology

It has been demonstrated that an active vaccination strategy with protein- or DNA-based epitope vaccines composed of the immunodominant self B cell epitope of amyloid-β₄₂ (Aβ₄₂) and a non-self T helper (Th) cell epitope is an immunotherapeutic approach to preventing or treating Alzheimer's disease (AD). As a DNA-based epitope vaccine, we used a plasmid encoding three copies of Aβ(1-11) and Th cell epitope, PADRE (p3Aβ(1-11)-PADRE). We have previously reported that three copies of component of complement C3d (3C3d) acts as a molecular adjuvant significantly enhancing immune responses in wild-type mice of the H2(b) haplotype immunized with p3Aβ(1-11)-PADRE. Here, we tested the efficacy of p3Aβ(1-11)-PADRE and the same vaccine fused with 3C3d (p3Aβ(1-11)-PADRE-3C3d) in a transgenic (Tg) mouse model of AD (Tg2576) of the H2(bxs) immune haplotype. The overall responses to both vaccines were very weak in Tg2576 mice despite the fact that the 3C3d molecular adjuvant significantly enhanced the anti-Aβ response to 3Aβ(1-11)-PADRE. Importantly, generation of low antibody responses was associated with the strain of amyloid precursor protein Tg mice rather than with a molecular adjuvant, as a p3Aβ(1-11)-PADRE-3C3d vaccine induced significantly higher antibody production in another AD mouse model, 3xTg-AD of the H2(b) haplotype. Finally, this study demonstrated that low concentrations of antibodies generated by both DNA vaccines were not sufficient for the reduction of Aβ pathology in the brains of vaccinated Tg2576 animals, confirming previous reports from preclinical studies and the AN-1792 clinical trials, which concluded that the concentration of anti-Aβ antibodies may be essential for the reduction of AD pathology.

Complement depletion with cobra venom factor delays acute cell-mediated rejection in pig-to-mouse corneal xenotransplantation

BACKGROUND

We have demonstrated earlier that porcine corneal xenografts underwent an acute cell-mediated rejection in mice despite the absence of T cells. In the present study, we investigated the effect of complement depletion by cobra venom factor (CVF) on the corneal xenograft rejection in a pig-to-mouse model.

METHODS

Porcine corneas were orthotopically transplanted into C57BL/6 (B6) and severe combined immunodeficiency (SCID) mice. For complement depletion, 25 microg of CVF (1 g/kg bodyweight) was injected intraperitoneally on the day before and 1, 3, 5, and 7 days after transplantation. Graft survival was clinically assessed by slit lamp biomicroscopy and the median survival time (MST) was calculated. The grafts were histologically evaluated serially after transplantation using antibodies against CD4, CD8, NK1.1, and F4/80.

RESULTS

The CVF treatment significantly prolonged the porcine corneal xenograft survival in both B6 (MST 9.4 vs. 15.5 days; P = 0.0011) and SCID mice (MST 16.4 vs. 20.5 days; P = 0.0474). Histologically, whereas macrophages and CD4(+) T cells were progressively infiltrated into porcine corneal grafts in CVF-untreated B6 mice, the infiltration by both cells was markedly delayed and decreased in the xenografts in CVF-treated B6 mice. Likewise, macrophage infiltration, which was prominent in rejected porcine xenografts in SCID mice, was also reduced in CVF-treated SCID mice.

CONCLUSIONS

Our results suggest that complement depletion by CVF delayed, although did not prevent, an acute cell-mediated rejection in a pig-to-mouse corneal xenotransplantation.

Modulation of B cell responses by Toll-like receptors

B lymphocytes are well known because of their key role in mediating humoral immune responses. Upon encounter with antigen and on cognate interaction with T cells, they differentiate into antibody-secreting plasma cells, which are critical for protection against a variety of pathogens. In addition to their antibody-production function, B cells are efficient antigen-presenting cells and express a variety of pathogen recognition receptors (PRRs). Engagement of these PRRs with their respective ligands results in cytokine and chemokine secretion and the upregulation of co-stimulatory molecules. These events constitute innate immune responses. Toll-like receptor (TLR) activation provides a third signal for B cell activation and is essential for optimal antigen-specific antibody responses. In some situations, TLR activation in B cells can result in autoimmunity. The purpose of this review is to provide some insights into the way that TLRs influence innate and adaptive B cell responses.

Complement-dependent transport of antigen into B cell follicles

Since the original proposal by Fearon and Locksley (Fearon and Locksley. 1996. Science 272: 50-53) that the complement system linked innate and adaptive immunity, there has been a rapid expansion of studies on this topic. With the advance of intravital imaging, a number of recent papers revealed an additional novel pathway in which complement C3 and its receptors enhance humoral immunity through delivery of Ag to the B cell compartment. In this review, we discuss this pathway and highlight several novel exceptions recently found with a model influenza vaccine, such as mannose-binding lectin opsonization of influenza and uptake by macrophages, and the capture of virus by dendritic cells residing in the medullary compartment of peripheral lymph nodes.

An allelic variant of Crry in the murine Sle1c lupus susceptibility interval is not impaired in its ability to regulate complement activation

The Sle1c subinterval on distal murine chromosome 1 confers loss of tolerance to chromatin. Cr2, which encodes complement receptors 1 and 2 (CR1/CR2; CD35/CD21), is a strong candidate gene for lupus susceptibility within this interval based on structural and functional alterations in its protein products. CR1-related protein/gene Y (Crry) lies 10 kb from Cr2 and encodes a ubiquitously expressed complement regulatory protein that could also play a role in the pathogenesis of systemic lupus erythematosus. Crry derived from B6.Sle1c congenic mice migrated at a higher m.w. by SDS-PAGE compared with B6 Crry, as a result of differential glycosylation. A single-nucleotide polymorphism in the first short consensus repeat of Sle1c Crry introduced a novel N-linked glycosylation site likely responsible for this structural alteration. Five additional single-nucleotide polymorphisms in the signal peptide and short consensus repeat 1 of Sle1c Crry were identified. However, the cellular expression of B6 and B6.Sle1c Crry and their ability to regulate the classical pathway of complement were not significantly different. Although soluble Sle1c Crry regulated the alternative pathway of complement more efficiently than B6 Crry, as a membrane protein, it regulated the alternative pathway equivalently to B6 Crry. These data fail to provide evidence for a functional effect of the structural alterations in Sle1c Crry and suggest that the role of Cr2 in the Sle1c autoimmune phenotypes can be isolated in recombinant congenic mice containing both genes.

The structure, genetic polymorphisms, expression and biological functions of complement receptor type 1 (CR1/CD35)

The complement system is comprised of soluble and cell surface associated proteins that recognize exogenous, altered, or potentially harmful endogenous ligands. In recent years, the complement system--particularly component C3 and its receptors--have been demonstrated to be a key link between innate and adaptive immunity. Complement receptor type 1 (CR1), the receptor for C3b/C4b complement peptides, has emerged as a molecule of immense interest in gaining insight to the susceptibility, pathophysiology, diagnosis, prognosis and therapy of such diseases. In this review, we wish to briefly bring forth the structure, genetic polymorphisms, expression and biological functions of CR1.

Abeta DNA vaccination for Alzheimer's disease: focus on disease prevention

Pre-clinical and clinical data suggest that the development of a safe and effective anti-amyloid-beta (Abeta) immunotherapy for Alzheimer's disease (AD) will require therapeutic levels of anti-Abeta antibodies, while avoiding proinflammatory adjuvants and autoreactive T cells which may increase the incidence of adverse events in the elderly population targeted to receive immunotherapy. The first active immunization clinical trial with AN1792 in AD patients was halted when a subset of patients developed meningoencephalitis. The first passive immunotherapy trial with bapineuzumab, a humanized monoclonal antibody against the end terminus of Abeta, also encountered some dose dependent adverse events during the Phase II portion of the study, vasogenic edema in 12 cases, which were significantly over represented in ApoE4 carriers. The proposed remedy is to treat future patients with lower doses, particularly in the ApoE4 carriers. Currently there are at least five ongoing anti-Abeta immunotherapy clinical trials. Three of the clinical trials use humanized monoclonal antibodies, which are expensive and require repeated dosing to maintain therapeutic levels of the antibodies in the patient. However in the event of an adverse response to the passive therapy antibody delivery can simply be halted, which may provide a resolution to the problem. Because at this point we cannot readily identify individuals in the preclinical or prodromal stages of AD pathogenesis, passive immunotherapy is reserved for those that already have clinical symptoms. Unfortunately those individuals have by that point accumulated substantial neuropathology in affected regions of the brain. Moreover, if Abeta pathology drives tau pathology as reported in several transgenic animal models, and once established if tau pathology can become self propagating, then early intervention with anti-Abeta immunotherapy may be critical for favorable clinical outcomes. On the other hand, active immunization has several significant advantages, including lower cost and the typical immunization protocol should be much less intrusive to the patient relative to passive therapy, in the advent of Abeta-antibody immune complex-induced adverse events the patients will have to receive immuno-supperssive therapy for an extended period until the anti Abeta antibody levels drop naturally as the effects of the vaccine decays over time. Obviously, improvements in vaccine design are needed to improve both the safety, as well as the efficacy of anti-Abeta immunotherapy. The focus of this review is on the advantages of DNA vaccination for anti-Abeta immunotherapy, and the major hurdles, such as immunosenescence, selection of appropriate molecular adjuvants, universal T cell epitopes, and possibly a polyepitope design based on utilizing existing memory T cells in the general population that were generated in response to childhood or seasonal vaccines, as well as various infections. Ultimately, we believe that the further refinement of our AD DNA epitope vaccines, possibly combined with a prime boost regime will facilitate translation to human clinical trials in either very early AD, or preferably in preclinical stage individuals identified by validated AD biomarkers.

Complement and its role in innate and adaptive immune responses

The complement system plays a crucial role in the innate defense against common pathogens. Activation of complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules. More recently, however, the role of complement in the immune response has been expanded due to observations that link complement activation to adaptive immune responses. It is now appreciated that complement is a functional bridge between innate and adaptive immune responses that allows an integrated host defense to pathogenic challenges. As such, a study of its functions allows insight into the molecular underpinnings of host-pathogen interactions as well as the organization and orchestration of the host immune response. This review attempts to summarize the roles that complement plays in both innate and adaptive immune responses and the consequences of these interactions on host defense.

The contribution of naturally occurring IgM antibodies, IgM cross-reactivity and complement dependency in murine humoral responses to pneumococcal capsular polysaccharides

Immunogenicity of 12 capsular polysaccharides (CPS) from Streptococcus pneumoniae did not correlate with pre-existing levels of natural IgM anti-CPS antibodies in mice. Immunization of mice with individual CPS, with the exception of type 14 (the only neutral CPS tested), increased serum IgM that also bound other CPS serotypes independent of structural similarity or commonly known contaminants. Surprisingly only IgM response to type 4 (which has a small immunodominant epitope) was dependent on either complement C3 or complement receptors CD35/CD21. IgG anti-CPS responses were infrequently induced, but critically dependent on complement. Our results have clarified the role of complement in the induction of IgM and IgG anti-CPS antibody responses in mice and have implications for CPS vaccine development.

Uncoupling CD21 and CD19 of the B-cell coreceptor

Complement receptors (CRs) CD21 and CD35 form a coreceptor with CD19 and CD81 on murine B cells that when coligated with the B-cell receptor lowers the threshold of activation by several orders of magnitude. This intrinsic signaling role is thought to explain the impaired humoral immunity of mice bearing deficiency in CRs. However, CRs have additional roles on B cells independent of CD19, such as transport of C3-coated immune complexes and regulation of C4 and C3 convertase. To test whether association of CR with CD19 is necessary for their intrinsic activation-enhancing role, knockin mice expressing mutant receptors, Cr2(Delta/Deltagfp), that bind C3 ligands but do not signal through CD19 were constructed. We found that uncoupling of CR and CD19 significantly diminishes survival of germinal center B cells and secondary antibody titers. However, B memory is less impaired relative to mice bearing a complete deficiency in CRs on B cells. These findings confirm the importance of interaction of CR and CD19 for coreceptor activity in humoral immunity but identify a role for CR in B-cell memory independent of CD19.

Enhanced humoral immune responses against T-independent antigens in Fc alpha/muR-deficient mice

IgM is an antibody class common to all vertebrates that plays a primary role in host defenses against infection. Binding of IgM with an antigen initiates the complement cascade, accelerating cellular and humoral immune responses. However, the functional role of the Fc receptor for IgM in such immune responses remains obscure. Here we show that mice deficient in Fc alpha/muR, an Fc receptor for IgM expressed on B cells and follicular dendritic cells (FDCs), have enhanced germinal center formation and affinity maturation and memory induction of IgG3(+) B cells after immunization with T-independent (TI) antigens. Moreover, Fc alpha/muR-deficient mice show prolonged antigen retention by marginal zone B (MZB) cells and FDCs. In vitro studies demonstrate that interaction of the IgM immune complex with Fc alpha/muR partly suppress TI antigen retention by MZB cells. We further show that downregulation of complement receptor (CR)1 and CR2 or complement deprivation by in vivo injection with anti-CR1/2 antibody or cobra venom factor attenuates antigen retention by MZB cells and germinal center formation after immunization with TI antigens in Fc alpha/muR(-/-) mice. Taken together, these results suggest that Fc alpha/muR negatively regulates TI antigen retention by MZB cells and FDCs, leading to suppression of humoral immune responses against T-independent antigens.

Defective B cell ontogeny and humoral immune response in mice prematurely expressing human complement receptor 2 (CR2, CD21) is similar to that seen in aging wild type mice

Mice prematurely expressing human CR2 (hCR2) in the B cell lineage have a defective B cell ontogeny and humoral immune response. We have previously determined altered tyrosine phosphorylation patterns within hCR2 transgenic mice, suggesting that irreversible changes in B cell signaling pathways had occurred, which could explain the B cell unresponsiveness associated with hCR2 transgene expression. In support of that assertion, we found that increasing antigen dose or addition of adjuvant had a minimal impact on the ability of B cells to respond to antigen. However, analysis of aged hCR2(high) mice (1 year plus) revealed that both B cell numbers, B cell sub-population distribution including expansion of a newly described B regulatory cell subset, and immune responses were comparable with age-matched hCR2 negative mice. Finally, we established that B cell unresponsiveness to antigen in aging wild type mice (1 year plus) was equivalent to that noted in 3-month-old hCR2(high) mice. This data provides evidence that 3-month-old hCR2(high) mice have a humoral immune system resembling aged mice and suggests that further examination of the precise molecular and cellular parallels between aged wild type mice and 3-month-old hCR2(high) mice could provide an important insight into the mechanisms which lead to B cell unresponsiveness in the aging immune system.

B cell acquisition of antigen in vivo

The fate of B lymphocytes is dictated in large part by cognate antigen and the environment in which it is encountered. Yet we are only now beginning to understand where and how B cells acquire antigen. Recent studies identify multiple pathways by which lymph-borne antigens enter the B cell follicles of LNs. Size is a major factor as particulate antigens and large IC are bound by subcapsular sinus macrophages. By contrast, small antigens (under 70kDa) are rapidly channeled into follicles via conduits secreted by fibroblastic reticular cells (FRC). Interestingly, the conduits not only deliver antigen to follicular dendritic cells (FDC) but also provide a rich source of B cell chemokine, that is, CXCL-13. Thus, the follicular conduits provide an 'antigen highway' for B cells trafficking within the LN. These new findings provide an important discovery in understanding how B cells acquire cognate antigen.

Complement and humoral immunity

The complement system was discovered almost a century ago as an important effector in antibody-dependent killing of microorganisms. Since this early period much was learned aboutthe biochemistry and structure of complement proteins and their function in mediating inflammation. More recently, a prominent role for complement was identified in linkage of innate and adaptive immunity. In this review, I will discuss our current understanding of the importance of complement in enhancing the humoral immune response to both model antigens and pathogens. As discussed below, it is evident that the complement system participates in marking of "foreign" pathogens and "presenting" them to B cells in a manner that enhances both antibody production and long-term memory. In this special issue of Vaccine, we see examples of how complement is critical in the immune response to bacterial and viral pathogens. Moreover, the finding that most organisms have co-evolved proteins to evade complement detection underscores its importance in host protection.

Complement and natural antibody are required in the long-term memory response to influenza virus

Complement, complement receptors and natural antibody (IgM) are important factors in the immune response against pathogens. Previous studies have indicated a role for C3, the complement receptors CD35/CD21 (CR1/CR2), and IgM in the immune response to influenza virus. Nevertheless, their contribution to the long-term memory response to this pathogen remains unknown. To elucidate this role, we characterized the secondary response on mice deficient of CR1/CR2 (Cr2-/-), C3 (C3-/-), secreted IgM (micros-/-) and the double knockout C3-/-micros-/-. Overall, our results suggest that C3, IgM and CR1/CR2 play crucial roles in the maintenance of long-term memory to influenza virus, possibly through the development of memory B cells and long-term antibody secretion.

Adaptive immune response in JAM-C-deficient mice: normal initiation but reduced IgG memory

We have recently shown that junctional adhesion molecule (JAM)-C-deficient mice have leukocytic pulmonary infiltrates, disturbed neutrophil homeostasis, and increased postnatal mortality. This phenotype was partially rescued when mice were housed in ventilated isolators, suggesting an inability to cope with opportunistic infections. In the present study, we further examined the adaptive immune responses in JAM-C(-/-) mice. We found that murine conventional dendritic cells express in addition to Mac-1 and CD11c also JAM-B as ligand for JAM-C. By in vitro adhesion assay, we show that murine DCs can interact with recombinant JAM-C via Mac-1. However, this interaction does not seem to be necessary for dendritic cell migration and function in vivo, even though JAM-C is highly expressed by lymphatic sinuses of lymph nodes. Nevertheless, upon immunization and boosting with a protein Ag, JAM-C-deficient mice showed decreased persistence of specific circulating Abs although the initial response was normal. Such a phenotype has also been observed in a model of Ag-induced arthritis, showing that specific IgG2a Ab titers are reduced in the serum of JAM-C(-/-) compared with wild-type mice. Taken together, these data suggest that JAM-C deficiency affects the adaptive humoral immune response against pathogens, in addition to the innate immune system.