Human complement receptor type 2 (CR2/CD21) transgenic mice provide an in vivo model to study immunoregulatory effects of receptor antagonists

A fusion protein that targets antigen-loaded extracellular vesicles to B cells enhances antigen-specific T cell expansion

Extracellular vesicles (EVs) have the potential to modulate immune responses via their cargo molecules and are being explored as vehicles in cancer immunotherapy. Dendritic cell-derived EVs can induce antigen-specific immune responses leading to reduced tumor burden. This response was shown to depend partially on B cells. EVs can be targeted to certain cells or tissues, and EVs from Epstein-Barr Virus (EBV) infected cells were shown to carry the EBV glycoprotein GP350 on their surface and target human CD21 (hCD21) on B cells. We therefore investigated whether targeting EVs to B cells via this mechanism could improve antigen-specific immune responses. A soluble fusion protein containing the phosphatidylserine-binding domain (C1C2) of lactadherin and hCD21-binding domain (D123) of GP350 was used to decorate and target EVs to B cells. D123-decorated EVs increased in vitro B cell targeting 5-fold compared to EVs decorated with a non-targeting control protein or undecorated EVs. Furthermore, in vivo, D123-decoration did not alter the biodistribution of EVs across organs but specifically targeted them to B cells in the spleen, blood and lymph nodes of hCD21-transgenic mice. Immunization with hCD21-targeted, OVA-loaded EVs resulted in a higher percentage of antigen-specific CD8+ T cells compared to untargeted EVs. Our data show that D123-decorated EVs efficiently target B cells and improve antigen-specific T cell responses in vivo, which could be explored in future therapeutic applications.

Insights Into the Structure-Function Relationships of Dimeric C3d Fragments

Cleavage of C3 to C3a and C3b plays a central role in the generation of complement-mediated defences. Although the thioester-mediated surface deposition of C3b has been well-studied, fluid phase dimers of C3 fragments remain largely unexplored. Here we show C3 cleavage results in the spontaneous formation of C3b dimers and present the first X-ray crystal structure of a disulphide-linked human C3d dimer. Binding studies reveal these dimers are capable of crosslinking complement receptor 2 and preliminary cell-based analyses suggest they could modulate B cell activation to influence tolerogenic pathways. Altogether, insights into the physiologically-relevant functions of C3d(g) dimers gained from our findings will pave the way to enhancing our understanding surrounding the importance of complement in the fluid phase and could inform the design of novel therapies for immune system disorders in the future.

New aspects in the regulation of human B cell functions by complement receptors CR1, CR2, CR3 and CR4

The involvement of complement in the regulation of antibody responses has been known for long. By now several additional B cell functions - including cytokine production and antigen presentation - have also been shown to be regulated by complement proteins. Most of these important activities are mediated by receptors interacting with activation fragments of the central component of the complement system C3, such as C3b, iC3b and C3d, which are covalently attached to antigens and immune complexes. This review summarizes the role of complement receptors interacting with these ligands, namely CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18) and CR4 (CD11c/CD18) expressed by B cells in health and disease. Although we focus on human B lymphocytes, we also aim to call the attention to important differences between human and mouse systems.

Revisiting the Coreceptor Function of Complement Receptor Type 2 (CR2, CD21); Coengagement With the B-Cell Receptor Inhibits the Activation, Proliferation, and Antibody Production of Human B Cells

The positive coreceptor function of complement receptor type 2 [CR2 (CD21)] on B cells is generally accepted, although its role in the enhancement of antibody production had only been proven in mice. The importance of this phenomenon prompted reinvestigation of the functional consequences of coclustering CD21 and the B cell receptor (BCR) on primary human cells. We found that, at non-stimulatory concentrations of anti-IgG/A/M, coclustering the BCR and CR2 enhanced the Ca²⁺ response, while activation marker expression, cytokine production, proliferation, and antibody production were all inhibited upon the coengagement of CR2 and BCR on human B cells. Thus, the “textbook dogma” claiming that C3d acts as an adjuvant to enhance humoral immunity is relevant only to mice and not to humans.

A salt-out strategy for purification of amphiphilic polymer-coated quantum dots

A purification strategy is presented that the OPA micelles are precipitated selectively in an OPA–QD solution by adding NaCl.

CD21 and FCRL5 form a receptor complex with robust B-cell activating capacity

The B-cell response to antigen is critically regulated by co-receptors. CD21 (complement receptor 2) amplifies the response to antigen linked to its ligands, specific C3 fragments. In contrast, human Fc receptor-like 5 (FCRL5), a novel IgG receptor, was reported to inhibit B-cell receptor (BCR) signaling. Here, we show that CD21 and FCRL5 physically associate, suggesting that immune complexes containing both C3 fragment and IgG could simultaneously engage the pre-assembled receptors. We found that activating signaling molecules such as CD19, active PLCγ2 and BTK were rapidly recruited to FCRL5 upon engagement, suggesting a novel activating function for FCRL5. We confirmed that FCRL5 through its ITIMs (immunoreceptor tyrosine-based inhibitory motif) inhibited BCR signaling in the absence of CD21 stimulation. In contrast, triple engagement of FCRL5, CD21 and the BCR led to a superior calcium response compared to CD21 and BCR co-stimulation, in both cell lines and tonsil B cells. Furthermore, the novel activating function was independent of established FCRL5 signaling motifs. While human peripheral B cells express either FCRL5 or CD21, we identified a sizable subset of tonsil B cells which co-express the two receptors. We propose that FCRL5 has dual signaling capacity, while CD21 co-engagement serves as molecular switch, converting FCRL5 from a negative to a positive co-receptor. In tissues, B cells that co-express FCRL5 and CD21 could robustly respond to IgG immune complexes loaded with C3 fragments.

Deficiencies in the CD19 complex

Signaling via the CD19-complex, consisting of CD19, CD81, CD21 and CD225, is critically important for B-cell development, differentiation and maturation. In this complex, each protein has its own distinct function. Over the past decade, 15 patients with antibody deficiency due to deficiencies in the CD19-complex have been described. These patients have deficiencies in different complex-members, all caused by either homozygous or compound heterozygous mutations. Although all patients had antibody deficiencies, the clinical phenotype was different per deficient protein. We aimed to provide an overview of what is known about the function of the different complex-members, knowledge from mouse-studies and to summarize the clinical phenotypes of the patients. Combining this knowledge together can explain why deficiencies in different members of the same complex, result in disease phenotypes that are alike, but not the same.

A novel C3d-containing oligomeric vaccine provides insight into the viability of testing human C3d-based vaccines in mice

The use of C3d, the final degradation product of complement protein C3, as a "natural" adjuvant has been widely examined since the initial documentation of its immunogenicity-enhancing properties as a consequence of binding to complement receptor 2. Subsequently it was demonstrated that these effects are most evident when oligomeric, rather than when monomeric forms of C3d, are linked to various test protein antigens. In this study, we examined the feasibility of enhancing the adjuvant properties of human C3d further by utilizing C4b-binding protein (C4BP) to provide an oligomeric arrayed scaffold fused to the model antigen, tetanus toxin C fragment (TTCF). High molecular weight, C3d-containing oligomeric vaccines were successfully expressed, purified from mammalian cells and used to immunize groups of mice. Surprisingly, anti-TTCF antibody responses measured in these mice were poor. Subsequently we established by in vitro and in vivo analysis that, in the presence of mouse C3, human C3d does not interact with either mouse or even human complement receptor 2. These data confirm the requirement to develop murine versions of C3d based adjuvant compounds to test in mice or that mice would need to be developed that express both human C3 and human CR2 to allow the testing of human C3d based adjuvants in mouse in any capacity.

Preparation of Monodisperse Hydrophilic Quantum Dots with Amphiphilic Polymers

Monodisperse hydrophilic quantum dots (QDs) are promising labeling materials for biomedical applications. However, the controllable preparation of monodisperse hydrophilic QDs with amphiphilic polymers remains a challenge. Herein, the molecular structures of amphiphilic polymers assembled on different-sized QDs are investigated. Both the experimental results and the molecular dynamics (MD) calculation suggest that the grafting ratio of amphiphilic polymers assembled on QDs increases as the size of QDs increases. Thus, the controllable preparation of different-sized monodisperse hydrophilic QDs can be achieved by simply varying the grafting ratio of amphiphilic molecules and applied in the simultaneous labeling of three tumor biomarkers.

Relative Impact of Complement Receptors CD21/35 (Cr2/1) on Scrapie Pathogenesis in Mice

Mammalian prion diseases are caused by prions, unique infectious agents composed primarily, if not solely, of a pathologic, misfolded form of a normal host protein, the cellular prion protein (PrP^C). Prions replicate without a genetic blueprint, but rather contact PrP^C and coerce it to misfold into more prions, which cause neurodegeneration akin to other protein-misfolding diseases like Alzheimer’s disease. A single gene produces two alternatively spliced mRNA transcripts that encode mouse complement receptors CD21/35, which promote efficient prion replication in the lymphoid system and eventual movement to the brain. Here we show that CD21/35 are high-affinity prion receptors, but mice expressing only CD21 die from prion disease sooner than CD35-expressing mice, which contain less prions early after infection and exhibit delayed terminal disease, likely due to their less organized splenic follicles. Thus, CD21 appears to be more important for defining splenic architecture that influences prion pathogenesis.

Complement receptors 1 and 2 (CR1/2 or CD35/CD21) recognize complement-opsonized antigens to initiate innate and adaptive immunity, respectively. CD35 stimulates phagocytosis on macrophages and antigen presentation on follicular dendritic cells (FDCs). CD21 helps activate B cells as part of the B cell coreceptor with CD19 and CD81. Differential splicing of transcripts from the mouse Cr2 gene generates isoforms with both shared and unique complement binding capacities and cell-type expression. In mouse models, genetic depletion of Cr2 causes either a delay or complete prevention of prion disease, but the relative importance of CD35 versus CD21 in promoting prion disease remains unknown. Here we show that both isoforms act as high-affinity cell surface prion receptors. However, mice lacking CD21 succumbed to terminal prion disease significantly later than mice lacking CD35 or wild-type and hemizygous mice. CD21-deficient mice contained fewer splenic prions than CD35 knockout mice early after infection that contributed to delayed prion neuroinvasion and terminal disease, despite forming follicular networks closer to proximal nerves. While we observed no difference in B cell networks, PrP^C expression, or number of follicles, CD21-deficient mice formed more fragmented, less organized follicular networks with fewer Mfge8-positive FDCs and/or tingible body macrophages (TBMφs) than wild-type or CD35-deficient mice. In toto, these data demonstrate a more prominent role for CD21 for proper follicular development and organization leading to more efficient lymphoid prion replication and expedited prion disease than in mice expressing the CD35 isoform.

IMPORTANCE Mammalian prion diseases are caused by prions, unique infectious agents composed primarily, if not solely, of a pathologic, misfolded form of a normal host protein, the cellular prion protein (PrP^C). Prions replicate without a genetic blueprint, but rather contact PrP^C and coerce it to misfold into more prions, which cause neurodegeneration akin to other protein-misfolding diseases like Alzheimer’s disease. A single gene produces two alternatively spliced mRNA transcripts that encode mouse complement receptors CD21/35, which promote efficient prion replication in the lymphoid system and eventual movement to the brain. Here we show that CD21/35 are high-affinity prion receptors, but mice expressing only CD21 die from prion disease sooner than CD35-expressing mice, which contain less prions early after infection and exhibit delayed terminal disease, likely due to their less organized splenic follicles. Thus, CD21 appears to be more important for defining splenic architecture that influences prion pathogenesis.

A new mouse anti-mouse complement receptor type 2 and 1 (CR2/CR1) monoclonal antibody as a tool to study receptor involvement in chronic models of immune responses and disease

Although reagents are available to block mouse complement receptor type 2 and/or type 1 (CR2/CR1, CD21/CD35) function in acute or short term models of human disease, a mouse anti-rat antibody response limits their use in chronic models. We have addressed this problem by generating in Cr2−/− mice a mouse monoclonal antibody (mAb 4B2) to mouse CR2/CR1. The binding of murine mAb 4B2 to CR2/CR1 directly blocked C3dg (C3d) ligand binding. In vivo injection of mAb 4B2 induced substantial down regulation of CR2 and CR1 from the B cell surface, an effect that lasted six weeks after a single injection of 2 mg of mAb. The 4B2 mAb was studied in vivo for the capability to affect immunological responses to model antigens. Pre-injection of mAb 4B2 before immunization of C57BL/6 mice reduced the IgG1 antibody response to the T-dependent antigen sheep red blood cells (SRBC) to a level comparable to that found in Cr2−/− mice. We also used the collagen-induced arthritis (CIA) model, a CR2/CR1-dependent autoimmune disease model, and found that mice pre-injected with mAb 4B2 demonstrated substantially reduced levels of pathogenic IgG2a antibodies to both the bovine type II collagen (CII) used to induce arthritis and to endogenous mouse CII. Consistent with this result, mice pre-injected with mAb 4B2 demonstrated only very mild arthritis. This reduction in disease, together with published data in CII-immunized Cr2−/− mice, confirm both that the arthritis development depends on CR2/CR1 receptors and that mAb 4B2 can be used to induce biologically relevant receptor blockade. Thus mAb 4B2 is an excellent candidate for use in chronic murine models to determine how receptor blockage at different points modifies disease activity and autoantibody responses.

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.

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.