Specific in vivo deletion of B-cell subpopulations expressing human immunoglobulins by the B-cell superantigen protein L

The Retropepsin-Type Protease APRc as a Novel Ig-Binding Protein and Moonlighting Immune Evasion Factor of Rickettsia

Rickettsiae are obligate intracellular Gram-negative bacteria transmitted by arthropod vectors. Despite their reduced genomes, the function(s) of the majority of rickettsial proteins remains to be uncovered. APRc is a highly conserved retropepsin-type protease, suggested to act as a modulator of other rickettsial surface proteins with a role in adhesion/invasion. However, APRc’s function(s) in bacterial pathogenesis and virulence remains unknown. This study demonstrates that APRc targets host serum components, combining nonimmune immunoglobulin (Ig)-binding activity with resistance to complement-mediated killing. We confirmed nonimmune human IgG binding in extracts of different rickettsial species and intact bacteria. Our results revealed that the soluble domain of APRc is capable of binding to human (h), mouse, and rabbit IgG and different classes of human Ig (IgG, IgM, and IgA) in a concentration-dependent manner. APRc-hIgG interaction was confirmed with total hIgG and normal human serum. APRc-hIgG displayed a binding affinity in the micromolar range. We provided evidence of interaction preferentially through the Fab region and confirmed that binding is independent of catalytic activity. Mapping the APRc region responsible for binding revealed the segment between amino acids 157 and 166 as one of the interacting regions. Furthermore, we demonstrated that expression of the full-length protease in Escherichia coli is sufficient to promote resistance to complement-mediated killing and that interaction with IgG contributes to serum resistance. Our findings position APRc as a novel Ig-binding protein and a novel moonlighting immune evasion factor of Rickettsia, contributing to the arsenal of virulence factors utilized by these intracellular pathogens to aid in host colonization.

A B-Cell Superantigen Induces the Apoptosis of Murine and Human Malignant B Cells

B-cell superantigens (Sags) bind to conserved sites of the VH or VL regions of immunoglobulin molecules outside their complementarity-determining regions causing the apoptosis of normal cognate B cells. No attempts to investigate whether B-cell Sags are able to induce the apoptosis of cognate malignant B cells were reported. In the present study we show that protein L (PpL), secreted by Finegoldia magna, a B-cell Sag which interacts with κ+ bearing cells, induces the apoptosis of murine and human κ+ lymphoma B cells both in vitro and in vivo. Apoptosis was not altered by caspase-8 inhibitor. No alterations in the levels of Bid, Fas and Fas-L were found suggesting that PpL does not activate the extrinsic pathway of apoptosis. The involvement of the intrinsic pathway was clearly indicated by: i) alterations in mitochondrial membrane potential (ΔΨm) both in murine and human lymphoma cells exposed to PpL; ii) decreased levels of apoptosis in the presence of caspase-9 inhibitor; iii) significant increases of Bim and Bax protein levels and downregulation of Bcl-2; iv) the translocation from the cytoplasm to the mitochondria of Bax and Bim pro-apoptotic proteins and its inhibition by caspase-9 inhibitor but not by caspase-8 inhibitor and v) the translocation of Bcl-2 protein from the mitochondria to the cytosol and its inhibition by caspase-9 inhibitor but not by caspase-8 inhibitor. The possibility of a therapeutic use of Sags in lymphoma/leukemia B cell malignancies is discussed.

Antibody light chain variable domains and their biophysically improved versions for human immunotherapy

We set out to gain deeper insight into the potential of antibody light chain variable domains (V_Ls) as immunotherapeutics. To this end, we generated a naïve human V_L phage display library and, by using a method previously shown to select for non-aggregating antibody heavy chain variable domains (V_Hs), we isolated a diversity of V_L domains by panning the library against B cell super-antigen protein L. Eight domains representing different germline origins were shown to be non-aggregating at concentrations as high as 450 µM, indicating V_L repertoires are a rich source of non-aggregating domains. In addition, the V_Ls demonstrated high expression yields in E. coli, protein L binding and high reversibility of thermal unfolding. A side-by-side comparison with a set of non-aggregating human V_Hs revealed that the V_Ls had similar overall profiles with respect to melting temperature (T_m), reversibility of thermal unfolding and resistance to gastrointestinal proteases. Successful engineering of a non-canonical disulfide linkage in the core of V_Ls did not compromise the non-aggregation state or protein L binding properties. Furthermore, the introduced disulfide bond significantly increased their T_ms, by 5.5–17.5 °C, and pepsin resistance, although it somewhat reduced expression yields and subtly changed the structure of V_Ls. Human V_Ls and engineered versions may make suitable therapeutics due to their desirable biophysical features. The disulfide linkage-engineered V_Ls may be the preferred therapeutic format because of their higher stability, especially for oral therapy applications that necessitate high resistance to the stomach’s acidic pH and pepsin.

Gram-positive anaerobic cocci--commensals and opportunistic pathogens

Among the Gram-positive anaerobic bacteria associated with clinical infections, the Gram-positive anaerobic cocci (GPAC) are the most prominent and account for approximately 25-30% of all isolated anaerobic bacteria from clinical specimens. Still, routine culture and identification of these slowly growing anaerobes to the species level has been limited in the diagnostic laboratory, mainly due to the requirement of prolonged incubation times and time-consuming phenotypic identification. In addition, GPAC are mostly isolated from polymicrobial infections with known pathogens and therefore their relevance has often been overlooked. However, through improvements in diagnostic and in particular molecular techniques, the isolation and identification of individual genera and species of GPAC associated with specific infections have been enhanced. Furthermore, the taxonomy of GPAC has undergone considerable changes over the years, mainly due to the development of molecular identification methods. Existing species have been renamed and novel species have been added, resulting in changes of the nomenclature. As the abundance and significance of GPAC in clinical infections grow, knowledge of virulence factors and antibiotic resistance patterns of different species becomes more important. The present review describes recent advances of GPAC and what is known of the biology and pathogenic effects of Anaerococcus, Finegoldia, Parvimonas, Peptoniphilus and Peptostreptococcus, the most important GPAC genera isolated from human infections.

In vivo VL-targeted microbial superantigen induced global shifts in the B cell repertoire

To subvert host defenses, some microbial pathogens produce proteins that interact with conserved motifs in V regions of B cell Ag receptor shared by large sets of lymphocytes, which define the properties of a superantigen. Because the clonal composition of the lymphocyte pool is a major determinant of immune responsiveness, this study was undertaken to examine the in vivo effect on the host immune system of exposure to a B cell superantigen, protein L (PpL), a product of the common commensal bacterial species, Finegoldia magna, which is one of the most common pathogenic species among Gram-positive anaerobic cocci. Libraries of Vκ L chain transcripts were generated from the spleens of control and PpL-exposed mice, and the expressed Vκ rearrangements were characterized by high-throughput sequencing. A total of 120,855 sequencing reads could be assigned to a germline Vκ gene, with all 20 known Vκ subgroups represented. In control mice, we found a recurrent and consistent hierarchy of Vκ gene usage, as well as patterns of preferential Vκ-Jκ pairing. PpL exposure induced significant targeted global shifts in repertoire with reduction of Vκ that contain the superantigen binding motif in all exposed mice. We found significant targeted reductions in the expression of clonotypes encoded by 14 specific Vκ genes with the predicted PpL binding motif. These rigorous surveys document the capacity of a microbial protein to modulate the composition of the expressed lymphocyte repertoire, which also has broad potential implications for host-microbiome and host-pathogen relationships.

The B-cell superantigen Finegoldia magna protein L causes pulmonary inflammation by a mechanism dependent on MyD88 but not B cells or immunoglobulins

OBJECTIVE AND DESIGN

To determine whether Finegoldia magna protein L (PL) causes lung inflammation and, if so, whether the response is dependent on its immunoglobulin (Ig)-binding B-cell superantigenic property.

MATERIAL

Pulmonary inflammatory reactions were analyzed at various time points after intratracheal administration of PL to various strains of mice.

RESULTS

PL caused peribronchial and perivascular inflammation that peaked at 18-24 h. Polymorphonuclear cells (PMNs) began to accumulate in bronchoalveolar lavage fluid (BALF) of PL-challenged mice by 4 h and accounted for >90% of leukocytes by 18-24 h. Inflammation was marked by the appearance of MIP-2, KC, TNF-α, and IL-6 in the BALF with peak levels attained 4 h after PL administration. PL-induced pulmonary inflammation was associated with increased airway hyper-reactivity following inhalation of methacholine. The inflammatory reaction was unabated in mice lacking B cells and immunoglobulins. In contrast, PL-induced inflammation was abrogated in MyD88-deficient mice. PL-induced responses required alveolar macrophages.

CONCLUSIONS

These results strongly suggest that PL-induced lung inflammation is dependent on an innate MyD88-dependent pathway rather than the Ig-binding properties of this microbial B cell superantigen. We propose that this pulmonary inflammatory reaction is caused by the interaction of PL with a Toll-like receptor expressed on alveolar macrophages.

Marginal zone B-cells, a gatekeeper of innate immunity

To maintain the integrity of an organism constantly challenged by pathogens, the immune system is endowed with a variety of cell types. B lymphocytes were initially thought to only play a role in the adaptive branch of immunity. However, a number of converging observations revealed that two B-cell subsets, marginal zone (MZ) and B1 cells, exhibit unique developmental and functional characteristics, and can contribute to innate immune responses. In addition to their capacity to mount a local antibody response against type-2 T-cell-independent (TI-2) antigens, MZ B-cells can participate to T-cell-dependent (TD) immune responses through the capture and import of blood-borne antigens to follicular areas of the spleen. Here, we discuss the multiple roles of MZ B-cells in humans, non-human primates, and rodents. We also summarize studies – performed in transgenic mice expressing fully human antibodies on their B-cells and in macaques whose infection with Simian immunodeficiency virus (SIV) represents a suitable model for HIV-1 infection in humans – showing that infectious agents have developed strategies to subvert MZ B-cell functions. In these two experimental models, we observed that two microbial superantigens for B-cells (protein A from Staphylococcus aureus and protein L from Peptostreptococcus magnus) as well as inactivated AT-2 virions of HIV-1 and infectious SIV preferentially deplete innate-like B-cells – MZ B-cells and/or B1 B-cells – with different consequences on TI and TD antibody responses. These data revealed that viruses and bacteria have developed strategies to deplete innate-like B-cells during the acute phase of infection and to impair the antibody response. Unraveling the intimate mechanisms responsible for targeting MZ B-cells in humans will be important for understanding disease pathogenesis and for designing novel vaccine strategies.

B cell development in GALT: role of bacterial superantigen-like molecules

Intestinal bacteria drive the formation of lymphoid tissues, and in rabbit, bacteria also promote development of the preimmune Ab repertoire and positive selection of B cells in GALT. Previous studies indicated that Bacillus subtilis promotes B cell follicle formation in GALT, and we investigated the mechanism by which B. subtilis stimulates B cells. We found that spores of B. subtilis and other Bacillus species, including Bacillus anthracis, bound rabbit IgM through an unconventional, superantigen-like binding site, and in vivo, surface molecules of B. anthracis spores promoted GALT development. Our study provides direct evidence that B cell development in GALT may be driven by superantigen-like molecules, and furthermore, that bacterial spores modulate host immunity.

B cell receptor affinity and B cell subset identity integrate to define the effectiveness, affinity threshold, and mechanism of anergy

In this study we show that BCR affinity and subset identity make unique contributions to anergy. Analysis of anti-Smith (Sm) B cells of different affinities indicates that increasing affinity improves anergy's effectiveness while paradoxically increasing the likelihood of marginal zone (MZ) and B-1 B cell differentiation rather than just follicular (FO) B cell differentiation. Subset identity in turn determines the affinity threshold and mechanism of anergy. Subset-specific affinity thresholds for anergy induction allow discordant regulation of low-affinity anti-Sm FO and MZ B cells and could account for the higher frequency of autoreactive MZ B cells than that of FO B cells in normal mice. The mechanism of anergy changes during differentiation and differs between subsets. This is strikingly illustrated by the observation that blockade of BCR-mediated activation of FO and MZ B cells occurs at different levels in the signaling cascade. Thus, attributes unique to B cells of each subset integrate with signals from the BCR to determine the effectiveness, affinity threshold, and mechanism of anergy.

Exploitation of host signaling pathways by B cell superantigens--potential strategies for developing targeted therapies in systemic autoimmunity

Some infectious agents produce molecules capable of interacting specifically with the immunoglobulin heavy- or light-chain variable regions, independently of the conventional-binding site. They are referred to as B cell superantigens (SAgs) and include protein A of Staphylococcus aureus (S. aureus), gp120 of HIV-1, and protein L of Peptostreptococcus magnus (P. magnus). In contrast to conventional antigens, B cell superantigens interact with conserved framework regions of immunoglobulins and can target a large proportion of B cells. In experimental models, they have been demonstrated to deplete B cell subsets responsible for innate functions, namely B-1a and marginal zone (MZ) B cells. As a result, the interactions of these superantigens with host cells impair the humoral immune response. In addition to providing clues toward understanding host-pathogen interactions and microbial pathogenesis, B cell superantigens represent potential therapeutic agents that could be used to specifically modulate expansion of B cell subsets in diseased subjects. In systemic autoimmune diseases, for example, there is activation and expansion of B cells that secrete pathogenic autoantibodies. Their depletion results in clinical improvement in both experimental animals and patients. Currently, attempts are being made to specifically deplete pathogenic autoantibody-producing B cells. Since B-1a and MZ B cells have been found to be expanded in autoimmune disorders, B cell superantigens, used alone or in combination with other biological agents, may have beneficial effects in autoimmune disease management.

Subversion of B lymphocyte tolerance by hydralazine, a potential mechanism for drug-induced lupus

Accumulating evidence indicates that epigenetic alterations contribute to exacerbated activation or deregulation of the mechanisms that maintain tolerance to self-antigens in patients with lupus, a systemic autoimmune disease that can be triggered by medications taken to treat a variety of conditions. Here, we tested the effect of hydralazine, an antihypertensive drug that triggers lupus, on receptor editing, a chief mechanism of B lymphocyte tolerance to self-antigens. Using mice expressing transgenic human Igs, we found that hydralazine impairs up-regulation of RAG-2 gene expression and reduces secondary Ig gene rearrangements. Receptor editing was also partially abolished in a dose-dependent manner by a specific inhibitor of MEK1/2. Adoptive transfer of bone marrow B cells pretreated with hydralazine or with a MEK inhibitor to naïve syngeneic mice resulted in autoantibody production. We conclude that, by disrupting receptor editing, hydralazine subverts B lymphocyte tolerance to self and contributes to generation of pathogenic autoreactivity. We also postulate that inhibition of the Erk signaling pathway contributes to the pathogenesis of hydralazine-induced lupus and idiopathic human lupus.

Direct impact of inactivated HIV-1 virions on B lymphocyte subsets

Although there is no convincing evidence that HIV infects primary B cells, marked changes in B cell responses have been described in HIV-1-infected subjects, including B cell repertoire perturbations, depression of B cell memory and paucity of CD5(+) B cells. As it is hard to assess the consequences of these in vitro and ex vivo observations in patients, the pathogenic mechanisms responsible for the B cell deficit are unclear, and direct and indirect effects of HIV-1 remain possible. To gain further insight into the impact of HIV-1 on the B cell compartment in vivo, we used XenoMouse mice, mice genetically engineered to express human antibodies with an absence of mouse antibody expression. In these transgenic animals, B cells expressing a virtually full human Ig repertoire develop, which allows investigation of the in vivo consequences of confronting B cells expressing human immunoglobulins with HIV-1. We found that soluble gp120 induced an inversion in the B-1a/B-1b cell ratios, without impacting B-2 cells or affecting substantially the T cell compartment. Virion treatment specifically and dramatically depressed B-1a cells, which represent the majority of B-1 cells in normal mice. The observed B cell changes were associated with a functional alteration of the humoral response to tetanus toxoid. Thus, the results reveal a capacity of HIV-1 to specifically impact a highly specialized B cell subpopulation. Because there is evidence that human IgM memory B cells are functionally equivalent to murine B-1a cells, our findings suggest that gp120 may have a direct deleting activity on B cell memory.

Unscrambling the role of human parvovirus B19 signaling in systemic autoimmunity

Despite enormous progress in understanding how the immune system works, the pathogenesis of autoimmune diseases still remains unclear. Growing evidence indicates that infectious agents can be potent initial triggers, subverting and exploiting host cell signaling pathways. This role is exemplified by the association of parvovirus B19 (B19) with human autoimmune disease. Infection with this common virus exhibits striking similarities with systemic autoimmune diseases, and can be associated with elevated serum autoantibody titers. The B19 virus produces proline-rich, 11-kDa proteins that have been implicated in modulation of host signaling cascades involved in virulence and pathogenesis. Additionally, B19 produces a non-structural protein (NS1) that functions as a transcription regulator by directly binding the p6 promoter and the Sp1/Sp3 transcription factors. The protein is also involved in DNA replication, cell cycle arrest and initiation of apoptotic damage, particularly in erythroid cells. When transfected to non-permissive cells, NS1 recruits the mitochondria cell death pathway. It is even more remarkable that NS1 functions as a trans-acting transcription activator for the IL6 promoter, up-regulating IL6 expression in host cells. Hence, B19 infection may play a pivotal role in triggering inflammatory disorders. By promoting apoptotic damage and trans-activating pro-inflammatory cytokine promoters, B19 may break the delicate balance between cell survival and apoptosis, and may contribute to immune deregulation. Understanding the mechanisms used by B19 to alter the cell signaling machinery may provide further insight into the mechanism by which autoimmune diseases develop.

Pathogenesis of B-cell superantigen-induced immune complex-mediated inflammation

Staphylococcal protein A (SpA) is representative of a new class of antigens, the B-cell superantigens (SAgs). These antigens bind to the Fab regions of immunoglobulin molecules outside their complementarity-determining regions. SpA, the best-studied B-cell SAg, reacts with the Fabs of most VH3+ immunoglobulins, which are expressed on 30 to 60% of human peripheral B cells. Therefore, B-cell SAgs like SpA have great potential to elicit inflammatory responses in vivo. We previously reported that the interaction of SpA with VH3+ immunoglobulin molecules leads to activation of the complement cascade and produces a histologic pattern of inflammation in the skin of a rabbit indicative of immune complex injury. To elucidate the cellular and molecular events contributing to this type of unconventional immune complex-mediated inflammation, we established a mouse peritoneal Arthus reaction model. Mice treated intravenously with human polyclonal immunoglobulin G (IgG), followed by intraperitoneal injection of SpA, showed neutrophil influx into the peritoneal cavity with peak numbers appearing at 8 h. This inflammatory reaction was dependent on the interaction of SpA with VH3+ IgG. Mast cells, FcgammaRIII, complement components, and tumor necrosis factor alpha play obligatory roles, and the reaction is associated with the local release of the CXC chemokines macrophage inflammatory protein 2 and KC. The data provide further compelling evidence for the induction of immune complex-mediated injury by a B-cell SAg and highlight important factors contributing to the pathogenesis of this novel type of inflammatory reaction.

Staphylococcal Protein A Deletes B-1a and Marginal Zone B Lymphocytes Expressing Human Immunoglobulins: An Immune Evasion Mechanism

Protein A (SpA) of Staphylococcus aureus is endowed with the capacity to interact with the H chain variable region (V(H)) of human Abs and to target >40% of B lymphocytes. To investigate whether this property represents a virulence factor and to determine the in vivo consequences of the confrontation of SpA with B lymphocytes, we used transgenic mice expressing fully human Abs. We found that administration of soluble SpA reduces B-1a lymphocytes of the peritoneal cavity and marginal zone B lymphocytes of the spleen, resulting in a markedly deficient type 2 humoral response. Single-cell PCR analysis and sequencing of the Ab V(H) gene repertoire revealed a significant reduction of V(H)3+ marginal zone B cells. Since the two B lymphocyte subsets targeted are involved in innate immune functions, our data suggest that crippling of humoral immunity by S. aureus represents an immune evasion mechanism that may aggravate recurrent infections.

B cell superantigens: a microbe's answer to innate-like B cells and natural antibodies

Marginal zone B cells and B-1 cells have been termed innate-like B cells as they express limited repertoires that play special roles in immune defenses against common infections. These B cells are the sources of natural antibodies and are capable of highly accelerated clonal responses that help counter blood-borne infections. We have characterized a class of microbial product with highly adapted binding interactions with host immunoglobulins/B cell receptors (BCRs), which enable the targeting of large supra-clonal sets of B cells for activation-associated apoptotic death. In recent studies, we have shown that all B cells with V region-targeted BCRs are susceptible. However, compared to follicular B cells, in vivo exposure preferentially causes innate-like B cells to undergo induced death with subsequent long-lasting supra-clonal depletion and immune tolerance. Based on these properties, it is likely that B cell superantigens influence the pathogenesis of some common infections, but also may provide novel therapeutic opportunities to treat B cell neoplastic and autoimmune diseases.

B-lymphocytes, innate immunity, and autoimmunity

Having evolved to generate a huge Ag-specific repertoire and to mount T cell-dependent responses and long-term memory, the B lymphocyte is a central player in the adaptive branch of immune defense. However, accumulating evidence indicates that B-1 cells of the peritoneal cavity and marginal zone (MZ) B cells of the spleen also can play innate-like immune functions. Their anatomical locations allow frequent Ag encounter. Secreting essentially germline-encoded, polyreactive Abs, and responding rapidly and vigorously to stimulation, these two B cell subsets have evolved to impart potentially protective responses. With their additional capacities to secrete factors that can directly mediate microbial destruction and to express Toll-like receptors (TLR), B cells provide an important link between the innate and adaptive branches of the immune system. Currently, the relevance of these innate-like B cells to the pathogenesis of autoimmune disease is the focus of investigation. In experimental models of autoimmunity, the sequestration of autoreactive B cells in the MZ has been proposed to be essential for the maintenance of self-tolerance. The low activation threshold of MZ B cells makes them particularly reactive to high loads and/or altered self-Ags, potentially exacerbating autoimmune disease. Their expansion in autoimmune models and their association with autoantibody secretion indicate that they may participate in tissue damage. The demonstration that B cell depletion therapies may represent a highly beneficial therapeutic goal in autoimmune disorders suggests that specific elimination of B-1 and MZ B cells may represent a more efficient immunointervention strategy in systemic autoimmunity.

Effect of the B cell superantigen protein A from S. aureus on the early lupus disease of (NZBxNZW) F1 mice

The (NZBxNZW) F(1) mouse develops a spontaneous autoimmune disease process with striking similarities to human systemic lupus erythematosus (SLE). In female (NZBxNZW) F(1) mice, the production of IgG antinuclear antibodies, including antibodies to double-stranded DNA (dsDNA), is associated with the development of a severe immune complex-mediated glomerulonephritis that results in death from renal failure in virtually all animals by 12 months of age. Since B-1 and marginal zone (MZ) cells represent a potential source of pathogenic antibodies and because B cell superantigens have been demonstrated to reduce B-1 and MZ cells in vivo, we tested the effect of repeated injections of the superantigen protein A (SpA) from S. aureus on the disease of this lupus model. We found that weekly intraperitoneal injections of SpA delay the progression of serum anti-DNA IgG and reduce proteinuria early in young female (NZBxNZW) F(1) mice. This superantigen also induced a specific depression in the numbers of peritoneal B-1 cells, as compared to mice treated with a control protein. These results support the role of B-1 cells in the development of the autoimmune disease in this mouse model and suggest that B cell superantigens may be useful in the management of autoimmune conditions.