B-cell superantigens: implications for selection of the human antibody repertoire

Cellular pathways and molecular events that shape autoantibody production in COVID-19

A hallmark of COVID-19 is the variety of complications that follow SARS-CoV-2 infection in some patients, and that target multiple organs and tissues. Also remarkable are the associations with several auto-inflammatory disorders and the presence of autoantibodies directed to a vast array of antigens. The processes underlying autoantibody production in COVID-19 have not been completed deciphered. Here, we review mechanisms involved in autoantibody production in COVID-19, multisystem inflammatory syndrome in children, and post-acute sequelae of COVID19. We critically discuss how genomic integrity, loss of B cell tolerance to self, superantigen effects of the virus, and extrafollicular B cell activation could underly autoantibody proaction in COVID-19. We also offer models that may account for the pathogenic roles of autoantibodies in the promotion of inflammatory cascades, thromboembolic phenomena, and endothelial and vascular deregulations.

B Cells at the Cross-Roads of Autoimmune Diseases and Auto-Inflammatory Syndromes

Whereas autoimmune diseases are mediated primarily by T and B cells, auto-inflammatory syndromes (AIFS) involve natural killer cells, macrophages, mast cells, dendritic cells, different granulocyte subsets and complement components. In contrast to autoimmune diseases, the immune response of patients with AIFS is not associated with a breakdown of immune tolerance to self-antigens. Focusing on B lymphocyte subsets, this article offers a fresh perspective on the multiple cross-talks between both branches of innate and adaptive immunity in mounting coordinated signals that lead to AIFS. By virtue of their potential to play a role in adaptive immunity and to exert innate-like functions, B cells can be involved in both promoting inflammation and mitigating auto-inflammation in disorders that include mevalonate kinase deficiency syndrome, Kawasaki syndrome, inflammatory bone disorders, Schnitzler syndrome, Neuro-Behçet's disease, and neuromyelitis optica spectrum disorder. Since there is a significant overlap between the pathogenic trajectories that culminate in autoimmune diseases, or AIFS, a more detailed understanding of their respective roles in the development of inflammation could lead to designing novel therapeutic avenues.

B lymphocytes, the gastrointestinal tract and autoimmunity

Under homeostatic conditions, bidirectional interactions between the gastrointestinal and the immune system allow production of both inflammatory and anti-inflammatory responses designed to prevent undesirable inflammation and to respond efficiently to potential insults. This balanced regulation can be disrupted in disorders that affect tissues remote to the gastrointestinal tract, as seen in autoimmune diseases. Recent reports have described a variety of B lymphocyte-mediated functions that likely contribute to gastrointestinal homeostasis to a greater extent than previously thought. Studies have shown that early B cell development takes place within the intestine, and that self-reactive B cells are rendered tolerant using mechanisms known to occur in the bone marrow, indicating that the gastrointestinal tract contributes to maintaining immune tolerance to self. Relatedly, continuous bacterial stimulation is essential for maintaining regulatory B cell functions and for mediating mucosal homeostasis. In studies of neuro-inflammation, intestinal IgA+ B cells, which constitute a prominent source of lymphocytes in the organism, can migrate to inflamed tissues and exert regulatory functions that attenuate inflammation in the central nervous system, indicating that, in addition to its local effects in the intestin, gut microbiota-B cell crosstalk can exert long-range beneficial effects. At the translational level, metabolites produced by gut microbiota can act as B cell-intrinsic epigenetic modulators, reducing inflammation in the skin and kidneys of mice suffering from experimental lupus. Given the significant impact of B cell-intestinal microbiota interactions, there is a momentum for improving our understanding of these pathways in autoinflammatory diseases and for designing novel therapeutic strategies for systemic autoimmune diseases where B cells play key roles.

Beyond Allotypes: The Influence of Allelic Diversity in Antibody Constant Domains

Polymorphic diversity in antibody constant domains has long been defined by allotypic motifs that cross react with the sera of other individuals. Improvements in sequencing technologies have led to the discovery of a large number of new allelic sequences that underlie this diversity. Many of the point mutations lie outside traditional allotypic motifs suggesting they do not elicit immunogenic responses. As antibodies play an important role in immune defense and biotechnology, understanding how this newly resolved diversity influences the function of antibodies is important. This review investigates the current known diversity of antibody alleles at a protein level for each antibody isotype as well as the kappa and lambda light chains. We focus on evidence emerging for how these mutations perturb antibody interactions with antigens and Fc receptors that are critical for function, as well as the influence this might have on the use of antibodies as therapeutics and reagents.

HIV gp120 Induces the Release of Proinflammatory, Angiogenic, and Lymphangiogenic Factors from Human Lung Mast Cells

Human lung mast cells (HLMCs) express the high-affinity receptor FcεRI for IgE and are involved in chronic pulmonary diseases occurring at high frequency among HIV-infected individuals. Immunoglobulin superantigens bind to the variable regions of either the heavy or light chain of immunoglobulins (Igs). Glycoprotein 120 (gp120) of HIV-1 is a typical immunoglobulin superantigen interacting with the heavy chain, variable 3 (V_H3) region of human Igs. The present study investigated whether immunoglobulin superantigen gp120 caused the release of different classes of proinflammatory and immunoregulatory mediators from HLMCs. The results show that gp120 from different clades induced the rapid (30 min) release of preformed mediators (histamine and tryptase) from HLMCs. gp120 also caused the de novo synthesis of cysteinyl leukotriene C₄ (LTC₄) and prostaglandin D₂ (PGD₂) from HLMCs. Incubation (6 h) of HLMC with gp120 induced the release of angiogenic (VEGF-A) and lymphangiogenic (VEGF-C) factors from HLMCs. The activating property of gp120 was mediated through the interaction with IgE V_H3⁺ bound to FcεRI. Our data indicate that HIV gp120 is a viral superantigen, which induces the release of different proinflammatory, angiogenic, and lymphangiogenic factors from HLMCs. These observations could contribute to understanding, at least in part, the pathophysiology of chronic pulmonary diseases in HIV-infected individuals.

Superantigenic Activation of Human Cardiac Mast Cells

B cell superantigens, also called immunoglobulin superantigens, bind to the variable regions of either the heavy or light chain of immunoglobulins mirroring the lymphocyte-activating properties of classical T cell superantigens. Protein A of Staphylococcus aureus, protein L of Peptostreptococcus magnus, and gp120 of HIV are typical immunoglobulin superantigens. Mast cells are immune cells expressing the high-affinity receptor for IgE (FcεRI) and are strategically located in the human heart, where they play a role in several cardiometabolic diseases. Here, we investigated whether immunoglobulin superantigens induced the activation of human heart mast cells (HHMCs). Protein A induced the de novo synthesis of cysteinyl leukotriene C₄ (LTC₄) from HHMCs through the interaction with IgE V_H3⁺ bound to FcεRI. Protein L stimulated the production of prostaglandin D₂ (PGD₂) from HHMCs through the interaction with κ light chains of IgE. HIV glycoprotein gp120 induced the release of preformed (histamine) and de novo synthesized mediators, such as cysteinyl leukotriene C₄ (LTC₄), angiogenic (VEGF-A), and lymphangiogenic (VEGF-C) factors by interacting with the V_H3 region of IgE. Collectively, our data indicate that bacterial and viral immunoglobulin superantigens can interact with different regions of IgE bound to FcεRI to induce the release of proinflammatory, angiogenic, and lymphangiogenic factors from human cardiac mast cells.

Ageing of the B-cell repertoire

Older people are more susceptible to infection, less responsive to vaccination and have a more inflammatory immune environment. Using spectratype analysis, we have previously shown that the B-cell repertoire of older people shows evidence of inappropriate clonal expansions in the absence of challenge, and that this loss of B-cell diversity correlates with poor health. Studies on response to vaccination, using both spectratyping and high-throughput sequencing of the repertoire, indicate that older responses to challenge are lacking in magnitude and/or delayed significantly. Also that some of the biologically significant differences may be in different classes of antibody. We have also previously shown that normal young B-cell repertoires can vary between different phenotypic subsets of B cells. In this paper, we present an analysis of immunoglobulin repertoire in different subclasses of antibody in five different populations of B cell, and show how the repertoire in these different groups changes with age. Although some age-related repertoire differences occur in naive cells, before exogenous antigen exposure, we see indications that there is a general dysregulation of the selective forces that shape memory B-cell populations in older people.

Human Immunodeficiency Virus Type 1 Cellular Entry and Exit in the T Lymphocytic and Monocytic Compartments: Mechanisms and Target Opportunities During Viral Disease

During the course of human immunodeficiency virus type 1 infection, a number of cell types throughout the body are infected, with the majority of cells representing CD4+ T cells and cells of the monocyte-macrophage lineage. Both types of cells express, to varying levels, the primary receptor molecule, CD4, as well as one or both of the coreceptors, CXCR4 and CCR5. Viral tropism is determined by both the coreceptor utilized for entry and the cell type infected. Although a single virus may have the capacity to infect both a CD4+ T cell and a cell of the monocyte-macrophage lineage, the mechanisms involved in both the entry of the virus into the cell and the viral egress from the cell during budding and viral release differ depending on the cell type. These host-virus interactions and processes can result in the differential targeting of different cell types by selected viral quasispecies and the overall amount of infectious virus released into the extracellular environment or by direct cell-to-cell spread of viral infectivity. This review covers the major steps of virus entry and egress with emphasis on the parts of the replication process that lead to differences in how the virus enters, replicates, and buds from different cellular compartments, such as CD4+ T cells and cells of the monocyte-macrophage lineage.

Staphylococcus aureus convert neonatal conventional CD4(+) T cells into FOXP3(+) CD25(+) CD127(low) T cells via the PD-1/PD-L1 axis

The gut microbiota provides an important stimulus for the induction of regulatory T (Treg) cells in mice, whether this applies to newborn children is unknown. In Swedish children, Staphylococcus aureus has become a common early colonizer of the gut. Here, we sought to study the effects of bacterial stimulation on neonatal CD4⁺ T cells for the induction of CD25⁺ CD127^low Treg cells in vitro. The proportion of circulating CD25⁺ CD127^low Treg cells and their expression of FOXP3, Helios and CTLA-4 was examined in newborns and adults. To evaluate if commensal gut bacteria could induce Treg cells, CellTrace violet-stained non-Treg cells from cord or peripheral blood from adults were co-cultured with autologous CD25⁺ CD127^low Treg cells and remaining mononuclear cells and stimulated with S. aureus. Newborns had a significantly lower proportion of CD25⁺ CD127^low Treg cells than adults, but these cells were Helios⁺ and CTLA-4⁺ to a higher extent than in adults. FOXP3⁺ CD25⁺ CD127^low T cells were induced mainly in neonatal CellTrace-stained non-Treg cells after stimulation with S. aureus. In cell cultures from adults, S. aureus induced CD25⁺ CD127^low T cells only if sorted naive CD45RA⁺ non-Treg cells were used, but these cells expressed less FOXP3 than those induced from newborns. Sorted neonatal CD25⁺ CD127^low T cells from S. aureus-stimulated cultures were still suppressive. Finally, blocking PD-L1 during stimulation reduced the induction of FOXP3⁺ CD25⁺ CD127^low T cells. These results suggest that newborns have a higher proportion of circulating thymically derived Helios⁺ Treg cells than adults and that S. aureus possess an ability to convert neonatal conventional CD4⁺ T cells into FOXP3⁺ CD25⁺ CD127^low Treg cells via the PD-1/PD-L1 axis.

Bioinformatic analysis of HIV-1 entry and pathogenesis

The evolution of human immunodeficiency virus type 1 (HIV-1) with respect to co-receptor utilization has been shown to be relevant to HIV-1 pathogenesis and disease. The CCR5-utilizing (R5) virus has been shown to be important in the very early stages of transmission and highly prevalent during asymptomatic infection and chronic disease. In addition, the R5 virus has been proposed to be involved in neuroinvasion and central nervous system (CNS) disease. In contrast, the CXCR4-utilizing (X4) virus is more prevalent during the course of disease progression and concurrent with the loss of CD4(+) T cells. The dual-tropic virus is able to utilize both co-receptors (CXCR4 and CCR5) and has been thought to represent an intermediate transitional virus that possesses properties of both X4 and R5 viruses that can be encountered at many stages of disease. The use of computational tools and bioinformatic approaches in the prediction of HIV-1 co-receptor usage has been growing in importance with respect to understanding HIV-1 pathogenesis and disease, developing diagnostic tools, and improving the efficacy of therapeutic strategies focused on blocking viral entry. Current strategies have enhanced the sensitivity, specificity, and reproducibility relative to the prediction of co-receptor use; however, these technologies need to be improved with respect to their efficient and accurate use across the HIV-1 subtypes. The most effective approach may center on the combined use of different algorithms involving sequences within and outside of the env-V3 loop. This review focuses on the HIV-1 entry process and on co-receptor utilization, including bioinformatic tools utilized in the prediction of co-receptor usage. It also provides novel preliminary analyses for enabling identification of linkages between amino acids in V3 with other components of the HIV-1 genome and demonstrates that these linkages are different between X4 and R5 viruses.

HIV-1 gp120 impairs the induction of B cell responses by TLR9-activated plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) play a central role in innate and adaptive immune responses to viral infections, including HIV type 1 (HIV-1). pDCs produce substantial quantities of type I IFN and proinflammatory cytokines upon stimulation via TLRs, specifically TLR7 or TLR9. The HIV-1 envelope glycoproteins, exemplified by the gp120 monomer, are the focus of vaccines aimed at inducing B cell responses. We have studied how the interactions of gp120 with various receptors on human pDCs affect the activation of these cells via TLR9 and their subsequent ability to stimulate B cells. We observed that IFN-α production by pDCs in response to TLR9, but not TLR7, stimulation was reduced by exposure to gp120. Specifically, gp120 inhibited the CpG-induced maturation of pDCs and their expression of TNF-α, IL-6, TLR9, IFN regulatory factor 7, and BAFF. Receptor-blocking and cross-linking studies showed that these inhibitory effects of gp120 were mediated by interactions with CD4 and mannose-binding C-type lectin receptors, but not with the chemokine receptors CCR5 and CXCR4. Of note is that gp120 inhibited the activation of B cells by TLR9-stimulated pDCs. Taken together, our data show that HIV-1 gp120 impairs pDC functions, including activation of B cell responses, and imply that TLR9 ligands may not be good adjuvants to use in combination with envelope glycoprotein vaccines.

Immunoglobulin analysis tool: a novel tool for the analysis of human and mouse heavy and light chain transcripts

Sequence analysis of immunoglobulin (Ig) heavy and light chain transcripts can refine categorization of B cell subpopulations and can shed light on the selective forces that act during immune responses or immune dysregulation, such as autoimmunity, allergy, and B cell malignancy. High-throughput sequencing yields Ig transcript collections of unprecedented size. The authoritative web-based IMGT/HighV-QUEST program is capable of analyzing large collections of transcripts and provides annotated output files to describe many key properties of Ig transcripts. However, additional processing of these flat files is required to create figures, or to facilitate analysis of additional features and comparisons between sequence sets. We present an easy-to-use Microsoft^® Excel^® based software, named Immunoglobulin Analysis Tool (IgAT), for the summary, interrogation, and further processing of IMGT/HighV-QUEST output files. IgAT generates descriptive statistics and high-quality figures for collections of murine or human Ig heavy or light chain transcripts ranging from 1 to 150,000 sequences. In addition to traditionally studied properties of Ig transcripts – such as the usage of germline gene segments, or the length and composition of the CDR-3 region – IgAT also uses published algorithms to calculate the probability of antigen selection based on somatic mutational patterns, the average hydrophobicity of the antigen-binding sites, and predictable structural properties of the CDR-H3 loop according to Shirai’s H3-rules. These refined analyses provide in-depth information about the selective forces acting upon Ig repertoires and allow the statistical and graphical comparison of two or more sequence sets. IgAT is easy to use on any computer running Excel^® 2003 or higher. Thus, IgAT is a useful tool to gain insights into the selective forces and functional properties of small to extremely large collections of Ig transcripts, thereby assisting a researcher to mine a data set to its fullest.

Corruption of human follicular B-lymphocyte trafficking by a B-cell superantigen

Protein A (SpA) of Staphylococcus aureus is known to target the paratope of immunoglobulins expressing V(H)3 genes, and to delete marginal zone B cells and B-1a in vivo. We have discovered that SpA endows S. aureus with the potential to subvert B-cell trafficking in the host. We found that SpA, whose Fc-binding site has been inactivated, binds essentially to naïve B cells and induces a long-lasting decrease in CXCR4 expression and in B-cell chemotaxis to CXCL12. Competition experiments indicated that SpA does not interfere with binding of CXCR4 ligands and does not directly bind to CXCR4. This conclusion is strongly supported by the inability of SpA to modulate clathrin-mediated CXCR4 internalization, which contrasts with the potent effect of anti-immunoglobin M (IgM) antibodies. Microscopy and biochemical experiments confirmed that SpA binds to the surface IgM/IgD complex and induces its clathrin-dependent internalization. Concomitantly, the SpA-induced signaling leads to protein kinase C-dependent CXCR4 downmodulation, suggesting that SpA impairs the recycling of CXCR4, a postclathrin process that leads to either degradation into lysozomes or de novo expression at the cell surface. In addition to providing novel insight into disruption of B-cell trafficking by an infectious agent, our findings may have therapeutic implications. Because CXCR4 has been associated with cancer metastasis and with certain autoimmune diseases, SpA behaves as an evolutionary tailored highly specific, chemokine receptor inhibitor that may have value in addition to conventional cytotoxic therapy in patients with various malignancies and immune-mediated diseases.

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.

Divergent human populations show extensive shared IGK rearrangements in peripheral blood B cells

We have analysed the transcribed immunoglobulin kappa (IGK) repertoire of peripheral blood B cells from four individuals from two genetically distinct populations, Papua New Guinean and Australian, using high-throughput DNA sequencing. The depth of sequencing data for each individual averaged 5,548 high-quality IGK reads, and permitted genotyping of the inferred IGKV and IGKJ germline gene segments for each individual. All individuals were homozygous at each IGKJ locus and had highly similar inferred IGKV genotypes. Preferential gene usage was seen at both the IGKV and IGKJ loci, but only IGKV segment usage varied significantly between individuals. Despite the differences in IGKV gene utilisation, the rearranged IGK repertoires showed extensive identity at the amino acid level. Public rearrangements (those shared by two or more individuals) made up 60.2% of the total sequenced IGK rearrangements. The total diversity of IGK rearrangements of each individual was estimated to range from just 340 to 549 unique amino acid sequences. Thus, the repertoire of unique expressed IGK rearrangements is dramatically less than previous theoretical estimates of IGK diversity, and the majority of expressed IGK rearrangements are likely to be extensively shared in individual human beings.

Immunoglobulins, antibody repertoire and B cell development

Swine share with most placental mammals the same five antibody isotypes and same two light chain types. Loci encoding lambda, kappa and Ig heavy chains appear to be organized as they are in other mammals. Swine differ from rodents and primates, but are similar to rabbits in using a single VH family (VH3) to encode their variable heavy chain domain, but not the family used by cattle, another artiodactyl. Distinct from other hoofed mammals and rodents, Ckappa:Clambda usage resembles the 1:1 ratio seen in primates. Since IgG subclasses diversified after speciation, same name subclass homologs do not exist among swine and other mammals unless very closely related. Swine possess six putative IgG subclasses that appear to have diversified by gene duplication and exon shuffle while retaining motifs that can bind to FcgammaRs, FcRn, C1q, protein A and protein G. The epithelial chorial placenta of swine and the precosial nature of their offspring have made piglets excellent models for studies on fetal antibody repertoire development and on the postnatal role of gut colonization, maternal colostrum and neonatal infection on the development of adaptive immunity during the "critical window" of immunological development. This chapter traces the study of the humoral immune system of this species through its various eras of discovery and compiles the results in tables and figures that should be a useful reference for educators and investigators.

Isolator and other neonatal piglet models in developmental immunology and identification of virulence factors

The postnatal period is a ‘critical window’, a time when innate and passive immunity protect the newborn mammal while its own adaptive immune system is developing. Neonatal piglets, especially those reared in isolators, provide valuable tools for studying immunological development during this period, since environmental factors that cause ambiguity in studies with conventional animals are controlled by the experimenter. However, these models have limited value unless the swine immune system is first characterized and the necessary immunological reagents developed. Characterization has revealed numerous features of the swine immune system that did not fit mouse paradigms but may be more generally true for most mammals. These include fetal class switch recombination that is uncoupled from somatic hypermutation, the relative importance of the molecular mechanisms used to develop the antibody repertoire, the role of gut lymphoid tissue in that process, and the limited heavy chain repertoire but diverse IgG subclass repertoire. Knowledge gained from studies of adaptive immunity in isolator-reared neonatal pigs suggests that isolator piglets can be valuable in identification of virulence factors that are often masked in studies using conventional animals.

Immunology

The concept of forbidden foods that should not be eaten goes back to the Garden of Eden and apart from its religious meanings it may also have foreshadowed the concept of foods that can provoke adverse reactions. Thus we could say that allergic diseases have plagued mankind since the beginning of life on earth. The prophet Job was affected by a condition that following the rare symptoms described by the Holy Bible might be identified as a severe form of atopic dermatitis (AD). The earliest record of an apparently allergic reaction is 2621 B.C., when death from stinging insects was first described by hieroglyphics carved into the walls of the tomb of Pharaoh Menes depicting his death following the sting of a wasp. In 79 A.D., the death of the Roman admiral Pliny the Elder was ascribed to the SO₂-rich gases emanating from the eruption of Mount Vesuvius. Hippocrates (460–377 B.C.) was probably the first to describe how cow’s milk (CM) could cause gastric upset and hives, proposing dietetic measures including both treatment and prevention for CM allergy.

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.

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.

HIV-1 envelope triggers polyclonal Ig class switch recombination through a CD40-independent mechanism involving BAFF and C-type lectin receptors

Switching from IgM to IgG and IgA is essential for antiviral immunity and requires engagement of CD40 on B cells by CD40L on CD4(+) T cells. HIV-1 is thought to impair CD40-dependent production of protective IgG and IgA by inducing progressive loss of CD4(+) T cells. Paradoxically, this humoral immunodeficiency is associated with B cell hyperactivation and increased production of nonprotective IgG and IgA that are either nonspecific or specific for HIV-1 envelope glycoproteins, including gp120. Nonspecific and gp120-specific IgG and IgA are sensitive to antiretroviral therapy and remain sustained in infected individuals with very few CD4(+) T cells. One interpretation is that some HIV-1 Ags elicit IgG and IgA class switch DNA recombination (CSR) in a CD40-independent fashion. We show that a subset of B cells binds gp120 through mannose C-type lectin receptors (MCLRs). In the presence of gp120, MCLR-expressing B cells up-regulate the CSR-inducing enzyme, activation-induced cytidine deaminase, and undergo CSR from IgM to IgG and IgA. CSR is further enhanced by IL-4 or IL-10, whereas Ab secretion requires a B cell-activating factor of the TNF family. This CD40L-related molecule is produced by monocytes upon CD4, CCR5, and CXCR4 engagement by gp120 and cooperates with IL-4 and IL-10 to up-regulate MCLRs on B cells. Thus, gp120 may elicit polyclonal IgG and IgA responses by linking the innate and adaptive immune systems through the B cell-activating factor of the TNF family. Chronic activation of B cells through this CD40-independent pathway could impair protective T cell-dependent Ab responses by inducing immune exhaustion.

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.

Immune receptor signaling, aging, and autoimmunity

With advancing age, the immune system undergoes changes that predispose to autoimmune reactivity. Aging reduces the efficiency of physical barriers, decreasing protection against invasive pathogens, and exposing previously hidden antigens in the body's own tissues. Self-antigens acquire alterations that increase their immunogenicity. In addition, the ability of innate immunity to eliminate infectious agents deteriorates, resulting in inappropriate persistence of immune stimulation and antigen levels exceeding the threshold for the activation of B or T cells. B cell turnover is reduced and numbers of naïve T cells decline to the advantage of increasing numbers of memory T cells. In parallel, the loss of co-stimulatory T cell molecules may increase reactivity of T cells, and render them less susceptible to downregulation. Since optimal immune reactivity requires a tight balance of transduction pathways in both T and B lymphocytes, and because these pathways are altered in systemic autoimmune diseases, we would like to propose that, with age, alterations of the immune receptor signaling machinery underlie the higher incidence of autoimmune phenomena in the elderly. Consistently, aging is associated with alterations in several components of the signaling complex in B cells, memory and naïve T cells, and a reduced activation of several lipid rafts-associated proteins. Because the coincidence of autoimmune disease with other ailments increases the burden of disease and limits therapeutic options in the aged, further investigation of these pathways in the elderly represents a challenge that will need to be addressed in order to devise effective preventive and therapeutic interventions.

Substitution patterns in alleles of immunoglobulin V genes in humans and mice

Immunoglobulins (Igs) constitute a subfamily of rapidly evolving proteins. It is postulated that this characteristic is due mainly to the participation of these proteins in highly diverse functions of recognition and defense. Although this vision of rapid evolution in Igs is widely accepted, various studies have demonstrated that diverse and contradictory forces not yet completely understood converge in the evolution of these receptors. In a recent study of the substitution patterns in the alleles that form the human IGHV locus, we found that the variation in genetic and structural information does not occur homogeneously among the different genes, nor among the regions and positions conforming said locus. In view of these results and of the importance of a better understanding of the basic evolutionary process in specific receptors (such as Igs) for both immunology and molecular evolution, it is important to explore the nature of the diversification process in these proteins in detail. In this work, therefore, we analyzed the substitution patterns in all the alleles reported for loci IGKV and IGLV in humans and mice, and we compared the results with those previously observed in the human IGHV locus. We found that the process of evolutionary variation of the Igs reflect the diversity of selective pressures operating on the different loci, genes, sub-regions and positions; for example, diversification through substitution is generally centered on CDRs, but only few positions inside the CDRs were frequently substituted. In spite of this general tendency, it is possible to observe differences in the degree of diversification among loci, families and genes. These tendencies to modify only certain attributes of IGV genes seem to be in agreement with differential strategies associated with the restrictions of the molecular immune recognition mechanism. The complexity of the evolutionary patterns observed in this study leads us to think that the predispositions observed herein may also be due in part to processes of DNA dynamics.

Novel Substitution Polymorphisms of Human Immunoglobulin VH Genes in Mexicans

It has been proposed that the defense and recognition functions of the immune system, especially those mediated by antibodies, require a great diversity of receptors. Nonetheless, functional and structural evidence has demonstrated the presence of restrictions, both in the use of the repertoire and in the recognition of antigens. Fifty-one functional genes have been described in the IghV locus; however, there is a variety of evidences indicating that only a small fraction of the immunoglobulin genes plays a central role in determining the fundamental properties of the antibody repertoire of the immune system. On the basis of this functional and structural information, we selected four IghV genes and characterized their polymorphism in a sample of Mexican individuals. We also analyzed the implications for the recognition mechanism of the substitutions found in the sequenced alleles. We found that diversification through allelism varies from segment to segment, both in the amount of alleles encountered and in the nature and distribution of mutations in the codifying zone, which might depend on its importance for the repertoire. Such functional characteristics may be useful in the interpretation of differential gene usage in certain physiological, ontological, and/or pathological conditions.

Structural analysis of substitution patterns in alleles of human immunoglobulin VH genes

The diversity in repertoires of antibodies (Abs) needed in response to the antigen challenge is produced by evolutionary and somatic processes. The mechanisms operating at a somatic level have been studied in great detail. In contrast, neither the mechanisms nor the strategies of diversification at an evolutionary level have yet been understood in similar detail. Particularly, the substitution patterns in alleles of immunoglobulin genes (Igs) have not been systematically studied. Furthermore, there is a scarcity of studies which link the analysis at a genetic level of the diversification of repertoires with the structural consequences at the protein level of the changes in DNA information. For the purpose of systematically characterizing the strategies of evolutionary diversification through sequence variation at alleles, in this work, we built a database for all the alleles of the IGHV locus in humans reported until now. Based on these data, we performed diverse analyses of substitution patterns and linked these results with studies at the protein level. We found that the sequence diversification in different alleles does not operate with equal intensity for all V genes. Our studies, both of the number of substitutions and of the type of amino acid change per sub-segment of the V-REGION evidenced differences in the selective pressure to which these regions are exposed. The implications of these results for understanding the evolutionary diversification strategies, as well as for the somatic generation of antibody repertoires are discussed.

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.

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

Some pathogens have evolved to produce proteins, called B-cell superantigens, that can interact with human immunoglobulin variable regions, independently of the combining site, and activate B lymphocytes that express the target immunoglobulins. However, the in vivo consequences of these interactions on human B-cell numbers and function are largely unknown. Using transgenic mice expressing fully human immunoglobulins, we studied the consequences of in vivo exposure of protein L of Peptostreptococcus magnus with human immunoglobulins. In the mature pool of B cells, protein L exposure resulted in a specific reduction of splenic marginal-zone B cells and peritoneal B-1 cells. Splenic B cells exhibited a skewed light-chain repertoire consistent with the capacity of protein L to bind specific kappa gene products. Remarkably, these two B-cell subsets are implicated in innate B-cell immunity, allowing rapid clearance of pathogens. Thus, the present study reveals a novel mechanism that may be used by some infectious agents to subvert a first line of the host's immune defense.

Effect of novel non-peptidic delta opioid receptor antagonists on human T and B cell activation

The effects of the antagonist naltrindole (NTI) on cells of the immune system have been largely studied although the mechanisms of action are still unclear. The aim of this study is to evaluate, in vitro, the immunomodulatory activity of four new delta-selective opioid compounds structurally related to naltrindole. The effects at different concentrations of these opioid antagonists on proliferative response were studied on normal human peripheral blood mononuclear cells (PBMC) stimulated with different stimuli: mitogens, the antigen PPD, the anti-CD3 monoclonal antibodies (mAb), the superantigen Staphylococcus aureus Cowan strain 1 (SAC) and alloantigens in the mixed lymphocyte cultures (MLR). The immunomodulatory capacity of these compounds was evaluated by determining the interleukin-2 (IL-2) release in mitogen activated PBMC. The present study shows that all the new delta opioid antagonists at 10(-5) M concentration are immunosuppressive. The inhibitory action is also evident at lower concentrations when anti-CD3 mAb and SAC were used as stimulators. In addition, the production of IL-2 was inhibited by the opioid treatment, but this might not be the only mechanism of action.

Down-modulation of the antigen receptor by a superantigen for human B cells

B cell superantigens (SAgs) have been implicated in human diseases by demonstrating non-clonotypic expansion of B cells bearing certain immunoglobulin variable region genes. One possibility is that, during infection with microorganisms secreting SAgs, these potent molecules might modulate BcR expression. To test this hypothesis, we investigated the potential effects of a SAg, protein L from Peptostreptococcus magnus, on antigen B cell receptor (BcR) surface expression in vitro. Using fluorescence microscopy, we found that this SAg induced down-regulation of BcR expression. This effect was time-, dose-, and temperature-dependent, and shedding of cell surface IgM molecules into the culture supernatant was not detected. These data demonstrate that SAg-mediated down-regulation of the BcR expression occurs primarily as a result of BcR internalization. In addition, two specific inhibitors of protein tyrosine kinases were found to retard the BcR modulation on the cell surface and inhibit SAg-induced receptor internalization, showing that tyrosine phosphorylation is required for subsequent internalization of mIg-ligand complexes. The down-modulation of BcR expression may have pathological consequences in patients infected with microorganisms secreting SAgs.

Subversion of B lymphocyte signaling by infectious agents

Infectious agents and their hosts interact in a complex manner, involving not only superficially apparent mechanisms, but also the signaling machinery that governs host cells responses. Thus, signaling events, surface molecule expression, and transcriptional control may be affected in various cell types, with profound consequences for the function of individual cells and organ systems. Studies of the biochemistry of cell signaling and cell invasion by infectious agents have begun to detail the interplay between elements of infectious organisms and the host at the molecular level. Consequently, the resulting interferences with lymphocyte signaling may disturb the function of the immune system. In B cells, alterations of immune receptor signaling has implications for human diseases. By affecting the mechanisms of the host's immune defense, this may not only lead to inadequate elimination of an infectious agent, but also to autoimmunity or neoplasia.

Application of the Spot method to the identification of peptides and amino acids from the antibody paratope that contribute to antigen binding

Overlapping peptide scans prepared by Spot synthesis have been used to map interaction sites in several systems. Here we report our experience with this approach to identify peptides from the variable parts of anti-hapten, anti-peptide and anti-protein antibodies that retain their specific antigen-binding capacity in the Spot format. In general, the identification by the Spot method of antigen-reactive peptides was confirmed by using soluble peptides which demonstrated antigen-binding capacity in ELISA or Biacore and, biological activity for some peptides derived from anti-CD4 antibodies. The Spot method was also used to map precisely key residues from the antibody paratope. The identification of critical residues from an anti-troponin I antibody of diagnostic interest is reported as well as the compiled results from the analysis of five other antibodies of various specificities. A critical assessment of our results is provided by comparing results obtained by our approach in the mapping of antibody residues critical for antigen binding with data from the literature concerning the structural analysis of antigen-antibody complexes.

Molecular determinants of the human antibody response to HIV-1: implications for disease control

Various aspects of the immune response to HIV-1 infection remain unclear. While seropositive subjects generally mount a strong humoral response, the antibodies produced are not effective in halting disease progression. Molecular characterization of the antibody repertoire specific for HIV-1 antigens represents an approach to further our understanding of the mechanisms involved in mounting a humoral immunity in this infection. Recently, the content, structure, and organization of the human immunoglobulin-variable gene loci have been elucidated and a number of laboratories have characterized the variable gene elements of human anti-HIV-1 antibodies derived from infected persons by cell fusion or by Epstein-Barr virus transformation. The results show evidence for extensive somatic mutations that lead to preferential amino acid substitutions in the hypervariable regions, an indication of an antigen-driven process. Multiple other molecular events also are engaged in generating antibody diversity, including various types of fusions of variable genes, usage of inverted diversity genes, and addition of extragenomic nucleotides. Most importantly, there is a paucity of antibodies expressing the major V(H)3 gene family, which could result from the capacity of gp120 to act as superantigen for human B cells. This V(H)3+ antibody deficit also has been observed in B cells isolated ex vivo from the patients. Since V(H)3+ antibodies play an essential role in immune defense against infections, the abnormalities observed in HIV-1 infection may predispose to opportunistic infections and further compromise the immune defense mechanisms of the subjects.

V(D)J germline gene repertoire analysis of monoclonal D antibodies and the implications for D epitope specificity

BACKGROUND

The D antigen is a highly immunogenic human RBC antigen. Alloimmunization against the D antigen produces high-affinity antibodies that cause hemolytic transfusion reactions and HDN.

STUDY DESIGN AND METHODS

Cloning and subsequent sequence analysis of 11 new samples of monoclonal anti-D was performed in an attempt to identify V(D)J germline gene usage. Sequences were compared and analyzed with 37 previously published samples of anti-D for identification of V(H) and V(L) pairings, canonical structures, and conformation of restricted germline gene usage.

RESULTS

The V(H) and V(L) pairings used by the new D MoAbs resulted in seven canonical combinations, three of which had not been described previously. Preferential usage of gene segments from the VH3 and VH4 families and of D3, D6, JH6, and DPK9 germline gene segments was also determined. Three samples of anti-D from different donors were found to use similar V(H) and V(kappa) germline genes, despite the fact that two of the antibodies recognized epD6/7 and the third recognized epD1. From the cumulative analysis of the anti-D IgG, 24 V(H) and V(L) gene pairings were identified, resulting in only 10 canonical structures.

CONCLUSIONS

Despite the potential for diversity, only a minority of V(H) and V(L) germline genes are used by anti-D. Consequently, V(H) and V(L) pairings and the resulting canonical structures are similarly restricted.

Mapping the B cell superantigen binding site for HIV-1 gp120 on a V(H)3 Ig

The emerging class of B cell superantigens includes HIV-1 gp120, which binds to many members of the V(H)3 Ig gene family. The present study addresses the structural features of V(H)3 antibodies conferring gp120 binding activity using a panel of recombinant full-length and Fab Ig proteins. Binding activity was fully conferred by the Fab portion of the Ig molecule. The V(H) region was the major determinant of binding; diverse light chains were permissive for gp120 binding. A series of recombinant V(H)3-V(H)1 chimeric molecules was created to analyze the contribution of different subregions of V(H)3 to gp120 binding. Hypervariable loop 1 (H1) substitution alone caused a 10-fold reduction in binding activity. The framework subregions (FR1, FR2 and FR3) and H2 also influenced binding, since substitutions of various combinations of these subregions conferred 10- to 100-fold binding reductions. We conclude that gp120 binding occurs through a non-conventional interaction involving multiple discontinuously arrayed residues spanning the V(H), and including roles in gp120 contact and favorable conformation of the V(H).

CD5 is A potential selecting ligand for B-cell surface immunoglobulin: a possible role in maintenance and selective expansion of normal and malignant B cells

Although the function of CD5 on B cells is unknown, previous studies suggested that CD5 interaction with V(H) framework regions of surface immunoglobulins (Igs) may contribute to survival and expansion of B cells. Here we used B-chronic lymphocytic leukemia (B-CLL) cells and transformed B-cell lines from normal and B-CLL patients to study CD5-Ig interactions. Immobilized Ig binds and permits isolation of CD5 from lysates of CD5-expressing cell lines. Immunoglobulins or Fab fragments of different V(H) families varied in their effectiveness as inhibitors of anti-CD5 staining of CLL cells, appendix and tonsil tissue sections. Human Ig also binds to purified recombinant CD5. We show here for the first time that the unconventional Ig-CD5 interaction maps to the extracellular CD5-D2 domain whereas conventional epitopes recognized by anti-CD5 antibodies are localized in the D1 domain of CD5. We propose that interactions of VH framework regions with CD5 as a ligand may maintain, select or expand normal, autoimmune or transformed B cells and also contribute to skewing of the normal V(H) repertoire.

VH gene usage by family members affected with chronic lymphocytic leukaemia

The excess risk of chronic lymphocytic leukaemia (CLL) in the first-degree relatives of affected patients suggests that familial CLL might constitute a useful model to study the pathogenesis of this disease, as has been demonstrated in numerous other neoplastic disorders. Previous studies have shown non-random utilization of immunoglobulin genes in CLL, some germline in sequence and others containing numerous somatic mutations. To investigate whether familial cases of CLL exhibit similarities in the composition of the B-cell receptor repertoire to the pattern expressed by CLL patients as a whole, we have studied 25 CLL patients belonging to 12 different families (four French and eight Italian), each of which contained at least two affected members. Among familial cases, VH gene segment utilization proved non-random and diverged from the frequencies previously reported among unrelated patients with CLL. Specifically, although the 4-34 and 5-51 gene segments were found repeatedly, the 1-69 and 4-39 gene segments were used sparingly and the 3-23 gene segment presented with increased frequency. Following the pattern detected in studies of unrelated patients, the single 1-69 expressing CLL contained an unmutated H chain sequence and included a long HCDR3 interval. In contrast, 3-23 containing H chains all used JH4, retained at most 93% homology with germline sequence, and included only short HCDR3 intervals. The vast majority of the CLL variable domains contained a high degree of somatic mutation and exhibited an excess of replacement mutations in the CDR intervals. These findings suggest that familial CLL cases may preferentially derive from B-cell progenitors that have responded to antigen.

Mechanisms of protein A superantigen-induced signal transduction for proliferation of mouse B cell

Apart from many of the biological properties of protein A (PA) of Staphylococcus aureus, it has been recognized recently as a B-cell superantigen. Therefore, we investigated the molecular mechanisms of PA superantigen-induced mice splenic B-cell proliferation. Treatment of resting B cells with PA-evoked cell proliferation. Binding of PA to B cells led to a cascade of signal transduction mechanisms involving tyrosine kinase that activated phospholipase C, which in turn activated protein kinase C (PKC), and translocated it from cytosol to membrane. Mitogen-activated protein (MAP) kinase has been found to be activated down-stream of PKC in this signal pathway, which ultimately caused an activation of serum-responsive factor (SRF). Inhibition at any step of this signaling cascade could block B-cell proliferation. PA could also stimulate the Bcl-2 gene expression at protein level thereby supporting the pro-proliferative effect of PA. Thus, the molecular mechanisms related to PA-induced B cell proliferation has been delineated in this report as tyrosine kinase > PLC > PKC > MAP kinase > SRF > Bcl-2. Knowledge gathered from these observations might be of immense help to study the immune cell proliferation as a part of immunoactivation process. Also, the development of suitable inhibitors of the signaling pathway outlined here might provide clues as to how to abrogate pathologic antibody production in many disease processes.

Structural Basis of the gp120 Superantigen-Binding Site on Human Immunoglobulins

B cell superantigens (SAg) interact with normal human nonimmune Igs (Igs), independently of the light chain isotype, and activate a large proportion of the B cell repertoire. Recently, the major envelope protein of HIV-1, gp120, was found to exhibit SAg-like properties for B cells with potential pathologic consequences for the infected host. This unconventional mode of interaction contrasts with its binding to immunization-induced Abs, which requires the tertiary structure of the heavy and light chain variable regions. In this report, we have examined the structural basis of the interaction between human Igs and gp120. We found that gp120 binding is restricted to Igs from the VH3 gene family and that the two VH genes 3-23 and 3-30, known to be overutilized during all stages of B cell development, frequently impart gp120 binding. We also provide evidence that the viral gp120 SAg can interact with only a subset of the human VH3+ Igs that can convey binding to the prototypic bacterial B cell SAg protein A from Staphylococcus aureus. Finally, we have identified amino acid positions present primarily in the first and third framework regions of the Ig heavy chain variable region, outside the conventional hypervariable loops, which correlate with gp120 binding. In a three-dimensional sequence-homology model, these residues partially overlap with the predicted SAg protein A binding site for VH3+ Igs.

Autoimmunity, immunodeficiency and mucosal infections: chronic intestinal inflammation as a sensitive indicator of immunoregulatory defects in response to normal luminal microflora

Despite the fact that target antigens and the genetic basis of several autoimmune diseases are now better understood, the initial events leading to a loss of tolerance towards self-components remain unknown. One of the most attractive explanations for autoimmune phenomena involves various infections as possible natural events capable of initiating the process in genetically predisposed individuals. The most accepted explanation of how infection causes autoimmunity is based on the concept of "molecular mimicry" (similarity between the epitopes of an autoantigen and the epitopes in the environmental antigen). Infectious stimuli may also participate in the development of autoimmunity by inducing an increased expression of stress proteins (hsp), chaperones and transplantation antigens, which leads to abnormal processing and presentation of self antigens. Superantigens are considered to be one of the most effective bacterial components to induce inflammatory reactions and to take part in the development and course of autoimmune mechanisms. It has long been known that defects in the host defense mechanism render the individual susceptible to infections caused by certain microorganisms. Impaired exclusion of microbial antigens can lead to chronic immunological activation which can affect the tolerance to self components. Defects in certain components of the immune system are associated with a higher risk of a development of autoimmune disease. The use of animal models for the studies of human diseases with immunological pathogenesis has provided new insights into the influence of immunoregulatory factors and the lymphocyte subsets involved in the development of disease. One of the most striking conclusion arising from work with genetically engineered immunodeficient mouse models is the existence of a high level of redundancy of the components of the immune system. However, when genes encoding molecules involved in T cell immunoregulatory functions are deleted, spontaneous chronic inflammation of the gut mucosa (similar to human inflammatory bowel disease) develops. Surprisingly, when such immunocompromised animals were placed into germfree environment, intestinal inflammation did not develop. Impairment of the mucosal immune response to the normal bacterial flora has been proposed to play a crucial role in the pathogenesis of chronic intestinal inflammation. The use of immunodeficient models colonized with defined microflora for the analysis of immune reactivity will shed light on the mode of action of different immunologically important molecules responsible for the delicate balance between luminal commensals, nonspecific and specific components of the mucosal immune system.

Selective deficit in antibodies specific for the superantigen binding site of gp120 in HIV infection

HIV infection is characterized by accelerated apoptosis and progressive loss of B cells. To see whether these abnormalities are related to the property of gp120 to act as a superantigen for VH3(+) B cells, we probed the temporal development of VH3(+) antibodies in HIV-1-infected subjects over a 7-year period. We found that VH3(+) antibodies specific for the gp120 superantigen binding site are deficient. Since VH3(+) antibodies impart protective responses to infectious agents, we quantified VH3(+) antibodies in serum samples from HIV-seropositive slow progressors and from patients who progressed to AIDS-related manifestations. We found that paucity in VH3(+) antibodies is a marker of rapid clinical decline. Remarkably, anti-gp160 VH3(+) antibodies showed a gradual decrease in progressors and, with time, varied depending on the viral load. We conclude that disease aggravation is associated with a decrease of the magnitude of the humoral response, that VH3(+) antibodies play an important role in protection, and that their underexpression may accelerate disease progression. We propose that vaccine preparations able to trigger VH3(+) antibodies might confer a better protection against HIV infection. This work also represents a novel mechanism of humoral deficiency resulting from the capacity of a viral antigen to affect an important subset of the B cell repertoire and to induce B cell death by apoptosis.

Selective alterations of the antibody response to HIV-1

HIV infection leads to progressive alterations of humoral immune functions, including B-cell hyperplasia, hypergammaglobulinemia, elevated autoantibody titers, a poor response to neoantigens and mitogens, polyclonal B-cell activation, monoclonal gammopathies, and a significant deterioration of the antigen-specific humoral response. There is also an important isotypic imbalance of the antibody (Ab) response in the systemic compartment and a profound modification of mucosal immune functions. These abnormalities may contribute to disease progression and development of opportunistic infections, despite the presence of serum-neutralizing anti-HIV Abs. Equally important are the abnormal selection mechanisms of the Ab repertoire that seem to be responsible for B-cell clonal deletions. The VH3 gene family, which encodes for approx 50% of immunoglobulins expressed by peripheral B-cells from normal adults, is underrepresented in human monoclonal antibodies to HIV-1 and in the peripheral B-cells of AIDS patients. These abnormalities, together with features of germinal center alteration, could be responsible for the clonal elimination of a subset of B-cells, and could contribute to HIV pathogenesis.

Staphylococcal Enterotoxin A Induces Survival of VH3-Expressing Human B Cells by Binding to the VH Region with Low Affinity

Staphylococcal enterotoxins (SE) are bacterial superantigens that bind to MHC class II molecules and to the Vβ-chain of the TCR, and subsequently activate T cells expressing specific Vβ regions. In this study, we have studied the effects of SEA on human B cell activation, and specifically the capacity of SEA to function as a B cell superantigen in vitro. We show herein that SEA failed to induce B cell proliferation and differentiation in the absence of T cells. However, SEA induced survival of B cells uniquely expressing VH3-containing IgM, independently of light chain utilization. The sequences of VH3 IgM gene products were determined and found to include a number of members of the VH3 family with a variety of different D and JH gene segments. Analysis of the sequences of VH3 gene products revealed possible sites in framework region 1 and/or framework region 3 that could be involved in SEA-mediated activation of VH3-expressing B cells. Binding studies showed that SEA interacts with the VH3 domain of Ig with low, but detectable affinity. These results indicate that SEA functions as a B cell superantigen by interacting with VH3 gene segments of Ig.

In Vivo Inflammatory Response to a Prototypic B Cell Superantigen: Elicitation of an Arthus Reaction by Staphylococcal Protein A

Staphylococcal protein A (SpA) is representative of a new class of Ags, the B cell superantigens (SAgs). These SAgs, unlike conventional Ags, bind to the Fab regions of Ig molecules outside their complementarity-determining regions. In addition, B cell SAgs can react with a substantial amount of a host’s serum Igs by virtue of their ability to interact with many members of an entire variable heavy chain (VH) or variable light chain gene family. For example, SpA reacts with the Fabs of most human Igs using heavy chains from the VH3 gene family (VH3+). Members of this gene family are expressed on 30 to 60% of human peripheral B cells. We sought to determine whether the interaction of a B cell SAg with its reactive Igs can elicit immune complex-mediated tissue injury. Using the Arthus reaction in rabbits as an in vivo model of immune complex-mediated tissue inflammation, we demonstrated that untreated rabbits, which were administered SpA intradermally (i.d.), do not develop a cutaneous inflammatory response. However, when rabbits were pretreated i.v. with human IgG (hIgG), i.d. injections of SpA induced an inflammatory response with the classical histologic features of an Arthus reaction. To determine whether this Arthus-like response occurred via a B cell superantigenic mechanism, the rabbits were pretreated with VH3-depleted hIgG and then were administered SpA i.d. We found that the induction of a prominent inflammatory response by SpA was dependent upon the presence of VH3+ molecules in the hIgG pretreatment. These results provide compelling evidence that an interaction of the B cell SAg, SpA, with its reactive (VH3+) IgGs leads to an immune complex-mediated inflammatory response in vivo.

Peptide models of immunological recognition: paratope dissection by multiple peptide synthesis

New approaches to obtain information about the residues of the antibody paratope involved in the interaction with antigen would be very useful to help perform rational mutagenesis of antibodies for improving sensitivity or selectivity in immunoassays. We have evaluated the possibility of dissecting the antibody paratope into large sets of short (12 residues) overlapping peptides to determine the contribution of each peptide to antigen binding. Our results show that the systematic analysis of the antigen-binding properties of heavy chain variable segment and light chain variable segment derived peptides of HyHEL-5, a model anti-lysozyme antibody, can be an experimental approach to the selection of paratope-derived peptides with antigen-binding activity. Detailed analysis of the contribution of each residue from each binding peptide permitted the identification of residues contributing to antigen binding. Of the 38 residues we identified, 22 corresponded to residues that had been shown by X-ray crystallography to be at the interface between HyHEL-5 and lysozyme. The peptide analysis we have developed is thus a way to map the subset of residues from the antibody paratope involved in antigen recognition. The same peptide approach was used to map the idiotope that an anti-idiotypic antibody recognized in the paratope of its cognate antibody.