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

Post-Transformation IGHV-IGHD-IGHJ Mutations in Chronic Lymphocytic Leukemia B Cells: Implications for Mutational Mechanisms and Impact on Clinical Course

Analyses of IGHV gene mutations in chronic lymphocytic leukemia (CLL) have had a major impact on the prognostication and treatment of this disease. A hallmark of IGHV-mutation status is that it very rarely changes clonally over time. Nevertheless, targeted and deep DNA sequencing of IGHV-IGHD-IGHJ regions has revealed intraclonal heterogeneity. We used a DNA sequencing approach that achieves considerable depth and minimizes artefacts and amplification bias to identify IGHV-IGHD-IGHJ subclones in patients with prolonged temporal follow-up. Our findings extend previous studies, revealing intraclonal IGHV-IGHD-IGHJ diversification in almost all CLL clones. Also, they indicate that some subclones with additional IGHV-IGHD-IGHJ mutations can become a large fraction of the leukemic burden, reaching numerical criteria for monoclonal B-cell lymphocytosis. Notably, the occurrence and complexity of post-transformation IGHV-IGHD-IGHJ heterogeneity and the expansion of diversified subclones are similar among U-CLL and M-CLL patients. The molecular characteristics of the mutations present in the parental, clinically dominant CLL clone (CDC) differed from those developing post-transformation (post-CDC). Post-CDC mutations exhibit significantly lower fractions of mutations bearing signatures of activation induced deaminase (AID) and of error-prone repair by Polη, and most of the mutations were not ascribable to those enzymes. Additionally, post-CDC mutations displayed a lower percentage of nucleotide transitions compared with transversions that was also not like the action of AID. Finally, the post-CDC mutations led to significantly lower ratios of replacement to silent mutations in VH CDRs and higher ratios in VH FRs, distributions different from mutations found in normal B-cell subsets undergoing an AID-mediated process. Based on these findings, we propose that post-transformation mutations in CLL cells either reflect a dysfunctional standard somatic mutational process or point to the action of another mutational process not previously associated with IG V gene loci. If the former option is the case, post-CDC mutations could lead to a lesser dependence on antigen dependent BCR signaling and potentially a greater influence of off-target, non-IG genomic mutations. Alternatively, the latter activity could add a new stimulatory survival/growth advantage mediated by the BCR through structurally altered FRs, such as that occurring by superantigen binding and stimulation.

Diversity of Functionally Distinct Clonal Sets of Human Conventional Memory B Cells That Bind Staphylococcal Protein A

Staphylococcus aureus, a common cause of serious and often fatal infections, is well-armed with secreted factors that disarm host immune defenses. Highly expressed in vivo during infection, Staphylococcal protein A (SpA) is reported to also contribute to nasal colonization that can be a prelude to invasive infection. Co-evolution with the host immune system has provided SpA with an Fc-antibody binding site, and a Fab-binding site responsible for non-immune superantigen interactions via germline-encoded surfaces expressed on many human BCRs. We wondered whether the recurrent exposures to S. aureus commonly experienced by adults, result in the accumulation of memory B-cell responses to other determinants on SpA. We therefore isolated SpA-specific class-switched memory B cells, and characterized their encoding VH : VL antibody genes. In SpA-reactive memory B cells, we confirmed a striking bias in usage for VH genes, which retain the surface that mediates the SpA-superantigen interaction. We postulate these interactions reflect co-evolution of the host immune system and SpA, which during infection results in immune recruitment of an extraordinarily high prevalence of B cells in the repertoire that subverts the augmentation of protective defenses. Herein, we provide the first evidence that human memory responses are supplemented by B-cell clones, and circulating-antibodies, that bind to SpA determinants independent of the non-immune Fc- and Fab-binding sites. In parallel, we demonstrate that healthy individuals, and patients recovering from S. aureus infection, both have circulating antibodies with these conventional binding specificities. These findings rationalize the potential utility of incorporating specially engineered SpA proteins into a protective vaccine.

B-cell development and pneumococcal immunity in vertically acquired HIV infection

OBJECTIVES

Many children with HIV infection now survive into adulthood. This study explored the impact of vertically acquired HIV in the era of antiretroviral therapy on the development of humoral immunity.

DESIGN

Natural and vaccine-related immunity to pneumococcus and B-cell phenotype was characterized and compared in three groups of young adults: those with vertically-acquired infection, those with horizontally acquired infection and healthy controls.

METHODS

Serotype-specific pneumococcal (Pnc) immunoglobulin M and G concentrations before and up to 1 year post-Pnc polysaccharide (Pneumovax) immunization were determined, and opsonophagocytic activity was analysed. B-cell subpopulations and dynamic markers of B-cell signalling, turnover and susceptibility to apoptosis were evaluated by flow cytometry.

RESULTS

HIV-infected patients showed impaired natural Pnc immunity and reduced humoral responses to immunization with Pneumovax; this was greatest in those viraemic at time of the study. Early-life viral control before the age of 10 years diminished these changes. Expanded populations of abnormally activated and immature B-cells were seen in both HIV-infected cohorts. Vertically infected patients were particularly vulnerable to reductions in marginal zone and switched memory populations. These aberrations were reduced in patients with early-life viral control.

CONCLUSION

In children with HIV, damage to B-cell memory populations and impaired natural and vaccine immunity to pneumococcus is evident in early adult life. Sustained viral control from early childhood may help to limit this effect and optimize humoral immunity in adult life.

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.

Dysfunctional B-cell responses during HIV-1 infection: implication for influenza vaccination and highly active antiretroviral therapy

Although HIV-1 infection does not directly target B cells, B-cell numbers are reduced and their function is impaired during HIV infection. Antibody titres against antigens previously encountered through vaccination or natural infection are low in patients with HIV. Intrinsic B-cell defects might be involved in the impairment of humoral immunity during early HIV infection. Abnormal T-cell activation and the altered expression of molecules involved in the B-cell homing process cause dysfunctional interaction between T and B cells in the germinal centres of lymphoid tissues, which might impair B-cell responses during HIV infection. Class-switch recombination is also impaired in individuals with HIV. Protective immune responses against T-cell-dependent antigens, including influenza antigens, rely on the production of neutralising antibodies. Impaired B-cell responses during HIV infection could therefore hamper the effectiveness of vaccinations against seasonal influenza or the new pandemic influenza A H1N1 vaccines in individuals with HIV. By maintaining B-cell responses, highly active antiretroviral therapy might improve the efficacy of influenza vaccines in individuals with HIV.

Neutralization of genetically diverse HIV-1 strains by IgA antibodies to the gp120-CD4-binding site from long-term survivors of HIV infection

OBJECTIVE

To identify an HIV epitope suitable for vaccine development.

DESIGN

Diverse HIV-1 strains express few structurally constant regions on their surface vulnerable to neutralizing antibodies. The mostly conserved CD4-binding site (CD4BS) of gp120 is essential for host cell binding and infection by the virus. Antibodies that recognize the CD4BS are rare, and one component of the CD4BS, the 421-433 peptide region, expresses B-cell superantigenic character, a property predicted to impair the anti-CD4BS adaptive immune response.

METHODS

IgA samples purified from the plasma of patients with HIV infection were analyzed for the ability to bind synthetic mimetics containing the 416-433 gp120 region and full-length gp120. Infection of peripheral blood mononuclear cells by clinical HIV isolates was measured by p24 ELISA.

RESULTS

IgA preparations from three patients with subtype B infection for 19-21 years neutralized heterologous, coreceptor CCR5-dependent subtype A, B, C, D, and AE strains with exceptional potency. The IgAs displayed specific binding of a synthetic 416-433 peptide mimetic dependent on recognition of the CD4-binding residues located in this region. Immunoadsorption, affinity chromatography, and mutation procedures indicated that HIV neutralization occurred by IgA recognition of the CD4BS.

CONCLUSION

These observations identify the 421-433 peptide region as a vulnerable HIV site to which survivors of infection can produce powerful neutralizing antibodies. This indicates that the human immune system can bypass restrictions on the adaptive B cell response to the CD4BS, opening the route to targeting the 421-433 region for attaining control of HIV infection.

Toward effective HIV vaccination: induction of binary epitope reactive antibodies with broad HIV neutralizing activity

We describe murine monoclonal antibodies (mAbs) raised by immunization with an electrophilic gp120 analog (E-gp120) expressing the rare ability to neutralize genetically heterologous human immunodeficiency virus (HIV) strains. Unlike gp120, E-gp120 formed covalent oligomers. The reactivity of gp120 and E-gp120 with mAbs to reference neutralizing epitopes was markedly different, indicating their divergent structures. Epitope mapping with synthetic peptides and electrophilic peptide analogs indicated binary recognition of two distinct gp120 regions by anti-E-gp120 mAbs, the 421-433 and 288-306 peptide regions. Univalent Fab and single chain Fv fragments expressed the ability to recognize both peptides. X-ray crystallography of an anti-E-gp120 Fab fragment revealed two neighboring cavities, the typical antigen-binding cavity formed by the complementarity determining regions (CDRs) and another cavity dominated by antibody heavy chain variable (V(H)) domain framework (FR) residues. Substitution of the FR cavity V(H) Lys-19 residue by an Ala residue resulted in attenuated binding of the 421-433 region peptide probe. The CDRs and V(H) FR replacement/silent mutation ratios exceeded the ratio for a random mutation process, suggesting adaptive development of both putative binding sites. All mAbs studied were derived from V(H)1 family genes, suggesting biased recruitment of the V gene germ line repertoire by E-gp120. The conserved 421-433 region of gp120 is essential for HIV binding to host CD4 receptors. This region is recognized weakly by the FR of antibodies produced without exposure to HIV, but it usually fails to induce adaptive synthesis of neutralizing antibodies. We present models accounting for improved CD4-binding site recognition and broad HIV neutralizing activity of the mAbs, long sought goals in HIV vaccine development.

Aerobic exercise training as a potential source of natural antibodies protective against human immunodeficiency virus-1

Despite the effectiveness of HAART in controlling HIV-1 replication, the emergence of drug-resistant viruses in infected patients and the severe side effects caused by the currently used drug regimens and the lack of an effective vaccine necessitate the continued search for new therapeutic strategies for prevention and therapy of HIV disease. Previously we reported that natural autoantibodies, recognizing peptide FTDNAKTI (peptide NTM1) derived from the C2 domain of HIV-1 gp120, contribute to the control of HIV disease. Here we demonstrated that sera from well-trained athletic (HIV-negative) subjects showed high reactivity with peptide NTM1. This result confirms that aerobic exercise training stimulates production of natural autoantibodies, which recognize peptide NTM1. Bioinformatics analysis indicates that these natural autoantibodies could slow down disease progression by blocking the superantigenic site on HIV-1 gp120. The results suggest that aerobic exercise training may be a promising non-toxic and inexpensive adjunctive anti-HIV therapy.

Catalytic antibodies to HIV: physiological role and potential clinical utility

Immunoglobulins (Igs) in uninfected humans recognize residues 421-433 located in the B cell superantigenic site (SAg) of the HIV envelope protein gp120 and catalyze its hydrolysis by a serine protease-like mechanism. The catalytic activity is encoded by germline Ig variable (V) region genes, and is expressed at robust levels by IgMs and IgAs but poorly by IgGs. Mucosal IgAs are highly catalytic and neutralize HIV, suggesting that they constitute a first line of defense against HIV. Lupus patients produce the Igs at enhanced levels. Homology of the 421-433 region with an endogenous retroviral sequence and a bacterial protein may provide clues about the antigen driving anti-SAg synthesis in lupus patients and uninfected subjects. The potency and breadth of HIV neutralization revives hopes of clinical application of catalytic anti-421-433 Igs as immunotherapeutic and topical microbicide reagents. Adaptive improvement of anti-SAg catalytic Igs in HIV infected subjects is not customary. Further study of the properties of the naturally occurring anti-SAg catalytic Igs should provide valuable guidance in designing a prophylactic vaccine that amplifies protective catalytic immunity to HIV.

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.

Antibodies to the superantigenic site of HIV-1 gp120: Hydrolytic and binding activities of the light chain subunit

Antibodies (Abs) to the superantigenic determinant of HIV gp120 (gp120(SAg)) are potential protective agents against HIV infection. We report that the light chain subunits of Abs cloned from lupus patients using phage library methods bind and hydrolyze gp120(SAg) independent of the heavy chain. Unlike frequent gp120(SAg) recognition by intact Abs attributable to V(H) domain structural elements, the isolated light chains expressed this activity rarely. Four light chains capable of gp120(SAg) recognition were identified by fractionating phage displayed light chains using peptide probes containing gp120 residues 421-433, a gp120(SAg) component. Three light chains expressed non-covalent gp120(SAg) binding and one expressed gp120(SAg) hydrolyzing activity. The hydrolytic light chain was isolated by covalent phage fractionation using an electrophilic analog of residues 421-433. This light chain hydrolyzed a reporter gp120(SAg) substrate and full-length gp120. Other peptide substrates and proteins were hydrolyzed at lower rates or not at all. Consistent with the expected nucleophilic mechanism of hydrolysis, the light chain reacted selectively and covalently with the electrophilic gp120(SAg) peptide analog. The hydrolytic reaction entailed a fast initial step followed by a slower rate limiting step, suggesting rapid substrate acylation and slow deacylation. All four gp120(SAg)-recognizing light chains contained sequence diversifications relative to their germline gene counterparts. These observations indicate that in rare instances, the light chain subunit can bind and hydrolyze gp120(SAg) without the participation of the heavy chain. The pairing of such light chains with heavy chains capable of gp120(SAg) recognition represents a potential mechanism for generating protective Abs with enhanced HIV binding strength and anti-viral proteolytic activity.

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.

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.

A point of view: HIV-1/AIDS is an allergy but CpG ODN treatments may inhibit virus replication and reactivate the adaptive immunity--hypothesis and implications

Reevaluation of the increase in the levels of IgE and IL-4 in sera of HIV-1 infected and AIDS patients led to the suggestion that AIDS resembles allergy. Studies on the properties of the viral shed gp120 revealed that it resemble environmental allergens in their ability to induce hematopoietic cells to release large amounts of Th2 cytokines, inhibitors of the patients adaptive immune response. Yet, induction of TLR9+ plasmacytoid DCs by CpG ODNs cause the release of type I interferons, inhibitors of HIV-1 replication and IL-4 release from hematopoietic cells. CpG ODN binding to TLR+ B cells inhibits IgE synthesis and reactivates the failing adaptive immunity. The possible use of CpG ODNs as treatment to patients is 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.

HIV-1 gp120 binding to dendritic cell receptors mobilize the virus to the lymph nodes, but the induced IL-4 synthesis by FcepsilonRI+ hematopoietic cells damages the adaptive immunity--a review, hypothesis, and implications

HIV-1 is equipped with the envelope gp160 glycoprotein for interaction with Langerhans cells (LCs) and dendritic cells (DCs), the members of the innate immune system, which confront the virus at the portal of virus entry in the human body. These cells are equipped with receptors by which they bind and endocytose the virus. The gp120 glycoprotein is used for binding to CD4 receptor and CCR5 co-receptor of T helper 2 (Th2) cells and the virions shed gp120 is able to induce FcepsilonRI+ hematopoietic cells to produce IL-4, which inactivate the host adaptive immune response. The properties of gp120s various functional domains are analyzed together with the regulatory viral proteins, which are involved in the damage to T and B cells during HIV-1 replication. The interaction of HIV-1 virions through their gp120 with LCs and DCs at the portal of virus entry will be discussed. A hypothesis will be presented that the understanding of the role of the different functional domains of gp120 in the life cycle of the virus and during AIDS will help in the design of approaches to prevent and abrogate HIV-1 infection and AIDS.

Naturally occurring proteolytic antibodies: selective immunoglobulin M-catalyzed hydrolysis of HIV gp120

We report the selective catalytic cleavage of the HIV coat protein gp120, a B cell superantigen, by IgM antibodies (Abs) from uninfected humans and mice that had not been previously exposed to gp120. The rate of IgM-catalyzed gp120 cleavage was greater than of other polypeptide substrates, including the bacterial superantigen protein A. The kinetic parameters of gp120 cleavage varied over a broad range depending on the source of the IgMs, and turnover numbers as great as 2.1/min were observed, suggesting that different Abs possess distinct gp120 recognition properties. IgG Abs failed to cleave gp120 detectably. The Fab fragment of a monoclonal IgM cleaved gp120, suggesting that the catalytic activity belongs to the antibody combining site. The electrophoretic profile of gp120 incubated with a monoclonal human IgM suggested hydrolysis at several sites. One of the cleavage sites was identified as the Lys(432)-Ala(433) peptide bond, located within the region thought to be the Ab-recognizable superantigenic determinant. A covalently reactive peptide analog (CRA) corresponding to gp120 residues 421-431 with a C-terminal amidino phosphonate diester mimetic of the Lys(432)-Ala(433) bond was employed to probe IgM nucleophilic reactivity. The peptidyl CRA inhibited the IgM-catalyzed cleavage of gp120 and formed covalent IgM adducts at levels exceeding a control hapten CRA devoid of the peptide sequence. These observations suggest that IgMs can selectively cleave gp120 by a nucleophilic mechanism and raise the possibility of their role as defense enzymes.

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.

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.

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.

A CCR5-dependent novel mechanism for type 1 HIV gp120 induced loss of macrophage cell surface CD4

Type 1 HIV gp120 is especially effective in disrupting immune cell function because it is able to cause dysregulation of both infected and uninfected cells. We report a novel CCR5-dependent mechanism of gp120-induced CD4 loss from macrophages. An M-tropic gp120, using CCR5, is able to induce 70% loss of cell surface CD4 from macrophages within an hour. This cell surface CD4 loss is more substantial and rapid than the 20% loss observed with T-tropic gp120(IIIB) by 3 h. The rapid and substantial CD4 loss induced by M-tropic gp120 is not observed on macrophages homozygous for the ccr5Delta32 mutation, which fail to express cell surface CCR5. We have used confocal imaging to show that gp120 and CD4 are internalized together by a process resembling receptor-mediated endocytosis, and that both proteins enter HLA-DR containing compartments of the macrophage. We have also shown by semiquantitative RT-PCR that, in response to CD4 loss from the cell surface, mRNA for CD4 is up-regulated and the intracellular pool of CD4 increases. CCR5 mRNA levels are also increased. It is proposed that internalization of self and viral protein and increased pools of intracellular CD4 could modulate Ag presentation efficiencies and have implications for the induction and maintenance of both productive immune responses and self-tolerance.

Impact of HIV-1 infection on VH3 gene repertoire of naive human B cells

B cells of the largest Ig variable heavy chain gene (VH) family, VH3, are reportedly decreased in patients with late stage HIV-1 disease. This deficit may contribute to their impaired responses to infections and vaccines. We confirmed that the VH3 family was underrepresented in serum IgM proteins, with a 45% decrease in patients with advanced HIV-1 disease. However, the proportion of VH3 within VH(1-6) IgM mRNA from peripheral B cells did not differ from that of control subjects (mean +/- SD, 57.1 +/- 9.7 vs 61.1 +/- 8. 7%). Similarly, within VH(1-6) IgD mRNA, which even more closely represents the unstimulated naive repertoire, the relative expression of VH3 mRNA was comparable in the two groups. Moreover, the frequency of individual genes within the VH3 family for IgD, particularly genes which encode putative HIV-1 gp120 binding sites, also was normal in HIV-1-infected patients. However, VH3 family expression for IgG mRNA was significantly decreased (17%) and VH4 IgG was increased (33%) relative to other VH families in advanced HIV-1-infected patients. Thus, the changes in VH family expression were more readily apparent in previously activated IgG "memory" B cell populations and, likely, in cells actively producing IgM rather than in resting naive cells. The presence of a relatively normal naive VH3 IgM and IgD mRNA repertoire in resting cells supports the prospect that with proper stimulation, particularly in conjunction with effective antiviral therapy, vigorous humoral immune responses to infections and vaccines may be elicited in this high-risk population.

gp120- and TNF-alpha-induced modulation of human B cell function: proliferation, cyclic AMP generation, Ig production, and B-cell receptor expression

BACKGROUND

It is well known that HIV-1 infection induces profound alterations in the immune system, including hyperactivation of B cells. TNF-alpha induces HIV-1 replication and immunodysregulation, including polyclonal B-cell activation.

OBJECTIVE

We sought to determine the effects of surface-binding HIV-1 envelope glycoprotein (gp120) and TNF-alpha on human B-cell function.

METHODS

HIV-1 seronegative peripheral blood human B cells were purified and activated by CD40 mAb and IL-4. In vitro studies of B-cell proliferation, cyclic AMP (cAMP) generation, receptor expression, and Ig production were performed.

RESULTS

gp120, an Ig superantigen, stimulated HIV-1 seronegative and HIV-1 seropositive human B-cell cAMP generation, proliferation, and Ig production. These gp120-induced B-cell responses were demonstrated to be specific as evidenced by the abrogation of the stimulatory response in the presence of anti-gp120 mAb, blocking of CD4 resulting in no change on gp120-induced B-cell responses, and the binding of gp120 in these B cells. TNF-alpha also stimulated cAMP generation, proliferation, and Ig production in B cells, and the binding of gp120 to these B cells stimulated by TNF-alpha further enhanced cell proliferation, cAMP generation, and Ig production. Antigenic expression of the B-cell receptor CD79b was down-regulated by gp120 but was not altered by the addition of TNF-alpha.

CONCLUSION

gp120 modulation of TNF-alpha-induced B-cell receptor- and cAMP-mediated signal transduction events may be involved in the B-cell abnormalities observed in HIV-1 infection.

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.