Cross talk between T and B cells generates B antigen-presenting cells able to induce inositol phosphate production in T cells responding to MIsa superantigens

Previous studies showed that activation of CD4⁺ T cells with mouse mammary tumor virus-encoded Mls^a superantigens induces strong proliferative responses and interleukin-2 production but fails to elicit typical early T cell receptor (TCR)-mediated signal transduction events, such as hydrolysis of polyphosphoinositides (PI) or an increase in intracellular calcium. Here we show that the failure of MIs^a antigen to activate PI hydrolysis applies when resting B cells are used as antigen-presenting cells (APC). By contrast, when MIs^a-bearing B cells are activated for 24h by exposure to lipopolysaccharide or, more importantly, to Mls^a-reactive T cells or anti-CD40 antibodies the cells develop the capacity to elicit easily detectable PI turnover. These studies demonstrate that, for B cells as APC, the initiation of certain TCR-associated signal transduction pathways can depend on activation of the APC. The data suggest that cross talk between T cells and resting B cells can suffice to generate competent B APC and lead to the delayed initiation of signaling pathways important in T cell responses.     

CD28/CTLA4-B7 interaction is dispensable for T cell stimulation by mouse mammary tumor virus superantigen but not for B cell differentiation and virus dissemination

B cells are the primary targets of infection for mouse mammary tumor virus (MMTV). However, for productive retroviral infection, T cell stimulation through the virally-encoded superantigen (SAG) is necessary. It activates B cells and leads to cell division and differentiation. To characterize the role of B cell differentiation for the MMTV life cycle, we studied the course of infection in transgenic mice deficient for CD28/CTLA4-B7 interactions (mCTLA4-Hγ1 transgenic mice). B cell infection occurred in CTLA4-Hγ1 transgenic mice as integrated proviral DNA could be detected in draining lymph node cells early after infection by polymerase chain reaction analysis. In mice expressing I-E, B cells were able to present the viral SAG efficiently to Vβ6⁺ T cells. These cells expanded specifically and were triggered to express the activation marker CD69. Further stages of progression of infection appeared to be defective. Kinetics experiments indicated that T and B cell stimulation stopped more rapidly than in control mice. B cells acquired an activated CD69⁺ phenotype, were induced to produce IgM but only partially switched to IgG secretion. Finally, the dissemination of infected cells to other lymph nodes and spleen was reduced and the peripheral deletion of Vβ6⁺ T cells was minimal. In contrast, in mice lacking I-E, T cell stimulation was also impaired and B cell activation undetectable. These data implicate B7-dependent cellular interactions for superantigenic T cell stimulation by low-affinity TCR ligands and suggest a role of B cell differentiation in viral dissemination and peripheral T cell deletion.     

A Vβ.2-specific superantigen from exogenous mouse mammary tumor virus carried by FM mice

A number of endogenous mouse mammary tumor virus (MMTV) proviruses encode superantigen that have the ability to stimulate T cells with a certain T cell receptor (TCR) β-chain variable region (Vβ) and to mediate the Vβ-specific clonal deletion. The tumorigenic milk-borne MMTV carried by C3H and GR mice also have superantigenic properties in vivo. In the present study we identified and characterized a novel Vβ8.2-specific superantigen of exogenous MMTV carried by FM mice. The open reading frame (ORF) in the 3′ long terminal repeat of the MMTV was cloned by polymerase chain reaction with primers corresponding to conserved regions spanning the ORF coding region. Sequence analysis of the ORF revealed that there is no sequence identical to those in other known MMTV in the carboxy terminus implicated in TCR Vβ recognition. Subcutaneous injection of the virus into adult BALB/c mice induced an approximately three- to fourfold enlargement of draining lymph nodes and a substantial increase of Vβ8.2⁺ CD4⁺ T cells in the lymph nodes within 6 days. The exposure of newborn BALB/c mice to the virus by foster nursing resulted in a marked deletion of Vβ8.2⁺ cells both in CD4⁺ and CD8⁺ T cells. Thus, a novel milk-borne MMTV in FM mice expresses strong superantigenic properties capable of stimulating Vβ8.2⁺ T cells. Vβ8.2⁺ T cells have been demonstrated to be frequently involved in recognition of conventional antigens and responsible for autoimmune diseases such as experimental allergic encephalomyelitis. Therefore, the MMTV (FM) may provide a new mouse model system for inducing immunodeficiency or autoimmune disease by retroviral infection.     

Delayed allograft rejection by T cell receptor Vβ8.1 transgenic mice peripherally tolerized to Mls-1

One commonly studied model system for peripheral tolerance is the antigen-specific unresponsiveness of T cells from mice previously inoculated with superantigens such as Mls-1^a. In this study, we used a TcR Vβ8.1 transgenic mouse model to investigate whether mice peripherally tolerized to Mls-1^a exhibit delayed skin allograft rejection. We report dramatic prolongation of skin allograft survival in Vβ8.1 transgenic but not in non-transgenic mice tolerized to Mls-1^a. Peripherally induced unresponsiveness to Mls-1^a can, therefore, be considered true tolerance.     

Influenza A virus hemagglutinin is a B cell-superstimulatory lectin

Influenza A viruses display T cell-independent polyclonal B cell-activating properties which are mediated by the B cell-superstimulatory envelope glycoprotein hemagglutinin (HA). In this report, the receptor-binding requirements for B cell activation by influenza viruses were expected. Neuraminidase treatment of resting mature B cells from BALB/c mice abrogated late (proliferation/immunoglobulin synthesis), early (up-regulation of cell surface markers, including CD25, B220, and B7-1) and veryearly events (homotypic adhesion) in virus-responding B lymphocytes. Similarly, pretreatment of murine responder cells with different inhibitors ofN-glycosylation (tunicamycin, deoxymannojirimycin) significantly suppressed subsequent B lymphocyte activation by HA, but not control responses to lipopolysaccharide or anti-μ. Assays with chimeric HA transfectants, expressing the loop region of epitope B (amino acids 155–160) of the globular head of H2 (high B cell-stimulatory subtype) or H3 (medium-stimulatory subtype) on the protein backbone of a low-stimulatory subtype (H1) failed to alter the B cell-stimulatory activity of the virus, suggesting that the hypervariable loop region is not crucial in determining the B cell-activating properties of the protein. Collectively, our results imply that the B cell-superstimulatory function of influenza virus HA is not mediated by a direct protein/protein interaction, but via binding of HA to terminal sialic acid residues on cell surface receptor glycoproteins. These findings identify the influenza virus HA glycoprotein as the first viral lectin with lymphocyte-activating properties.     

T cells made deficient in interleukin-2 production by exposure to staphylococcal enterotoxin B in vivo are primed for interferon-γ and interleukin-10 secretion

The superantigen staphylococcal enterotoxin B (SEB) induces a defect in interleukin (IL)-2 production by T cells expressing specific T cell receptor Vβ domains. The present study was undertaken to determine the capacity of T cells, made deficient in IL-2 production by exposure to SEB in vivo, to secrete interferon (IFN)-γ and IL-10 and to induce pathology upon SEB rechallenge. For this purpose, BALB/c mice received two intraperitoneal injections of 100 μg SEB with a 48-h interval. First, we compared peak serum levels of IL-2, IFN-γ and IL-10 after SEB rechallenge with those measured after a single SEB injection in control mice. The expected defect in IL-2 production in SEB-pretreated mice was associated with a major increase in IL-10 and IFN-γ levels which were about fivefold higher than in controls. Experiments in mice depleted of CD4⁺ or CD8⁺ cells as well as studies in which purified T cell populations were rechallenged with SEB in vitro indicated that both CD4⁺ and CD8⁺ cells from SEB-pretreated mice were primed for IL-10 and IFN-γ production. Furthermore, SEB-pretreated mice were sensitized to the toxic effects of the superantigen as indicated by a 30-70% lethality rate (vs. 0% in naive mice) within 48 h after SEB rechallenge. IFN-γ was involved in the lethal syndrome as it could be prevented by injection of neutralizing anti-IFN-γ monoclonal antibody. We conclude that SEB-reactive T cells made deficient for the production of IL-2 by exposure to SEB in vivo are primed for IFN-γ and IL-10 production, and that IFN-γ up-regulation is involved in the shock syndrome occurring upon SEB rechallenge.     

Respiratory syncytial virus (RSV) evades the human adaptive immune system by skewing the Th1/Th2 cytokine balance toward increased levels of Th2 cytokines and IgE, markers of allergy—a review

Infection of infants in their first year of life, children and elderly people with the respiratory syncytial virus (RSV) endangers the life of the patient. An attempt to develop a formalin-inactivated RSV (FI-RSV) vaccine during the 1960s resulted in an aggravated infection in immunized children, leading to hospitalization, while infection of non-immunized children produced much milder symptoms. The reason for this remained an enigma, one which was gradually solved over the last decade by many researchers who studied the molecular biology of RSV infection of respiratory ciliary cells. Clinical studies of RSV-infected patients indicated increased levels of Th2 cytokines and IgE in the patients’ sera, suggesting that an allergy-like condition developed during infection. The biomarkers of allergy caused by endogenous or environmental allergens include a marked increase of the Th2 cytokine IL-4 and IgE non-neutralizing antibodies to the allergen. The way allergens trigger allergy was deciphered recently, and will be discussed later. Studies of RSV infection led to the suggestion that RSV patients suffer from allergy prior to RSV infection, a concept that was later abandoned. Studies on HIV-1 [Y. Becker, Virus Genes 28, 319–331 (2005)] research led me to the hypothesis that since HIV-1 infection induces a marked increase of IL-4 and IgE in serum, an allergy-like condition, the AIDS stage is the result of an allergen motif that is embedded in the shed viral gp120 molecules. It is hypothesized that the viral-soluble G glycoprotein (sG) contains a T cell superantigen (Tsag) that is capable of binding to the V_H3 domain of IgE/FcεRI⁺ hematopoietic cells, basophils, mast cells and monocytes, similar to the case of allergens, and that this aggregation causes these innate system cells to degranulate and release large amounts of Th2 cytokines (IL-4, IL-5, IL-10, IL-13) into the blood. The way these Th2 cytokines skew the Th1/Th2 balance toward Th2 > Th1 will be discussed. The aim of the present review is to base RSV pathogenicity on the numerous very good analyses of the virus genes and to suggest a therapeutic approach to treatment that is directed at preventing the inhibitory effects of Th2 cytokines on the adaptive immune system of the patients, instead of inhibiting RSV replication by antivirals. The review of the molecular research on the role of the viral fusion (F) and attachment (G) glycoproteins of RSV provided information on their role in the virus infection: early in infection the F glycoprotein induces Th1 cells to release the Th1 cytokines IL-2, IL-12 and IFN-γ to activate precursors CTLs (pCTLs) to become anti-RSV CTLs. The G and sG glycoproteins attach to FKNR1⁺ ciliary respiratory epithelial cells as well as directly to eosinophils to the lungs. The sG T cell antigen can also induce the release of large amounts of Th2 cytokines from CD4⁺ T cells and from FCεRI⁺ mast cells, basophils and monocytes. By comparison to HIV-1 gp120 it is possible to show that in the G and sG proteins the T cell antigen resembles the CD4⁺ T cell superantigen (=allergen) domain of HIV-1 gp120 which aggregates with IgE/FCεRI⁺ hematopoietic cells. The increased IL-4 level in the serum inhibits the adaptive immune response: IL-4Rα⁺ Th1 cells stop Th1 cytokine synthesis and IL-4Rα⁺ B cells stop the synthesis of antiviral IgG and IgA and switch to IgE synthesis. In addition, the hematopoietic cells release histamine and prostaglandin which induce wheezing. The gradual increase of sG molecules creates a gradient of fractalkine (FKN) which directs IL-5-activated eosinophils to the lungs of the patient.     

Effect of cell:cell competition and BAFF expression on peripheral B cell tolerance and B-1 cell survival in transgenic mice expressing a low level of Igkappa-reactive macroself antigen

In mice carrying a synthetic Igkappa-reactive superantigen ("kappa macroself antigen"), low level expression induced split peripheral B cell tolerance in the sIgkappa+ compartment, with striking reductions in follicular and marginal zone (MZ) B cells and the retention of significant numbers of sIgkappa+ B-1a but not B-1b cells in the peritoneum. Here, we characterize the transgenic line pKkappa with this split tolerance phenotype and assess the effects of B cell competition and the survival cytokine BAFF (B cell activating factor belonging to the TNF family) on peripheral tolerance. In pKkappa mice the surviving peritoneal and splenic kappa+ B cells were largely lost in mice carrying one copy of the human Ckappa exon in place of the mouse version, a maneuver that generates additional antigen non-reactive competitor B cells in this model. Furthermore, overexpression of BAFF suppressed kappa-macroself antigen-induced deletion and promoted production of both IgM,kappa and IgA,kappa antibodies in mice with normal Igkappa alleles but not in mice carrying one copy of the human Ckappa allele. These findings suggest that BAFF overexpression has minimal effects on the survival of autoreactive B cells in a polyclonal immune system and that B cell:B cell competition plays a potent role in suppressing the survival of B-1 and splenic B cells with excessive autoreactivity.