Endogenous Superantigens Shape Response to Exogenous Superantigens

Endogenous superantigen-mediated thymic negative selection resulted in a paucity of mature T cells bearing T-cell receptor (TCR) Vβ8 in the periphery. Consequently, the magnitude of immune response to exogenous superantigen staphylococcal enterotoxin B, which activates TCR Vβ8⁺ T cells, was significantly reduced and conferred protection from superantigen-induced mortality.     

Superantigens initiate cognate CD4+ T cell/ B cell interactions leading to early activation and proliferation of B cells

Dimerization or even multimerization of various receptors is commonly required for signal transduction. We report here that clustering of major histocompatibility complex class II molecules in human B cells by biotinylated staphylococcal enterotoxin A (SEA) cross-linked with avidin induces an increase in the level of intracellular calcium. This response was abolished by prior treatment with protein tyrosine kinase (PTK) inhibitors, suggesting that SEA-triggered calcium mobilization in B cells is probably dependent on the activation of PTK. The implication of PTK in SEA-induced early B cell activation was then confirmed by demonstrating that cross-linked SEA induces a significant increase in the level of tyrosine phosphorylation in B cells. The requirement of biotinavidin cross-linking in SEA-induced calcium mobilization in B cells can be fulfilled by the addition CD4⁺ T cells, suggesting a role for CD4 molecules. Using the murine CD4^? T cell hybridoma 3DT, or its derivative 11B3 transfected with human CD4 that both express SEA-specific TCR, we confirmed the CD4 requirement for B cell calcium mobilization and that both specific TCR and CD4 molecules are required in early events of B cell activation induced by SEA. The role of CD4 in SEA-induced B cell proliferation was then investigated. SEA-stimulated B cells proliferated in the presence of CD4⁺ T cells, whereas no response was observed in the presence of CD8⁺ T cells. The addition of clone 11B3 CD4⁺ T cells failed to fulfill the requirement of CD4⁺ T cells in SEA-induced B cell proliferation, indicating the possible involvement of other CD4⁺ T cell surface molecules in this response. This issue is currently under investigation.     

Macrophages are related to goblet cell hyperplasia and induce MUC5B but not MUC5AC in human bronchus epithelial cells

Airway goblet cell hyperplasia (GCH)--detectable by mucin staining--and abnormal macrophage infiltrate are pathological features present in many chronic respiratory disorders. However, it is unknown if both factors are associated. Using in-vivo and in-vitro models, we investigated whether macrophages are related with GCH and changes in mucin immunophenotypes. Lung sections from Sprague-Dawley rats treated for 48?h with one intra-tracheal dose of PBS or LPS (n=4-6 per group) were immunophenotyped for rat-goblet cells, immune, and proliferation markers. Human monocyte-derived macrophages (MDM) were pre-treated with or without LPS, immunophenotyped, and their supernatant, as well as cytokines at levels equivalent to supernatant were used to challenge primary culture of normal human bronchus epithelial cells (HBEC) in air-liquid interface, followed by MUC5B and MUC5AC mucin immunostaining. An association between increased bronchiolar goblet cells and terminal-bronchiolar proliferative epithelial cells confirmed the presence of GCH in our LPS rat model, which was related with augmented bronchiolar CD68 macrophage infiltration. The in-vitro experiments have shown that MUC5AC phenotype was inhibited when HBEC were challenged with supernatant from MDM pre-treated with or without LPS. In contrast, TNF-α and interleukin-1β at levels equivalent to supernatant from LPS-treated MDM increased MUC5AC. MUC5B was induced by LPS, supernatant from LPS-treated MDM, a mix of cytokines including TNF-α and TNF-α alone at levels present in supernatant from LPS-treated MDM. We demonstrated that macrophages are related with bronchiolar GCH, and that they induced MUC5B and inhibited MUC5AC in HBEC, suggesting a role for them in the pathogenesis of airway MUC5B-related GCH.     

Superantigens.

Over the past three years superantigens have come to the forefront of immunological research. Studies in a number of laboratories have indicated that superantigens play a central role in shaping the T-cell repertoire in the development of tolerance, autoimmunity and immunodeficiency. This is in addition to their classic role in the pathogenesis of a number of infectious diseases.     

Superantigens.

The endogenous superantigens (the enigmatic minor lymphocyte-stimulating antigens) have been identified; they are encoded by integrated mouse mammary tumor viruses. The retroviral superantigens appear to be transmembrane glycoproteins, and their highly variable extracellular carboxyl terminus is responsible for V beta interaction. In spite of intensive efforts the precise structure-function relationship for the superantigens is not yet clear. The most important consequences of the introduction of the superantigens in vivo are shock and T-cell depletion and anergy. The search for novel superantigens related to human diseases has started.     

Searching for Superantigens

Superantigens comprise a large group of viral and bacterial proteins that stimulate T lymphocyte proliferation without regard for the antigenic specificity of the T cells but dependent on the composition of the variable part of the β chain of the T cell receptor. Superantigens induce T cell proliferation dependent on class IIMHC molecules on antigen presenting cells but do not require processing. Major subfamilies of Superantigens include the viral Superantigens, the bacterial pyrogenic toxin Superantigens, and other bacterial Superantigens. Two major approaches have been taken to identify superantigen association with human diseases: a) assessing Vβ T cell receptor skewing in peripheral blood or tissues of patients with illnesses, b) recognition of toxic shock syndrome and related illnesses which are likely to be caused by Superantigens.     

B Cell Superantigens: Potential Modifiers of the Normal Human BCell Repertoire

Staphylococcal protein A (SPA), HIV gp120, and staphylococcal enterotoxins (SE) are B cell superantigens that induce V_H specific B cell responses. In addition, the red blood cell antigens, i/I, have some features of a B cell superantigen. Binding of SPA, SE and HIV gp120 are V_H family specific, whereas binding of i/I is V_H gene specific. SPA and HIV gp120 function by stimulating V_H3-expressing B cells, whereas SE appear to function by enhancing survival of the appropiate V_H-expressing B cells. Moreover, HIV gp120 has been shown to delete VH3-expressing B cells. In this review, we describe evidence that shows how these superantigens may play a role in shaping the normal B cell repertoire.     

TCL1 oncogene expression in B cell subsets from lymphoid hyperplasia and distinct classes of B cell lymphoma

Activation of the TCL1 oncogene has been implicated in T cell leukemias/lymphomas and recently was associated with AIDS diffuse large B cell lymphomas (AIDS-DLBCL). Also, in nonmalignant lymphoid tissues, antibody staining has shown that mantle zone B cells expressed abundant Tcl1 protein, whereas germinal center (GC; centrocytes and centroblasts) B cells showed markedly reduced expression. Here, we analyze isolated B cell subsets from hyperplastic tonsil to determine a more precise pattern of Tcl1 expression with development. We also examine multiple B cell lines and B lymphoma patient samples to determine whether different tumor classes retain or alter the developmental pattern of expression. We show that TCL1 expression is not affected by Epstein-Barr virus (EBV) infection and is high in na?ve B cells, reduced in GC B cells, and absent in memory B cells and plasma cells. Human herpesvirus-8 infected primary effusion lymphomas (PEL) and multiple myelomas are uniformly TCL1 negative, whereas all other transformed B cell lines tested express moderate to abundant TCL1. This observation supports the hypothesis that PEL, like myeloma, usually arise from post-GC stages of B cell development. Tcl1 protein is also detected in most na?ve/GC-derived B lymphoma patient samples (23 of 27 [85%] positive), whereas most post-GC-derived B lymphomas lack expression (10 of 41 [24%] positive). These data indicate that the pattern of Tcl1 expression is distinct between na?ve/GC and post-GC-derived B lymphomas (P < 0.001) and that the developmental pattern of expression is largely retained. However, post-GC-derived AIDS-DLBCL express TCL1 at a frequency equivalent to na?ve/GC-derived B lymphomas in immune-competent individuals (7 of 9 [78%] positive), suggesting that TCL1 down-regulation is adversely affected by severe immune system dysfunction. These findings demonstrate that TCL1 expression in B cell lymphoma usually reflects the stage of B cell development from which they derive, except in AIDS-related lymphomas.     

Clonal rearrangements of immunoglobulin genes and progression to B cell lymphoma in cutaneous lymphoid hyperplasia.

Cutaneous lymphoid hyperplasia (CLH) is a disorder characterized by the development of one or more skin lesions containing dense lymphoid infiltrates that exhibit the histopathologic features of a benign, reactive process. Nevertheless, some cases have been associated with the subsequent development of clinically overt lymphomas. This suggests that monoclonal populations may exist in some cases of CLH and that these cases may represent a subset more likely to evolve into lymphoma. To determine if such a subset of CLH can be distinguished, Southern blot analysis of DNA was used to study the immunogenotypic features of lesions from 14 patients with clinical, histopathologic, and immunopathologic findings characteristic of CLH. Five cases exhibited detectable clonal rearrangements of immunoglobulin genes. Furthermore, one of these five cases evolved into overt diffuse large cell lymphoma of B cell lineage during a 2-year follow-up of recurrent disease at the original cutaneous site. The immunoglobulin gene rearrangements of this lymphoma were identical to those of the prior CLH lesion. There was no evidence of detectable t(14;18) chromosomal translocations or clonal rearrangements of the beta gene of the T cell receptor in any case. It was concluded that CLH can be divided into two subsets based on the presence or absence of a clonal B cell population, and that overt lymphoma can arise from the former subset and contain the same B cell clone identified in the pre-existent CLH lesion.     

Superantigens in Human Disease

Superantigens have been implicated in a wide variety of human diseases. Yet, solid evidence for their role in pathogenesis is available only for Toxic Shock Syndrome and a few other conditions. This evidence is critically reviewed herein.     

Superantigens: structure-function relationships

Superantigens are a class of highly potent immuno-stimulatory molecules produced by Staphylococcus aureus and Streptococcus pyogenes. These toxins possess the unique ability to interact simultaneously with MHC class II molecules and T-cell receptors, forming a trimolecular complex that induces profound T-cell proliferation. The resultant massive cytokine release causes epithelial damage and leads to capillary leak and hypotension. The staphylococcal superantigens are designated staphylococcal enterotoxins A, B, C (and antigenic variants), D, E, and the recently discovered enterotoxins G to Q, and toxic shock syndrome toxin-1. The streptococcal superantigens include the pyrogenic exotoxins A (and antigenic variants), C, G-J, SMEZ, and SSA. Superantigens are implicated in several diseases including toxic shock syndrome, scarlet fever and food poisoning; and their function appears primarily to debilitate the host sufficiently to permit the causation of disease. Structural studies over the last 10 years have provided a great deal of information regarding the complex interactions of these molecules with their receptors. This, combined with the wealth of new information from genomics initiatives, have shown that, despite their common molecular architecture, superantigens are able to crosslink MHC class II molecules and T-cell receptors by a variety of subtly different ways through the use of various structural regions within each toxin.