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Bacterial superantigens such as staphylococcal enterotoxin-A (SEA) have been implicated in the pathogenesis of psoriasis vulgaris. Major histocompatibility complex (MHC) class II molecules are high affinity receptors for SEA, and T cells found in psoriatic skin lesions express high levels of MHC class II. Here we address the question of whether SEA can directly activate psoriatic T cells in the absence of professional antigen-presenting cells. We show that SEA induces i) tyrosine phosphorylation of several proteins, ii) downregulation of the T-cell receptor (TCR), and iii) production of interferon-gamma (IFN-gamma), but not autocrine mitogenesis in CD8-positive T clones obtained from skin lesions of a patient with psoriasis vulgaris. Psoriatic T cells do not respond to SEA molecules if mutations are introduced in the TCRbeta- or in both the two MHC class II alpha- and beta-binding sites of SEA. Mutations in only one of the two MHC class II binding sites of SEA has different effects on T-cell activation. Thus, SEA molecules with a mutation in the MHC class II beta-binding site induce protein tyrosine phosphorylation, but not IFN-gamma production or co-stimulation of cytokine-mediated proliferation. In contrast, SEA with a mutation in the MHC class II alpha-binding site induces IFN-gamma and a qualitatively changed tyrosine phosphorylation profile. Both mutations delete the co-stimulatory effect on cytokine-mediated proliferation. This suggests that both MHC class II binding sites are involved in the autopresentation of SEA by psoriatic T cells. In conclusion, we provide evidence that SEA directly activates MVHC class H-positive psoriatic T-cell lines to produce IFN-gamma, a key cytokine in the pathogenesis of psoriasis vulgaris.

Dimerization of MHC class II molecules on the cell surface of human THP-1 monocytic cell line is a requirement for staphylococcal superantigen (SAG)-induced cytokine gene expression. The capacities of various SAG to induce this response are governed by their modes of interaction with MHC class II molecules. Staphylococcal enterotoxin A (SEA), with its two binding sites, dimerizes MHC class II molecules and subsequently induces cytokine gene expression in THP-1 cells. Here, we demonstrate that staphylococcal enterotoxin D (SED) and staphylococcal enterotoxin E (SEE) induce, similarly, IL-1β and TNF-α gene expression in these cells. Using mutated toxins that lost their binding site with the MHC class II α- or β-chain, we demonstrate that this response is also mediated by the dimerization of MHC class II molecules through two binding sites. Furthermore, SED forms Zn2+-dependent homodimers that allow multiple modes of MHC class II clustering, including ligation of α-chains (α/α), β-chains (β/β), or the α- and β-chains of two different class II molecules. The β/β interaction following Zn2+-dependent SED/SED homodimer formation seems to be mediated by the appearance of a novel binding site on SED that interacts with histidine 81 of the MHC class II β-chain. The different modes of SED interactions also influence SED-induced T cell activation where simultaneous ligation of the α- and β-chains is essential for optimal response. These various modes of SED binding may be used to preserve bivalency regardless of variability in the MHC class II α/β/peptide complexes.

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We have investigated the effect of the superantigen staphylococcal enterotoxin A (SEA) on the balance between T-cell response and non-responsiveness in T-cell receptor (TcR) V beta 3 transgenic mice. One injection of SEA resulted in a substantial activation of TcR V beta 3+ cells, whereas T cells from mice injected with repeated doses of SEA displayed a diminished response to a subsequent in vitro challenge. The reduced responsiveness became apparent when SEA was injected multiple times with short intervals. Proliferation and cytokine production in anergized T cells were severely reduced when stimulated with SEA in vitro, whereas cytotoxic T lymphocyte (CTL) activity remained unaffected. The dichotomy between these functions was examined in vitro with respect to different T-cell subsets. The total number of CD4+ T cells was reduced in the hyporesponsive spleens, compatible with cell deletion. The remaining CD4+ TcR V beta 3+ T cells showed anergy of all tested functions and did not respond to exogenous interleukin-2 (IL-2). In contrast, there was an expansion of CD8+ TcR V beta 3+ T cells with an intact cytotoxic activity. The in vitro proliferation and production of cytokines in the CD8+ compartment was impaired, but could be partially restored in the presence of exogenously added IL-2. Analysis of the cytokine response to SEA in vivo showed that IL-2 and tumour necrosis factor (TNF) were mainly produced by CD4+ T cells, while interferon-gamma (IFN-gamma) was predominantly released by CD8+ T cells. Induction of anergy resulted in a reduction of IL-2 and TNF mRNA levels, frequencies of producing cells as well as serum protein content. In contrast, there was only a moderate influence on the IFN-gamma level in vivo. The results suggest that SEA-induced hyporesponsiveness involves CD4+ cell deletion and a failure to produce cytokines in the remaining CD4+ T-cell compartment, while IFN-gamma production and cytotoxicity in the CD8+ T-cell compartment stay relatively intact.

The staphylococcal enterotoxins (SE) comprise a family of structurally related phage-encoded bacterial proteins, which are the most potent mitogens known for murine and human T lymphocytes. In this report we describe a novel cytotoxic mechanism, where SE directs human CD3+ T lymphocytes to mediate strong cytotoxicity against target cells of irrelevant nominal specificity. The SE-dependent cellular cytotoxicity (SDCC) occurred at picomolar concentrations of SE and involved the initial binding of the SE to the target cells and subsequent triggering of the cytotoxic T cells. SDCC was induced by SEA, SEB, SEC1 and SED, which indicates that this is a common property conserved among all SE. Certain antibodies to the HLA-DR molecule efficiently blocked SDCC. Major histocompatibility complex (MHC) class II+ RAJI cells and HLA-DR-transfected murine L cells were sensitive to SDCC, whereas the MHC class II- RJ.2.2.5 RAJI cell mutant and untransfected L cells were completely resistant to SDCC. These results demonstrate that the MHC class II antigen is the target molecule in SDCC. HLA-DR molecules acted as receptors for SE and the complex was recognized by T lymphocytes in a polyclonal fashion. SDCC was mediated by allospecific cytotoxic CD8+ T cells, by cloned CD8+ T cells and by fresh human peripheral blood mononuclear cells. The SDCC phenomenon provides a rapid, potent and specific mechanism for elimination of HLA-DR+ target cells. We suggest that SDCC is an important combat strategy, employed by the bacteria to avoid specific MHC class II antigen-dependent immune recognition, by inducing T-cell dependent autologous lysis of MHC class II-expressing cells.