V beta 8+CD4-CD8- subpopulation induced by staphylococcal enterotoxin B

The cellular orchestra in skin allergy; are differences to lung and nose relevant?

PURPOSE OF REVIEW

It has been a long lasting question that although a similar peripheral allergen-specific immune response has been observed, why some patients show only atopic dermatitis, rhinitis and asthma alone or their combinations. The answer resides in the propensity of resident tissue cells and local antigen-presenting cells and T cells for developing an allergic inflammatory immune response. Antigen-presenting cells introduce processed allergens to T helper lymphocytes, where a decision of developing different types of T cell immunity is given under the influence of several cytokines, chemokines, costimulatory signals and regulatory T cells.

RECENT FINDINGS

We focused in this review article on effector T cell subsets, which have been recently described such as Th9, Th17 cells and Th22 cells, which are characterized by their IL-9 and IL-10, IL-17 (or IL-17A) and IL-22 expression, respectively together with other proinflammatory cytokines, which coordinate local tissue inflammation. Both naturally occurring CD4+CD25+ regulatory T (Treg) cells and inducible populations of allergen-specific, IL-10-secreting Treg type 1 cells inhibit allergen-specific effector cells and have been shown to play a central role in the maintenance of peripheral homeostasis and the establishment of controlled immune responses in allergic inflammatory tissues.

SUMMARY

Better understanding and characterization of newly described effector cell subsets and their interaction between antigen presenting cells and resident tissue cells will enlighten our knowledge on the mechanisms of allergic diseases.

Epicutaneous sensitization with superantigen induces allergic skin inflammation

BACKGROUND

Atopic dermatitis (AD) is characterized by skin infiltration with eosinophils and lymphocytes and expression of Th2 cytokines in acute skin lesions. The skin of patients with AD is frequently colonized with enterotoxin-secreting strains of Staphylococcus aureus. Staphylococcal enterotoxins have been implicated in the exacerbations of the inflammatory skin lesions in patients with AD.

OBJECTIVE

We sought to determine whether epicutaneous (EC) sensitization of mice with staphylococcal enterotoxin B (SEB) results in allergic skin inflammation.

METHODS

BALB/c mice were EC-sensitized with SEB. Their skin was examined for allergic inflammation and cytokine expression, and their splenocytes were examined for cytokine secretion in response to SEB.

RESULTS

EC sensitization with SEB elicited a local, cutaneous, inflammatory response characterized by dermal infiltration with eosinophils and mononuclear cells and increased mRNA expression of the Th2 cytokine IL-4 but not of the Th1 cytokine IFN-gamma. EC-sensitized mice mounted a systemic Th2 response to SEB evidenced by elevated total and SEB-specific IgG1 and IgE. Although EC sensitization with SEB resulted in selective depletion of SEB-specific T-cell receptor Vbeta8+ cells from the spleen and sensitized skin, splenocytes from SEB-sensitized mice secreted relatively more IL-4 and less IFN-gamma than did saline-sensitized controls, consistent with Th2 skewing of the systemic immune response to the superantigen.

CONCLUSION

These results suggest that EC exposure to superantigens skews the immune response toward Th2 cells, leading to allergic skin inflammation and increased IgE synthesis that are characteristic of AD.

Cytokine network and dysregulated apoptosis in atopic dermatitis

Activation and skin-selective homing of peripheral blood memory/effector T cells and effector functions in the skin represent sequential immunological events in the pathogenesis of atopic dermatitis (AD). T cells infiltrating the skin utilize the cutaneous lymphocyte-associated antigen (CLA) and other receptors to recognize and cross the vascular endothelium. In the peripheral blood of AD patients, both CD4+ and CD8 subsets of CLA+CD45RO+ T cells are in an activated state with high CD25, HLA-DR, and CD40-ligand expression. They express upregulated Fas and Fas-ligand and undergo activation-induced apoptosis. After homing to skin these T cells form dermal infiltrates which play a key role in the pathogenesis of the disease. Skin-infiltrating T cells in AD are protected from activation-induced cell death, although they express both Fas and Fas-ligand. They are protected from apoptosis by cytokines such as IL-2, IL-4, and IL-15 and extracellular matrix components such as fibronectin and transferrin. CLA+, skin-homing T cells may play a role in peripheral blood eosinophilia and hyper IgE production by high IL-5 and IL-13 expression, respectively. These T cells secrete IFN-gamma in the skin, which upregulates Fas on keratinocytes and renders them susceptible to apoptosis. Keratinocyte apoptosis is induced by Fas-ligand, either soluble or expressed on the surface of T cells, leading to eczema formation. Here we discuss the mechanisms of skin-selective T cell homing and activation, and emphasize the concept of dysregulated apoptosis of T cells, eosinophils, and keratinocytes as essential pathogenetic episodes in AD and other eczematous disorders.

Immune regulation in atopic dermatitis

Atopic dermatitis is a chronic inflammatory skin disease with a pathogenesis of complex immune dysregulation and interplay of genetic, environmental and psychological factors. Activation and skin-selective homing of peripheral-blood T cells, and effector functions in the skin, represent sequential immunological events in the pathogenesis of atopic dermatitis. Both CD4(+) and CD8(+) T cells bearing the cutaneous-lymphocyte-associated antigen represent activated memory/effector T cell subsets and induce IgE, mainly via IL-13, and prolong eosinophil lifespan, mainly via IL-5. Dysregulated apoptosis in skin-homing T cells and keratinocytes contributes to the elicitation and progress of atopic dermatitis. T cell survival is enhanced in the skin by cytokines and extracellular-matrix proteins. These activated T cells induce keratinocyte apoptosis, leading to eczema formation.

Skin Homing (Cutaneous Lymphocyte-Associated Antigen-Positive) CD8+ T Cells Respond to Superantigen and Contribute to Eosinophilia and IgE Production in Atopic Dermatitis

In allergic inflammations of the skin, activation of CD4+ T cells was demonstrated to play an important role; however, a minor role for CD8+ T cells is implied. In the present study, we compared cutaneous lymphocyte-associated Ag (CLA)-expressing CD4+ and CD8+ subsets, which were isolated from peripheral blood and lesional skin biopsies in atopic dermatitis (AD) patients. We demonstrated that CD8+CLA+ T cells proliferate in response to superantigen and are as potent as CD4+CLA+ T cells in IgE induction and support of eosinophil survival. In atopic skin inflammation, the existence of high numbers of CD4+ and CD8+ T cells was demonstrated by immunohistochemistry and by culturing T cells from skin biopsies. In peripheral blood, both CD4+ and CD8+ subsets of CLA+CD45RO+ T cells were in an activated state in AD. The in vivo-activated CLA+ T cells of both subsets spontaneously released an IL-5- and IL-13-dominated Th2 type cytokine pattern. This was confirmed by intracytoplasmic cytokine staining immediately after isolation of the cells from peripheral blood. In consequence, both CD4+ and CD8+, CLA+ memory/effector T cells induced IgE production by B cells mainly by IL-13, and enhanced eosinophil survival in vitro by delaying eosinophil apoptosis, mainly by IL-5. These results indicate that in addition to the CD4+ subset, the CD8+CLA+ memory/effector T cells are capable of responding to superantigenic stimulation and play an important role in the pathogenesis of AD.

Requirement of I-E molecule for thymocyte apoptosis induced by staphylococcal enterotoxin B in vivo

In vivo administration of bacterial superantigen staphylococcal enterotoxin B (SEB) to BALB/c mice led to thymus atrophy resulting from thymocyte apoptosis. In this study, we demonstrated that SEB induced a substantial reduction in thymocyte numbers in BALB/c, B10. D2 (H-2(d) haplotype), B10.BR, C3H/HeJ, C3H/HeN (H-2(k)), and (BALB/c x B6)F1 (H-2(dxb)), but caused little or no effect in I-E- strains such as B6, B10, A.BY (H-2(b)), and A.SW (H-2(s)) mice. Elimination of CD4(+)CD8(+) cells predominantly accounted for the thymocyte loss, although the numbers of other subpopulations may also be reduced. Thymocyte apoptosis was shown by an increase in the level of DNA fragmentation in BALB/c but not in B6 mice after SEB administration. Treatment with anti-I-Ed monoclonal antibody to BALB/c mice blocked SEB-induced thymocyte apoptosis when anti-I-Ad exerted less effect. In contrast to SEB, staphylococcal enterotoxin A led to comparable levels of thymus atrophy in BALB/c and B6 mice. Studies on the surface marker expression indicated that CD25 expression was upregulated on BALB/c mouse thymocytes but with only a moderate increase in B6 mice. The CD4(+)CD8(+) cells were the major (>90%) population that expressed elevated levels of CD25 in BALB/c mice. An increase in the expression of TCRalphabeta, CD3, and CD69 surface markers was also observed on thymocytes from BALB/c mice, but not from I-E- strains. The differential response of I-E+ and I-E- mice to SEB may be exploited as a model for the study of apoptosis in the thymus.