Bacterial Polysaccharides, Endotoxins, and Immunomodulation

Turning the inside out: the microbiology of atopic dermatitis

Allergy is on the rise worldwide. The hygiene hypothesis of atopic diseases linked microbes with atopic dermatitis (AD) both as drivers and modulators of skin pathology. The earlier literature favoured an inside-outside model of AD where an immunological abnormality compounded by a gut microbiota dysbiosis is the primary event. Probiotic intervention trials with lactobacilli and bifidobacteria as well as the application of bifidogenic oligosaccharide prebiotics showed indeed promising clinical results, but no consistent gut microbiota dysbiosis could be linked with AD. An alternative hypothesis known as outside-inside model of AD considers a genetic skin barrier effect compounded by a skin microbiota dysbiosis as primary pathogenic event. Cultivation microbiology has demonstrated strong skin colonization with superantigen-encoding Staphylococcus aureus in AD patients; microbiota and molecular microbiome analyses demonstrated that S. aureus abundance fluctuates and parallels clinical symptoms. In a mouse model, δ-toxin of S. aureus induced mast cell degranulation, leading to AD-like symptoms. Mutant mice developing AD symptoms showed increased skin colonization with S. aureus; antibiotic treatment alleviated the symptoms. Clinical trials showed that various treatments reducing S. aureus skin load also reduced AD symptoms, suggesting S. aureus as a potential critical driver of AD and a target for antimicrobial interventions other than antibiotics.

Effect of exopolysaccharide V2-7, isolated from Halomonas eurihalina, on the proliferation in vitro of human peripheral blood lymphocytes

The immunomodulatory activity of the exopolysaccharide V2-7, a sulfated polymer excreted by the moderately halophilic bacteria Halomonas eurihalina, was studied in vitro. [3H]thymidine incorporation and flow-cytometry measurements showed that this exopolysaccharide enhanced the unspecific proliferation of human lymphocytes in response to the presence of anti-CD3 monoclonal antibody. It was effective at concentrations of less than 1 microg/ml, maximum activity being achieved at 0.2 microg/ml.

CD34(+) hematopoietic stem cells exert accessory function in lipopolysaccharide-induced T cell stimulation and CD80 expression on monocytes

CD34(+) hematopoietic stem cells, which circulate in peripheral blood with very low frequency, exert essential accessory function during lipopolysaccharide (LPS)-induced human T lymphocyte activation, resulting in interferon gamma production and proliferation. In contrast, stimulation of T cells by "conventional" recall antigens is not controlled by blood stem cells. These conclusions are based on the observation that depletion of CD34(+) blood stem cells results in a loss of LPS-induced T cell stimulation as well as reduced expression of CD80 antigen on monocytes. The addition of CD34-enriched blood stem cells resulted in a recovery of reactivity of T cells and monocytes to LPS. Blood stem cells could be replaced by the hematopoietic stem cell line KG-1a. These findings may be of relevance for high risk patients treated with stem cells or stem cell recruiting compounds and for patients suffering from endotoxin-mediated diseases.

Stimulation of Human T Lymphocytes by LPS Is MHC Unrestricted, But Strongly Dependent on B7 Interactions

Recently, we have shown that LPS is a potent inducer of human T cell proliferation and lymphokine production. However, the activation of T cells by LPS has been demonstrated to be monocyte dependent and to require direct cell-to-cell contact. Here, we investigated the role of monocytes as accessory cells and the requirement for costimulatory signals in more detail. We found that the accessory cell activity of monocytes during LPS-induced T cell proliferation is characterized by the following features: LPS-primed monocytes are competent stimulators of T cell proliferation; interaction of LPS with monocytes during the priming step is dependent on CD14 and is sensitive to ammonia; monocyte/T cell interactions are not MHC restricted but are strongly dependent on interactions of CD28 and/or CTLA-4 on T cells and their ligands CD80 and/or CD86 on monocytes. CD80 seems to be crucial for the activation of T cells by monocytes, since monocytes expressing CD86 but not CD80 after LPS stimulation were unable to stimulate T cells; IL-12, at least as a costimulatory factor, but not IL-15, is important in LPS-induced T cell proliferation. Taken together, our results indicate that LPS acts neither as a mitogen, nor as a superantigen, nor as an Ag. The activation of human T cells by LPS requires the help of accessory functions by primed monocytes and is MHC unrestricted but needs costimulatory signals via CD28 and/or CTLA-4.

Effects of the adjuvant MPL + TDM on tumor challenge in the B16 mouse melanoma model

Mice were injected three times at 2 week intervals with the adjuvant MPL + TDM or phosphate-buffered saline. Two weeks after the last injection, all mice were challenged subcutaneously with a tumorgenic dose of B16 tumor and observed for 40 days. The adjuvant hastened the time at which tumors appeared (P = 0.02).