Ganglioside-exposed dendritic cells inhibit T-cell effector function by promoting regulatory cell activity.

Tumour pathogenesis is characterized by an immunosuppressive microenvironment that limits the development of effective tumour-specific immune responses. This is in part the result of tumour-dependent recruitment and activation of regulatory cells, such as myeloid-derived suppressor cells and regulatory T cells in the tumour microenvironment and draining lymph nodes. Shedding of gangliosides by tumour cells has immunomodulatory properties, suggesting that gangliosides may be a critical factor in initiating an immunosuppressive microenvironment. To better define the immunomodulatory properties of gangliosides on antigen-specific T-cell activation and development we have developed an in vitro system using ganglioside-treated murine bone-marrow-derived dendritic cells to prime and activate antigen-specific CD4(+) T cells from AND T-cell receptor transgenic mice. Using this system, ganglioside treatment promotes the development of a dendritic cell population characterized by decreased CD86 (B7-2) expression, and decreased interleukin-12 and interleukin-6 production. When these cells are used as antigen-presenting cells, CD4 T cells are primed to proliferate normally, but have a defect in T helper (Th) effector cell development. This defect in Th effector cell responses is associated with the development of regulatory T-cell activity that can suppress the activation of previously primed Th effector cells in a contact-dependent manner. In total, these data suggest that ganglioside-exposed dendritic cells promote regulatory T-cell activity that may have long-lasting effects on the development of tumour-specific immune responses.

CD4 co-receptor dependent signaling promotes competency for re-stimulation induced cell death of effector T cells.

The elimination of activated T cells by FAS-mediated signaling is an important immunoregulatory mechanism used to maintain homeostasis and prevent tissue damage. T cell receptor-dependent signals are required to confer sensitivity to FAS-mediated re-stimulation-induced cell death (RICD), however, the nature of these signals is not well understood. In this report, we show, using T cells from CD4-deficient mice reconstituted with a tail-less CD4 transgene, that CD4-dependent signaling events are a critical part of the competency signal required for RICD. This is in part due to defects in FAS receptor signaling complex formation as shown by decreased recruitment of FAS and caspase 8 into lipid rafts following antigen re-stimulation in the absence of CD4-dependent signals. In addition, in the absence of CD4-dependent signals, effector T cells have a selective defect in IL-2 secretion following peptide re-stimulation, while provision of exogenous IL-2 during re-stimulation partially restores susceptibility to RICD. Importantly, IL-2 production and proliferation after primary peptide stimulation is comparable between wild type and CD4-deficient T cells indicating that the requirement for CD4-dependent signaling events for IL-2 production is developmentally regulated and is particularly critical in previously activated effector T cells. In total, our results indicate that CD4 co-receptor dependent signaling events specifically regulate effector T cell survival and function. Further, these data suggest that CD4-dependent signaling events may protect against the decreased IL-2 production and resistance to cell death seen during chronic immune stimulation.

A cell-impermeable cyclosporine A derivative reduces pathology in a mouse model of allergic lung inflammation.

Although the main regulators of leukocyte trafficking are chemokines, another family of chemotactic agents is cyclophilins. Intracellular cyclophilins function as peptidyl-prolyl cis-trans isomerases and are targets of the immunosuppressive drug cyclosporine A (CsA). Cyclophilins can also be secreted in response to stress factors, with elevated levels of extracellular cyclophilins detected in several inflammatory diseases. Extracellular cyclophilins are known to have potent chemotactic properties, suggesting that they might contribute to inflammatory responses by recruiting leukocytes into tissues. The objective of the present study was to determine the impact of blocking cyclophilin activity using a cell-impermeable derivative of CsA to specifically target extracellular pools of cyclophilins. In this study, we show that treatment with this compound in a mouse model of allergic lung inflammation demonstrates up to 80% reduction in inflammation, directly inhibits the recruitment of Ag-specific CD4(+) T cells, and works equally well when delivered at 100-fold lower doses directly to the airways. Our findings suggest that cell-impermeable analogs of CsA can effectively reduce inflammatory responses by targeting leukocyte recruitment mediated by extracellular cyclophilins. Specifically blocking the extracellular functions of cyclophilins may provide an approach for inhibiting the recruitment of one of the principal immune regulators of allergic lung inflammation, Ag-specific CD4(+) T cells, into inflamed airways and lungs.