The modalities of this tolerance induction might be considered as mirroring innate immunity and so be described as ‘innate tolerance’. CD1d-restricted immune responses should also be considered within such a group of tolerance effectors. CD1d is a non-classical major histocompatibility class 1-like molecule that primarily presents either find more microbial or endogenous glycolipid antigens to T cells involved in innate immunity. CD1d-restricted T cells comprise NKT cells and a subpopulation of γδ T cells expressing the Vγ4 T-cell receptor. In particular, activated NKT cells secrete large quantities

of cytokines that both help control infection and modulate the developing adaptive immune response. However, NKT cells can also promote Treg-cell activation[75] and the chronic in vivo stimulation of NKT often leads to a Th2 bias in the immune response and promotes the generation of tolerogenic dendritic cells. Selleckchem AP24534 Furthermore, with similar modalities to MSC and macrophages, reagents have been identified that, by interacting with CD1d, differently bias Th-cell

responses.[76] One of the best examples in which effectors of such ‘innate tolerance’ are actively recruited is cancer. Tumour cells evade immune system recognition not only by mutating antigenic epitopes initially recognized by host immune surveillance, but also and especially by creating an environment that is extremely potent at inhibiting immune responses in a non-specific fashion. Fibroblasts[77] and immunosuppressive myelomonocytic cells[78] heavily infiltrate the tumour process and facilitate the activation of ‘adaptive tolerance’ effectors like Treg cells.[45] Within this context, it is plausible to surmise a major role of MSC because of their

ability to polarize and activate Thymidine kinase immunosuppressive networks as summarized in this review. This hypothesis gains support also by a recent set of data elegantly generated using a transgenic mouse in which stromal cells could be depleted. The depletion of cells expressing fibroblast activation protein-α caused rapid hypoxic necrosis of both cancer and stromal cells in immunogenic tumours by a process involving IFN-γ and TNF-α.[79] Mesenchymal stromal cells can also contribute to the tumour-related immune impairment because they produce TGF-β, which can suppress or alter the activation, maturation and differentiation of both innate and adaptive immune cells.[80] In addition, TGF-β has an important role in the differentiation and induction of Treg cells. Furthermore, in the presence of IL-6, also produced by MSC, TGF-β induces the differentiation of IL-17-producing CD4+ Th17 cells, which may have tumour-promoting activities.[81] An interesting proposal for a ‘tissue-based’ approach to the regulation of the immune response has been recently put forward by Matzinger and Kamala.