First, CD54 expression on HSCs acting as third-party veto cells may lead to the redistribution of its ligand lymphocyte function-associated antigen 1 (LFA-1), which is important

for the transmission KU-60019 price of TCR signals,26 away from the TCR interacting with peptide-loaded MHC molecules on the APCs. This would ultimately lead to a failure of the T cells to become activated. This assumption is supported by the following observations: T cells undergo a weak initial stimulation, which is indicated by the up-regulation of CD69 and the release of small amounts of cytokines, and T cells ultimately are not sufficiently stimulated to enter the cell cycle or a differentiation program to become effector T cells. Second, establishing a close interaction between HSCs and T cells through CD54 may allow mediators with short-range activity to exert a regulatory function. However, we did not find evidence for the involvement of classic immune-regulatory molecules such as IL-6, IL-10, TGF-β, and retinoic acid (not shown). Yet, close physical interactions may also allow for the exchange of regulatory molecules through nanopores or exosomes, as recently described for Tregs

selleck chemical in the suppression of DC function.27 A common feature of all these attempts to explain the immune-regulatory function of CD54 is that it is not expressed on the same cell presenting the antigen. In other words, CD54 expression in trans seems to have immune-regulatory effects, whereas CD54 expression in cis promotes the development of T cell immunity. This dichotomy can explain the apparently contradictory functions of CD54 in promoting inflammation and T cell

immunity and impeding T cell activation. The third-party veto function of HSCs portrayed here represents a novel form of immune regulation that has not been described so far. It is clearly distinct from the clonal deletion of already activated T cells reported previously for HSCs,16 and it does not depend on inhibitory molecules such as IL-10 and TGF-β. However, it bears a resemblance to T cell anergy, which is MCE公司 triggered by incomplete stimulation through APCs.25 The development of the HSC veto function involves initial mutual interactions with T cells stimulated by APCs. This eventually results in T cells being completely inhibited from proliferating and entering a differentiation program by mechanisms that need to be addressed in future studies. A previous study identified a function of IFN-γ in inducing B7-H1 expression, which mediates the HSC-induced protection of islet grafts from T cell–mediated rejection.28 We also observed a contribution of IFN-γ to the regulation of CD54 on HSCs, which influences subsequent veto function (data not shown), and this is consistent with a general contribution of IFN-γ to the immune-regulatory capacity of HSCs. It is important to note that the HSC veto function does not affect T cell viability.