Respiratory Syncytial Virus triggers immune tolerance through induction of tolerogenic dendritic cells and expansion of regulatory T cells.
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Dendritic cells have the ability to control the balance between immunity and tolerance. Upon viral exposure, Dendritic Cells (DCs) steadily detect pathogens and exert their antigen presentation function to induce adaptive T cell response. Respiratory Syncytial Virus (RSV) is an important respiratory pathogen in infants and young children worldwide. Here we show that RSV exposure polarizes DC maturation to a tolerogenic state. RSV exposed DCs (RSV-DCs) are unable to prime allogeneic CD4+ T cell proliferation and cytokine production. Strikingly, RSV exposed DCs are able to efficiently inhibit on-going Mixed Leukocyte Reaction (MLR) in trans. Phenotypic characterization of RSV-DCs indicates that they express a variety of surface inhibitory molecules and secrete high amount of the cytokine IL-10. Autocrine IL-10 receptor signaling is required for tolerogenic conversion. A direct comparison with pharmacologically generated tolerogenic DCs indicates RSV-DCs are much more potent at inhibiting CD4+ T cell alloproliferation. Furthermore, we find that RSV-DCs propagate their tolerogenic signal through expansion of regulatory T cells (Tregs). RSV- DCs induce the selective expansion of CD4+/CD25+/FoxP3+/CTLA+/GITR+ Tregs in the bulk T cell population. These Tregs are able to inhibit on-going MLR in trans, indicating their functional potency. An analysis of the non-proliferating target CD4+ T cells indicates that they are in a state of phenotypic and functional anergy. These T cells express anergy markers and are unresponsive to secondary anti-CD3/CD28 restimulation. Interaction of B7 negative co-stimualtor PD-L1 and its receptor PD-1 are required for Treg expansion and function as blockade led to a reversal of anergy induction in the target population. These in vitro observations led us to investigate the impact of RSV-DCs on immune tolerance in vivo. We did preliminary studies demonstrating RSV exposure induces BALB/c bone marrow derived DCs (BM-DCs) tolerogenic characterized by inhibition of C57BL/6 CD4 T cell alloproliferation and upregulation of mouse PD-L1 molecule. Similarly to what we observed in human cells, RSV-mouse DCs are able to expand a population of CD4+CD25+FoxP3+ regulatory T cells. These cells with immune suppressive function can then be adoptively transferred into murine models of autoimmune diseases and organ transplantion to suppress antigen-specific immune activation in vivo.