Project Summary Sensitivity to UV light affects ~80% of lupus patients and can exacerbate local skin disease and lead to systemic disease flares. The molecular pathways predisposing lupus patients to photosensitivity and the mechanisms driving the inflammatory responses to UV light are poorly understood. We showed that a single exposure to UV light leads to a rapid type I interferon (IFN- I) response in human and murine skin in vivo. That UV light is a potent stimulus of the IFN-I response is particularly relevant as lupus patients (~75%) have a high IFN-I signature in both the skin tissue and the peripheral blood cells, indicative of worse disease. Therefore, there is a critical need to discover what regulates the IFN-I production in response to UV light and in lupus skin. Recent studies showed that deficiency in VISTA result in lupus-like skin disease and accelerate systemic disease in lupus mouse models. The observed pathologies were accompanied by an increase in the IFN-I signature, suggesting VISTA can modulate the IFN-I response. Indeed, we demonstrated that an agonistic anti-VISTA antibody inhibits the IFN-I response in human monocytes. While findings that the transcriptional profile of cells from lupus patients resembles that of murine VISTA-/- cells suggest an important role for VISTA in lupus pathogenesis, how VISTA modulates the IFN-I response in lupus skin and in response to UV light is unknown. Here, we hypothesize that VISTA negatively regulates the IFN-I response to UV light and that VISTA activity in keratinocytes modulates the baseline and UV-triggered IFN-I production. To test these hypotheses, we will pursue the following specific aims: In Aim 1, we will define how VISTA regulates local and systemic IFN-I response to UV light in vivo. We predict that the IFN-I response to UV light in the skin and blood of VISTA-/- mice will be of greater magnitude and/or duration, compared to VISTA-sufficient controls. We will examine the therapeutic potential of targeting VISTA to modulate the IFN-I response to UV light using an agonistic anti-human VISTA monoclonal antibody in humanized VISTA knock-in mice. In Aim 2, we will characterize UV-light triggered IFN-I production by keratinocytes and define keratinocyte VISTA function. The cellular source and class of IFN-I in UV light exposed skin is unknown as is the function of VISTA expressed by keratinocytes. Given the rapid IFN-b production in the skin after UV exposure, we will investigate the contribution of keratinocytes as the likely source of IFN-Is in UV light-exposed skin of normal and lupus mouse strains. We will also define the keratinocyte-specific VISTA function in vivo in VISTA conditional knockout mice and in primary lupus keratinocytes.