Project Summary Our understanding of the specific cell subsets that are being depleted by vedolizumab and other effective agents within the intestine remains superficial. This is in part due to technical limitations which had hampered the unbiased interrogation of cellular and molecular composition of the human intestinal lamina propria. Recently, these technological challenges have been overcome by the development of mass cytometry, which enables the simultaneous quantification of up to 40 surface and/or intracellular markers on cell subsets. We have recently adapted existing mass cytometry methodology previously employed mainly for peripheral blood cell analyses, to allow for the interrogation of intestinal lamina propria cells. We have developed, tested and optimized a panel of 37 immune cell markers that allow us to simultaneously identify all major immune cell populations, as well as numerous sub-populations, intracellular cytokines and transcription factors. Here we will prospectively assess the effects of vedolizumab on the cellular composition of intestine by performing deep immunophenotyping of immune cell subsets, prior to and after the drug’s administration. We will then determine whether changes on cell subsets correlate with clinical, endoscopic or histologic parameters of disease activity. These studies will test the validity of mass cytometry outcomes as surrogate markers for the efficacy of drugs that target lymphocyte traffic (vedolizumab, etrolizumab, anti- MAdCAM-1, ozanimod) and may directly shed light on the specific mechanism of action of these traffic-targeted therapies.