In-Clinic Latanoprost As a Novel Vaccine Adjuvant Abstract Discovering in-clinic compounds with potent adjuvant properties for novel, effective and translational vaccines is a high-priority research area. Through screening the NIH Clinical Collection, we have identified the in-clinic drug Latanoprost triggering expression of proinflammatory mediators in skin, increasing skin-infiltration of dendritic cell (DC) subsets, and enhancing induction of antigen-specific IFNγ+CD8 T cells and protective immunity to the microneedle array cutaneous vaccination. In intradermal and intramuscular vaccinations, Latanoprost promotes antigen-specific antibodies (Ab), including the effective anti-pathogen IgG subclass. We hypothesize that Latanoprost functions as a novel T cell- and Ab-promoting adjuvant to enhance vaccine efficacy with two aims: 1) determine Latanoprost’s vaccine adjuvant effects via DC subsets, 2) evaluate Latanoprost’s adjuvant effects to skin and intramuscular vaccinations. If successful, our discovery of Latanoprost as a new, safe, and effective T cell- and Ab-promoting adjuvant to skin and intramuscular vaccinations would have a clinical significance on repurposing this in-clinic drug in immunizations.