Abstract The goal of this project is to establish the experimental system to decipher the effect of environmental and microbe-derived aryl hydrocarbon receptor (AHR) ligands on the perinatal immune system. AHR signaling has been shown to play a role in the differentiation of both effector (Th17, Th22) and regulatory T cells (Foxp3+ and Tr1). In our recent study, we demonstrate that AHR signaling plays an essential role also in the development of human effector T cells that produce a mast cell growth factor IL-9 (Th9). It is now well established that microbiome-derived AHR ligands play significant roles in the immunological homeostasis between the microbes and the host. These AHR ligands can have a profound effect on T cell functions and human health. In this project, we will decipher how the microbiome and environment-derived AHR ligands change the response of T cells in the perinatal conditions. The infant immune system is highly tolerogenic compared to the adult immune system. This immunosuppressive state is needed to prevent inflammation and disease following the encounter with environmental microorganisms after birth and establish symbiotic conditions with commensal microorganisms. Hence, disruption of the tolerogenic environment in the perinatal immune system could change the host immune function and antigen receptor repertoire. Changes in the immune repertoire will alter the compositions of the microbiome in turn. The effect could have long-lasting influences on the compositions of the microbiome and hence can be a factor that changes the developmental original of health and disease (DOHaD). We have recently demonstrated that vitamin D suppresses the expression of AHR. Our further characterization of T cells activated in the presence of active vitamin showed that vitamin D upregulates immune checkpoint molecule CTLA4 and PD-L1. Co-expression of CTLA4 and PD-L1 was found among T cells with immunosuppressive functions. Conversely, stimulation of T cells with a microbe derived AHR ligand cause a marked decrease in immune checkpoint expression. Together, the data suggest that the balance between vitamin D and AHR ligand can dictate the outcomes of T cell activation and overall immune responses. In this project, we will establish two experiment systems to study the immune checkpoint modulation by microbial metabolites. One is the system to study the immune checkpoint regulation mechanism, the other is to determine the overall functional consequences of vitamin D and microbe-derived AHR ligands. Completion of this project will help to elucidate the AHR ligand effect on host- microbe/environment interactions and human immune responses.