Abstract This resubmission is in response to PAR-18-333 “Understanding the Early Development of the Immune System”. Increasing evidence suggests that commensal bacteria play an important role in health and disease, including in pregnancy. Using advanced methodology, recent studies have indicated that the fetus may not live in a sterile environment. Our preliminary data using antibiotics suggest that commensal bacteria play an important role, during pregnancy, in shaping the early development of fetal immune tolerance to islet autoimmunity that may manifest later in life of the offspring. This has led us to hypothesize that removing the low number of commensal bacteria in the uterus may regulate the early development of fetal immune system and fetal immune programming to be tolerant or not, to islet autoimmunity. The overall goal of our proposal is to address an important gap in our knowledge and understanding of the role of maternal microbiota in influencing fetal immune system development and immune tolerance. We propose three specific aims to test our different but inter-connected hypotheses: Aim 1: To test the hypothesis that the mode of birth delivery contributes to the development/programming of the immune system of the neonates, which later affects development of immune tolerance later in life in non obese diabetic (NOD) mice; Aim 2: To test the hypothesis that fetal immune system development is associated with the presence of maternal commensal bacteria, using germ free (GF) NOD mice; Aim 3: To determine whether an oral commensal, which expresses an islet autoantigen mimic peptide, will influence immune tolerance to self antigen and autoimmunity, by maternal-fetal transfer and subsequent fetal immune system development in GF mice.