Breastfeeding is associated with immunological benefits that persist beyond infancy, including reduced risk of respiratory infection. While these benefits are often attributed to immunoglobulins and antimicrobial compounds, human milk is also rich in T cells whose function is unknown. A central goal of my research career is to elucidate the protective function of breastmilk T cells against infant respiratory infection. To this end, I propose to investigate the respiratory-mammary axis of cellular immunity and its role in the establishment, retention, and response of tissue resident memory T cells (TRM) in the lactating breast. I hypothesize that i) maternal exposure to respiratory infections results in priming of T cell populations that go on to seed TRM in both the breast and the respiratory tract and ii) antigen exposure via saliva of the nursing infant drives persistence of TRM in the lactating breast. I will use samples from a cohort of mother infant pairs, which includes breastmilk cells (BMC), nasal mucosal cells (NMC), and peripheral blood mononuclear cells (PBMC). In Aim 1, I will investigate whether T cells resident to the upper respiratory tract and lactating breast are derived from a shared cellular population by defining the transcriptional and clonal overlap of T cells from BMC and NMC using single cell RNA sequencing with paired T cell receptor (TCR)αβ sequencing. In Aim 2, I will evaluate the contribution of infant respiratory infection and coinciding maternal and infant respiratory infection on the frequency and functional state of breastmilk T cell subsets. To do this, I will enroll mother-infant pairs seeking care at Seattle Children’s Urgent Care clinics with PCR-confirmed infant SARS-CoV-2 or influenza A. BMC from acute and convalescent timepoints will be evaluated by high-parameter flow cytometry for T cell immunophenotyping. In Aim 3, I will interrogate breastmilk antigen-specific TRM responses in the setting of maternal and infant respiratory