The role of CD69 in established resident memory T cells - Resident memory T cells (TRM) remain at a site of infection long after disease has been resolved. TRM form a critical first line of defense against re-infection, but TRM can be pathogenic in the context of recurrent autoimmune inflammation. The canonical protein used to identify TRM is the membrane-bound C-type lectin CD69. Naïve T cells up-regulate CD69 within hours of stimulation by type I interferons or antigen recognition, and hence CD69 has long been used as an “early activation marker.” However, TRM are memory cells, and by definition have not been recently activated. It remains unknown why established TRM express high levels of surface CD69. Early after a naïve T cell has been activated, CD69 performs several key functions. First, CD69 retains the T cell in the lymph node by antagonizing sphingosine 1-phosphate receptor 1 (S1PR1), a G-protein-coupled receptor that guides T cell exit from lymph nodes. This causes a T cell activated by cytokines to reside longer in the lymph node to search for its cognate antigen, and a T cell activated by antigen to reside longer in the lymph node to complete its activation. Second, CD69 may amplify signals through cytokine receptors or the T cell receptor. CD69 cross-linking induces elevation of intracellular calcium, and our preliminary data implicate CD69 signaling in activation of Akt and Erk. Our central hypothesis is that there is an inverse relationship between the speed of T cell activation and the ability of a T cell to circulate, mediated by the antagonism between CD69 and S1PR1. Thus CD69+ resident memory T cells stay at likely sites of pathogen re-entry and respond very quickly upon re-infection, while S1PR1+ circulating T cells arrive more slowly but contribute to a sustained response. Consistent with this hypothesis, our preliminary data suggest that early after re-challenge, Cd69-/- TRM fail to recruit inflammatory monocytes efficiently to the skin. Furthermore, we have developed Cd69f/f mice in order to interrogate the role of CD69 specifically in TRM, avoiding confounding effects of CD69 expression in T cell development and initial activation. We will test our hypothesis in two aims. We will define the role of CD69 in skin-resident CD8 TRM at steady-state, asking whether CD69 expression on TRM regulates their numbers, localization, or transcriptional profile at baseline. We will then define the role of CD69 in skin-resident CD8 TRM after re-challenge, assessing changes in leukocyte recruitment and vascular permeability; whether any change reflects changes in chemokine secretion or endothelial adhesion molecules; and whether this in turn reflects altered cytokine secretion or other key properties of TRM.
