PROJECT SUMMARY In type 1 diabetes (T1D), autoimmunity is established and self-perpetuating within the islets at diagnosis. Autoreactive T cells in T1D patients destroy endogenous and grafted beta cells, yet there is a gap in knowledge about the mechanisms by which T cell pathogenesis is modulated in the islets. Evidence from mouse models of T1D and human samples suggest that myeloid cells play an essential role in this process. The myeloid compartment represents the largest immune population in non-diabetic and many T1D human islets. Islet myeloid cells can contribute to islet destruction or protection, yet a significant gap in knowledge remains about how the islet myeloid compartment plays such divergent roles in islet autoimmunity during T1D progression. We have shown that Mertk signaling in islet myeloid cells suppresses autoreactive T cell responsiveness to antigen and prevents rapid progression of T1D. Mertk mediates apoptotic cell uptake (efferocytosis), and its signaling is immunoregulatory. However, we do not yet understand the mechanisms by which Mertk signaling in islet myeloid cells suppresses the ability of autoreactive T cells to respond to locally presented antigen. Our scRNA-seq analyses of myeloid cells from the islets of non-diabetic and T1D organ donors support that, similar to our mouse data, efferocytosis is enhanced and antigen processing and presentation are suppressed during the active period of disease following T1D onset. Thus, our overarching hypothesis is that in the islets during T1D, stimulatory myeloid cell subsets promote pathogenic T cell functions through antigenic stimulation and inhibitory islet myeloid cell subsets suppress the pathogenic T cells in a manner that is dependent upon Mertk mediated efferocytosis within the pancreatic islet. To begin to address the gaps in knowledge about islet myeloid cell function, we propose the following aims: Aim 1: Elucidate the mechanisms by which Mertk expressing myeloid cells modulate effector and regulatory T cell responses in the islets. Aim 2: Characterize the myeloid cell subsets that actively present antigen to effector and regulatory T cells in mouse islets and how this is altered by Mertk signaling. Aim 3: Determine the human islet myeloid subsets that perform Mertk-mediated efferocytosis and drive T cell signaling in human islets. The successful completion of this proposal will result in: (1) an improved understanding of the mechanisms used by Mertk- expressing islet myeloid cells to suppress the islet T cell response, (2) an enhanced understanding of pancreatic islet myeloid cell subsets and pathways that drive pathogenic T cell activation versus those that regulate islet autoimmunity and (3) identification of novel therapeutic targets to potentially skew the autoim- mune T cell response toward protective immunity in type 1 diabetes.