Summary Transplantation T critical Macrophages are constantly exposed to damage- and pathogen-associated molecular patterns (DAMPs and PAMPs). These stimuli directly induce the release of proinflammatory molecules, but they also promote epigenetic and metabolic modifications, leading to a long-lasting increase in macrophage functional responsiveness (trained immunity). The identification of therapies to target trained immunity is an unmet need in transplant medicine. is the ideal treatment f or end-stage organ failure, but long-term graft survival is still insufficient. and B cells have a key role in acute rejection, but recent evidence indicates that also innate immune cells are in alloimmune response. This erythropoietin grant renewal builds logically on the results generated during the first funding cycle showing that (EPO)has immunomodulating activity in murine and human cells. We showed that EPO promotes conversion of naïve T cells into regulatory T cells (Treg) in mice and humans, leading to prolonged graft survival and reduced severity of multiple autoimmune disease models in mice. Our new preliminary data indicate that EPO has also inhibitory effects on macrophage training and hinders the proinflammatory program of previously trained cells. This effect, that occurs through a direct interaction with the EPO receptor on myeloid cells, leads to prolonged graft survival in mice that underwent training. The present proposal will address the mechanisms responsible for these effects using in vivo and in vitro systems. We will also test the role of endogenous EPO on trained immunity. The results of this research project will provide new insight on the role of macrophage training in alloimmune responses and how EPO counteracts them. The data have the potential to unravel cellular and molecular pathways that are important in the pathophysiology of rejection and to identify new therapeutic targets. The fact that EPO is FDA-approved, makes our findings easier to be translated into the clinic.