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Although Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) have been approved in multiple diseases, including BRCA1/2 mutant breast cancer, responses are usually transient requiring the deployment of combination therapies for optimal efficacy. Here we thus explore mechanisms underlying sensitivity and resistance to PARPi using two intrinsically PARPi sensitive (T22) and resistant (T127) syngeneic murine breast cancer models in female mice. We demonstrate that tumor associated macrophages (TAM) potentially contribute to the differential sensitivity to PARPi. By single-cell RNA-sequencing, we identify a TAM_C3 cluster, expressing genes implicated in anti-inflammatory activity, that is enriched in PARPi resistant T127 tumors and markedly decreased by PARPi in T22 tumors. Rps19/C5aR1 signaling is selectively elevated in TAM_C3. C5aR1 inhibition or transferring C5aR1hi cells increases and decreases PARPi sensitivity, respectively. High C5aR1 levels in human breast cancers are associated with poor responses to immune checkpoint blockade. Thus, targeting C5aR1 may selectively deplete pro-tumoral macrophages and engender sensitivity to PARPi and potentially other therapies. PARP inhibitors (PARPi) have been approved for the treatment of metastatic triple-negative breast cancer (BC), however resistance and recurrence are often observed. Here, in preclinical models of BRCA1/2 wild type and homologous recombination competent BC, the authors show that C5aR1-positive tumor associated macrophages are associated with PARPi-resistance, suggesting targeting C5aR1 as a therapeutic option.

Several unique waves of ?? T cells are generated solely in the fetal/neonatal thymus, whereas additional ?? T cell subsets are generated in adults. One intriguing feature of ?? T cell development is the coordination of differentiation and acquisition of effector function within the fetal thymus; however, it is less clear whether this paradigm holds true in adult animals. In this study, we investigated the relationship between maturation and thymic export of adult-derived ?? thymocytes in mice. In the Rag2pGFP model, immature (CD24+) ?? thymocytes expressed high levels of GFP whereas only a minority of mature (CD24-) ?? thymocytes were GFP+ Similarly, most peripheral GFP+ ?? T cells were immature. Analysis of ?? recent thymic emigrants (RTEs) indicated that most ?? T cell RTEs were CD24+ and GFP+, and adoptive transfer experiments demonstrated that immature ?? thymocytes can mature outside the thymus. Mature ?? T cells largely did not recirculate to the thymus from the periphery; rather, a population of mature ?? thymocytes that produced IFN-? or IL-17 remained resident in the thymus for at least 60 d. These data support the existence of two populations of ?? T cell RTEs in adult mice: a majority subset that is immature and matures in the periphery after thymic emigration, and a minority subset that completes maturation within the thymus prior to emigration. Additionally, we identified a heterogeneous population of resident ?? thymocytes of unknown functional importance. Collectively, these data shed light on the generation of the ?? T cell compartment in adult mice.