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Background PARP inhibitors (PARPi) have significantly improved outcomes for women with ovarian cancer. This is attributed to primary effects on DNA repair pathways that result in synthetic lethality in homologous recombination deficient (HRD) tumor cells. Evidence that PARPi also have immunomodulatory effects presents opportunities to extend treatment benefit. While tumor-intrinsic mechanisms that enhance tumor immunogenicity have been identified, less is known about direct effects of PARPi on T cells. Since PARPi are primarily used in the maintenance setting when tumor burden is low, tumor-extrinsic effects may predominate in the majority of patients. We hypothesized that PARPi could impact T cell function through effects on stimulation-induced transcription and proliferation.Methods Conventional naïve (Tconv) and Foxp3-eGFP+ (Tregs) T cells were isolated from the lymph nodes and spleen of healthy female mice and stimulated in vitro in the presence or absence of olaparib (PARPi). Flow cytometry was used to assess proliferation (CellTrace), DNA damage (p-H2AX), cell cycle progression (DAPI staining and Edu incorporation) and differentiation (T-bet, GATA3, RORγT and Foxp3) under polarizing conditions (Th1, Th2, Th17 and iTreg, respectively).Results Although T cells from healthy animals do not have HRD, PARPi restricted proliferation of Tconv after stimulation. Tconv stimulated in the presence of PARPi had elevated levels of DNA damage compared to non-treated Tconv. Notably, stimulated Tconv had an accumulation of cells halted in G2 (DAPI-high, Edu-low) when treated with PARPi. When compared across cell divisions, the effect was most pronounced in non-divided cells. Similar results were seen in analyses of Treg. PARPi also restricted T cell differentiation under Th-polarizing conditions, although subset-specific transcription factors were still expressed.Conclusions PARPi treatment alters the proliferative capacity of newly-activated T cells through the accumulation of DNA damage and stalling of the cell cycle. Interestingly, the effect is diminished as cells divide, suggesting an active repair mechanism. While we attribute the reduction in the proportion of differentiated cells stimulated under polarizing conditions to direct effects on cell divisions, we cannot rule out the possibility that PARPi directly affects T cell function.