PARP Inhibitors Supercharge Stem Cell Therapy for Inflammatory Diseases

Mesenchymal stem/stromal cells (MSCs) hold enormous promise for treating autoimmune and hyperinflammatory diseases, but their therapeutic potency depends on proper 'licensing' by inflammatory signals in the tissue microenvironment. Understanding how to amplify this licensing process is a key goal in regenerative medicine. This study, published in Stem Cell Research & Therapy, identifies PARP1 as an unexpected brake on MSC immunosuppressive function and demonstrates that releasing this brake with clinically approved PARP inhibitors produces substantially more potent therapeutic cells.

The research team treated human umbilical cord-, dental pulp-, and mouse bone marrow-derived MSCs with inflammatory cytokines IFNγ and TNFα (10 ng/ml each, 24 hours) in the presence or absence of PARP inhibitors rucaparib or olaparib, as well as using PARP1 genetic knockout mice. Both pharmacological inhibition and genetic deletion of PARP1 consistently elevated mRNA and protein expression of immunosuppressive mediators—iNOS (in mouse MSCs), IDO1 (in human MSCs), TSG-6, PD-L1, and chemokines CXCL9, CXCL10, and CXCL11—compared to cytokine stimulation alone. RNA sequencing confirmed broad upregulation of immunoregulatory gene networks.

Functionally, MSCs treated with PARP inhibitors showed markedly enhanced ability to suppress T cell proliferation in co-culture assays using CFSE-labeled splenocytes and PBMCs. In vivo, pre-conditioned MSCs (IFNγ/TNFα ± PARP inhibitor) were tested in two established mouse models. In the ConA-induced acute liver injury model, PARP-inhibitor-pretreated MSCs reduced serum aminotransferase levels and liver histological damage significantly more than control-primed MSCs. In the DSS-induced colitis model, mice receiving PARP-inhibitor-primed MSCs showed reduced disease activity index scores, better body weight maintenance, and less colon tissue inflammation.

Mechanistically, the team found that PARP inhibition increased phosphorylation of STAT1 at tyrosine 701 (pY701-STAT1) in cytokine-stimulated MSCs. STAT1 is a transcription factor critical for driving iNOS, IDO1, and PD-L1 expression in activated MSCs. Blocking STAT1 with the inhibitor nifuroxazide or genetic STAT1 knockout abolished the enhancement of immunosuppressive factor expression seen with PARP inhibition, confirming that pY701-STAT1 is the key downstream effector. Interestingly, PARP inhibition did not affect STAT1 total protein levels, only its phosphorylation state, suggesting a post-translational regulatory node.

These findings are conceptually significant: while PARP inhibitors are known to dampen pro-inflammatory responses in macrophages, they paradoxically amplify anti-inflammatory responses in MSCs—both effects being beneficial in inflammatory disease contexts. The study also raises the possibility that patients already receiving PARP inhibitors (e.g., cancer patients on olaparib) may experience altered MSC biology, a consideration for combination therapies. The use of FDA-approved PARP inhibitors provides a clear translational pathway, though human clinical validation remains needed.