A novel CDK8/19 inhibitor RO8323 mitigates allograft rejection through dual mechanisms of action to modulate regulatory T cell and myeloid cell

Regulatory T (Treg) cells are pivotal in maintaining immune homeostasis through suppression of effector T (Teff) cells, making their therapeutic modulation a promising strategy for treating autoimmune and inflammatory diseases. CDK8/19 inhibitors promote Treg cell differentiation by upregulating Foxp3 expression in both naive and memory/effector T cells. In this study we identified a novel dual CDK8/19 inhibitor RO8323 and systematically dissected the mechanism of CDK8/19-mediated immunoregulation. RO8323 inhibited CDK8 and CDK19 with IC₅₀ values of 2 nM and 3 nM, respectively, displaying >100-fold kinome selectivity. In the in vitro and in vivo experimental settings, we demonstrated that RO8323 selectively enhanced Treg differentiation while suppressing Teff. Furthermore, RO8323 exerted anti-inflammatory effects on myeloid cells by selectively upregulating IL-10 production but not proinflammatory cytokines (TNF-α, IL-6, and IL-12) following TLR agonist activation. In the DBA/2 → BALB/c cGVHD model, administration of RO8323 (3 mg·kg^-1·d^-1, i.g.) from day 7 to day 49 displayed significant therapeutic potential by reducing clinical severity scores and enhancing immune reconstitution -a finding reported for the first time in this context. Complementary studies using an ear-heart transplantation model revealed that administration of RO8323 (3, 10 mg·kg^-1·d^-1, i.g.) dose-dependently prolonged cardiac allograft survival accompanied by increased Treg frequencies. These results not only elucidate the immunomodulatory mechanisms of CDK8/19 inhibition but also highlight its translational value for managing alloimmune responses such as GVHD and transplant rejection.