Sato et al. developed mouse tumorigenic organoid lines that resembled the basal-squamous subtype of the human urothelial carcinoma. In mice implanted with the organoids, anti-PD-1 and anti-CTLA-4 combination (ICB) led to tumor rejection and formation of immunological memory in a manner dependent on CD4+ T cells, but not CD8+ T cells. Tumor rejection did not rely on the expression of MHC class I, MHC class II, or Ifngr1 in the tumor. Mechanistically, ICB expanded IFNγ-producing Th1 CD4+ T cells in the tumor and draining lymph nodes, and IFNγ impaired tumor growth independent of other CD4+ T cell functions.
Immune checkpoint blockade (ICB) provides clinical benefit to a minority of patients with urothelial carcinoma (UC). The role of CD4+ T cells in ICB-induced antitumor activity is not well defined; however, CD4+ T cells are speculated to play a supportive role in the development of CD8+ T cells that kill tumor cells after recognition of tumor antigens presented by MHC class I. To investigate the mechanisms of ICB-induced activity against UC, we developed mouse organoid-based transplantable models that have histologic and genetic similarity to human bladder cancer. We found that ICB can induce tumor rejection and protective immunity with these systems in a manner dependent on CD4+ T cells but not reliant on CD8+ T cells. Evaluation of tumor infiltrates and draining lymph nodes after ICB revealed expansion of IFN-gamma-producing CD4+ T cells. Tumor cells in this system express MHC class I, MHC class II, and the IFN-gamma receptor (Ifngr1), but none were necessary for ICB-induced tumor rejection. IFN-gamma neutralization blocked ICB activity, and, in mice depleted of CD4+ T cells, IFN-gamma ectopically expressed in the tumor microenvironment was sufficient to inhibit growth of tumors in which the epithelial compartment lacked Ifngr1. Our findings suggest unappreciated CD4+ T cell-dependent mechanisms of ICB activity, principally mediated through IFN-gamma effects on the microenvironment.