IFN-γ–driven skewing towards Th1 over Th17 differentiation underlies CRS and neutropenia in CAR-T therapy

Chimeric antigen receptor T cell (CAR-T) therapy has led to significant improvements in patient survival. However, a subset of patients experience high-grade toxicities, including cytokine release syndrome (CRS) and immune cell-associated hematological toxicity (ICAHT). We utilized IL-2Ra knockout mice to model toxicities with elevated levels of IL-6, IFN-γ, and TNF-α and increased M1-like macrophages. Onset of CRS was accompanied by a reduction in peripheral blood neutrophils due to disruption of bone marrow neutrophil homeostasis characterized by an increase in apoptotic neutrophils and a decrease in proliferative and mature neutrophils. Both nontumor-bearing and Em-ALL tumor-bearing mice recapitulated the cooccurrence of CRS and neutropenia. IFN-γ-blockade alleviated CRS and neutropenia without affecting CAR-T efficacy. Mechanistically, a Th1-Th17 imbalance was observed to drive cooccurrence of CRS and neutropenia in an IFN-γ-dependent manner leading to decreased IL-17A and G-CSF, neutrophil production, and neutrophil survival. In patients, we observed an increase in the IFN-γ-to-IL-17A ratio in the peripheral blood during high-grade CRS and neutropenia. We have uncovered a biological basis for ICAHT and provide support for the use of IFN-γ blockade to reduce both CRS and neutropenia.