Plasma IFN-γ and IL-6 levels correlate with peripheral T-cell numbers but not toxicity in RCC patients treated with CAR T-cells

•Authors prefer in vivo CAR T cell enumeration by flow cytometry over qPCR.•Peripheral blood T cell numbers correlated with plasma levels of IFN-γ and IL-6.•In CAR T cell treatment predictors of T-cell persistence are lacking.

Autologous T-cells genetically modified to express a chimeric antigen receptor (CAR) against carboxy-anhydrase-IX (CAIX) were administered to twelve patients with CAIX-positive metastatic renal cell carcinoma. Here, we questioned whether plasma cytokine levels following treatment or in vitro cytokine production from the T-cell infusion products could serve as predictors for peripheral T-cell persistence or in vivo T-cell activity. We demonstrated that CAR surface as well as gene expression are down-regulated following T-cell infusion, and that peripheral numbers of CAR T-cells are best captured by flow cytometry and not by qPCR. Numbers of CAR T-cells in blood correlated with plasma levels of IFN-γ and IL-6, but not with any of the other cytokines tested. Plasma IFN-γ or IL-6 levels did not correlate with liver enzyme values. Thus, out of 27 cytokines tested, IFN-γ and IL-6 levels in plasma are potential surrogate markers for CAR T-cell persistence in solid tumors.