Although a majority of patients will achieve a complete response following a single infusion of CD19-targeted CAR-modified T cells (CD19 CAR T cells)<sup>2-4</sup>, the broad applicability of this treatment is hampered by severe cytokine release syndrome (CRS), which is characterized by fever, hypotension and respiratory insufficiency associated with elevated serum cytokines, including interleukin-6 (IL-6)<sup>2,5</sup>.