It has previously been reported that patients with macrophage activation syndrome (MAS)-like manifestations following CAR-T cell therapy have elevated serum ferritin, but decreased serum C reactive protein (CRP).38 Here, we observed decreased serum CRP in E3K CAR-T cell treated mice, suggesting that E3K CAR-T cell associated toxicity is linked to macrophage activation. Here, CRP is linked to macrophage activation syndrome.