Chimeric antigen receptor (CAR) T cells are genetically engineered to recognize and eradicate cancer cells. CAR T cell therapy shows promising results in treating patients with hematological malignancies but it can also lead to undesirable side effects, such as Cytokine Release Syndrome (CRS). CRS is triggered by CAR T cell-based activation of monocytes, which leads to massive release of cytokines, especially IL-1b and IL-6. The monocytes are activated by direct interaction with the CAR T cells via the CD40 ligand (CD40L) – CD40 receptor axis or via uptake of GM-CSF released from the activated CAR T cells. Current mouse xenotransplantation models are able to predict the efficacy of CAR T cells but not necessarily CRS. To overcome this limitation, we established a cell-based in vitro assay that involves the co-culturing of leukemic B cells with primary CD19-targeting CAR T cells and monocytes from the same donor to monitor both CAR T cell activity and cytokine release. We show that upon activation, the CD19-targeting CAR T cells upregulated CD40L and GM-CSF expression, which in turn activated the isogenic monocytes to secrete IL-1b and IL-6. CRISPR-Cas9 based disruption of the CD40L and/or GM-CSF encoding loci in CAR T cells did not affect the antitumor activity of the engineered lymphocytes but significantly reduced IL-1b and IL-6 secretion from the monocytes. We conclude that our cell-based assay is able to model CRS in vitro, allowing us to evaluate gene-edited CAR T cells pertaining their antitumor activity as well as their propensity to trigger CRS.