GM-CSF neutralization in vitro enhances CAR-T cell proliferation in the presence of monocytes and does not impair CAR-T–cell effector function. (A) Lenzilumab neutralizes CAR-T cell–produced GM-CSF in vitro compared with isotype-control treatment, as assayed by a multiplex assay after 3 days of culture with CART19 cells in media alone or CART19 cells cocultured with NALM6; n = 2 experiments, 2 replicates per experiment, a representative experiment is depicted. (B) GM-CSF–neutralizing antibody treatment did not inhibit the ability of CAR-T cells to proliferate, as assayed by a carboxyfluorescein diacetate succinimidyl ester flow cytometry proliferation assay of live CD3 cells; n = 3 donors, 2 replicates per donor, a representative experiment at the 3-day time point is depicted. (C) Lenzilumab enhanced the proliferation of CART19 cells compared with isotype-control–treated with CART19 cells when cocultured with monocytes; n = 3 donors at the 3-day time point, 2 replicates per donor. (D) Lenzilumab treatment did not inhibit cytotoxicity of CART19 cells or UTD T cells when cultured with NALM6; n = 3 donors, 2 replicates per donor, a representative experiment at the 48-hour time point is depicted. All data are mean ± SEM. ***P < .001, ****P < .0001, Student t test. Alone, CART19 cells in media alone; MOLM13, CART19+MOLM13; ns, P > .05, lenzilumab vs isotype-control treatment using the Student t test; NALM6, CART19+NALM6; PMA/ION, CART19 cells plus 5 ng/mL PMA and 0.1 μg/mL ionomycin.