1057 - Combining CD19 CAR T cells with T cell-engaging bispecific antibody treatment enhances efficacy in murine models of CLL and ex vivo patient cultures

Failure of adaptive immunity and immunotherapy such as immune checkpoint blockade (ICB) and chimeric antigen receptor (CAR) T cell therapy has been linked to dysfunctional effector T cells in chronic lymphocytic leukemia (CLL) patients. Chronic exposure to tumor (neo)antigens in an immunosuppressive microenvironment results in T cell exhaustion with limited reactivation upon immunotherapy. Despite an increasing number of effective tumor-targeting treatment options for CLL, patients are frequently suffering from refractory disease when malignant cells resistant to standard therapy emerge. CLL is still considered incurable with an urgent clinical need for additional treatment modalities for therapy-resistant cases. We therefore aimed to exploit CAR T cell therapy in combination with additional immunostimulation to overcome the lack of response in CLL as opposed to other B cell malignancies, such as acute lymphoblastic leukemia (ALL), where CD19-CAR T cells are an effective and safe treatment strategy.

Clinically relevant drugs were screened for candidates improving CAR T cell therapy in an in vitro system based on several B cell non-Hodgkin lymphoma (B-NHL) cell lines cocultured with healthy donor-derived CD19-CAR T cells or non-transduced (NT) control T cells for four days. Bispecific antibody (BsAb) treatment targeting CD20 as tumor antigen and CD3 on T cells was included in the screening, aiming to maximize tumor cell killing via combined forces of CD19-CAR T cells and bystander endogenous T cells engaged to kill CD20-positive cells. In this model system, the addition of BsAb had only minor effects on CAR T cell expansion and tumor cell killing, but was highly beneficial for NT T cell proliferation and cytotoxicity. We hypothesized that BsAb-mediated activation of non-transduced endogenous T cells in the physiological context may complement CAR T cell therapy and improve overall anti-tumor response.

The fully immunocompetent Eµ-TCL1 mouse model of CLL was used to evaluate the potential of this combination approach in vivo. Monotherapies with either murine CD20-BsAb or CD19-CAR T cells were established in this model, both demonstrating dose-dependent efficacy in terms of CLL cell killing. Following lymphodepletion via low dose irradiation, the CAR T cell treatment was titrated to define an injected cell number with suboptimal treatment efficacy leading to relapse of animals after initial response to mimic key challenges in the clinical reality. Combination of CAR T therapy with subsequent injections of BsAb resulted in reduced tumor load and prolonged survival compared to both monotherapies. Intriguingly, preliminary data indicate that this higher efficacy is accompanied by improved CAR T cell expansion under BsAb therapy. In the long term, not only bystander T cells, but also CAR T cells may hence benefit from repeated BsAb stimulation.

To validate these findings in a relevant ex vivo setting mimicking the cellular composition of CLL patient lymph nodes, the combination treatment was tested in primary lymph node-derived samples of CLL (n = 4) and other B-NHL (n = 20) patients containing both malignant B cells and tumor-experienced T cells. Addition of CD20-BsAb enhanced overall tumor cell killing in cocultures with CAR T cells and improved activation and proliferation of autologous T cells marked by increased percentages of CD25+, Granzyme B+ and Ki67+ cells. Therefore, CAR T cell-mediated cytotoxicity can be complemented by BsAb-induced tumor cell killing mediated by autologous T cells in the ex vivo setting of primary lymph node cultures.

In conclusion, our data suggest that combination of CAR T cell and BsAb therapies can enhance therapeutic efficacy in CLL and other B-NHL. Both bystander T cell activation and improved long term expansion or persistence of CAR T cells with repeated BsAb restimulation seem to contribute to this improved tumor control.