The University of Pennsylvania is a leader in chimeric antigen receptor (CAR) T cell research and in translating basic research findings into successful clinical trials. CAR T cell therapies are designed to attack and destroy cancer by harnessing a patient’s own immune system. Researchers extract the patient’s T cells, modify them outside of the body so that they recognize tumor cells, and reintroduce the modified T cells back into the patient’s blood. The goal of this therapy is to provide a safe, highly specific, and long-term anti-tumorigenic system to prevent cancer relapse. Unprecedented clinical responses to CD19-specific CAR T cell therapy in patients with aggressive blood cancers have generated public awareness of T cell engineering and provide much future promise. One of the goals for T cell immunotherapy is to treat solid tumor cancers, which are more difficult to target with T cells than blood cancers.
From a young age, recent GTV alumna Jenessa Smith has been fascinated with science. Having close family members affected by cancer motivated Jenessa to pursue translational research and to join the laboratory of Dr. Daniel Powell at Penn. In a paper recently published in Molecular Therapy, Jenessa and colleagues examined the in vivo efficacy of a novel B7-H4 CAR T cell therapy designed to treat ovarian cancer. B7-H4 is a transmembrane protein that negatively regulates T cell immunity and is overexpressed in a number of malignancies, particularly in ovarian cancers. Previous scientific reports provided evidence, albeit controversial, for the absence or relatively low expression of B7-H4 protein in non-oncogenic tissues, making B7-H4 a promising target for CARs. Anti-B7-H4 antibody-drug conjugates did not cause overt toxicity in previous murine studies, which further motivated the preclinical research of B7-H4 CAR T cell therapy spearheaded by Jenessa.
Jenessa and colleagues generated B7-H4 CAR constructs by cloning anti-B7-H4 single chain variable fragments into available CAR lentiviral vectors. These newly generated CAR vectors were efficiently transduced into primary T cells and selectively bound both murine and human recombinant B7-H4 proteins. Subsequent in vitro studies confirmed the capability of B7-H4 CARs to properly target and lyse B7-H4-expressing cells. Following these results, Jenessa and colleagues proceeded to test the efficacy of their CAR-T vectors in an established mouse model of ovarian cancer. Nude mice bearing human ovarian tumor xenografts received either B7-H4 CAR T cell therapy or were treated with control CAR T cells. Supporting the team’s hypothesis, the B7-H4 CAR treatment led to a significant reduction of ovarian tumor burden and spoke to the promise of B7-H4 treatment as a novel therapy to treat ovarian cancer. Further observations, however, revealed some negative side effects. While the mice with B7-H4 CAR T cell treatment had a positive anti-oncogenic response, they exhibited clear signs of overall toxicity: mice were underweight, lethargic, and dehydrated. What could account for such lethal side effects? Jenessa and colleagues excluded the graft-versus-host-disease-like pathology as the cause of the observed toxicity, and speculated that B7-H4 CAR T cells might have targeted non-cancerous tissues in mice.
To test that hypothesis, the team examined B7-H4 expression in cancerous and normal cells using a new B7-H4 antibody to verify previous reports of B7-H4 expression in both murine and human tissues. As expected, B7-H4 was overexpressed in engrafted ovarian tumor xenografts. However, Jenessa and colleagues also observed widespread expression of endogenous B7-H4 in non-cancerous mouse and human tissues, contrary to previous reports. B7-H4 was mostly enriched in ductal and mucosal epithelial cells of multiple tissues, including esophagus, trachea, salivary gland, and liver. Additionally, B7-H4 was localized at or near sites of tissue damage and lymphocytic infiltration in mice receiving B7-H4 CAR T cell treatment. The latter finding further suggested that the CAR-induced toxicity likely occurred as the result of inauspicious attack mounted against B7-H4 expressed in non-cancerous tissues. This posed safety concerns, so B7-H4-specific CAR T cells in their current design could not be pursued in clinical applications.
(hover over the image for the legend) Illustration by Lindsey Weed
It is clear that safety remains a paramount concern for the advancement of CAR T cell research. Yet, CAR T cell therapy holds promise for academic and medical communities. Jenessa, among many, has hope, and her work at Penn is a valuable contribution to the field. She explains that “in the future [B7-H4 CAR T cell therapy] could provide a unique opportunity for preclinical evaluation of safety approaches that limit CAR-mediated toxicity after tumor destruction in vivo.” Overall, Jenessa’s research provides a promising yet sobering view of both the current advances and ongoing challenges that the booming field of T cell immunotherapy faces.
Jenessa continues to study CAR T cells as a research scientist at Poseida Therapeutics and feels “extremely lucky to be a part of this very exciting and groundbreaking field.”