Contribution of Bcl-2 Inhibitor Venetoclax Toward Anti-CD19 CAR T Cell Efficacy in Relapsed/Refractory Diffuse Large B Cell Lymphoma
Authors: Mandeville Taylor K , Mavis Cory , Gu Juan , Olejniczak Scott , Paragh Gyorgy , Dey Prasenjit , Hernandez-Ilizaliturri Francisco J. 
Published: November 2021

Abstract
Introduction
Clinical outcomes of anti-CD19 chimeric antigen receptor T-cell (CAR-T CD19) therapy for the treatment of relapsed/refractory Diffuse Large B Cell lymphoma (r/r DLBCL) must be improved, as patients undergoing CAR-T CD19 therapy can expect a cure rate of only 41% and a median survival of just 6.3 months if unresponsive. Of patients presenting with naïve DLBCL, 35% will advance to third-line treatment with CAR-T CD19 therapy. Poor clinical responses to CAR-T CD19 therapy and a small window of survival for unresponsive patients highlights the critical need for improvement of CAR-T CD19 therapy in the setting of r/r DLBCL. Overexpression of anti-apoptotic Bcl-2 protein has been characterized in r/r DLBCL, and contributes to the disease's aggressive phenotype. Aberrant expression of the Bcl-2 protein proves targetable by an FDA-approved small molecule inhibitor which mimics the role of pro-apoptotic BH3 proteins, known as Venetoclax. Here, we investigate the effect of Venetoclax on CAR-T CD19 cell immunophenotype, effector function, and cytotoxicity upon co-culture with a lab-generated model of immortalized rituximab/chemo-resistant B cell lymphomas (Raji4RH) which recapitulate the resistant phenotype of r/r DLBCL seen in patients.
Methods
PBMCs were isolated and activated from apheresis samples from consenting adults (healthy donors) using the Miltenyi Biotec TransACT human CD3/CD28 Kit. Cells were transduced with lentiviral vectors encoding for 2nd generation CAR constructs 24 hours post-activation. Transduced cells were cultured in RPMI media supplemented with hIL-2 (1ng/mL) and hIL-7 (10ng/mL). At day 14, cells were cultured with Raji4RH and Venetoclax in either combined or solitary cultures. Co-culture cytotoxicity assays were performed by pre-labeling target cells with permeant dye (Thermo Fisher Scientific CellTrace Blue Proliferation Kit), then incubating with CAR-T CD19 cells (GFP labeled) at various effector:target ratios and Venetoclax concentrations for 24 hours. Sample wells were stained with viability dye and acquired on BD LSR II Cytometer. CAR-T CD19 cell immunophenotype was identified following 24-hour exposure of Venetoclax to CAR T cell populations at various concentrations. Following exposure, samples were stained with viability dye, CD3, CD4, CD8, CD45RA, CD45RO, CD62L, CCR7, CD25, and CD95. Frequencies of T REG, T Central Memory, T naive, and T Stem Cell Memory populations were calculated. CAR-T CD19 cell effector function was elucidated by activating (24 hours), then exposing CAR-T CD19 cells to concentrations of Venetoclax for 24 hours. Brefeldin A was added to culture for the remaining 4 hours of incubation. Cells were collected and stained intracellularly for IFNγ, Perforin, and Granzyme B; and extracellularly for CD3, CD4, and CD8. Sample acquisition for effector function assay was performed on BD LSRII Cytometer.
Results
Concurrent in vitro CAR-T CD19 administration and Venetoclax exposure significantly increases Raji4RH cell death as opposed to single agent cohorts. CAR-T CD19 cell viability is unaffected by Venetoclax at concentrations which confer synergy, and a predominantly CD8+ population is favored. Finally, Venetoclax appears to significantly increase the frequency of T N/T SCM and T CM within exposed cultures, while affording a 21% decrease in T REG frequency. Additionally, the notable increase in T N/T SCM and T CM phenotypes following exposure to Venetoclax did not arise as a consequence of decreased cellular viability.
Conclusion
The combination of Venetoclax exposure and αCD19 CAR T cell administration demonstrates synergy when employed in the context of rituximab-resistant lymphoma cells, which may be explained by the effect of the BH3 mimetic's exposure on CAR-T CD19 cell immunophenotype. The ability to manipulate Bcl-2 protein expression within CAR-T CD19 cells affords insight into the role that the Bcl-2 family pathway plays within CAR-T CD19 cell biology. In addition to answering fundamental questions of CAR T cell biology, the combination of Venetoclax and CAR-T CD19 therapy may provide a solution to the observed clinical gap, in which CAR-T CD19 therapy alone cures less than half of all patients who advance to this third line treatment regimen.
Figure 1 Figure 1.

Disclosures
No relevant conflicts of interest to declare.