Presence of Endogenous TCR Antigen in Vivo Attenuates Efficacy of Anti-CD19 Targeted CAR T Cell Therapy
Authors: Yang Yinmeng , Chien Christopher Daniel , Jacoby Elad , Qin Haiying , Haso Waleed , Fry Terry J 
Published: October 2019

Abstract
Adoptive therapy using T cells genetically engineered to express chimeric antigen receptors (CAR) has proven extremely effective against acute lymphoblastic leukemia (ALL) in clinical trials with the use of anti-CD19 CAR T cells. Most CAR T cell protocols use autologous T cells, which are then activated, transduced with the anti-CD19 CAR, and expanded ex-vivo before infusion back into the patient. This approach minimizes the risk of graft-versus-host disease (GVHD) even in allogeneic transplant recipients, due to tolerization of the donor T cell repertoire in the recipient. However, many patients have heavy disease burden and lymphopenia due to previous treatments, which makes the isolation of healthy T cells difficult. Thus, centers are exploring the potential of allogeneic T cell donors and the possibility of universal T cell donors for CAR-based therapy including the use of virus-specific T cells. In these cases, in addition to the chimeric receptor specificity, the transduced T cell population will also have reactivity against target antigens through the endogenous TCR. However, little is known about the impact of signaling of the endogenous TCR on CAR T cell activity, particularly in vivo. To test this, we used a syngeneic transplantable ALL murine model, E2aPBx, in which CD19 CAR T cells can effectively eradicate ALL. CD4 (Marilyn) and CD8 (Matahari) T cells from syngeneic HY-TCR transgenic donors specific for the minor histocompatibility male antigen, HY, were used as CAR T cell donors to control for endogenous TCR reactivity. Splenic T cells isolated from Matahari, Marilyn, or B6 mice were activated ex-vivo using anti-CD3/anti-CD28 beads, with the addition of IL2 and IL7. T cells were transduced with a retroviral vector expressing a murine CAR composed of anti-CD19 scfv/CD28/CD3ζ on days two and three. CAR T cells are evaluated in vitro by CD107a degranulation assay and INF gamma ELISA. In response to HY peptide alone or HY+CD19- line M39M, transduced CD8 HY (Matahari) cells produced IFN gamma and expressed CD107a whereas transduced CD4 HY (Marilyn) cells only produced IFN gamma. Interestingly, in response to CD19+HY- ALL, both Matahari and Marilyn expressed CD107a and produced IFN gamma indicating that CD4 T cells can acquire CD8-like lytic activity when stimulated through a CAR receptor. When CD19 CAR transduced Marilyns and Mataharis were stimulated in the presence of HY and CD19, CD8 Mataharis had an attenuated effect against CD19, suggesting that the presence of antigen activated TCR adversely affects the potency of the CAR receptor. Efficacy of the HY and polyclonal CAR T cells were next tested in-vivo in male and female B6 mice. Mice were given 1E6 E2aPBx ALL leukemia cells on day 1, and received 500 rads sub-lethal total body irradiation on day 4 as a lymphodepleting regimen. On day 5, mice were given a low (1E5) or high (5E6) dose of CAR T cells. There was a statistically significant (p=0.0177) improvement in the survival of female versus male mice after treatment with the CD4+ HY specific anti-CD19 CAR T cells, and female mice that received HY anti-CD19 CAR T cells survived longer than untreated control females (p=0.01). Remarkably, the survival of male mice that received HY anti-CD19 CAR T cells was statistically worse than untreated control males (p=0.008). This suggests that the presence of TCR antigen negatively impacts the function of CAR T cells. Furthermore, in a separate experiment using an equally mixed population of Marilyn (CD4+) and Matahari (CD8+) HY specific T cells, males has a statistically significantly (p=0.0116) worse survival compared to females after receiving 5E5 HY specific T cells. In conclusion, simultaneous stimulation through both CAR and TCR results in attenuated cytokine production and degranulation by CD8 T cells. In vivo, in the presence of the endogenous TCR antigen, both CD4 and CD8 CAR T cells are less potent at eradicating leukemia. These have implications for the development of universal donors for CAR T cell therapy.

Disclosures
No relevant conflicts of interest to declare.