Abstract 3974: Evaluating the susceptibility of solid tumors to chimeric antigen receptor modified T cell therapies.
Authors: Long Adrienne H. , Highfill Steven L. , Haso Waleed M. , Orentas Rimas J. , Mackall Crystal L. 
Published: August 2014

Abstract
Adoptive cell therapy with tumor infiltrating lymphocytes (TIL) has been successfully used as a treatment in metastatic melanoma. However, TIL are not present in the majority of solid cancers. To overcome this barrier, T cells can be transduced with chimeric antigen receptors (CARs) to produce tumor specific T cells for adoptive immunotherapy. CD19 CAR T cell therapies have demonstrated activity against B cell malignancies in both pre-clinical and clinical studies, but CARs targeting solid tumors have been less active in clinical trials and we observed less success when targeting solid tumors with GD2 CAR therapies in preclinical models. We hypothesize that the observed difference in anti-tumor efficacy may be due to 1) a more hostile microenvironment within solid tumors compared to hematologic malignancies, and/or 2) variable CAR potency. To normalize for differences in CAR potency we created a CD19 expressing osteosarcoma cell line, thus allowing use of the well-characterized CD19 CAR to explore the susceptibility of solid malignancies to adoptive immunotherapy.
The GD2 expressing 143B osteosarcoma cell line was transfected to express CD19 (143-CD19) and susceptibility to GD2 and CD19 CAR therapies was compared. 51Cr release experiments demonstrated that both GD2 and CD19 CAR T cells were similarly cytolytically active against 143B and 143B-CD19 cells, respectively. The in vivo susceptibility was then compared using two xenograft models. NSG mice were injected IM with 5e5 143B or 143B-CD19. The first was an early treatment model, where 3 days following tumor inoculation, mice received 5e6 GD2 CAR, CD19 CAR or mock T cells IV. In mice treated with GD2 CAR T cells, no difference in tumor growth or survival was observed. However, mice inoculated with 143B-CD19 and treated with CD19 CAR T cells showed improved survival (p&lt;0.01) and complete eradication of their CD19+ disease. CD19 CAR T cells were next tested in a late treatment model: NSG mice were injected with 1e6 143B-CD19. On day 19, mice were treated IV with 1e7 CD19 CAR or mock T cells. By day 28, tumors of CD19 CAR treated mice had shrunk to 50% the size of day 19 tumors, while mock treated mice had progressed to 2 cm x 2 cm (p&lt;0.01). Thus, the CD19 CAR induced impressive regression of established &gt;1 cm x 1 cm 143B-CD19 tumors. We further observed that human solid tumors expand murine myeloid derived suppressor cells in this system, which bear a hallmark phenotype and suppress human T cells. We conclude that despite the presence of an immunosuppressive solid tumor microenvironment, appropriate choice of antigen and CAR design can lead to regression in established solid tumors. Our ongoing work focuses on using this platform to more clearly define the CAR T cell characteristics that correlate to anti-solid tumor efficacy in vivo, as well as explore how modulating the immunosuppressive solid tumor microenvironment can enhance efficacy of otherwise ineffective CAR treatments.
Citation Format: Adrienne H. Long, Steven L. Highfill, Waleed M. Haso, Rimas J. Orentas, Crystal L. Mackall. Evaluating the susceptibility of solid tumors to chimeric antigen receptor modified T cell therapies. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3974. doi:10.1158/1538-7445.AM2013-3974