Kinetics of Tumor Destruction by Chimeric Antigen Receptor-modified T Cells

Abstract

The use of chimeric antigen receptor (CAR)-modified T cells as a therapy for hematologic malignancies and solid tumors is becoming more widespread. However, the infusion of a T-cell product targeting a single tumor-associated antigen may lead to target antigen modulation under this selective pressure, with subsequent tumor immune escape. With the purpose of preventing this phenomenon, we have studied the impact of simultaneously targeting two distinct antigens present on tumor cells: namely mucin 1 and prostate stem cell antigen, both of which are expressed in a variety of solid tumors, including pancreatic and prostate cancer. When used individually, CAR T cells directed against either tumor antigen were able to kill target-expressing cancer cells, but tumor heterogeneity led to immune escape. As a combination therapy, we demonstrate superior antitumor effects using both CARs simultaneously, but this was nevertheless insufficient to achieve a complete response. To understand the mechanism of escape, we studied the kinetics of T-cell killing and found that the magnitude of tumor destruction depended not only on the presence of target antigens but also on the intensity of expression-a feature that could be altered by administering epigenetic modulators that upregulated target expression and enhanced CAR T-cell potency.

Improving Chimeric Antigen Receptor-Modified T Cell Function by Reversing the Immunosuppressive Tumor Microenvironment of Pancreatic Cancer

“…Though several pancreatic cancer-targeted CARs have been developed (carcinoembryonic antigen [CEA], 33 mesothelin, 34 Her2/ neu, 35 and PSCA 9,12 ), the immunosuppressive tumor microenvironment, which can impair the proliferative capacity and in vivo persistence of the infused T cells poses a major challenge to successful immunotherapy. 19,21,22 To overcome this barrier, a number of groups have combined CAR T cells with other therapies designed to modulate the tumor microenvironment including checkpoint inhibitors and oncolytic viruses.…”

“…12 This transgenic molecule could be stably expressed on the surface of activated T cells (mean 78% ± 6%; n = 5; Figures 1B and 1C), enabling them to specifically kill PSCA-expressing target cells, including CAPAN-1, a pancreatic cancer cell line, and K562 cells modified to transgenically express PSCA (K562-PSCA) (74% ± 4% and 73% ± 6% specific lysis, respectively; 10:1 E:T ratio, n = 4). This killing was antigen-specific as confirmed by the lack of killing of PSCAÀ targets (K562-NT, 7% ± 4% specific lysis).…”

“…12 To assess transduction efficiency, we genetically modified activated T cells with the 4/7 ICR and measured expression of both the IL-4 receptor and mOrange. Figure 2B shows a representative example, with 56.2% of cells expressing both markers.…”

“…To generate secondgeneration CAR (2G), CD28 co-stimulatory endodomain was added to the 1G CAR construct between the G1-CH2CH3 and zeta domain. 12 …”

“…As a therapeutic alternative, we developed a PSCA-directed CAR and demonstrated that transgenic cells were able to efficiently and selectively eliminate antigen-expressing tumors in both in vitro and in vivo xenograft mouse models. 12 However, malignant cells comprise only 10%-40% of the tumor, 13,14 while the rest is a complex desmoplastic matrix consisting of extracellular proteins, stellate cells, and immunomodulatory cells that produce cytokines including interleukin-4 (IL-4), IL-10, and transforming growth factor b (TGF-b) that promote fibrogenesis, support tumor growth and protect malignant cells from immune destruction. [15][16][17][18][19][20] Thus, in order to provide clinical benefit to patients with PDAC, it is likely that the tumor-targeting T cells will require additional engineering to enable them to withstand this hostile environment.…”