Driving CARs on the uneven road of antigen heterogeneity in solid tumors

Abstract

Uniform and strong expression of CD19, a cell surface antigen, on cells of B-cell lineage is unique to hematologic malignancies. Tumor-associated antigen (TAA) targets in solid tumors exhibit heterogeneity with regards to intensity and distribution, posing a challenge for chimeric antigen receptor (CAR) T-cell therapy. Novel CAR designs, such as dual TAA-targeted CARs, tandem CARs, and switchable CARs, in conjunction with inhibitory CARs, are being investigated as means to overcome antigen heterogeneity. In addition to heterogeneity in cancer-cell antigen expression, the key determinants for antitumor responses are CAR expression levels and affinity in T cells. Herein, we review CAR T-cell therapy clinical trials for patients with lung or pancreatic cancers, and provide detailed translational strategies to overcome antigen heterogeneity.

Figure 1

(A) CAR structure: First generation CARs contain an antigen recognition domain fused with an intracellular activation domain. Second and third generation CARs integrate one or two co-stimulatory signals such as CD28, 4-1BB, or OX40. (B) Novel strategies to augment the antitumor efficacy of CARs: CAR T cells rendered resistant to immune checkpoint blockade by co-expression of a dominant negative receptor, armored CARs that secret antitumor-potentiating cytokines, tandem CARs that express two linked scFvs to recognize different antigens, and switchable CARs that recognize a tagged epitope on therapeutic antibodies binding to the cell surface antigen on cancer cells. Co-expression of two CARs enables T cells to simultaneously recognize two TAAs. iCARs (inhibitory CARs that inhibit T-cell activation) express an intracellular inhibitory domain that is fused with an extracellular scFv that recognizes a “safety antigen” expressed on normal cells. CAR, chimeric antigen receptor; scFv, single chain variable fragment; TAA, tumor-associated antigen

Figure 2

(A) Multiplex immunofluorescent staining of a human lung adenocarcinoma that demonstrates heterogeneous antigen expression of MSLN and MUC16 on tumor cells. (B) Addressing TAA heterogeneity in solid tumors: (1) Single TAA-targeted CAR T-cell therapy may result in antigen escape or the outgrowth of tumor cells that either express very low levels of TAA (below CAR T-cell activation threshold) or do not express the targeted TAA. Targeting two TAAs simultaneously, either by co-administration of CAR T cells targeting different antigens (2) or using a TanCAR (3), can mitigate tumor escape. A broad spectrum of TAAs can be targeted simultaneously with switchable CAR-transduced T cells (4). CAR, chimeric antigen receptor; MSLN, mesothelin; MUC16, mucin 16; TAA, tumor-associated antigen; TanCAR, tandem CAR