It is very effective to use more sensitive engineered T cells to deal with tumor recurrence, such as acute lymphoblastic leukemia (ALL), which can usually provide a lasting response. However, although CAR T cells have high efficacy, such as those targeting CD19 in B cell malignant tumors, which may cause 70% to 90% of the lasting response, they do recur (Sterner and Sterner, 2021).
It is known that one process emphasizing disease recurrence is the reduction or complete loss of therapeutic targets in cancer cells after treatment with single antigen targeting CAR T cells. For example, CAR antigen escape occurs after treatment for CD19 or BCMA. After CAR T cell therapy targeting IL13Ra2, a similar phenomenon will appear in solid tumors (Sterner and Sterner, 2021).
CAR antigen escape may occur through a variety of mechanisms, including antigen mutation or alternative splicing, epitope masking, low antigen density and lineage switching. As Sadelain shared, several methods can be used to deal with the problem of reduced antigen density in tumors, including the use of dual specific CARs or multiple CARs with different signal transmission characteristics. Otherwise, the co expression of CAR assisting molecules (such as chimeric costimulatory receptors) may help to improve the low antigen recognition of T cells.
The mechanism leading to the escape of CAR antigen.
“Drug resistance mechanism related to antigen loss after treatment of CD19 CAR T cells. (A) CAR T cells bind to tumor related antigens and represent anti-tumor activity. (B) CAR T cells cannot bind to antigens due to CD19 CAR mutation or alternative splicing, so cancer cells may be resistant to treatment. (C) CAR T cells cannot attack cancer cells because CD19 CAR binds to CD19 antigen and subsequently covers CD19 epitopes. (D) The down regulated antigen density can prevent cancer cells from antagonizing CD19 CAR T cell therapy. (E) The surface antigen changes from the lymphatic lineage to the bone marrow lineage, thus preventing CD19 CAR T cells from binding to cancer cells. ” Retrieved from Song et al. without modification. two thousand and nineteen
Beyond the sensitivity of CAR T cells
Dr. Sadelain’s team compared the antigen sensitivity threshold of T cells modified by CRISPR/Cas9 based tools and AAV vector gene delivery engineering. Specifically, by targeting T cell receptors α T cells modified by constant chain (TRAC) loci produce TRAC CARs and newly developed HLA I independent T cell receptors (TRAC-HIT) or HIT T cells (Eyquem et al. 20217, Mansilla Soto et al. 2022). Both TRAC-CAR (CD3 independent) and TRAC-HIT (CD3 dependent) receptors redirect T-cell receptor antigen specificity and support HLA independent antigen binding.
The research team found that HIT T cells are more sensitive among T cells that detect CD19 through TRAC-HIT or TRAC-CAR T receptors, and can effectively detect and target cells that express low levels of CD19 in vitro, even as low as 20 CD19 molecules. In contrast, TRAC-CAR T cells have a higher threshold in detecting and inducing cell lysis, and cannot effectively target cells expressing less than 200 CD19 molecules.