Precision tumour targeting using costimulatory-only chimeric antigen receptors maintains efficacy without toxicity
Fisher. J, Abramowski. P, Don. N, Anderson. J
UCL Institute of Child Health, London, UK
On-target off-tumour toxicity constrains the development of immunotherapies targeting highly expressed tumour antigens, producing challenging and sometimes fatal toxicity. The strength of CD3ζ-containing chimeric antigen receptors expressed in αβT cells, having a “single switch” to induce antigen-specific killing, is also a fundamental limitation as it offers no differentiation between antigen displayed on tumour and antigen displayed on healthy tissue. This limits suitability of tumour-specific antigens in order to balance the needs of both safety and efficacy.
Vδ2+ γδT cells differentiate between healthy and transformed cells in an MHC-independent manner determined by differing expression of TCR ligands (phosphoantigen) on stressed versus healthy cells. Their native TCR is therefore not redundant in the tumour context and need not be replaced with an artificial construct. They can be easily expanded from peripheral blood or apheresis product to clinically useful numbers. We designed a series of chimeric antigen receptors containing only costimulatory motifs in the endodomain (Costimulation-only CARs) to augment Vδ2 γδT cell activity and bypass tumour immune-escape mechanisms.
Using GD2 and CD33 as model antigens, we demonstrate in both solid and haematological systems that γδT cells expressing a “Costimulation-only” CAR retain anti-tumour cytotoxicity but show no activity against cells which do not engage the γδTCR. Precise control of cytokine production is achievable with selective stimulus of CAR and/or TCR. Anti-tumour efficacy is maintained with no toxicity against healthy cells with equivalent antigen expression.
“Costimulation-only” CAR expressing γδT cells have a phenotype favourable for adoptive transfer with significantly lower exhaustion marker expression than cells expressing a conventional CD3ζ-CD28 CAR, and are capable of mounting a secondary expansion following antigen re-challenge.
This approach offers a means of safely recapitulating immunotherapy against tumour antigens previously rejected on grounds of on-target off-tumour toxicity.