Elimination of CAR-engineered lymphocytes by autologous anti-CAR T cells

The adoptive transfer of chimeric antigen receptor (CAR) T cells represents one of the fastest growing areas in cancer immunotherapy. Although gene-modified cells have shown unparalleled antitumor efficiency in patients suffering from hematological malignancies, highly potent CAR T cells can cause severe and partly life-threatening on-target and off-target effects including of cytokine release syndrome and neurological toxicity. Consequently, there is an increasing demand for developing effective strategies to selectively eliminate gene-modified cells in vivo.
One possible approach represents the insertion of a targetable moiety into CAR T cells. In that regard, we previously characterized a small peptide epitope (E-tag) derived from the human nuclear La protein which we incorporated into the extracellular spacer region of CARs. Based on a monoclonal anti-La antibody recognizing this epitope, we generated a CAR construct for specific binding and depletion of E-tag-labelled CAR T cells.
In flow cytometry-based cytotoxicity assays, T cells redirected via the novel CAR construct selectively eliminated E-tag-expressing CAR T cells whilst cells lacking this epitope on their surface were not attacked. Interestingly, T cell killing was reciprocal and dependent on an intracellular signaling domain as well as the effector to target cell ratio. Our studies further indicate that T cells expressing high CAR levels were more efficiently depleted than T cells with low CAR expression. In addition, CD4+ and CD8+ target cells were equally well eliminated by both CD4+ and CD8+ effector T cells.
Altogether, we here provide an approach for specific and efficient depletion of overactive CAR T cells in case patients experience severe side effects. The E-tag can easily be included into all CARs irrespective of the targeted tumor antigen and represents a promising tool to improve safety of cell-based immunotherapies.