Addition of the C-terminus of CD6 to a chimeric antigen receptor enhances cytotoxicity and does not compromise expression.

T cells expressing chimeric antigen receptors (CARs) are a promising new cancer immunotherapy that has now reached the clinic. CARs are synthetic receptors that redirect T cells towards a tumour-associated antigen and activate them through various fused signalling regions, for example derived from CD3ζ, 4-1BB or CD28. Analysis of the optimal combination of CAR components including signalling domains is not yet comprehensive and may vary with the particular application. The C-terminus of the T-cell surface receptor CD6 is critical for its co-stimulatory effects and signals through two phospho-tyrosine motifs that bind to the intracellular adaptor proteins GADS and SLP-76. Addition of the C terminus of CD6 did not compromise CAR expression, showing it was a stable moiety that can be used independently of the native receptor. A third-generation CAR containing 4-1BB, CD3ζ and the C terminus of CD6 (4-1BBz-CD6) enhanced interferon-γ release and cytotoxicity when compared with the second-generation 4-1BB CD3ζ (4-1BBz) CAR. The CD6 C terminus is a valuable addition to potential components for modular design of CARs to improve effector function, particularly cytotoxicity.