CD28 cosignalling does not affect the activation threshold in a chimeric antigen receptor-redirected T-cell attack.

摘要

Adoptive immunotherapy of cancer using chimeric antigen receptor (CAR)-engineered T cells with redirected specificity showed efficacy in recent trials. In preclinical models, 'second-generation' CARs with CD28 costimulatory domain in addition to CD3zeta performed superior in redirecting T-cell effector functions and survival. Whereas CD28 costimulation sustains physiological T-cell receptor (TCR)-CD3 activation of naive T cells, the impact of CD28 cosignalling on the threshold of CAR-mediated activation of pre-stimulated T cells without B7-CD28 recruitment remained unclear. Using CARs of different binding affinities, but same epitope specificity, we demonstrate that CD28 cosignalling neither lowered the antigen threshold nor the binding affinity for redirected T-cell activation. 'Affinity ceiling' above which increase in affinity does not increase T-cell activation was not altered. Accordingly, redirected tumor cell killing depended on the binding affinity but was likewise effective for CD3zeta and CD28-CD3zeta CARs. In contrast to CD3zeta, CD28-CD3zeta CAR-driven activation was not increased further by CD28-B7 engagement. However, CD28 cosignalling, which is required for interleukin-2 induction could not be replaced by high-affinity CD3zeta CAR binding or high-density antigen engagement. We conclude that CD28 CAR cosignalling does not alter the activation threshold but redirects T-cell effector functions.