Dissecting molecular mechanisms underlying T cell co-potentiation when targeting the TCR/CD3 complex with anti-CD3 Fab fragments

Abstract

Abstract A cognate antigen engagement of TCR induces a conformational change in the CD3 complex (CD3Δc) that is required for productive T cell activation. CD3Δc fails when TCRs interact with poorly immunogenic antigens that do not active T cells. Anti-CD3 monovalent Fab fragments bound to the CD3 complex mimic CD3Δc, producing T cell “co-potentiation” when associated with TCRs bound to weak antigens that enhances T cell activation, neither stimulating non-engaged T cells, nor interrupting T cell responses to strong antigens. Previously, we showed T cell co-potentiation can be exploited as a novel immunotherapeutic principle by reducing melanoma burden in B6 mice treated with anti-CD3 Fabs. TCR signaling initiation requires CD3Δc for the exposure of a conserved sequence in the cytosolic domain of CD3ɛ rich in prolines (PRS) that enables the recruitment of adaptor protein Nck. The mutation of cysteines in the CXXC motif at the extracellular domain of CD3ɛ inhibits the conformational change across the plasma membrane that results on PRS accessibility. Here, using two strains of B6 mice that express mutations in either the CXXC or the PRS motifs in CD3ɛ, we study the potential contribution of each motif in T cell co-potentiation. Our results from these studies favor the involvement of the CXXC and PRS motifs in the molecular mechanism supporting the co-potentiation of T cell activation by anti-CD3 Fab fragments. Supported by MU Start-up funds and NIH grants: NCI, U01 CA244314; NIAID, R01 AI097187