T cell activation by recombinant FcepsilonRI gamma-chain immune receptors: an extracellular spacer domain impairs antigen-dependent T cell activation but not antigen recognition

T cells can be endowed with antigen specificity by grafting with a chimeric receptor consisting of an extracellular antigen binding moiety (scFv) derived from an antibody and an intracellular signaling domain. Conflicting data exist on the impact of an extracellular spacer domain between the antigen binding and the signaling domain with respect to cellular activation. Here, we recorded conjugate formation and antigen-driven cellular activation of T cells grafted with receptor molecules that contain the same antigen binding site (anti-CD30 HRS3-scFv) and signaling domain (FcepsilonRI gamma-chain), however, with and without an IgG1 CH2CH3 (Fc) spacer domain between the scFv and transmembrane moiety. Receptors of both configurations mediate equally efficient conjugate formation between receptor grafted T cells and antigen-positive target cells. Specific signaling by the spacer containing receptor, however, is blocked by five- to 10-fold lower concentrations of soluble antigen than by the spacer-less receptor indicating a higher avidity of the spacer containing receptor to soluble antigen. In contrast, cellular activation upon binding to antigen-positive cells is mediated more efficiently by the spacer-less receptor. This demonstrates that the extracellular spacer domain impairs antigen-dependent cellular activation by the chimeric immune receptor, but not intercellular conjugate formation.