Heterodimeric complex formation with CD8 and TCR by bispecific antibody sustains paracrine IL-2-dependent growth of CD3⁺ CD8⁺ T cells

During physiologic activation of mature CD8⁺ T cells, TCR and CD8 bind to the same Ag-complexed MHC class I molecule. Thereby, close proximity is induced between CD8 and the TCR/CD3 complex. During this engagement, CD8 may deliver TCR-independent signals via its associated protein tyrosine kinase, p56^lck. We studied the potential biologic effects of close association between CD8 and TCR/ CD3 complexes by using a bispecific antibody (bsAb) directed against both TCR and CD8 molecules. This hybrid hybridoma (quadroma)-produced bsAb binds as a monomeric molecule to CD3⁺ CD8⁺ but not CD3⁺ CD4⁺ T cells. The bsAb proved capable of inducing the cytotoxic effector function of cloned CD3⁺ CD8⁺ T cells but not of CD3⁺ CD4⁺ T cells. When the bsAb was presented to resting T cells by monocytes, proliferation of the CD3⁺ CD4⁺ but not the CD3⁺ CD8⁺ subset of T lymphocytes was induced. Parental anti-TCR antibody induced vigorous growth of cells of both subsets. Essentially identical results were obtained when bsAb was presented in an immobilized fashion. The unresponsiveness of the CD3⁺ CD8⁺ T cells with respect to mitogenesis could be restored by exogenous rIL-2. The data suggest that bsAb-induced activation differs from activation by monospecific anti-TCR antibody. The former appears to more closely mimic physiologic Ag-induced signaling, because it leads to a similar paracrine IL-2-dependent growth pattern. The bsAb may, therefore, be instrumental in studying T cell signaling pathways, in particular the role of CD8-associated p56^lck therein.