A previously unrecognized large fraction of cytotoxic lymphocyte precursors is present in CD4 + human peripheral blood T cells

We describe a limiting dilution (LD) culture system in which cell sorter-purified CD4 + (and CD8 +) peripheral blood T cells are cocultured with irradiated, anti-CD3 mab-producing OKT3 hybridoma cells. Under these conditions, one out of 2–3 CD4 + (and CD8 +) T cells is induced to clonal proliferation. In striking contrast to previously described LD culture systems, every growing CD4 + cell clone displayed cytotoxic activity when tested in a lectin-facilitated 51Cr release assay against P815 target cells. This contrasts with the development of cytotoxic CD4 + T cells in alloantigen-stimulated LD cultures, where only one out of 15–20 proliferating CD4 + cells killed P815 in the presence of PHA, and one out of 300–500 proliferating CD4 + cells displayed alloantigen-specific cytotoxic activity. Furthermore, we have established antigen-specific proliferating CD4 + T cell clones which do not exert antigen-specific cytotoxicity but can be cytotoxic when crosslinked to target cells via lectin or monoclonal antibody (anti-CD3, anti-TCR). Our results show that a previously unrecognized large fraction (at least 30–50%) of all peripheral blood CD4 + T cells can give rise to cytotoxic effector cells. The mode of CD4 + T cell activation (OKT3 hybridoma versus alloantigen) thus determines whether the intrinsic cytotoxic capacity of CD4 + T cells is functionally activated or not.