Cloned L3T4⁺ T lymphocytes protect mice against Listeria monocytogenes by secreting IFN-γ

Murine T lymphocyte clones sensitized to Listeria monocytogenes were developed to investigate specific mechanisms of T cell-mediated immunity. The clones were of the Thy-1.2⁺, L3T4⁺, Lyt-2^- phenotype and proliferated in a dose response fashion to heat-killed Listeria. Cloned T lymphocytes injected intravenously protected nonimmune mice against L. monocytogenes challenge as determined by spleen and liver bacterial numbers. Supernatants, produced by stimulating clones with heat-killed Listeria for 48 h, also afforded protection against L. monocytogenes. The clonal supernatants contained significant quantities of IFN-γ and CSF. IFN-γ production was Ag specific and occurred within 24 h of stimulation. CSF production by clones was increased four- to sixfold over baseline as determined by a bone marrow colony-forming assay and was Ag specific. When the IFN-γ in supernatants was neutralized with a specific mAb, protection afforded by the supernatants was lost. These data indicate that one mechanism for Ag-specific, T lymphocyte-mediated protection against L. monocytogenes is the release of IFN-γ.