‘Ignorance’ of Antigen‐Specific Murine CD4 + and CD8 + T Cells is Overruled by Lipopolysaccharide and Leads to Specific Induction of IFN‐γ *

Lipopolysaccharide (LPS) can activate human and murine T cells in vivo and in vitro . Here we analysed the effects of LPS on T cells with defined specificities in T‐cell receptor (TCR)‐transgenic systems. LPS rapidly induced high amounts of interferon (IFN)‐γ in a subpopulation of purified T cells from DO11.10 (OVA 323–339 /H2‐A d ) and OT‐1 (OVA 257–264 /H2‐K b ) mice when coincubated with antigen‐pulsed peritoneal exudate cells (PECs). LPS induced IFN‐γ in T cell cultures even when the number of antigenic major histocompatibility complex (MHC) class‐I complexes was too small to stimulate the T cells. LPS, thus, overruled the unresponsiveness of the otherwise ‘antigen‐ignorant’ T cells. The release of IFN‐γ strictly correlates with the PECs' ability to produce interleukin (IL)‐12. In contrast to the induction of IFN‐γ, antigen‐specific IL‐2 secretion and proliferation of T cells were rather decreased in the presence of LPS. Only very few IFN‐γ‐secreting natural killer (NK) cells and natural killer T (NKT) cells in the given experimental system could be detected using intracellular fluorescence‐activated cell sorter (FACS) staining. Taken together, our results indicate that LPS has the potential to activate quiescent T cells and to specifically induce IFN‐γ in CD4 and CD8 T cells. This may have direct consequences for the activation of autoreactive T cells following bacterial infections.