Diesel Exhaust Particles Suppress In Vivo IFN-γ Production by Inhibiting Cytokine Effects on NK and NKT Cells

Diesel exhaust particles (DEP) have strong, selective Th2 adjuvant activity when inhaled with conventional Ags. We used a novel technique for measuring in vivo cytokine production to investigate possible mechanisms by which DEP might promote a Th2 response. Injection of DEP i.p. stimulated IL-6 secretion, but failed to increase IL-4, IL-10, or TNF-α secretion, and decreased basal levels of IFN-γ. When injected with or before LPS, DEP had little effect on the LPS-induced TNF-α responses, but partially inhibited the LPS-induced IL-10 response and strongly inhibited the LPS-induced IFN-γ response. DEP also inhibited the IFN-γ responses to IL-12, IL-12 plus IL-18, IL-2, and poly(I · C). DEP treatment had little effect on the percentages of NK and NKT cells in the spleen, but inhibited LPS-induced IFN-γ production by splenic NK and NKT cells. In contrast, DEP failed to inhibit the IFN-γ response by anti-CD3 mAb-activated NKT cells. Taken together, these observations suggest that DEP inhibit Toll-like receptor ligand-induced IFN-γ responses by interfering with cytokine signaling pathways that stimulate NK and NKT cells to produce IFN-γ. Our observations also suggest that DEP may promote a Th2 response by stimulating production of inflammatory cytokines while simultaneously inhibiting production of IFN-γ, and raise the possibility that the same mechanisms contribute to the association between DEP exposure and asthma.