Lipid A directly inhibits IL‐4 production by murine Th2 cells but does not inhibit IFN‐γ production by Th1 cells

Lipopolysaccharide (LPS) is known to be an immunopotentiator but its effect on cytokine production by Th1 and Th2 cells is unknown. We found that high amounts of LPS, its lipid A moiety, and a lipid A analog all induced a decrease in IL‐4 production and an increase in IFN‐γ production when given to keyhole limpet hemocyanin (KLH)‐restimulated lymph node cells prepared from KLH‐primed mice. Lipid A was similarly found to inhibit IL‐4 production by purified CD4+ T cells and Th2 clones activated with immobilized anti‐CD3ϵ and anti‐CD28 antibodies, suggesting that the inhibition is not indirectly mediated through effects on antigen‐presenting cells. No inhibitory effect of lipid A was observed on IFN‐γ production by a Th1 clone. Production of both IL‐4 by the Th2 clones and IFN‐γ by the Th1 clone were inhibited by the immunosuppressive agent cyclosporin A. These findings indicate that lipid A can directly inhibit IL‐4 production by CD4+ T cells without inhibiting the production of IFN‐γ. Lipid A may therefore become a useful tool to study the intracellular events that differentiate Th1 and Th2 cells.