Leishmania mexicana promastigotes inhibit macrophage IL-12 production via TLR-4 dependent COX-2, iNOS and arginase-1 expression

The effects of Leishmania mexicana metacyclic promastigotes upon MAP kinase signalling in mouse bone marrow macrophages and subsequent expression of the disease regulatory proteins iNOS and COX-2 were studied. At a ratio of 5:1, promastigotes caused a marked increase in phosphorylation of the three major MAP kinases, ERK, p38 and JNK. MAP kinase signalling was substantially reduced in TLR-4−/− but not TLR-2−/− deficient macrophages and completely abolished in double TLR-2/4−/− macrophages. A similar outcome was observed using cysteine peptidase B deficient amastigotes. Furthermore, whilst promastigotes had no independent effect on iNOS or COX-2 expression, they prolonged the induction of these proteins stimulated by LPS and enhanced PGE2 and NO production. Induction of COX-2 and iNOS was also TLR-4 dependent. Blockade of either PGE2 or NO production with indomethacin or l-NAME reversed promastigote inhibition of LPS induced IL-12 production. Promastigotes also increased macrophage arginase-1 expression and enhanced arginase activity, both of which were substantially reduced in TLR-4 but not TLR-2 deficient macrophages. Surprisingly, arginase inhibition by Nor-NOHA also caused a reversal of promastigote mediated inhibition of macrophage IL-12 production. These data demonstrate for the first time the role of TLR-4 in mediating the effects of L. mexicana promastigotes on MAP kinase activation, up-regulation of COX-2, iNOS as well as arginase-1 expression in macrophages and further shows that PGE2, NO and arginase activity all contribute substantially to the inhibition of host cell IL-12 production.

► TLR-4 activation by Leishmania mexicana promastigotes and CPB-deficient amastigotes.
► Prolonged, TLR-4 dependent iNOS and COX-2 expression by L. mexicana promastigotes.
► Enhanced TLR-4 dependent-arginase-1 expression.
► Regulation of IL-12 induction by an arginase-1 dependent mechanism.