Inhibition of Lipopolysaccharide-Induced Macrophage IL-12 Production by Leishmania mexicana Amastigotes: The Role of Cysteine Peptidases and the NF-κB Signaling Pathway

Infection with lesion-derived Leishmania mexicana amastigotes inhibited LPS-induced IL-12 production by mouse bone marrow-derived macrophages. This effect was associated with expression of cysteine peptidase B (CPB) because amastigotes of CPB deletion mutants had limited ability to inhibit IL-12 production, whereas preincubation of cells with a CPB inhibitor, cathepsin inhibitor IV, was able to suppress the effect of wild-type amastigotes. Infection with wild-type amastigotes resulted in a time-dependent proteolytic degradation of IκBα and IκBβ and the related protein NF-κB. This effect did not occur with amastigotes of CPB deletion mutants or wild-type promastigotes, which do not express detectable CPB. NF-κB DNA binding was also inhibited by amastigote infection, although nuclear translocation of cleaved fragments of p65 NF-κB was still observed. Cysteine peptidase inhibitors prevented IκBα, IκBβ, and NF-κB degradation induced by amastigotes, and recombinant CPB2.8, an amastigote-specific isoenzyme of CPB, was shown to degrade GST-IκBα in vitro. LPS-mediated IκBα and IκBβ degradation was not affected by these inhibitors, confirming that the site of degradation of IκBα, IκBβ, and NF-κB by the amastigotes was not receptor-driven, proteosomal-mediated cleavage. Infection of bone marrow macrophages with amastigotes resulted in cleavage of JNK and ERK, but not p38 MAPK, whereas preincubation with a cysteine peptidase inhibitor prevented degradation of these proteins, but did not result in enhanced protein kinase activation. Collectively, our results suggest that the amastigote-specific cysteine peptidases of L. mexicana are central to the ability of the parasite to modulate signaling via NF-κB and consequently inhibit IL-12 production.