Cystatins of filarial nematodes up‐regulate the nitric oxide production of interferon‐γ‐activated murine macrophages

Summary Cystatins of two filarial nematodes were studied with regard to their capacity to up‐regulate the production of nitric oxide (NO) in vitro, and the effects were analysed. Recombinant cystatin of the human pathogenic filaria Onchocerca volvulus and of the rodent filaria Acanthocheilonema viteae significantly enhanced the NO production of interferon (IFN)‐γ‐activated macrophages of BALB/c and C3H/HeJ mice. Truncated cystatins lacking the N‐terminal protease inhibitory active site, and showing marginal protease inhibitory activity, up‐regulated the NO production to the same extent as the full‐length proteins, indicating that the effect on the NO production is independent of cysteine protease inhibition. NO did not contribute to the suppression of proliferative T cell responses exerted by filarial cystatins, as shown in other studies, since NO synthase inhibitors did not restore proliferative responses. The up‐regulation of NO production induced by filarial cystatins was partly dependent on the production of interleukin‐10 and tumour necrosis factor‐α, since depletion of both cytokines by antibodies led to a diminution of the enhanced NO production by 22–48%. Our data suggest that filarial cystatins are potent triggers of the production of NO, a mediator which was shown to have a role as an effector molecule against filarial worms in vitro and in vivo.