Immunomodulatory peptide from cystatin, a natural cysteine protease inhibitor, against leishmaniasis as a model macrophage disease.

Cystatin, a natural cysteine protease inhibitor, has strong antileishmanial activity, which is due to its potential to induce nitric oxide (NO) generation from macrophages. Cysteine protease-inhibitory activity and NO-up-regulatory activity correspond to different regions, as revealed by the dissection of cystatin cDNA into nonoverlapping fragments. By using synthetic overlapping peptides, the NO-up-regulatory activity was found to be confined to a 10-mer sequence. In addition to having reasonable inhibitory effects on amastigote multiplication within macrophages (50% inhibitory concentration, 5.2 microg/ml), 97 and 93% suppression, respectively, of liver and spleen parasite burdens was achieved with the 10-mer peptide at a dose of 0.5 mg/kg of body weight/day, given consecutively for 4 days along with a suboptimal dose of gamma interferon in a 45-day mouse model of visceral leishmaniasis. Peptide treatment modulated the levels of cytokine secretion by infected splenocytes, with increased levels of interleukin-12 and tumor necrosis factor alpha and increased inducible NO synthase production, and also resulted in resistance to reinfection. The generation of a natural peptide from cystatin with robust immunomodulatory potential may therefore provide a promising therapeutic agent for macrophage-associated diseases.