During chronic
filariasis, parasite-specific cellular responsiveness is profoundly down-regulated.
Cystatins, a group of
cysteine protease inhibitors, have been implicated in this suppressive activity. In an attempt to investigate the effects of
cystatins in vivo, we isolated and expressed a 14 kDa
protein of the rodent filaria Litomosoides sigmodontis with substantial homologies to
cystatins from human pathogenic filariae.
Cystatin was detected in
antigen preparations of several developmental stages of L. sigmodontis, as well as in the supernatants of in vitro cultured adult worms. On closer examination, L. sigmodontis
cystatin (Ls-
Cystatin) migrated as two separate bands at 14 and 15 kDa. When
cystatin was introduced into the peritoneal cavity of C57BL/6 mice via micro-osmotic pumps, the production of
nitric oxide was profoundly reduced upon microfilarial challenge and, at the same time, synthesis of
TNF-alpha mRNA became up-regulated. Furthermore,
antigen-specific proliferative response of spleen cells to circulating L. sigmodontis microfilariae was significantly diminished in the presence of
cystatin, whereas the antibody production was not suppressed. In vaccination trials, using the L. sigmodontis/BALB/c mouse model of
filariasis, L. sigmodontis
cystatin did not generate protective effects in terms of adult worm recovery, however, lower numbers of patent
infections, i.e. less
infections with microfilaraemia were observed in vaccinated animals. These results suggested that
cystatin acts as an immunomodulatory molecule during the course of a filarial
infection, and its neutralisation might contribute to generate protective immune responses.