Fasciola hepatica cathepsin L cysteine proteinase suppresses Bordetella pertussis-specific interferon-γ production in vivo

We have previously demonstrated that Fasciola hepatica infection significantly reduced Bordetella pertussis-specific interferon (IFN)-γ production in mice coinfected with B. pertussis or immunized with a pertussis whole cell vaccine (Pw). In the present study, we have identified parasite molecules capable of mimicking this suppressive effect of F. hepatica. Parenteral injection of mice with culture medium in which adult F. hepatica were maintained (excretory/secretory, ES, products) suppressed B. pertussis-specific IFN-γ production in mice immunized with Pw. The suppressive effect of ES was abrogated by coinjecting ES with the cysteine proteinase inhibitor, Z-Phe-Ala-diazo-methylketone. Furthermore, purified cathepsin L proteinase (FheCL), a major component of ES products, was capable of suppressing IFN-γ production. The suppressive effect of FheCL was attenuated in interleukin (IL)-4 defective (IL-4^-/-) mice. Therefore, FheCL released by F. hepatica is involved in the suppression of Th1 immune responses and this suppression may be dependent upon IL-4.