Helminth Cysteine Proteases Inhibit TRIF-dependent Activation of Macrophages via Degradation of TLR3

Sheila Donnelly,Sandra M O’Neill,Colin M Stack,Mark W Robinson,Lynne Turnbull,Cynthia Whitchurch,John P Dalton

doi:10.1074/jbc.m109.060368

Abstract

Helminth pathogens prepare a Th2 type immunological environment in their hosts to ensure their longevity. They achieve this by secreting molecules that not only actively drive type 2 responses but also suppress type 1 responses. Here, we show that the major cysteine proteases secreted from the helminth pathogens Fasciola hepatica (FheCL1) and Schistosoma mansoni (SmCB1) protect mice from the lethal effects of lipopolysaccharide by preventing the release of inflammatory mediators, nitric oxide, interleukin-6, tumor necrosis factor alpha, and interleukin-12, from macrophages. The proteases specifically block the MyD88-independent TRIF-dependent signaling pathway of Toll-like receptor (TLR)4 and TLR3. Microscopical and flow cytometric studies, however, show that alteration of macrophage function by cysteine protease is not mediated by cleavage of components of the TLR4 complex on the cell surface but occurs by degradation of TLR3 within the endosome. This is the first study to describe a parasite molecule that degrades this receptor and pinpoints a novel mechanism by which helminth parasites modulate the innate immune responses of their hosts to suppress the development of Th1 responses.

Highlights

To establish successful chronic infections, helminth parasites induce potent Th2 immune responses, which can only occur when Th1 cytokines are suppressed [1]
We have previously shown that the rapid suppression of inter- showed that treatment of mice with the major secreted cathepferon-␥-associated inflammatory responses during infection sin B1 (SmB1) protease of the helminth S. mansoni was with F. hepatica is related to the secretion of FheCL1 cysteine capable of suppressing macrophage proinflammatory cytokine proteases [14]
We found that FheCL1-treated macrophages secreted significantly lower levels of IL-12, TNF␣, and nitrite when exposed to poly(I1⁄7C) for 12 h compared with macrophages taken from phosphatebuffered saline (PBS)-treated or FheCL1Gly26-treated controls (Fig. 4A and data not shown)

Summary

To whom correspondence should be addressed

Enzyme activity ascribable to the papain family of cysteine proteases (clan CA, family C1), such as cathepsin L and B, has been identified as a major component of the secretions of almost all helminth parasites of humans, livestock, and companion animals [7, 8]. These proteases are pivotal to the parasitic lifestyle by mediating fundamental processes such as host invasion, acquisition of nutrients, and the migration through host tissues (reviewed in Ref. 9). This study, describes a novel means by which parasites alter innate immune cell function and prevent the establishment of potent Th1-driven inflammatory responses that would lead to their elimination

EXPERIMENTAL PROCEDURES

RESULTS

Findings

DISCUSSION