Amblyomma americanum serpin 41 (AAS41) inhibits inflammation by targeting chymase and chymotrypsin

Ticks inject serine protease inhibitors (serpins) into their feeding sites to evade serine protease-mediated host defenses against tick-feeding. This study describes two highly identitical (97%) but functionally different Amblyomma americanum tick saliva serpins (AAS41 and 46) that are secreted at the inception of tick-feeding. We show that AAS41, which encodes a leucine at the P1 site inhibits inflammation system proteases: chymase (SI = 3.23, Ka = 5.6 ± 3.7X103M−1 s−1) and α-chymotrypsin (SI = 3.18, Ka = 1.6 ± 4.1X104M−1 s−1), while AAS46, which encodes threonine has no inhibitory activity. Similary, rAAS41 inhibits rMCP-1 purified from rat peritonuem derived mast cells. Consistently, rAAS41 inhibits chymase-mediated inflammation induced by compound 48/80 in rat paw edema and vascular permeability models. Native AAS41/46 proteins are among tick saliva immunogens that provoke anti-tick immunity in repeatedly infested animals as revealed by specific reactivity with tick immune sera. Of significance, native AAS41/46 play critical tick-feeding functions in that RNAi-mediated silencing caused ticks to ingest significantly less blood. Importantly, monospecific antibodies to rAAS41 blocked inhibitory functions of rAAS41, suggesting potential for design of vaccine antigens that provokes immunity to neutralize functions of this protein at the tick-feeding site. We discuss our findings with reference to tick-feeding physiology and discovery of effective tick vaccine antigens. •Amblyomma americanum secretes two highly identical (97% identity) serpins (AAS 41 and 46) that have different functions•Amblyomma americanum utilizes AAS41 to block host inflammation defense by disrupting chymase and chymotrypsin functions•Functions of AAS 41 and 46 are critical to tick feeding success as revealed by RNAi mediated silencing•Both AAS 41 and 46 are among immunogens that provoke anti-tick immunity in repeatedly infested rabbits.