Mast Cells and Basophils in the Defense against Ectoparasites: Efficient Degradation of Parasite Anticoagulants by the Connective Tissue Mast Cell Chymases

Abstract

Ticks, lice, flees, mosquitos, leeches and vampire bats need to prevent the host’s blood coagulation during their feeding process. This is primarily achieved by injecting potent anticoagulant proteins. Basophils frequently accumulate at the site of tick feeding. However, this occurs only after the second encounter with the parasite involving an adaptive immune response and IgE. To study the potential role of basophils and mast cells in the defense against ticks and other ectoparasites, we produced anticoagulant proteins from three blood-feeding animals; tick, mosquito, and leech. We tested these anticoagulant proteins for their sensitivity to inactivation by a panel of hematopoietic serine proteases. The majority of the connective tissue mast cell proteases tested, originating from humans, dogs, rats, hamsters, and opossums, efficiently cleaved these anticoagulant proteins. Interestingly, the mucosal mast cell proteases that contain closely similar cleavage specificity, had little effect on these anticoagulant proteins. Ticks have been shown to produce serpins, serine protease inhibitors, upon a blood meal that efficiently inhibit the human mast cell chymase and cathepsin G, indicating that ticks have developed a strategy to inactivate these proteases. We show here that one of these tick serpins (IRS-2) shows broad activity against the majority of the mast cell chymotryptic enzymes and the neutrophil proteases from human to opossum. However, it had no effect on the mast cell tryptases or the basophil specific protease mMCP-8. The production of anticoagulants, proteases and anti-proteases by the parasite and the host presents a fascinating example of an arms race between the blood-feeding animals and the mammalian immune system with an apparent and potent role of the connective tissue mast cell chymases in the host defense.

Mast Cell and Basophil Granule Proteases - In Vivo Targets and Function

“…Almost an identical pattern of cleavage was seen for these two anticoagulants indicating that the connective tissue MC proteases cleave the anticoagulants of a broad array of different ectoparasites ( 92 ). Interestingly, the mucosal MC proteases were in general inactive against these anticoagulant proteins indicating a high selectivity of these proteases in their target specificity ( 92 ). Connective tissue MCs are primarily located in the skin which is the primary site of infection for these ectoparasites, indicating a functional adaptation of the cleavage specificity to targets of importance for the particular type and tissue location of the MC.…”

“…We tested their activity on the thrombin inhibitors hirudin from Hirudum medicinalis and on anophelin from the malaria vector Anopheles gambiae. Almost an identical pattern of cleavage was seen for these two anticoagulants indicating that the connective tissue MC proteases cleave the anticoagulants of a broad array of different ectoparasites (92). Interestingly, the mucosal MC proteases were in general inactive against these anticoagulant proteins indicating a high selectivity of these proteases in their target specificity (92).…”

“…In contrast to our expectations, we found that mMCP-8 had no effect on this tick anticoagulants. However, all the connective tissue MC chymases tested, including human-, dog-, rat-, hamster- and opossum chymases did very efficiently degrade this anticoagulant protein, indicating a potent role of the MC chymases in the defense against this ectoparasite ( 92 ). We then continued this analysis by analyzing the role of this panel of hematopoietic serine proteases on anticoagulants from two other ectoparasites, leeches and mosquitos.…”

Serpins in Tick Physiology and Tick-Host Interaction

“…Thus, RmS-3 may be a key component in modulating the early steps of inflammatory reactions by blocking the chymase which is generated during mast cell activation ( Coutinho et al., 2020 ). Chymase also appears to be crucial for the degradation of tick anticoagulants, so its inhibition should help the tick to maintain blood fluidity ( Fu et al., 2021 ). A recent study showed that the serpin IRS-2 of I. ricinus can inactivate almost all connective tissue chymases from a range of animals, including human, hamster, rat, dog, and opossum, as well as mucosal mast cell proteases, rat blood vessel chymases, and also neutrophil proteases.…”

“…A recent study showed that the serpin IRS-2 of I. ricinus can inactivate almost all connective tissue chymases from a range of animals, including human, hamster, rat, dog, and opossum, as well as mucosal mast cell proteases, rat blood vessel chymases, and also neutrophil proteases. However, this serpin fails to inactivate mast cell tryptases and the basophil-specific protease mMCP-8 ( Fu et al., 2021 ). The first study of the tick serpin IRS-2 disclosed the protein as having a preferential specificity for chymase and cathepsin G and as having a significant anti-inflammatory effect in vivo by reducing swelling and neutrophil migration into inflamed tissues, while a later study showed that IRS-2 reduced spirochete Borrelia burgdorferi -induced IL-6 production in splenic dendritic cells ( Chmelar et al., 2011 ; Páleníková et al., 2015 ).…”