Lufaxin, a protein found in the saliva of the blood-feeding sand fly, may simultaneously inhibit both the complement and coagulation cascades and serve as a model for the development of bispecific therapeutics to treat complement-related diseases in which thrombosis is a prominent manifestation, such as such as paroxysmal nocturnal hemoglobinuria (PNH). The findings are published in the journal Blood.

Researchers performed surface plasmon resonance experiments and cryogenic electron microscopy to describe the structure and mechanism of the potent complement and coagulation inhibitor.

According to the study results, lufaxin was found to have a crystal structure with a novel all β-sheet fold containing 2 domains. The inhibitor also demonstrated its ability to stabilize the binding of the serine protease factor B to C3b, yet it did not interact with either C3b or serine protease factor B alone.

It did, however, bind via its N-terminal domain at an interface containing elements of both C3b and FB. The occupation of this spot ensures the closed conformation of the serine protease factor B and prevents its proteolytic activation by the protease factor D.

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“The structure of lufaxin is not like that of other naturally occurring proteinaceous fXa inhibitors including various serpins and the tick anticoagulant peptide,” Andersen and colleagues wrote.

Furthermore, lufaxin could bind with fXa at a separate site in its C-terminal domain while being bound with C3bB. The cryogenic electron microscopy structure of a C3bB-lufaxin-fXa complex confirmed that the protein can bind both targets at the same time as well as inhibit the activity of fXa through “substrate-like binding of a C-terminal peptide at the active site as well as other interactions in this region.”

“The co-occurrence of complement and fibrin deposition in many complement-related disorders, suggests that concomitant inhibition of complement and coagulation may be an effective avenue for treatment,” the study authors noted.

“The two pathways are activated reciprocally in that tissue damage caused by the [membrane attack complex (MAC)] results in the generation of procoagulant surfaces and subsequently activated coagulation proteases are capable of activating complement in a non-canonical manner.”

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Many therapeutic inhibitors target the C3bB molecule because it is the precursor of C3bBb, the central convertase of the alternative pathway of complement activation. This blockage prevents the accumulation of C3b and anaphylatoxin C3a as well as the creation of intermediates such as anaphylatoxin C5a and components of the MAC.


Andersen JF, Lei H, Strayer EC, et al. A bispecific inhibitor of complement and coagulation blocks activation in complementopathy models via a novel mechanism. Blood. Published online March 22, 2023. doi:10.1182/blood.2022019359