Abstract
Borrelia miyamotoi, a relapsing fever spirochete transmitted by Ixodid ticks causes B. miyamotoi disease (BMD). To evade the human host´s immune response, relapsing fever borreliae, including B. miyamotoi, produce distinct variable major proteins. Here, we investigated Vsp1, Vlp15/16, and Vlp18 all of which are currently being evaluated as antigens for the serodiagnosis of BMD. Comparative analyses identified Vlp15/16 but not Vsp1 and Vlp18 as a plasminogen-interacting protein of B. miyamotoi. Furthermore, Vlp15/16 bound plasminogen in a dose-dependent fashion with high affinity. Binding of plasminogen to Vlp15/16 was significantly inhibited by the lysine analog tranexamic acid suggesting that the protein–protein interaction is mediated by lysine residues. By contrast, ionic strength did not have an effect on binding of plasminogen to Vlp15/16. Of relevance, plasminogen bound to the borrelial protein cleaved the chromogenic substrate S-2251 upon conversion by urokinase-type plasminogen activator (uPa), demonstrating it retained its physiological activity. Interestingly, further analyses revealed a complement inhibitory activity of Vlp15/16 and Vlp18 on the alternative pathway by a Factor H-independent mechanism. More importantly, both borrelial proteins protect serum sensitive Borrelia garinii cells from complement-mediated lysis suggesting multiple roles of these two variable major proteins in immune evasion of B. miyamotoi.
Highlights
Borrelia miyamotoi, a relapsing fever spirochete transmitted by Ixodid ticks causes B. miyamotoi disease (BMD)
To detect binding of plasminogen, purified H is6-tagged borrelial proteins were immobilized on microtiter plates (5 μg/ml each) and the protein–protein interaction was assessed by using a polyclonal anti-plasminogen antibody
That Vlp15/16 displayed a strong affinity for plasminogen and that the nature of the interaction is partially mediated by lysine residues but not influenced by ionic strength
Summary
A relapsing fever spirochete transmitted by Ixodid ticks causes B. miyamotoi disease (BMD). Multiple episodes of recurrent high fever, the hallmark presentation of relapsing fever, are thought to be caused by the humoral immune responses to spirochete surface antigens[24], the decrease and resurgence of Borrelia populations, including B. miyamotoi, producing antigenically variant surface proteins[11,25,26,27]. This system consists of polymorphic genes encoding for immunodominant variable major proteins (Vmps), which are dispersed as silent promoter-less vmp gene cassettes on archival linear plasmids[25,26]. Owing to its broad substrate specificity, plasmin degrades many components of the extracellular matrix, matrix metalloproteases and complement c omponents[37]
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