Leptospirosis is a zoonotic disease of global importance. It is caused by a Gram-negative bacterial spirochete of the genus Leptospira .

Leptospira, a zoonotic pathogen, is known to infect various hosts and can establish persistent infection. This remarkable ability of bacteria is attributed to its potential to modulate (activate or evade) the host immune response by exploiting its surface proteins. We have identified and characterized the domain of the variable region of Leptospira immunoglobulin-like protein A (LAV) involved in immune modulation. The 11th domain (A11) of the variable region of LigA (LAV) induces a strong TLR4 dependent innate response leading to subsequent induction of humoral and cellular immune responses in mice. A11 is also involved in acquiring complement regulator FH and binds to host protease Plasminogen (PLG), there by mediating functional activity to escape from complement-mediated killing. The deletion of A11 domain significantly impaired TLR4 signaling and subsequent reduction in the innate and adaptive immune response. It also inhibited the binding of FH and PLG thereby mediating killing of bacteria. Our study discovered an unprecedented role of LAV as a nuclease capable of degrading Neutrophil Extracellular Traps (NETs). This nuclease activity was primarily mediated by A11. These results highlighted the moonlighting function of LigA and demonstrated that a single domain of a surface protein is involved in modulating the host innate immune defenses, which might allow the persistence of Leptospira in different hosts for a long term without clearance.

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Leptospira, a zoonotic pathogen is known to infect a variety of hosts and capable of establishing persistent infection. This remarkable ability of bacteria is attributed to its potential to evade or modulate the host immune response by exploiting its surface proteins. We have identified and characterized the domain of Leptospira immunoglobulin-like protein A (LigA) that is involved in modulating the host innate immune response. We identified that the 11th domain (A11) of the variable region of LigA (LAV) induces strong TLR4 dependent innate response in mouse macrophages via signalling through MAP kinase pathway leading to the production of pro-inflammatory cytokines (IL-6 and TNF-α) and expression of costimulatory molecules (CD80, CD86, CD40) and maturation marker (MHC-II). A11 is also involved in acquiring complement regulators like FH, C4b binding protein and Plasminogen and mediating functional activity to escape from both classical and alternate pathways of complement-mediated killing. The deletion of A11 significantly impaired TLR4 signalling and subsequent activation of innate immune cells and also inhibited the binding of complement regulators leading to the killing of bacteria. Our study discovered an unprecedented role of LAV as nuclease capable of degrading Neutrophil Extracellular Traps (NETs). This nuclease activity was mediated by A11 and was inhibited with anti-LAV antibodies. These results highlight the moonlighting function of LigA and demonstrates that a single domain of a surface protein is involved in evading a myriad of host innate immune defences, which might allow the persistence of Leptospira in different hosts for a long term without clearance.