The lipoprotein LipL32, an enigma of leptospiral biology

Spirochetes of the genus Leptospira are responsible for the widespread, but neglected, zoonotic disease leptospirosis. In recent years there have been major advances in the understanding of leptospiral biology and the molecular basis of pathogenesis, yet the protein LipL32 remains an enigma of leptospiral biology. LipL32 (also known as Hap1) is an outer membrane, surface exposed lipoprotein and is the most abundant protein in the cell. LipL32 is found exclusively in pathogenic leptospires, is highly conserved, expressed in vivo and is highly immunogenic. These features make this protein of great interest from a pathogenesis and vaccine development point of view. Functional studies have shown that LipL32 binds to several extracellular matrix proteins, including laminin, multiple collagens and fibronectin, and the plasma proteins plasminogen and fibronectin. However, despite these indications for a role in adhesion to host tissues, a transposon mutant deficient for LipL32 had normal virulence in both the acute and colonisation models of infection and displayed normal adhesion to extracellular matrix in vitro. LipL32 stimulates a strong antibody response during natural infection of humans and a wide range of animal species. However, vaccination studies using LipL32 have yielded mixed results, despite the use of a wide range of vaccine vectors, adjuvants and protein modifications. Further work is clearly required to fully elucidate the role of this protein in the biology of Leptospira.