Culture-attenuated pathogenic Leptospira lose the ability to survive complement lytic activity due to decreased C4BP uptake.

Pathogenic Leptospira spp. evade the complement system by capturing soluble regulators of the alternative, classical, and lectin pathways - such as factor H, C4BP, and vitronectin - via proteins on their surface. By capturing these regulators, Leptospira can disrupt the complement activation cascade, thereby preventing opsonization by C3b/iC3b and lysis by the membrane attack complex (MAC). The ability of low-passage pathogenic Leptospira strains - LOCaS46 (L. interrogans sv Canicola), LOVe30 (L. interrogans sv Icterohaemorrhagiae), and MOCA45 (L. santarosai sv Tarassovi) - to bind C4BP was evaluated and compared to their corresponding culture-attenuated (LOCaS46, LOVe30, and MOCA45) and to the saprophytic Patoc I strain of Leptospira biflexa sv Patoc. Binding to C4BP was assessed by ELISA and confirmed by Western blot, the expression level of the genes for C4BP-binding proteins was evaluated by RT-qPCR, and the survival of the Leptospira spp. strains in normal human serum (NHS) was estimated to assess complement resistance. Results showed that culture attenuated (CA) strains had a lower capacity for binding to C4BP, and surviving to NHS as compared to low-passage (LP) strains. Also, transcription level of the genes for the C4BP-binding proteins LigA, LigB, LcpA, enolase and Lsa23, was lower in the CA strains than in their corresponding LP strains. This suggest that reduction of the ability to capture C4BP in culture attenuated (CA) strains is due at least in part to lower expression of C4BP-binding proteins, affecting the evasion of classical and lectin pathways of the complement system and therefore the capability of survival in NHS.