Calcium binds to leptospiral immunoglobulin-like protein, LigB, and modulates fibronectin binding

Fibronectin binds to and induces conformational change in a disordered region of leptospiral immunoglobulin-like protein B

Leptospira interrogans is a pathogenic spirochete that causes disease in both humans and animals. LigB (Leptospiral immunoglobulin-like protein B) contributes to the binding of Leptospira to extracellular matrix proteins such as fibronectin (Fn), fibrinogen, laminin, and collagen. A high affinity Fn-binding region of LigB has been recently localized to LigBCen2, which contains the partial eleventh and full twelfth immunoglobulin-like repeats (LigBCen2R) and 47 amino acids of the non-repeat region (LigBCen2NR) of LigB. In this study, LigBCen2NR was shown to bind to the N-terminal domain (NTD) of Fn (K(D) = 379 nm) by an enzyme-linked immunosorbent assay and isothermal titration calorimetry. Interestingly, this sequence was not observed to adopt secondary structure by far UV circular dichroism or by differential scanning calorimetry, in agreement with computer-based secondary structure predictions. A low partition coefficient (K(av)) measured with gel permeation chromatography, a high hydrodynamic radius (R(h)) measured with dynamic light scattering, and the insensitivity of the intrinsic viscosity to guanidine hydrochloride treatment all suggest that LigBCen2NR possesses an extended and disordered structure. Two-dimensional (15)N-(1)H HSQC NMR spectra of intact LigBCen2 in the absence and presence of NTD are consistent with these observations, suggesting the presence of both a beta-rich region and an unstructured region in LigBCen2 and that the latter of these selectively interacts with NTD. Upon binding to NTD, LigBCen2NR was observed by CD to adopt a beta-strand-rich structure, suggestive of the known beta-zipper mode of NTD binding.

A novel fibronectin type III module binding motif identified on C-terminus of Leptospira immunoglobulin-like protein, LigB

Infection by pathogenic strains of Leptospira hinges on the pathogen's ability to adhere to host cells via extracellular matrix such as fibronectin (Fn). Previously, the immunoglobulin-like domains of Leptospira Lig proteins were recognized as adhesins binding to N-terminal domain (NTD) and gelatin binding domain (GBD) of Fn. In this study, we identified another Fn-binding motif on the C-terminus of the Leptospira adhesin LigB (LigBCtv), residues 1708-1712 containing sequence LIPAD with a beta-strand and nascent helical structure. This motif binds to 15th type III modules (15F(3)) (K(D)=10.70 microM), and association (k(on)=600 M(-1)s(-1)) and dissociation (k(off)=0.0129 s(-1)) rate constants represents a slow binding kinetics in this interaction. Moreover, pretreatment of MDCK cells with LigB(1706-1716) blocked the binding of Leptospira by 39%, demonstrating a significant role of LigB(1706-1716) in cellular adhesion. These data indicate that the LIPAD residues (LigB(1708-1712)) of the Leptospira interrogans LigB protein bind 15F(3) of Fn at a novel binding site, and this interaction contributes to adhesion to host cells.

Repeated domains of leptospira immunoglobulin-like proteins interact with elastin and tropoelastin

Leptospira spp., the causative agents of leptospirosis, adhere to components of the extracellular matrix, a pivotal role for colonization of host tissues during infection. Previously, we and others have shown that Leptospira immunoglobulin-like proteins (Lig) of Leptospira spp. bind to fibronectin, laminin, collagen, and fibrinogen. In this study, we report that Leptospira can be immobilized by human tropoelastin (HTE) or elastin from different tissues, including lung, skin, and blood vessels, and that Lig proteins can bind to HTE or elastin. Moreover, both elastin and HTE bind to the same LigB immunoglobulin-like domains, including LigBCon4, LigBCen7'-8, LigBCen9, and LigBCen12 as demonstrated by enzyme-linked immunosorbent assay (ELISA) and competition ELISAs. The LigB immunoglobulin-like domain binds to the 17th to 27th exons of HTE (17-27HTE) as determined by ELISA (LigBCon4, K(D) = 0.50 microm; LigBCen7'-8, K(D) = 0.82 microm; LigBCen9, K(D) = 1.54 microm; and LigBCen12, K(D) = 0.73 microm). The interaction of LigBCon4 and 17-27HTE was further confirmed by steady state fluorescence spectroscopy (K(D) = 0.49 microm) and ITC (K(D) = 0.54 microm). Furthermore, the binding was enthalpy-driven and affected by environmental pH, indicating it is a charge-charge interaction. The binding affinity of LigBCon4D341N to 17-27HTE was 4.6-fold less than that of wild type LigBCon4. In summary, we show that Lig proteins of Leptospira spp. interact with elastin and HTE, and we conclude this interaction may contribute to Leptospira adhesion to host tissues during infection.

Immunogenicity and protective efficacy of recombinant Leptospira immunoglobulin-like protein B (rLigB) in a hamster challenge model

Leptospiral immunoglobulin-like protein (LigB) was truncated into conserved (LigBcon) and variable (varB1, varB2) fragments and expressed as GST/His-tag fusion proteins. Four-week-old hamsters were immunized with equal amounts of each fragment individually or combined in alum adjuvant at days 0 and 21 and subsequently challenged three weeks after the booster with 2.5 LD(50) live virulent Leptospira interrogans serovar Pomona. Our results demonstrate that immunization with LigB produced strong humoral immune responses as revealed by high titers against each fragment and significant enhancement in Th2 cytokines (IL-4, IL-10). A significant activation of CMI is revealed by enhanced proliferation of lymphocytes and up regulation of Th1 cytokines (IL-12p40, IFN-gamma) was also noted. Of the peptides studied, rLigBcon was able to impart maximum protection (71%), followed by rVarB1 (54%), whereas rVarB2 was not able to impart a significant level of protection (33%) against lethal infection as revealed by enhanced survival and reduced severity of histopathological lesions in vital organs (viz. kidney, liver, spleen) of the immunized animals. Moreover, concurrent administration of all three fragments significantly enhanced the protective efficacy of the vaccine (83%). Overall, our results clearly demonstrate that LigB has emerged as novel protective antigen that can be used in future subunit vaccines against leptospirosis.