Evaluation of LipL32 and LigA/LigB Knockdown Mutants in Leptospira interrogans Serovar Copenhageni: Impacts to Proteome and Virulence

Identification of equine mares as reservoir hosts for pathogenic species of Leptospira

Equine leptospirosis can result in abortion, stillbirth, neonatal death, placentitis, and uveitis. Horses can also act as subclinical reservoir hosts of infection, which are characterized as asymptomatic carriers that persistently excrete leptospires and transmit disease. In this study, PCR and culture were used to assess urinary shedding of pathogenic Leptospira from 37 asymptomatic mares. Three asymptomatic mares, designated as H2, H8, and H9, were PCR-positive for lipL32, a gene specific for pathogenic species of Leptospira. One asymptomatic mare, H9, was culture-positive, and the recovered isolate was classified as L. kirschneri serogroup Australis serovar Rushan. DNA capture and enrichment of Leptospira genomic DNA from PCR-positive, culture-negative samples determined that asymptomatic mare H8 was also shedding L. kirschneri serogroup Australis, whereas asymptomatic mare H2 was shedding L. interrogans serogroup Icterohaemorrhagiae. Sera from all asymptomatic mares were tested by the microscopic agglutination test (MAT) and 35 of 37 (94.6%) were seropositive with titers ranging from 1:100 to 1:3200. In contrast to asymptomatic mares, mare H44 presented with acute spontaneous abortion and a serum MAT titer of 1:102,400 to L. interrogans serogroup Pomona serovar Pomona. Comparison of L. kirschneri serogroup Australis strain H9 with that of L. interrogans serogroup Pomona strain H44 in the hamster model of leptospirosis corroborated differences in virulence of strains. Since lipopolysaccharide (LPS) is a protective antigen in bacterin vaccines, the LPS of strain H9 (associated with subclinical carriage) was compared with strain H44 (associated with spontaneous abortion). This revealed different LPS profiles and immunoreactivity with reference antisera. It is essential to know what species and serovars of Leptospira are circulating in equine populations to design efficacious vaccines and diagnostic tests. Our results demonstrate that horses in the US can act as reservoir hosts of leptospirosis and shed diverse pathogenic Leptospira species via urine. This report also details the detection of L. kirschneri serogroup Australis serovar Rushan, a species and serotype of Leptospira, not previously reported in the US.

Global proteome of the saprophytic strain Leptospira biflexa and comparative analysis with pathogenic strain Leptospira interrogans uncover new pathogenesis mechanisms

Leptospira is a genus of bacteria that includes free-living saprophytic species found in water or soil, and pathogenic species, which are the etiologic agents of leptospirosis. Besides all the efforts, there are only a few proteins described as virulence factors in the pathogenic strain L. interrogans. This work aims to perform L. biflexa serovar Patoc1 strain Paris global proteome and to compare with the proteome database of pathogenic L. interrogans serovar Copenhageni strain Fiocruz L1-130. We identified a total of 2327 expressed proteins of L. biflexa by mass spectrometry. Using the Get Homologues software with the global proteome of L. biflexa and L. interrogans, we found orthologous proteins classified into conserved, low conserved, and specific proteins. Comparative bioinformatic analyses were performed to understand the biological functions of the proteins, subcellular localization, the presence of signal peptide, structural domains, and motifs using public softwares. These results lead to the selection of 182 low conserved within the saprophyte, and 176 specific proteins of L. interrogans. It is anticipated that these findings will indicate further studies to uncover virulence factors in the pathogenic strain. This work presents for the first time the global proteome of saprophytic strain L. biflexa serovar Patoc, strain Patoc1. SIGNIFICANCE: The comparative analysis established an array of specific proteins in pathogenic strain that will narrow down the identification of immune protective proteins that will help fight leptospirosis.

Proteomic profiles of Leptospira borgpetersenii serovar Hardjo strains JB197 and HB203 cultured at different temperatures

Leptospirosis is a global zoonotic disease affecting humans, domestic, and wild animals. Leptospira are typically shed in the urine of reservoir hosts which persist in suitable environments where incidental host transmission occurs after direct contact with infected urine or contaminated environments. Interestingly, serologically identical L. borgpetersenii serovar Hardjo strains JB197 and HB203 show divergent disease severity in the hamster model; JB197 causes severe acute infection while HB203 causes persistent chronic infection. Historically, serovar Hardjo was limited to culture at 29 °C, but utilization of HAN media allows propagation from host tissues at 37 °C. Here, the proteome of strains JB197 and HB203 were characterized after culture from experimentally challenged hamsters at 29 °C and 37 °C. Comparative analyses of JB197 and HB203 samples cultured at 29 °C yielded 425 significantly differentially expressed (DE) proteins, while strains at 37 °C yielded 613 DE proteins including prominent outer membrane proteins and known virulence factors. In agreement, membrane protein GO terms were identified by STRING network analyses along with numerous metabolic KEGG pathways consistent with condition differences. Within strain, JB197 cultured at 29 °C vs 37 °C identified 529 DE proteins, while HB203 identified 524 DE proteins. Investigating differential protein profiles provide insights into strain specific behaviors with implications for better understanding host-pathogen interactions, disease transmission, and response to environmental conditions which can contribute to vaccine development, diagnostic improvement, and ultimately leptospirosis control. SIGNIFICANCE: Leptospirosis is a devastating zoonotic disease affecting humans, wild and domestic animals around the globe. Different species and serovars of Leptospira can affect various animal host species differently; for instance, a serovar that is asymptomatic in the rat may cause severe disease in a dog or human. These differences in host response are not only found at the species and serovar level for Leptospira, but also at the strain level. A prime example comes from strains JB197 and HB203, both species L. borgpetersenii, both serovar Hardjo. Interestingly, JB197 causes a severe acute infection in the hamster while HB203 causes an asymptomatic chronic infection. Understanding these unique relationships between pathogen and host species is important, especially in the context of prevention technologies such as vaccine design, where the strain of Leptospira used as a bacterin might have different efficiencies in different hosts. In this study, proteomic profiles of strains JB197 and HB203 were analyzed, and results revealed diverse protein expression profiles of outer membrane proteins, as well as proteins functioning in motility and growth.

Characterization of cellular immune response in hamsters immunized with recombinant vaccines against leptospirosis based on LipL32:LemA:LigAni chimeric protein

In the last 20 years, various research groups have endeavored to develop recombinant vaccines against leptospirosis to overcome the limitations of commercially available bacterins. Numerous antigens and vaccine formulations have been tested thus far. However, the analysis of cellular response in these vaccine formulations is not commonly conducted, primarily due to the scarcity of supplies and kits for the hamster animal model. Our research group has already tested the Q1 antigen, a chimeric protein combining the immunogenic regions of LipL32, LemA, and LigANI, in recombinant subunit and BCG-vectored vaccines. In both strategies, 100 % of the hamsters were protected against clinical signs of leptospirosis. However, only the recombinant BCG-vectored vaccine provided protection against renal colonization. Thus, the objective of this study is to characterize the cellular immune response in hamsters immunized with different vaccine formulations based on the Q1 antigen through transcriptional analysis of cytokines. The hamsters were allocated into groups and vaccinated as follows: recombinant subunit (rQ1), recombinant BCG (rBCG:Q1), and saline and BCG Pasteur control vaccines. To assess the cellular response induced by the vaccines, we cultured and stimulated splenocytes, followed by RNA extraction from the cells and analysis of cytokines using real-time PCR. The results revealed that the recombinant subunit vaccine elicited a Th2-type response, characterized by the expression of cytokines IL-10, IL-1α, and TNF-α. This pattern closely resembles the cytokines expressed in severe cases of leptospirosis. On the other hand, the rBCG-vectored vaccine induced a Th1-type response with significant up-regulation of IFN-γ. These findings suggest the involvement of the cellular response and the IFN-γ mediated inflammatory response in the sterilizing immunity mediated by rBCG. Therefore, this study may assist future investigations in characterizing the cellular response in hamsters, aiming to elucidate the mechanisms of efficacy and establish potential correlates of protection.

LipL41 and LigA/LigB Gene Silencing on a LipL32 Knockout Leptospira interrogans Reveals the Impact of Multiple Mutations on Virulence

Leptospirosis is a global zoonosis caused by pathogenic bacteria of the genus Leptospira. The application of the CRISPR/Cas9 system has facilitated the generation of mutants and subsequent evaluation of phenotypes. Since DNA breaks induced by RNA-guided Cas9 nuclease are lethal to Leptospira, different methodologies were implemented to overcome this limitation. Initially, CRISPR interference (CRISPRi) was employed to create knockdown mutants, utilizing a catalytically inactive Cas9 (dCas9). Subsequently, the co-expression of CRISPR/Cas9 and a DNA repair system from Mycobacterium smegmatis enabled the generation of scarless knockout mutants. We eliminated plasmids from the lipL32 knockout L. interrogans strain and further achieved multiple gene mutations via gene silencing in this knockout background. Strains lacking both LipL41 and LipL32 and LigA, LigB, and LipL32, were evaluated. The absence of proteins LipL32 and LipL41 had no effect on leptospiral virulence. On the other hand, mutants lacking LigA, LigB, and LipL32 were unable to cause acute disease. The expanded apparatus for genetic manipulation of pathogenic leptospires via the CRISPR/Cas9 system has allowed the evaluation of multiple mutations upon leptospiral virulence. This work shows that LipL32 and LipL41 are not required for acute disease and consolidates LigA and LigB proteins as virulence factors.

Evaluation of Leptospira interrogans knockdown mutants for LipL32, LipL41, LipL21, and OmpL1 proteins

Introduction

Leptospirosis is a worldwide zoonosis caused by pathogenic and virulent species of the genus Leptospira, whose pathophysiology and virulence factors remain widely unexplored. Recently, the application of CRISPR interference (CRISPRi) has allowed the specific and rapid gene silencing of major leptospiral proteins, favoring the elucidation of their role in bacterial basic biology, host-pathogen interaction and virulence. Episomally expressed dead Cas9 from the Streptococcus pyogenes CRISPR/Cas system (dCas9) and single-guide RNA recognize and block transcription of the target gene by base pairing, dictated by the sequence contained in the 5' 20-nt sequence of the sgRNA.

Methods

In this work, we tailored plasmids for silencing the major proteins of L. interrogans serovar Copenhageni strain Fiocruz L1-130, namely LipL32, LipL41, LipL21 and OmpL1. Double- and triple-gene silencing by in tandem sgRNA cassettes were also achieved, despite plasmid instability.

Results

OmpL1 silencing resulted in a lethal phenotype, in both L. interrogans and saprophyte L. biflexa, suggesting its essential role in leptospiral biology. Mutants were confirmed and evaluated regarding interaction with host molecules, including extracellular matrix (ECM) and plasma components, and despite the dominant abundance of the studied proteins in the leptospiral membrane, protein silencing mostly resulted in unaltered interactions, either because they intrinsically display low affinity to the molecules assayed or by a compensation mechanism, where other proteins could be upregulated to fill the niche left by protein silencing, a feature previously described for the LipL32 mutant. Evaluation of the mutants in the hamster model confirms the augmented virulence of the LipL32 mutant, as hinted previously. The essential role of LipL21 in acute disease was demonstrated, since the LipL21 knockdown mutants were avirulent in the animal model, and even though mutants could still colonize the kidneys, they were found in markedly lower numbers in the animals' liver. Taking advantage of higher bacterial burden in LipL32 mutant-infected organs, protein silencing was demonstrated in vivo directly in leptospires present in organ homogenates.

Discussion

CRISPRi is now a well-established, attractive genetic tool that can be applied for exploring leptospiral virulence factors, leading to the rational for designing more effective subunit or even chimeric recombinant vaccines.

Leptospiral imelysin (LIC_10713) is secretory, immunogenic and binds to laminin, fibronectin, and collagen IV

Leptospirosis is a widespread zoonotic disease caused by pathogenic Leptospira. Early and accurate diagnosis is the prime step in managing the disease. Secretory proteins of Leptospira remain distinguished for diagnosis due to their availability as soluble proteins in the serum and their interaction with the host immune response due to their extracellular presence. This study presents the cloning, expression, purification, and characterization of imelysin or LruB (LIC_10713), a putative leptospiral protein. We report that the localization of imelysin showed its presence in the inner membrane and in the culture supernatant. The imelysin was upregulated under in vitro physiological conditions of infection. The LIC_10713 interacted significantly with laminin, fibronectin, collagen type I, and collagen type IV in a dose-dependent manner. Phylogenetic analysis showed that LIC_10713 is predominately found in the pathogenic species of Leptospira, and the GxHxxE motif of imelysin-like proteins is represented as the amino acid sequence G_WH_AIE. Also, immunoglobulins in leptospirosis-infected patients recognize recombinant-LIC_10713 with 100% specificity and 90.9% sensitivity. The secretion nature, abundance, upregulation, binding to ECM components, and immunogenicity determine LIC_10713 as an important molecule that can be used as an anti-leptospirosis measure. KEY POINTS: • The imelysin-like protein (LIC_10713) of Leptospira is a secretory protein • The protein LIC_10713 can bind ECM molecules • The LIC_10713 is mainly found in pathogenic leptospires • The anti-LIC_10713 antibody from human serum can detect the r-LIC_10713.

LigA formulated in AS04 or Montanide ISA720VG induced superior immune response compared to alum, which correlated to protective efficacy in a hamster model of leptospirosis

Leptospirosis is a zoonotic disease of global importance. The current vaccine provides serovar-specific and short-term immunity and does not prevent bacterial shedding in infected animals. Subunit vaccines based on surface proteins have shown to induce protection in an animal model. However, these proteins were tested with non-clinical adjuvants and induced low to moderate protective efficacy. We formulated a variable region of Leptospira immunoglobulin-like protein A (LAV) in clinical adjuvants, AS04 and Montanide ISA720VG, and then evaluated the immune response in mice and protective efficacy in a hamster model. Our results show that animals immunized with LAV-AS04 and LAV-Montanide ISA720VG (LAV-M) induced significantly higher levels of LAV-specific antibodies than LAV-Alum. While LAV-Alum induced Th2 response with the induction of IgG1 and IL-4, AS04 and LAV-M induced a mixed Th1/Th2 response with significant levels of both IgG1/IL-4 and IgG2c/IFN-γ. Both LAV-AS04 and LAV-M induced the generation of a significantly higher number of cytotoxic T cells (CTLs). The immune response in LAV-AS04- and LAV-M-immunized animals was maintained for a long period (>180 days) with the generation of a significant level of B- and T-cell memory. The strong immune response by both vaccines correlated to enhanced recruitment and activation of innate immune cells particularly DCs at draining lymph nodes and the formation of germinal centers (GCs). Furthermore, the immune response generated in mice correlated to protective efficacy in the hamster model of leptospirosis. These results indicate that LAV-AS04 and LAV-M are promising vaccines and can be further evaluated in clinical trials.

Characterization of novel nuclease and protease activities among Leptospiral immunoglobulin-like proteins

Bacterial immunoglobulin-like (BIg) domain containing proteins play a variety of biological functions. Leptospiral Immunoglobulin-like (Lig) proteins are well-known virulence factors located on the surface of the pathogenic Leptospira that act during adhesion, invasion, and immune evasion. The Lig proteins have many roles and have been designated as multifaceted proteins. However, the hydrolyzing function of Lig proteins is not yet investigated in detail. Here, we report novel in-vitro nuclease and protease activities in the Ig-like domain of LigA protein. All Ig-like domains were able to cleave DNA in the presence of a divalent ion, but not RNA. Site-directed mutagenesis revealed Mg⁺² binding residues in the Ig-like domain of LigA7. The basis of novel nuclease activity may be associated with protein adopting different conformation in the presence of divalent ions and substrate as investigated by change of intrinsic fluorescence. The docking of a stretch of double-strand DNA shows the binding on the positive surface of the protein. In addition, the protein is also observed to cleave a general protease substrate, β-casein, in our experimental condition. Our results proposed that the novel functions may be associated with neutrophil extracellular Trap (NET) evasion. Overall this study enhances the basic knowledge of non-nuclease proteins involved in the DNA cleavage activity and makes the foundation to explore its in-vivo activity in pathogenic Leptospira and other pathogens as well. Moreover, this information may be utilized to develop preventive strategies to interfere with Leptospira immune evasion.

A Novel Breakthrough in Leptospira spp. Mutagenesis: Knockout by Combination of CRISPR/Cas9 and Non-homologous End-Joining Systems

Leptospirosis is of general concern as it is a widespread zoonotic disease caused by pathogenic species of the genus Leptospira, although this genus also includes free-living saprophytic strains. Understanding the pathophysiology of leptospirosis is still in its infancy even after several years of its discovery, because of the lack of effective genetic tools. The use of the Streptococcus pyogenes CRISPR/Cas9 system and its variations have pushed the leptospirosis research forward, relying on the simplicity of the technique. However, the lethality of double-strand breaks (DSB) induced by the RNA-guided Cas9 enzyme has limited the generation of knockout mutants. In this work, we demonstrated sustained cell viability after concurrent expression of CRISPR/Cas9 and Mycobacterium tuberculosis non-homologous end-joining components in a single-plasmid strategy in L. biflexa. Scarless mutations resulting in null phenotypes could be observed in most of the colonies recovered, with deletions in the junctional site ranging from 3 to almost 400 bp. After plasmid curing by in vitro passages in a medium without antibiotic, selected marker-free and targeted mutants could be recovered. Knockout mutants for LipL32 protein in the pathogen L. interrogans could be obtained using M. smegmatis NHEJ machinery, with deletions ranging from 10 to 345 bp. In conclusion, we now have a powerful genetic tool for generating scarless and markerless knockout mutants for both saprophytic and pathogenic strains of Leptospira.