Immunization with the Haemophilus ducreyi hemoglobin receptor HgbA with adjuvant monophosphoryl lipid A protects swine from a homologous but not a heterologous challenge

Infections caused by Haemophilus ducreyi: one organism, two stories

SUMMARYChancroid, a sexually transmitted infection caused by Haemophilus ducreyi, is characterized by painful genital ulcers (GU) and inguinal lymphadenitis. H. ducreyi was recently described as a major cause of non-sexually transmitted cutaneous ulcers (CU) on the lower legs in children in yaws-endemic regions. This review explores the relationship between CU and GU strains of H. ducreyi; their clinical presentation, diagnosis, epidemiology, and treatment; and how findings from a human challenge model relate to GU and CU. We contrast the decline of GU with the persistence of CU caused by H. ducreyi. Factors such as transmission dynamics, control, and elimination efforts are discussed. Syndromic management and targeted treatment of sex workers can eradicate chancroid, while skin colonization by CU strains and environmental factors may necessitate topical treatments or vaccination for CU eradication. Efforts should focus on identifying additional reservoirs of CU strains, improving hygiene, and eliminating asymptomatic colonization to eradicate this painful infection in children.

Microbial Risks Caused by Livestock Excrement: Current Research Status and Prospects

Livestock excrement is a major pollutant yielded from husbandry and it has been constantly imported into various related environments. Livestock excrement comprises a variety of microorganisms including certain units with health risks and these microorganisms are transferred synchronically during the management and utilization processes of livestock excrement. The livestock excrement microbiome is extensively affecting the microbiome of humans and the relevant environments and it could be altered by related environmental factors as well. The zoonotic microorganisms, extremely zoonotic pathogens, and antibiotic-resistant microorganisms are posing threats to human health and environmental safety. In this review, we highlight the main feature of the microbiome of livestock excrement and elucidate the composition and structure of the repertoire of microbes, how these microbes transfer from different spots, and they then affect the microbiomes of related habitants as a whole. Overall, the environmental problems caused by the microbiome of livestock excrement and the potential risks it may cause are summarized from the microbial perspective and the strategies for prediction, prevention, and management are discussed so as to provide a reference for further studies regarding potential microbial risks of livestock excrement microbes.

Interactions of the Skin Pathogen Haemophilus ducreyi With the Human Host

The obligate human pathogen Haemophilus ducreyi causes both cutaneous ulcers in children and sexually transmitted genital ulcers (chancroid) in adults. Pathogenesis is dependent on avoiding phagocytosis and exploiting the suppurative granuloma-like niche, which contains a myriad of innate immune cells and memory T cells. Despite this immune infiltrate, long-lived immune protection does not develop against repeated H. ducreyi infections—even with the same strain. Most of what we know about infectious skin diseases comes from naturally occurring infections and/or animal models; however, for H. ducreyi, this information comes from an experimental model of infection in human volunteers that was developed nearly three decades ago. The model mirrors the progression of natural disease and serves as a valuable tool to determine the composition of the immune cell infiltrate early in disease and to identify host and bacterial factors that are required for the establishment of infection and disease progression. Most recently, holistic investigation of the experimentally infected skin microenvironment using multiple “omics” techniques has revealed that non-canonical bacterial virulence factors, such as genes involved in central metabolism, may be relevant to disease progression. Thus, the immune system not only defends the host against H. ducreyi, but also dictates the nutrient availability for the invading bacteria, which must adapt their gene expression to exploit the inflammatory metabolic niche. These findings have broadened our view of the host-pathogen interaction network from considering only classical, effector-based virulence paradigms to include adaptations to the metabolic environment. How both host and bacterial factors interact to determine infection outcome is a current focus in the field. Here, we review what we have learned from experimental H. ducreyi infection about host-pathogen interactions, make comparisons to what is known for other skin pathogens, and discuss how novel technologies will deepen our understanding of this infection.

A Class I Haemophilus ducreyi Strain Containing a Class II hgbA Allele Is Partially Attenuated in Humans: Implications for HgbA Vaccine Efficacy Trials

Haemophilus ducreyi causes chancroid and is a major cause of cutaneous ulcers in children. Due to environmental reservoirs, both class I and class II H. ducreyi strains persist in cutaneous ulcer regions of endemicity following mass drug administration of azithromycin, suggesting the need for a vaccine. The hemoglobin receptor (HgbA) is a leading vaccine candidate, but its efficacy in animal models is class specific. Controlled human infection models can be used to evaluate vaccines, but only a class I strain (35000HP) has been characterized in this model. As a prelude to evaluating HgbA vaccines in the human model, we tested here whether a derivative of 35000HP containing a class II hgbA allele (FX548) is as virulent as 35000HP in humans. In eight volunteers infected at three sites with each strain, the papule formation rate was 95.8% for 35000HP versus 62.5% for FX548 (P = 0.021). Excluding doses of FX548 that were ≥2-fold higher than those of 35000HP, the pustule formation rate was 25% for 35000HP versus 11.7% for FX548 (P = 0.0053). By Western blot analysis, FX548 and 35000HP expressed equivalent amounts of HgbA in whole-cell lysates and outer membranes. The growth of FX548 and 35000HP was similar in media containing hemoglobin or hemin. By whole-genome sequencing and single-nucleotide polymorphism analysis, FX548 contained no mutations in open reading frames other than hgbA We conclude that by an unknown mechanism, FX548 is partially attenuated in humans and is not a suitable strain for HgbA vaccine efficacy trials in the model.

Salmonella enterica serovar Choleraesuis vector delivering SaoA antigen confers protection against Streptococcus suis serotypes 2 and 7 in mice and pigs

Streptococcus suis is one of the major pathogens that cause economic losses in the swine industry worldwide. However, current bacterins only provide limited prophylactic protection in the field. An ideal vaccine against S. suis should protect pigs against the clinical diseases caused by multiple serotypes, or at least protect against the dominant serotype in a given geographic region. A new recombinant Salmonella enterica serotype Choleraesuis vaccine vector, rSC0011, that is based on the regulated delayed attenuation system and regulated delayed antigen synthesis system, was developed recently. In this study, an improved recombinant attenuated Salmonella Choleraesuis vector, rSC0016, was developed by incorporating a sopB mutation to ensure adequate safety and maximal immunogenicity. In the spleens of mice, rSC0016 colonized less than rSC0011. rSC0016 and rSC0011 colonized similarly in Peyer's patches of mice. The recombinant vaccine rSC0016(pS-SaoA) induced stronger cellular, humoral, and mucosal immune responses in mice and swine against SaoA, a conserved surface protein that is present in many S. suis serotypes, than did rSC0011(pS-SaoA) without sopB or rSC0018(pS-SaoA), which is an avirulent, chemically attenuated vaccine strain. rSC0016(pS-SaoA) provided 100% protection against S. suis serotype 2 in mice and pigs, and full cross-protection against SS7 in pigs. This new vaccine vector provides a foundation for the development of a universal vaccine against multiple serotypes of S. suis in pigs.

The anti-influenza M2e antibody response is promoted by XCR1 targeting in pig skin

XCR1 is selectively expressed on a conventional dendritic cell subset, the cDC1 subset, through phylogenetically distant species. The outcome of antigen-targeting to XCR1 may therefore be similar across species, permitting the translation of results from experimental models to human and veterinary applications. Here we evaluated in pigs the immunogenicity of bivalent protein structures made of XCL1 fused to the external portion of the influenza virus M2 proton pump, which is conserved through strains and a candidate for universal influenza vaccines. Pigs represent a relevant target of such universal vaccines as pigs can be infected by swine, human and avian strains. We found that cDC1 were the only cell type labeled by XCR1-targeted mCherry upon intradermal injection in pig skin. XCR1-targeted M2e induced higher IgG responses in seronegative and seropositive pigs as compared to non-targeted M2e. The IgG response was less significantly enhanced by CpG than by XCR1 targeting, and CpG did not further increase the response elicited by XCR1 targeting. Monophosphoryl lipid A with neutral liposomes did not have significant effect. Thus altogether M2e-targeting to XCR1 shows promises for a trans-species universal influenza vaccine strategy, possibly avoiding the use of classical adjuvants.

Immunization with the Haemophilus ducreyi trimeric autotransporter adhesin DsrA with alum, CpG or imiquimod generates a persistent humoral immune response that recognizes the bacterial surface

The Ducreyi serum resistance A (DsrA) protein of Haemophilus ducreyi belongs to a large family of multifunctional outer membrane proteins termed trimeric autotransporter adhesins responsible for resistance to the bactericidal activity of human complement (serum resistance), agglutination and adhesion. The ability of DsrA to confer serum resistance and bind extracellular matrix proteins lies in its N-terminal passenger domain. We have previously reported that immunization with a recombinant form of the passenger domain of DsrA, rNT-DsrA, in complete/incomplete Freund's adjuvant, protects against a homologous challenge in swine. We present herein the results of an immunogenicity study in mice aimed at investigating the persistence, type of immune response, and the effect of immunization route and adjuvants on surrogates of protection. Our results indicate that a 20 μg dose of rNT-DsrA administered with alum elicited antisera with comparable bacterial surface reactivity to that obtained with complete/incomplete Freund's adjuvant. At that dose, high titers and bacterial surface reactivity persisted for 211 days after the first immunization. Administration of rNT-DsrA with CpG or imiquimod as adjuvants elicited a humoral response with similar quantity and quality of antibodies (Abs) as seen with Freund's adjuvant. Furthermore, intramuscular administration of rNT-DsrA elicited high-titer Abs with significantly higher reactivity to the bacterial surface than those obtained with subcutaneous immunization. All rNT-DsrA/adjuvant combinations tested, save CpG, elicited a Th2-type response. Taken together, these findings show that a 20 μg dose of rNT-DsrA administered with the adjuvants alum, CpG or imiquimod elicits high-quality Abs with reactivity to the bacterial surface that could protect against an H. ducreyi infection.

Defining Potential Vaccine Targets of Haemophilus ducreyi Trimeric Autotransporter Adhesin DsrA

Haemophilus ducreyi is the causative agent of the sexually transmitted genital ulcer disease chancroid. Strains of H. ducreyi are grouped in two classes (I and II) based on genotypic and phenotypic differences, including those found in DsrA, an outer membrane protein belonging to the family of multifunctional trimeric autotransporter adhesins. DsrA is a key serum resistance factor of H. ducreyi that prevents binding of natural IgM at the bacterial surface and functions as an adhesin to fibronectin, fibrinogen, vitronectin, and human keratinocytes. Monoclonal antibodies (MAbs) were developed to recombinant DsrA (DsrA(I)) from prototypical class I strain 35000HP to define targets for vaccine and/or therapeutics. Two anti-DsrAI MAbs bound monomers and multimers of DsrA from genital and non-genital/cutaneous H. ducreyi strains in a Western blot and reacted to the surface of the genital strains; however, these MAbs did not recognize denatured or native DsrA from class II strains. In a modified extracellular matrix protein binding assay using viable H. ducreyi, one of the MAbs partially inhibited binding of fibronectin, fibrinogen, and vitronectin to class I H. ducreyi strain 35000HP, suggesting a role for anti-DsrA antibodies in preventing binding of H. ducreyi to extracellular matrix proteins. Standard ELISA and surface plasmon resonance using a peptide library representing full-length, mature DsrAI revealed the smallest nominal epitope bound by one of the MAbs to be MEQNTHNINKLS. Taken together, our findings suggest that this epitope is a potential target for an H. ducreyi vaccine.

Identification and analysis of three virulence-associated TonB-dependent outer membrane receptors of Pseudomonas fluorescens

Pseudomonas fluorescens is a Gram-negative bacterium that can infect a wide range of farmed fish. However, very little is known about the virulence mechanism of P. fluorescens as a fish pathogen. In this study, we identified and analyzed 3 TonB-dependent outer membrane receptors (TDRs) from a pathogenic P. fluorescens strain isolated from fish. In silico analysis revealed that all 3 proteins (named Tdr1 to 3) possess structural domains typical of TDRs. Quantitative real time RT-PCR analysis showed that tdr1, tdr2, and tdr3 expressions were upregulated under iron-depleted conditions. Compared to the wild type, mutants defective in tdr1, tdr2, and tdr3 were retarded in growth to different extents. Infection in a turbot Scophthalmus maximus model showed that all 3 mutants were impaired in their ability to desseminate into and colonize host tissues. In addition, the tdr1 and tdr3 mutants exhibited significantly reduced virulence. When used as subunit vaccines, purified recombinant proteins of Tdr1, Tdr2, and, in particular, Tdr3 elicited significant protection in turbot against lethal P. fluorescens challenge. The vaccinated fish produced specific serum antibodies, which, when incubated with P. fluorescens, blocked infection of P. fluorescens in fish cells. Together these results indicate that Tdr1, Tdr2, and Tdr3 are iron-regulated factors that participate in bacterial virulence and induce protective immunity as subunit vaccines.

The Haemophilus ducreyi trimeric autotransporter adhesin DsrA protects against an experimental infection in the swine model of chancroid

Adherence of pathogens to cellular targets is required to initiate most infections. Defining strategies that interfere with adhesion is therefore important for the development of preventative measures against infectious diseases. As an adhesin to host extracellular matrix proteins and human keratinocytes, the trimeric autotransporter adhesin DsrA, a proven virulence factor of the Gram-negative bacterium Haemophilus ducreyi, is a potential target for vaccine development. A recombinant form of the N-terminal passenger domain of DsrA from H. ducreyi class I strain 35000HP, termed rNT-DsrAI, was tested as a vaccine immunogen in the experimental swine model of H. ducreyi infection. Viable homologous H. ducreyi was not recovered from any animal receiving four doses of rNT-DsrAI administered with Freund's adjuvant at two-week intervals. Control pigs receiving adjuvant only were all infected. All animals receiving the rNT-DsrAI vaccine developed antibody endpoint titers between 3.5 and 5 logs. All rNT-DsrAI antisera bound the surface of the two H. ducreyi strains used to challenge immunized pigs. Purified anti-rNT-DsrAI IgG partially blocked binding of fibrinogen at the surface of viable H. ducreyi. Overall, immunization with the passenger domain of the trimeric autotransporter adhesin DsrA accelerated clearance of H. ducreyi in experimental lesions, possibly by interfering with fibrinogen binding.

A vaccine of L2 epitope repeats fused with a modified IgG1 Fc induced cross-neutralizing antibodies and protective immunity against divergent human papillomavirus types

Current human papillomavirus (HPV) major capsid protein L1 virus-like particles (VLPs)-based vaccines in clinic induce strong HPV type-specific neutralizing antibody responses. To develop pan-HPV vaccines, here, we show that the fusion protein E3R4 consisting of three repeats of HPV16 L2 aa 17–36 epitope (E3) and a modified human IgG1 Fc scaffold (R4) induces cross-neutralizing antibodies and protective immunity against divergent HPV types. E3R4 was expressed as a secreted protein in baculovirus expression system and could be simply purified by one step Protein A affinity chromatography with the purity above 90%. Vaccination of E3R4 formulated with Freunds adjuvant not only induced cross-neutralizing antibodies against HPV pseudovirus types 16, 18, 45, 52, 58, 6, 11 and 5 in mice, but also protected mice against vaginal challenges with HPV pseudovirus types 16, 45, 52, 58, 11 and 5 for at least eleven months after the first immunization. Moreover, vaccination of E3R4 formulated with FDA approved adjuvant alum plus monophosphoryl lipid A also induced cross-neutralizing antibodies against HPV types 16, 18 and 6 in rabbits. Thus, our results demonstrate that delivery of L2 antigen as a modified Fc-fusion protein may facilitate pan-HPV vaccine development.

Mutational analysis of hemoglobin binding and heme utilization by a bacterial hemoglobin receptor

Iron is an essential nutrient for most living organisms. To acquire iron from their environment, Gram-negative bacteria use TonB-dependent transporters that bind host proteins at the bacterial surface and transport iron or heme to the periplasm via the Ton machinery. TonB-dependent transporters are barrel-shaped outer membrane proteins with 22 transmembrane domains, 11 surface-exposed loops, and a plug domain that occludes the pore. To identify key residues of TonB-dependent transporters involved in hemoglobin binding and heme transport and thereby locate putative protective epitopes, the hemoglobin receptor of Haemophilus ducreyi HgbA was used as a model of iron/heme acquisition from hemoglobin. Although all extracellular loops of HgbA are required by H. ducreyi to use hemoglobin as a source of iron/heme, we previously demonstrated that hemoglobin binding by HgbA only involves loops 5 and 7. Using deletion, substitution, and site-directed mutagenesis, we were able to differentiate hemoglobin binding and heme acquisition by HgbA. Deletion or substitution of the GYEAYNRQWWA region of loop 5 and alanine replacement of selected histidines affected hemoglobin binding by HgbA. Conversely, mutation of the phenylalanine in the loop 7 FRAP domain or substitution of the NRQWWA motif of loop 5 significantly abrogated utilization of heme from hemoglobin. Our findings show that hemoglobin binding and heme utilization by a bacterial hemoglobin receptor involve specific motifs of HgbA.

Haemophilus ducreyi-induced interleukin-10 promotes a mixed M1 and M2 activation program in human macrophages

During microbial infection, macrophages are polarized to classically activated (M1) or alternatively activated (M2) cells in response to microbial components and host immune mediators. Proper polarization of macrophages is critical for bacterial clearance. To study the role of macrophage polarization during Haemophilus ducreyi infection, we analyzed a panel of macrophage surface markers in skin biopsy specimens of pustules obtained from experimentally infected volunteers. Lesional macrophages expressed markers characteristic of both M1 and M2 polarization. Monocyte-derived macrophages (MDM) also expressed a mixed M1 and M2 profile of surface markers and cytokines/chemokines upon infection with H. ducreyi in vitro. Endogenous interleukin 10 (IL-10) produced by infected MDM downregulated and enhanced expression of several M1 and M2 markers, respectively. Bacterial uptake, mediated mainly by class A scavenger receptors, and activation of mitogen-activated protein kinase and phosphoinositide 3-kinase signaling pathways were required for H. ducreyi-induced IL-10 production in MDM. Compared to M1 cells, IL-10-polarized M2 cells displayed enhanced phagocytic activity against H. ducreyi and similar bacterial killing. Thus, IL-10-modulated macrophage polarization may contribute to H. ducreyi clearance during human infection.

On the evolution of the sexually transmitted bacteria Haemophilus ducreyi and Klebsiella granulomatis

Haemophilus ducreyi and Klebsiella (Calymmatobacterium) granulomatis are sexually transmitted bacteria that cause characteristic, persisting ulceration on external genitals called chancroid and granuloma inguinale, respectively. Those ulcers are endemic in developing countries or exist, as does granuloma inguinale, only in some geographic "hot spots."H. ducreyi is placed in the genus Haemophilus (family Pasteurellacae); however, this phylogenetic position is not obvious. The multiple ways in which the bacterium may be adapted to its econiche through specialized nutrient acquisitions; defenses against the immune system; and virulence factors that increase attachment, fitness, and persistence within genital tissue are discussed below. The analysis of K. granulomatis phylogeny demonstrated a high degree of identity with other Klebsiella species, and the name K. granulomatis comb. nov. was proposed. Because of the difficulty in growing this bacterium on artificial media, its characteristics have not been sufficiently defined. More studies are needed to understand bacterial genetics related to the pathogenesis and evolution of K. granulomatis.

Genome-wide expression profiling and mutagenesis studies reveal that lipopolysaccharide responsiveness appears to be absolutely dependent on TLR4 and MD-2 expression and is dependent upon intermolecular ionic interactions

Lipid A (a hexaacylated 1,4' bisphosphate) is a potent immune stimulant for TLR4/MD-2. Upon lipid A ligation, the TLR4/MD-2 complex dimerizes and initiates signal transduction. Historically, studies also suggested the existence of TLR4/MD-2-independent LPS signaling. In this article, we define the role of TLR4 and MD-2 in LPS signaling by using genome-wide expression profiling in TLR4- and MD-2-deficient macrophages after stimulation with peptidoglycan-free LPS and synthetic Escherichia coli lipid A. Of the 1396 genes significantly induced or repressed by any one of the treatments in the wild-type macrophages, none was present in the TLR4- or MD-2-deficient macrophages, confirming that the TLR4/MD-2 complex is the only receptor for endotoxin and that both are required for responses to LPS. Using a molecular genetics approach, we investigated the mechanism of TLR4/MD-2 activation by combining the known crystal structure of TLR4/MD-2 with computer modeling. According to our murine TLR4/MD-2-activation model, the two phosphates on lipid A were predicted to interact extensively with the two positively charged patches on mouse TLR4. When either positive patch was abolished by mutagenesis into Ala, the responses to LPS and lipid A were nearly abrogated. However, the MyD88-dependent and -independent pathways were impaired to the same extent, indicating that the adjuvant activity of monophosphorylated lipid A most likely arises from its decreased potential to induce an active receptor complex and not more downstream signaling events. Hence, we concluded that ionic interactions between lipid A and TLR4 are essential for optimal LPS receptor activation.

Passive immunization with a polyclonal antiserum to the hemoglobin receptor of Haemophilus ducreyi confers protection against a homologous challenge in the experimental swine model of chancroid

Haemophilus ducreyi, the etiologic agent of chancroid, has an obligate requirement for heme. Heme is acquired by H. ducreyi from its human host via TonB-dependent transporters expressed at its bacterial surface. Of 3 TonB-dependent transporters encoded in the genome of H. ducreyi, only the hemoglobin receptor, HgbA, is required to establish infection during the early stages of the experimental human model of chancroid. Active immunization with a native preparation of HgbA (nHgbA) confers complete protection in the experimental swine model of chancroid, using either Freund's or monophosphoryl lipid A as adjuvants. To determine if transfer of anti-nHgbA serum is sufficient to confer protection, a passive immunization experiment using pooled nHgbA antiserum was conducted in the experimental swine model of chancroid. Pigs receiving this pooled nHgbA antiserum were protected from a homologous, but not a heterologous, challenge. Passively transferred polyclonal antibodies elicited to nHgbA bound the surface of H. ducreyi and partially blocked hemoglobin binding by nHgbA, but were not bactericidal. Taken together, these data suggest that the humoral immune response to the HgbA vaccine is protective against an H. ducreyi infection, possibly by preventing acquisition of the essential nutrient heme.

Rapid divergence of two classes of Haemophilus ducreyi

Haemophilus ducreyi, the etiologic agent of chancroid, expresses variants of several key virulence factors. While previous reports suggested that H. ducreyi strains formed two clonal populations, the differences between, and diversity within, these populations were unclear. To assess their variability, we examined sequence diversity at 11 H. ducreyi loci, including virulence and housekeeping genes, augmenting published data sets with PCR-amplified genes to acquire data for at least 10 strains at each locus. While sequences from all 11 loci place strains into two distinct groups, there was very little variation within each group. The difference between alleles of the two groups was variable and large at 3 loci encoding surface-exposed proteins (0.4 < K(S) < 1.3, where K(S) is divergence at synonymous sites) but consistently small at genes encoding cytoplasmic or periplasmic proteins (K(S) < 0.09). The data suggest that the two classes have recently diverged, that recombination has introduced variant alleles into at least 3 distinct loci, and that these alleles have been confined to one of the two classes. In addition, recombination is evident among alleles within, but not between, classes. Rather than clones of the same species, these properties indicate that the two classes may form distinct species.