Acinetobacter baumannii rOmpA vaccine dose alters immune polarization and immunodominant epitopes

Biology of Acinetobacter baumannii: Pathogenesis, Antibiotic Resistance Mechanisms, and Prospective Treatment Options

Acinetobacter baumannii is undoubtedly one of the most successful pathogens responsible for hospital-acquired nosocomial infections in the modern healthcare system. Due to the prevalence of infections and outbreaks caused by multi-drug resistant A. baumannii, few antibiotics are effective for treating infections caused by this pathogen. To overcome this problem, knowledge of the pathogenesis and antibiotic resistance mechanisms of A. baumannii is important. In this review, we summarize current studies on the virulence factors that contribute to A. baumannii pathogenesis, including porins, capsular polysaccharides, lipopolysaccharides, phospholipases, outer membrane vesicles, metal acquisition systems, and protein secretion systems. Mechanisms of antibiotic resistance of this organism, including acquirement of β-lactamases, up-regulation of multidrug efflux pumps, modification of aminoglycosides, permeability defects, and alteration of target sites, are also discussed. Lastly, novel prospective treatment options for infections caused by multi-drug resistant A. baumannii are summarized.

Current advances and challenges in the development of Acinetobacter vaccines

Acinetobacter baumannii is a major cause of healthcare-associated infections worldwide with high morbidity and mortality. The clinical treatment of A. baumannii infections has become increasingly difficult because of the rapid emerging of multidrug and extremely drug resistant strains. Thus, there is an urgent need for the development of novel intervention strategies to combat this multidrug-resistant pathogen. Vaccine is one of the most effective medical measures for infection control and is likely to overcome the development of multidrug resistance by A. baumannii. Here we discussed the recent advances and potential challenges in development of A. baumannii vaccines with a focus on the 3 most important steps in the preclinical vaccine development: antigen selection, immune correlates of protection, and animal models for efficacy evaluation.

Mucosal immunization with purified OmpA elicited protective immunity against infections caused by multidrug-resistant Acinetobacter baumannii

Multidrug-resistant Acinetobacter baumannii (A. baumannii) is a rapidly emerging pathogen causing infections with high mortality rates due to inadequate medical treatment. New ways to prevent and treat such infections are of a critical medical need. In this study, intranasal vaccination with A. baumannii outer membrane protein A (OmpA) induced both systemic and mucosal antibodies. After challenge intraperitoneally by clinical strains of multidrug-resistant A. baumannii, mice immunized with OmpA had a significantly higher survival rate than control mice. The OmpA protein level tested positive by western blot in clinical strains of A. baumannii. Furthermore, characterization of human sera for anti-OmpA immunoglobulin G (IgG) antibody levels demonstrated that OmpA protein was immunogenic in healthy individuals and patients with A. baumannii invasive infections. In conclusion, to the best of our knowledge, this is the first study protective efficacy of mucosal immunization with OmpA as a protein antigen against multidrug-resistant A. Baumannii.

Acinetobacter baumannii Infection and IL-17 Mediated Immunity

Acinetobacter baumannii is a significant cause of severe hospital-acquired infections with a recent rise in multidrug-resistant infections involving traumatic wounds of military personnel. The interleukin-17 (IL-17) pathway is essential for neutrophil recruitment in response to a variety of pathogens, while the control of A. baumannii infection is known to be dependent on neutrophils. This suggests that IL-17 may play an important role in A. baumannii infection; however, this has yet to be studied. Here, we summarize the recent advances in understanding the host-pathogen interaction of A. baumannii and propose a potential role of the IL-17 pathway in generating a protective immune response.

Immunization with a 22-kDa outer membrane protein elicits protective immunity to multidrug-resistant Acinetobacter baumannii

A. baumannii infections are becoming more and more serious health issues with rapid emerging of multidrug and extremely drug resistant strains, and therefore, there is an urgent need for the development of nonantibiotic-based intervention strategies. This study aimed at identifying whether an outer membrane protein with molecular weight of about 22 kDa (Omp22) holds the potentials to be an efficient vaccine candidate and combat A. baumannii infection. Omp22 which has a molecule length of 217 amino acids kept more than 95% conservation in totally 851 reported A. baumannii strains. Recombinant Omp22 efficiently elicited high titers of specific IgG in mice. Both active and passive immunizations of Omp22 increased the survival rates of mice, suppressed the bacterial burdens in the organs and peripheral blood, and reduced the levels of serum inflammatory cytokines and chemokines. Opsonophagocytosis assays showed in vitro that Omp22 antiserum had highly efficient bactericidal activities on clonally distinct clinical A. baumannii isolates, which were partly complements-dependent and opsonophagocytic killing effects. Additionally, administration with as high as 500 μg of Omp22 didn’t cause obvious pathological changes in mice. In conclusion, Omp22 is a novel conserved and probably safe antigen for developing effective vaccines or antisera to control A. baumannii infections.

Pathogenic Acinetobacter: from the Cell Surface to Infinity and Beyond

The genus Acinetobacter encompasses multiple nosocomial opportunistic pathogens that are of increasing worldwide relevance because of their ability to survive exposure to various antimicrobial and sterilization agents. Among these, Acinetobacter baumannii, Acinetobacter nosocomialis, and Acinetobacter pittii are the most frequently isolated in hospitals around the world. Despite the growing incidence of multidrug-resistant Acinetobacter spp., little is known about the factors that contribute to pathogenesis. New strategies for treating and managing infections caused by multidrug-resistant Acinetobacter strains are urgently needed, and this requires a detailed understanding of the pathobiology of these organisms. In recent years, some virulence factors important for Acinetobacter colonization have started to emerge. In this review, we focus on several recently described virulence factors that act at the bacterial surface level, such as the capsule, O-linked protein glycosylation, and adhesins. Furthermore, we describe the current knowledge regarding the type II and type VI secretion systems present in these strains.

Bioinformatic prediction of the antigenic epitopes of recombinant ferritin of Echinococcus granulosus

Echinococcosis is a zoonotic parasitic disease affecting humans and other mammals, which is mainly caused Echinococcus at larval stages. It is predominantly endemic in Chinese pasture regions, including Xinjiang, Qinghai, Gansu and Ningxia. The aim of the present study was to predict the T‑ and B‑combined epitopes of Echinococcus granulosus (Eg). ferritin, and to analyze its secondary structure using online software. Prediction of the T‑ and B‑combined epitopes of Eg. ferritin was performed using IEDB, SYFPEITHI and LEPS software, which are used to identify common areas of T‑ and B‑cells. The results of the present study identified several potential antigenic epitopes of Eg. ferritin, including seven B‑cell antigen epitope amino acid sequences with high values: 8‑16, 54‑61, 70‑75, 80‑90, 103‑109, 117‑124 and 167‑173; and four T‑cell antigen epitope amino acid sequences with high values: 85‑93, 105‑113, 133‑141 and 157‑165. Furthermore, a combined epitope region comprising an 105‑109 amino acid sequence was identified. In conclusion, using bioinformatic methods, the present study confirmed the existence of Eg. ferritin on four T‑cell antigen epitopes, seven B‑cell antigen epitopes, and one T‑ and B‑combined epitope region. These findings provide significant information for further investigation of the antigenicity of Eg. ferritin and the development of highly efficient epitope vaccines.

Cryopreservation of virulent Acinetobacter baumannii to reduce variability of in vivo studies

Background

Microbiological assays require accurate and reproducible preparation of bacterial inocula. Inocula prepared on different days by different individuals can vary significantly from experiment to experiment. This variance is particularly problematic for Gram-negative bacterial infections, for which threshold effects can result in marked variations in host outcome with minor differences in inocula.

Results

We compared the accuracy of traditional methods versus using frozen stocks for preparing Acinetobacter baumannii inocula for infection in mice. Standard inoculum preparation resulted in substantial variability, both with respect to the actual inocula achieved compared to the targeted inocula, and with respect to the in vivo outcome resulting from similar inocula. Cryopreservation of the bacteria resulted in no significant decrement in growth of the bacteria. Furthermore, preparation of multiple infectious inocula from a frozen stock significantly improved the accuracy of the achieved inocula, and resulted in more reproducible in vivo outcomes from infection. Frozen stocks reduced inter-experiment variability associated with inoculum preparation, displayed no significant loss of growth capacity, and maintained virulence, increasing the reliability of infection.

Conclusions

Frozen stocks require considerably less time to prepare and enhance reproducibility of in vivo experimental results when infecting with A. baumannii.

Immunological evaluation of OMV(PagL)+Bap(1-487aa) and AbOmpA(8-346aa)+Bap(1-487aa) as vaccine candidates against Acinetobacter baumannii sepsis infection

Acinetobacter baumannii is an important nosocomial pathogen that causes a high morbidity and mortality rate in infected patients with sepsis form. The surface exposed virulence proteins and serum resistance factors helping to dissemination of this bacterium to bloodstream are the most promising vaccine candidates against this microorganism. In this project we immunologically evaluated OMV(PagL)+Bap(1-487aa) and AbOmpA (8-346aa)+Bap(1-487aa) as combination forms as well as Bap(1-487aa), AbOmpA(8-346aa) and OMV(PagL) singly, with addition of alum adjuvant as vaccine candidates. The titers of total IgG, IgG1 and IgG2c as well as concentration of IL-4 and IFN-γ and survival rates were measured in a C57BL/6 murine model with disseminated sepsis. The ratio of IgG1/IgG2c and profile of IL-4/IFN-γ in OMV (PagL)+Bap (1-487aa) formulation shows the humoral and cellular immune responses have been induced robustly and have created a full protection against A. baumannii ATCC 19606 and MDR AB-44 strains. We found that the two combination vaccine candidates were protective and induced both Th1 and Th2 responses.

Host fate is rapidly determined by innate effector-microbial interactions during Acinetobacter baumannii bacteremia

BACKGROUND

Acinetobacter baumannii is one of the most antibiotic-resistant pathogens. Defining mechanisms driving pathogenesis is critical to enable new therapeutic approaches.

METHODS

We studied virulence differences across a diverse panel of A. baumannii clinical isolates during murine bacteremia to elucidate host-microbe interactions that drive outcome.

RESULTS

We identified hypervirulent strains that were lethal at low intravenous inocula and achieved very high early, and persistent, blood bacterial densities. Virulent strains were nonlethal at low inocula but lethal at 2.5-fold higher inocula. Finally, relatively avirulent (hypovirulent) strains were nonlethal at 20-fold higher inocula and were efficiently cleared by early time points. In vivo virulence correlated with in vitro resistance to complement and macrophage uptake. Depletion of complement, macrophages, and neutrophils each independently increased bacterial density of the hypovirulent strain but insufficiently to change lethality. However, disruption of all 3 effector mechanisms enabled early bacterial densities similar to hypervirulent strains, rendering infection 100% fatal.

CONCLUSIONS

The lethality of A. baumannii strains depends on distinct stages. Strains resistant to early innate effectors are able to establish very high early bacterial blood density, and subsequent sustained bacteremia leads to Toll-like receptor 4-mediated hyperinflammation and lethality. These results have important implications for translational efforts to develop therapies that modulate host-microbe interactions.

The role of filamentous hemagglutinin adhesin in adherence and biofilm formation in Acinetobacter baumannii ATCC19606(T)

Filamentous hemagglutinin adhesins (FHA) are key factors for bacterial attachment and subsequent cell accumulation on substrates. Here an FHA-like Outer membrane (OM) adhesin of Acinetobacter baumannii ATCC19606(T) was displayed on Escherichia coli. The candidate autotransporter (AT) genes were identified in A. baumannii ATCC19606(T) genome. The exoprotein (FhaB1) and transporter (FhaC1) were produced independently within the same cell (FhaB1C1). The fhaC1 was mutated. In vitro adherence to epithelial cells of the recombinant FhaB1C1 and the mutant strains were compared with A. baumanni ATCC19606(T). A bivalent chimeric protein (K) composed of immunologically important portions of fhaB1 (B) and fhaC1 (C) was constructed. The mice vaccinated with chimeric protein were challenged with A. baumannii ATCC19606(T) and FhaB1C1 producing recombinant E. coli. Mutations in the fhaC1 resulted in the absence of FhaB1 in the OM. Expression of FhaB1C1 enhanced the adherence of recombinant bacteria to A546 bronchial cell line. The results revealed association of FhaB1 with bacterial adhesion and biofilm formation. Immunization with a combination of recombinant B and K proteins proved protective against A. baumanni ATCC19606(T). The findings may be applied in active and passive immunization strategies against A. baumannii.

Immunization against multidrug-resistant Acinetobacter baumannii effectively protects mice in both pneumonia and sepsis models

OBJECTIVE

Acinetobacter baumannii is considered the prototypical example of a multi- or pan- drug-resistant bacterium. It has been increasingly implicated as a major cause of nosocomial and community-associated infections. This study proposed to evaluate the efficacy of immunological approaches to prevent and treat A. baumannii infections.

METHODS

Mice were immunized with outer membrane vesicles (OMVs) prepared from a clinically isolated multidrug-resistant strain of A. baumannii. Pneumonia and sepsis models were used to evaluate the efficacy of active and passive immunization with OMVs. The probable effective mechanisms and the protective potential of clonally distinct clinical isolates were investigated in vitro using an opsonophagocytic assay.

RESULTS

Intramuscular immunization with OMVs rapidly produced high levels of OMV-specific IgG antibodies, and subsequent intranasal challenge with A. baumannii elicited mucosal IgA and IgG responses. Both active and passive immunization protected the mice from challenges with homologue bacteria in a sepsis model. Bacterial burden in bronchoalveolar lavage fluids (BALF), lung, and spleen, inflammatory cell infiltration in BALF and lung, and inflammatory cytokine accumulation in BALF was significantly suppressed in the pneumonia model by both active and passive immunization strategies. The antisera from immunized mice presented with significant opsonophagocytic activities in a dose-dependent manner against not only homologous strains but also five of the other six clonally distinct clinical isolates.

CONCLUSIONS

Utilizing immunological characteristics of outer membrane proteins to elevate protective immunity and circumvent complex multidrug-resistance mechanisms might be a viable approach to effectively control A. baumannii infections.

Intranasal immunization protects against Acinetobacter baumannii-associated pneumonia in mice

Multidrug-resistant Acinetobacter baumannii has become an important causative agent of healthcare associated infections. Hospital- and community-acquired pneumonia is the most common clinical manifestation of A. baumannii infection worldwide and is often associated with high mortality. Most experimental vaccine studies to date have evaluated vaccines against systemic A. baumannii infections following systemic immunization. We recently demonstrated that a mouse model of respiratory A. baumannii infection using the strain LAC-4 results in disease progression that is similar to that observed in humans. Here we used this model in conjunction with an inactivated whole cell vaccine to evaluate the feasibility of developing protective mucosal vaccines against respiratory A. baumannii infection and to investigate the potential mechanism of protection of such vaccines. Our results showed that intranasal immunization with formalin-killed whole cells of the LAC-4 strain elicited mucosal and systemic antigen-specific immune responses, and protected mice against lethal intranasal or intraperitoneal challenges. Compared to naïve mice, immunized mice had significantly fewer bacteria in their lungs, and the pathogen was barely detectable in blood and spleens at 24h post challenge, indicating the ability of immunized mice to control extrapulmonary dissemination of the pathogen. Mechanistic studies using gene-deficient mice, neutropenic mice, or passive immunization showed that B cells and neutrophils, but not FcRγ, played crucial roles in the protection against respiratory A. baumannii challenge of intranasally immunized mice whereas passive transfer of hyperimmune sera only prolonged the survival time of challenged mice by 48 h. These results provide immunological insights for the rational design of novel mucosal vaccines to protect against respiratory A. baumannii infection and demonstrate the feasibility to develop such vaccines.

The prediction of T- and B-combined epitope and tertiary structure of the Eg95 antigen of Echinococcus granulosus

Echinococcosis, also known as hydatid disease, is a type of zoonotic parasitic disease caused by the Echinococcus larvae infection. The disease is severely harmful to both humans and animals. Research and development of an epitope vaccine is crucial. To determine the dominant epitopes of the Eg95 antigen, the tertiary structure and the T- and B-combined epitope of the Eg95 protein for Echinococcus granulosus were predicted and analyzed in the present study. The tertiary structure of the Eg95 protein was predicted using the 3DLigandsite server and RasMol software. The T- and B-combined epitope of the Eg95 antigen was analyzed using the DNAStar (V5.0), IEDB, SYFPEITHI and BIMAS. Tertiary structure prediction results showed that there were potential epitopes in Eg95 antigen. Bioinformatics analysis revealed the T- and B-combined epitopes of Eg95 antigen. Four and six T- and B-combined epitopes induced immune responses in humans and mice. Additionally, four T- and B-combined epitopes induced immune responses in both humans and mice. The tertiary structure and T- and B-combined epitopes of the Eg95 protein were also determined. The results obtained in the present study may be beneficial in the investigation of Eg95 antigenicity and the development of dominant epitope vaccines.