Innate Immune Responses to Acinetobacter baumannii in the Airway

Anti-OmpA antibodies as potential inhibitors of Acinetobacter baumannii biofilm formation, adherence to, and proliferation in A549 human alveolar epithelial cells

Outer membrane protein A (OmpA) is a critical virulence factor in Acinetobacter baumannii, influencing adhesion, biofilm formation, host immune response, and host cell apoptosis. We investigated the invasion of A549 alveolar epithelial cells by A. baumannii and examined how anti-OmpA antibodies impact these interactions. OmpA was expressed and purified, inducing anti-OmpA antibodies in BALB/c mice. The potential toxicity of OmpA was evaluated in mice by analyzing histology from six organs. A549 cells were exposed to A. baumannii strains 19606 and a clinical isolate. Using cell culture and light microscopy, we scrutinized the effects of anti-OmpA sera on serum resistance, adherence, internalization, and proliferation of A. baumannii in A549 cells. The viability of A549 cells was assessed upon exposure to live A. baumannii and anti-OmpA sera. OmpA-induced antibody demonstrated potent bactericidal effects on both strains of A. baumannii. Both strains formed biofilms, which were reduced by anti-OmpA serum, along with decreased bacterial adherence, internalization, and proliferation in A549 cells. Anti-OmpA serum improved the survival of A549 cells post-infection. Pre-treatment with cytochalasin D hindered bacterial internalization, highlighting the role of actin polymerization in invasion. Microscopic examination revealed varied interactions encompassing adherence, apoptosis, membrane alterations, vacuolization, and damage. A549 cells treated with anti-OmpA serum exhibited improved structures and reduced damage. The findings indicate that A. baumannii can adhere to and proliferate within epithelial cells with OmpA playing a pivotal role in these interactions, and the complex nature of these interactions shapes the intricate course of A. baumannii infection in host cells.

Virulence Factors and Pathogenicity Mechanisms of Acinetobacter baumannii in Respiratory Infectious Diseases

Acinetobacter baumannii (A. baumannii) has become a notorious pathogen causing nosocomial and community-acquired infections, especially ventilator-associated pneumonia. This opportunistic pathogen is found to possess powerful genomic plasticity and numerous virulence factors that facilitate its success in the infectious process. Although the interactions between A. baumannii and the pulmonary epitheliums have been extensively studied, a complete and specific description of its overall pathogenic process is lacking. In this review, we summarize the current knowledge of the antibiotic resistance and virulence factors of A. baumannii, specifically focusing on the pathogenic mechanisms of this detrimental pathogen in respiratory infectious diseases. An expansion of the knowledge regarding A. baumannii pathogenesis will contribute to the development of effective therapies based on immunopathology or intracellular signaling pathways to eliminate this harmful pathogen during infections.

Yinhuapinggan granule ameliorates lung injury caused by multidrug-resistant Acinetobacter baumannii via inhibiting NF-κB/NLRP3 pathway

Yinhuapinggan granule (YHPG) is a traditional Chinese medicine prescription with rich clinical experience for the treatment of colds and coughs. The aim of this study is to investigate the protective effect of YHPG on multidrug-resistant (MDR) Acinetobacter baumannii (A. baumannii) infection in vivo and its potential anti-inflammatory mechanism. BALB/c mice were intranasally inoculated with MDR A. baumannii strain to establish the pneumonia infection model, and received intraperitoneally cyclophosphamide to form immunosuppression before attack. YHPG (6, 12 and 18 g/kg) was administered by gavage once a day for 3 consecutive days after infection. The protective effect of YHPG was evaluated by lung index, spleen index, thymus index, pathological changes of lung tissue and inflammatory factors (IL-1β, IL-6 and TNF-α) in serum. The expression of key targets of NF-κB/NLRP3 signaling pathway in vivo was analyzed by immunohistochemistry, immunofluorescence, reverse transcription quantitative PCR (RT-qPCR) and Western blot. The results showed that YHPG improved the lung index and its inhibition rate, immune organ indexes and lung pathological changes in infected mice, and significantly reduced IL-1β, IL-6 and TNF-α levels in serum. In addition, YHPG significantly down-regulated the mRNA and protein expression of NF-κB p65, NLRP3, ASC, Caspase-1, TNF-α, IL-6 and IL-1β in mice lung tissue. The results of the current study demonstrated that YHPG has significant protective effects on mice infected with MDR A.baumannii, which may be related to the regulation of inflammatory factors and NF-κB/NLRP3 signaling pathway, indicating that YHPG has a wide range of clinical application value and provides a theoretical basis for its treatment of MDR A.baumannii infection.

Replicative Acinetobacter baumannii strains interfere with phagosomal maturation by modulating the vacuolar pH

Bacterial pneumonia is a common infection of the lower respiratory tract that can afflict patients of all ages. Multidrug-resistant strains of Acinetobacter baumannii are increasingly responsible for causing nosocomial pneumonias, thus posing an urgent threat. Alveolar macrophages play a critical role in overcoming respiratory infections caused by this pathogen. Recently, we and others have shown that new clinical isolates of A. baumannii, but not the common lab strain ATCC 19606 (19606), can persist and replicate in macrophages within spacious vacuoles that we called Acinetobacter Containing Vacuoles (ACV). In this work, we demonstrate that the modern A. baumannii clinical isolate 398, but not the lab strain 19606, can infect alveolar macrophages and produce ACVs in vivo in a murine pneumonia model. Both strains initially interact with the alveolar macrophage endocytic pathway, as indicated by EEA1 and LAMP1 markers; however, the fate of these strains diverges at a later stage. While 19606 is eliminated in an autophagy pathway, 398 replicates in ACVs and are not degraded. We show that 398 reverts the natural acidification of the phagosome by secreting large amounts of ammonia, a by-product of amino acid catabolism. We propose that this ability to survive within macrophages may be critical for the persistence of clinical A. baumannii isolates in the lung during a respiratory infection.

Replicative Acinetobacter baumannii strains interfere with phagosomal maturation by modulating the vacuolar pH

Bacterial pneumonia is a common infection of the lower respiratory tract that can afflict patients of all ages. Multidrug-resistant strains of Acinetobacter baumannii are increasingly responsible for causing nosocomial pneumonias, thus posing an urgent threat. Alveolar macrophages play a critical role in overcoming respiratory infections caused by this pathogen. Recently, we and others have shown that new clinical isolates of A. baumannii , but not the common lab strain ATCC 19606 (19606), can persist and replicate in macrophages within spacious vacuoles that we called A cinetobacter C ontaining V acuoles (ACV). In this work, we demonstrate that the modern A. baumannii clinical isolate 398, but not the lab strain 19606, can infect alveolar macrophages and produce ACVs in vivo in a murine pneumonia model. Both strains initially interact with the alveolar macrophage endocytic pathway, as indicated by EEA1 and LAMP1 markers; however, the fate of these strains diverges at a later stage. While 19606 is eliminated in an autophagy pathway, 398 replicates in ACVs and are not degraded. We show that 398 reverts the natural acidification of the phagosome by secreting large amounts of ammonia, a by-product of amino acid catabolism. We propose that this ability to survive within macrophages may be critical for the persistence of clinical A. baumannii isolates in the lung during a respiratory infection.

Neutrophil and Macrophage Response in Acinetobacter Baumannii Infection and Their Relationship to Lung Injury

Acinetobacter baumannii (AB) infection has become a threat to global public health. AB is one of the main pathogens causing nosocomial infections, especially ventilator-associated pneumonia. AB easily develops resistance against a variety of antibiotics, which makes the treatment of AB infections difficult. Therefore, it is necessary to study new treatment plans like anti-infection immunity. Both animal models of AB infection and in vitro cell experiments show that macrophages are activated in the early stage of the immune response and regulate the recruitment of neutrophils, thus playing a role in clearing AB. AB components and the immune responses they induce can lead to injury of the infected organ, mostly in the lungs. Understanding the response of innate immunity to ABs at different stages after infection and the relationship between the response and lung injury can help to develop new immunotherapy methods and prevent lung injury. This article provides a comprehensive review of the response of neutrophils and macrophages to AB infection and their association with lung injury to develop effective therapies for AB infection and prevent lung injury.

Dynamic Interaction Between Mucosal Immunity and Microbiota Drives Nose and Pharynx Homeostasis of Common Carp (Cyprinus carpio) After SVCV Infection

The crosstalk between the immune system and microbiota drives an amazingly complex mutualistic symbiosis. In mammals, the upper respiratory tract acts as a gateway for pathogen invasion, and the dynamic interaction between microbiota and mucosal immunity on its surface can effectively prevent disease development. However, the relationship between virus-mediated mucosal immune responses and microbes in lower vertebrates remains uncharacterized. In this study, we successfully constructed an infection model by intraperitoneally injecting common carp (Cyprinus carpio) with spring viremia of carp virus (SVCV). In addition to the detection of the SVCV in the nose and pharynx of common carp, we also identified obvious histopathological changes following viral infection. Moreover, numerous immune-related genes were significantly upregulated in the nose and pharynx at the peak of SVCV infection, after which the expression levels decreased to levels similar to those of the control group. Transcriptome sequencing results revealed that pathways associated with bacterial infection in the Toll-like receptor pathway and the Nod-like receptor pathway were activated in addition to the virus-related Rig-I-like receptor pathway after SVCV infection, suggesting that viral infection may be followed by opportunistic bacterial infection in these mucosal tissues. Using 16S rRNA gene sequencing, we further identified an upward trend in pathogenic bacteria on the mucosal surface of the nose and pharynx 4 days after SVCV infection, after which these tissues eventually reached new homeostasis. Taken together, our results suggest that the dynamic interaction between mucosal immunity and microbiota promotes the host to a new ecological state.

Early Alterations of Lymphocyte Subsets in Acute Respiratory Distress Syndrome Caused by Acinetobacter baumannii Pneumonia: A Prospective Observational Study

Background: To prospectively observe the early alterations of lymphocyte subsets in ARDS caused by Acinetobacter baumannii.

Methods: ARDS patients admitted to our ICU between January 1, 2017 and May 30, 2020 were selected. We enrolled all the pulmonary ARDS caused by Acinetobacter baumannii pneumonia who required mechanical ventilation or vasopressors. All the available clinical data, follow up information and lymphocyte subsets were recorded.

Results: Eighty-seven of all the 576 ARDS patients were enrolled. The 28-day mortality of the enrolled patients was 20.7% (18/87). The T lymphocyte count (452 vs. 729 cells/ul, P = 0.004), especially the CD8⁺ T lymphocyte count (104 vs. 253 cells/ul, P = 0.002) was significantly lower in non-survivors, as were counts of the activated T cell subsets (CD8⁺CD28⁺ and CD8⁺CD38⁺). The CD8⁺ T cell count was an independent risk factor for 28-day mortality, and a cutoff value of 123 cells/ul was a good indicator to predict the prognosis of ARDS caused by Acinetobacter baumannii pneumonia, with sensitivity of 74.6% and specificity of 83.3% (AUC 0.812, P < 0.0001).

Conclusions: Lower CD8⁺ T cell count was associated with higher severity and early mortality in ARDS patients caused by Acinetobacter baumannii pneumonia, which could be valuable for outcome prediction.

Chimeric vaccine designs against Acinetobacter baumannii using pan genome and reverse vaccinology approaches

Acinetobacter baumannii (A. baumannii), an opportunistic, gram-negative pathogen, has evoked the interest of the medical community throughout the world because of its ability to cause nosocomial infections, majorly infecting those in intensive care units. It has also drawn the attention of researchers due to its evolving immune evasion strategies and increased drug resistance. The emergence of multi-drug-resistant-strains has urged the need to explore novel therapeutic options as an alternative to antibiotics. Due to the upsurge in antibiotic resistance mechanisms exhibited by A. baumannii, the current therapeutic strategies are rendered less effective. The aim of this study is to explore novel therapeutic alternatives against A. baumannii to control the ailed infection. In this study, a computational framework is employed involving, pan genomics, subtractive proteomics and reverse vaccinology strategies to identify core promiscuous vaccine candidates. Two chimeric vaccine constructs having B-cell derived T-cell epitopes from prioritized vaccine candidates; APN, AdeK and AdeI have been designed and checked for their possible interactions with host BCR, TLRs and HLA Class I and II Superfamily alleles. These vaccine candidates can be experimentally validated and thus contribute to vaccine development against A. baumannii infections.

Role of immunodeficiency in Acinetobacter baumannii associated pneumonia in mice

Background

Acinetobacter baumannii (A. baumannii) has become one of the most important opportunistic pathogens inducing nosocomial pneumonia and increasing mortality in critically ill patients recently. The interaction between A. baumannii infection and immune response can influence the prognosis of A. baumannii related pneumonia. The target of the present study was to investigate the role of immunodeficiency in A. baumannii induced pneumonia.

Methods

Male BALB/c mice were randomly divided into the normal immunity control (NIC) group, normal immunity infection (NIA) group, immune compromised control (CIC) group, and immune compromised infection (CIA) group (n = 15 for each group). Intraperitoneal injection of cyclophosphamide and intranasal instillation of A. baumannii solution were used to induce compromised immunity and murine pneumonia, respectively. The mice were sacrificed at 6 and 24 h later and the specimens were collected for further tests. Seven-day mortality of mice was also assessed.

Results

After A. baumannii stimulation, the recruitment of neutrophils in mice with normal immunity increased sharply (P = 0.030 at 6 h), while there was no significant raise of neutrophil counts in mice with compromised immune condition (P = 0.092 at 6 h, P = 0.772 at 24 h). The Th cell polarization presented with pulmonary interleukin (IL)-4 and interferon (IFN)-γ level in response to the A. baumannii in CIA group were significantly depressed in comparison with in NIA group (IFN-γ: P = 0.003 at 6 h; P = 0.001 at 24 h; IL-4: P < 0.001 at 6 h; P < 0.001 at 24 h). The pulmonary conventional dendritic cell accumulation was even found to be inhibited after A. baumannii infection in immunocompromised mice (P = 0.033). Correspondingly, A. baumannii associated pneumonia in mice with compromised immunity caused more early stage death, more severe histopathological impairment in lung.

Conclusion

A. baumannii could frustrate the immune response in immunocompromised conditions, and this reduced immune response is related to more severe lung injury and worse outcome in A. baumannii induced pneumonia.

Acinetobacter baumannii: An Ancient Commensal with Weapons of a Pathogen

Acinetobacter baumannii is regarded as a life-threatening pathogen associated with community-acquired and nosocomial infections, mainly pneumonia. The rise in the number of A. baumannii antibiotic-resistant strains reduces effective therapies and increases mortality. Bacterial comparative genomic studies have unraveled the innate and acquired virulence factors of A. baumannii. These virulence factors are involved in antibiotic resistance, environmental persistence, host-pathogen interactions, and immune evasion. Studies on host–pathogen interactions revealed that A. baumannii evolved different mechanisms to adhere to in order to invade host respiratory cells as well as evade the host immune system. In this review, we discuss current data on A. baumannii genetic features and virulence factors. An emphasis is given to the players in host–pathogen interaction in the respiratory tract. In addition, we report recent investigations into host defense systems using in vitro and in vivo models, providing new insights into the innate immune response to A. baumannii infections. Increasing our knowledge of A. baumannii pathogenesis may help the development of novel therapeutic strategies based on anti-adhesive, anti-virulence, and anti-cell to cell signaling pathways drugs.

Host Innate Immune Responses to Acinetobacter baumannii Infection

Acinetobacter baumannii has emerged as a major threat to global public health and is one of the key human pathogens in healthcare (nosocomial and community-acquired)-associated infections. Moreover, A. baumannii rapidly develops resistance to multiple antibiotics and is now globally regarded as a serious multidrug resistant pathogen. There is an urgent need to develop novel vaccines and immunotherapeutics as alternatives to antibiotics for clinical management of A. baumannii infection. However, our knowledge of host immune responses to A. baumannii infection and the identification of novel therapeutic targets are significantly lacking. This review highlights the recent advances and critical gaps in our understanding how A. baumannii interacts with the host innate pattern-recognition receptors, induces a cascade of inflammatory cytokine and chemokine responses, and recruits innate immune effectors (such as neutrophils and macrophages) to the site of infection for effective control of the infection. Such knowledge will facilitate the identification of new targets for the design and development of effective therapeutics and vaccines to fight this emerging threat.

Biological sex influences susceptibility to Acinetobacter baumannii pneumonia in mice

Acinetobacter baumannii (A. baumannii) is an extremely versatile multidrug-resistant pathogen with a very high mortality rate; therefore, it has become crucial to understand the host response during its infection. Given the importance of mice for modeling infection and their role in preclinical drug development, equal emphasis should be placed on the use of both sexes. Through our studies using a murine model of acute pneumonia with A. baumannii, we observed that female mice were more susceptible to infection. Likewise, treatment of male mice with estradiol increased their susceptibility to infection. Analysis of the airway compartment revealed enhanced inflammation and reduced neutrophil and alveolar macrophage numbers compared with male mice. Depletion of either neutrophils or alveolar macrophages was important for bacterial clearance; however, depletion of alveolar macrophages further exacerbated female susceptibility because of severe alterations in metabolic homeostasis. Our data highlight the importance of using both sexes when assessing host immune pathways.