Anti-LPS IgA and IgG Can Inhibit Serum Killing of Pseudomonas aeruginosa in Patients with Cystic Fibrosis

Antibody-Mediated Serum Resistance Protects Pseudomonas aeruginosa During Bloodstream Infections

BACKGROUND

Pseudomonas aeruginosa is a frequent pathogen isolated from bacterial bloodstream infection (BSI) and is associated with high mortality. To survive in the blood, P aeruginosa must resist the bactericidal action of complement (ie, serum killing). Antibodies usually promote serum killing through the classical complement pathway; however, "cloaking antibodies" (cAbs) have been described, which paradoxically protect bacteria from serum killing. The relevance of cAbs in P aeruginosa BSI is unknown.

METHODS

Serum and P aeruginosa were collected from a cohort of 100 patients with BSI. Isolates were tested for sensitivity to healthy control serum (HCS). cAb prevalence was determined in sera. Patient sera were mixed with HCS to determine if killing of the matched isolate was inhibited.

RESULTS

Overall, 36 patients had elevated titers of cAbs, and 34 isolates were sensitive to HCS killing. Fifteen patients had cAbs and HCS-sensitive isolates; of these patients, 14 had serum that protected their matched bacteria from HCS killing. Patients with cAbs were less likely to be neutropenic or have comorbidities.

CONCLUSIONS

cAbs are prevalent in patients with P aeruginosa BSI and allow survival of otherwise serum-sensitive bacteria in the bloodstream. Generation of cAbs may be a risk factor for the development of BSI.

Mechanisms of antibody-dependent enhancement of infectious disease

Antibody-dependent enhancement (ADE) of infectious disease is a phenomenon whereby host antibodies increase the severity of an infection. It is well established in viral infections but ADE also has an underappreciated role during bacterial, fungal and parasitic infections. ADE can occur during both primary infections and re-infections with the same or a related pathogen; therefore, understanding the underlying mechanisms of ADE is critical for understanding the pathogenesis and progression of many infectious diseases. Here, we review the four distinct mechanisms by which antibodies increase disease severity during an infection. We discuss the most established mechanistic explanation for ADE, where cross-reactive, disease-enhancing antibodies bound to pathogens interact with Fc receptors, thereby enhancing pathogen entry or replication, ultimately increasing the total pathogen load. Additionally, we explore how some pathogenic antibodies can shield bacteria from complement-dependent killing, thereby enhancing bacterial survival. We interrogate the molecular mechanisms by which antibodies can amplify inflammation to drive severe disease, even in the absence of increased pathogen replication. We also examine emerging roles for autoantibodies in enhancing the pathogenesis of infectious diseases. Finally, we discuss how we can leverage these insights to improve vaccine design and future treatments for infectious diseases.

Hit 'em Where It Hurts: Gram-Negative Bacterial Lipopolysaccharide as a Vaccine Target

Infections with antimicrobial-resistant (AMR) bacteria pose an increasing threat to the ability to perform surgical procedures, organ transplantation, and treat cancer among many other medical conditions. There are few new antimicrobials in the development pipeline. Vaccines against AMR Gram-negative bacteria may reduce the use of antimicrobials and prevent bacterial transmission. This review traces the origins of lipopolysaccharide (LPS)-based vaccines against Gram-negative bacteria, the role of O polysaccharides and LPS core regions as potential vaccine targets, the development of new vaccine technologies, and their application to vaccines in current development.

Pseudomonas aeruginosa and the Complement System: A Review of the Evasion Strategies

The increasing emergence of multidrug resistant isolates of P. aeruginosa causes major problems in hospitals worldwide. This concern is particularly significant in bloodstream infections that progress rapidly, with a high number of deaths within the first hours and without time to select the most appropriate treatment. In fact, despite improvements in antimicrobial therapy and hospital care, P. aeruginosa bacteremia remains fatal in about 30% of cases. The complement system is a main defensive mechanism in blood against this pathogen. This system can mark bacteria for phagocytosis or directly lyse it via the insertion of a membrane attack complex in the bacterial membrane. P. aeruginosa exploits different strategies to resist complement attack. In this review for the special issue on “bacterial pathogens associated with bacteriemia”, we present an overview of the interactions between P. aeruginosa and the complement components and strategies used by this pathogen to prevent recognition and killing by the complement system. A thorough understanding of these interactions will be critical in order to develop drugs to counteract bacterial evasion mechanisms.

Structural Characterization and Evaluation of an Epitope at the Tip of the A-Band Rhamnan Polysaccharide of Pseudomonas aeruginosa

Pseudomonas aeruginosa produces a variety of cell surface glycans. Previous studies identified a common polysaccharide (PS) antigen often termed A-band PS that was composed of a neutral d-rhamnan trisaccharide repeating unit as a relatively conserved cell surface carbohydrate. However, nuclear magnetic resonance (NMR) spectra and chemical analysis of A-PS preparations showed the presence of several additional components. Here, we report the characterization of the carbohydrate component responsible for these signals. The carbohydrate antigen consists of an immunogenic methylated rhamnan oligosaccharide at the nonreducing end of the A-band PS. Initial studies performed with the isolated antigen permitted the production of conjugates that were used to immunize mice and rabbits and generate monoclonal and polyclonal antibodies. The polyclonal antibodies were able to recognize the majority of P. aeruginosa strains in our collection, and three monoclonal antibodies were generated, one of which was able to recognize and facilitate opsonophagocytic killing of a majority of P. aeruginosa strains. This monoclonal antibody was able to recognize all P. aeruginosa strains in our collection that includes clinical and serotype strains. Synthetic oligosaccharides (mono- to pentasaccharides) representing the terminal 3-O-methyl d-rhamnan were prepared, and the trisaccharide was identified as the antigenic determinant required to effectively mimic the natural antigen recognized by the broadly cross-reactive monoclonal antibody. These data suggest that there is considerable promise in this antigen as a vaccine or therapeutic target.