(527) A paradoxical role of antibodies to Pseudomonas aeruginosa in cystic fibrosis chronic pulmonary infections

Background: Pseudomonas aeruginosa is a multi-drug resistant, opportunistic pathogen which causes recalcitrant pulmonary infections in individuals with cystic fibrosis (CF). Once chronic infection is established, P. aeruginosa cannot currently be eradicated with available therapeutics and results in increased morbidity and mortality. Our laboratory discovered a subset of patients with P. aeruginosa lung infections to have high titers of O-antigen specific IgG2 and IgA which instead of protecting against infection, inhibited complement-mediated serum killing of infecting strains. These were termed ‘cloaking antibodies’ (cAbs) and were associated with worse lung function in non-CF bronchiectasis and associated with an increased risk of chronic lung allograft dysfunction in lung transplant recipients. Additionally, on three separate occasions we demonstrated that removal of these cAbs via plasmapheresis and supplementation with donor intravenous immunoglobulin (IV-Ig) offers profound clinical benefits and elimination of infecting multi-drug resistant P. aeruginosa. More recently, cAbs to P. aeruginosa were identified in the serum of 32% of individuals with CF, however their clinical importance in this disease setting remains unclear. Here, we aimed to investigate the in vivo relevance of cAbs within CF lung secretions at the site of infection and determine their impact on disease.

Methods: We collected health data, serum, sputum, and isolated infecting P. aeruginosa (n = 128) from 43 individuals with CF. Serum and sputum were screened for IgG2, IgM, IgA, IgA1 and IgA2 to lipopolysaccharide (LPS) from three common O-antigen serotypes of P. aeruginosa (O1, O3 & O6) via ELISA, and their ability to kill infecting isolates determined via serum bactericidal assays. Results were compared to multiple parameters of disease including forced expiratory volume in the first second (FEV1) and body mass index (BMI).

Results: We found 30.3% of patients had high titers of P. aeruginosa LPS-specific IgG2 or IgA in their serum that could inhibit complement killing of their cognate bacterial isolate, corroborating previous findings. In contrast, only high titers of P. aeruginosa LPS-specific IgA were detected in CF sputum which directly correlated to levels within serum. However, only three patient’s antibody levels were sufficient to inhibit bacterial complement killing within sputum. Independently of cAbs, increased P. aeruginosa LPS-specific IgA and not total IgA correlated to worse lung function (FEV1) in both serum and sputum.

Conclusions: These findings indicate dysregulation in the humoral immune response to P. aeruginosa in CF. Whilst the presence of cAbs at the site of infection was limited compared to within serum, our results indicate that P. aeruginosa LPS-specific IgA may play a role in CF pathophysiology and infection persistence via other antibody-mediated disease mechanisms warranting further investigation. Understanding the role of pathogen-specific antibody responses in CF is crucial for the development of novel therapeutics and make treatments such as plasmapheresis and donor IV-Ig more readily available that have proven to be useful to treat multi-drug resistant P. aeruginosa in the context of cAbs.

Acknowledgements: This work was supported by the Cystic Fibrosis Foundation (US) Pilot & Feasibility Award 2020, The Prince Charles Hospital Foundation New Investigator grant 2020 and The Australian Cystic Fibrosis Research Trust (ACFRT) PhD top up scholarship 2021.

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