Immunoglobulin G subclass switching impacts sensitivity of an immunoassay targeting Francisella tularensis lipopolysaccharide

The CDC Tier 1 select agent Francisella tularensis is a small, Gram-negative bacterium and the causative agent of tularemia, a potentially life-threatening infection endemic in the United States, Europe and Asia. Currently, there is no licensed vaccine or rapid point-of-care diagnostic test for tularemia. The purpose of this research was to develop monoclonal antibodies (mAbs) specific to the F. tularensis surface-expressed lipopolysaccharide (LPS) for a potential use in a rapid diagnostic test. Our initial antigen capture ELISA was developed using murine IgG3 mAb 1A4. Due to the low sensitivity of the initial assay, IgG subclass switching, which is known to have an effect on the functional affinity of a mAb, was exploited for the purpose of enhancing assay sensitivity. The ELISA developed using the IgG1 or IgG2b mAbs from the subclass-switch family of 1A4 IgG3 yielded improved assay sensitivity. However, surface plasmon resonance (SPR) demonstrated that the functional affinity was decreased as a result of subclass switching. Further investigation using direct ELISA revealed the potential self-association of 1A4 IgG3, which could explain the higher functional affinity and higher assay background seen with this mAb. Additionally, the higher assay background was found to negatively affect assay sensitivity. Thus, enhancement of the assay sensitivity by subclass switching is likely due to the decrease in assay background, simply by avoiding the self-association of IgG3.

In vivo Distribution and Clearance of Purified Capsular Polysaccharide from Burkholderia pseudomallei in a Murine Model

Burkholderia pseudomallei is the causative agent of melioidosis, a severe infection prominent in northern Australia and Southeast Asia. The “gold standard” for melioidosis diagnosis is bacterial isolation, which takes several days to complete. The resulting delay in diagnosis leads to delayed treatments, which could result in death. In an attempt to develop better methods for early diagnosis of melioidosis, B. pseudomallei capsular polysaccharide (CPS) was identified as an important diagnostic biomarker. A rapid lateral flow immunoassay utilizing CPS-specific monoclonal antibody was developed and tested in endemic regions worldwide. However, the in vivo fate and clearance of CPS has never been thoroughly investigated. Here, we injected mice with purified CPS intravenously and determined CPS concentrations in serum, urine, and major organs at various intervals. The results indicate that CPS is predominantly eliminated through urine and no CPS accumulation occurs in the major organs. Immunoblot analysis demonstrated that intact CPS was excreted through urine. To understand how a large molecule like CPS was eliminated without degradation, a 3-dimenational structure of CPS was modeled. The predicted CPS structure has a rod-like shape with a small diameter that could allow it to flow through the glomerulus of the kidney. CPS clearance was determined using exponential decay models and the corrected Akaike Information Criterion. The results show that CPS has a relatively short serum half-life of 2.9 to 4.4 hours. Therefore, the presence of CPS in the serum and/or urine suggests active melioidosis infection and provides a marker to monitor treatment of melioidosis.

An outer membrane component, capsular polysaccharide (CPS), is a virulence factor expressed by many Gram-negative bacteria including Burkholderia pseudomallei, the causative agent of melioidosis. Recently, B. pseudomallei CPS was identified as a useful diagnostic biomarker, leading to the development of a lateral flow immunoassay (LFI) targeting CPS for B. pseudomallei detection. In this current work, we studied the in vivo fate of CPS using a murine model, to better understand the clinical applications and potential limitations of the LFI. Interestingly, we found that B. pseudomallei CPS has a unique set of characteristics (as compared to other bacterial capsule antigens) including rapid kidney clearance from serum, no deposition in major internal organs, and ability to be cleared without degradation. Clinically, these findings suggest that CPS may be a potential biomarker for detecting active melioidosis and monitoring melioidosis treatment outcome. Additionally, urine may be used as a non-invasive sample for detecting melioidosis.