Species-Specific Immunoassay for Diagnosis and Tracking Treatment Response in Foot and Ankle Infection

Category: Basic Sciences/Biologics; Diabetes Introduction/Purpose: Staphylococcus aureus is the major pathogen foot and ankle infections and osteomyelitis (>50%). There are currently no sensitive and specific diagnostic tools for monitoring a pathogen’s ongoing infection or providing prognostic measures. We have developed a novel immunoassay for S. aureus, and have applied this to diagnose and monitor its infectivity. We hypothesize that: 1) the species-specific immunoassay can serve as a reliable diagnostic tool for S. aureus foot and ankle infections and 2) the immunoassay provides a measure of treatment response and prognosis of clinical outcome to antibiotics therapy for S. aureus foot and ankle infections. Methods: From July 2015 to July 2019, 83 infected diabetic foot ulcer (DFU) patients undertaking surgical treatment were recruited. Blood were drawn from subjects at initial, 4-week, 8-week, and 12-week after surgery. Clinical wound healing status was determined by a fellowship-trained orthopaedic foot and ankle surgeon. Serum antibodies and plasmablast cultured antibodies (newly synthesized antibodies: NSA) were harvested. Eight unique S. aureusantigens from distinct functional classes were used for the immunoassay. All serum and NSA samples were run on a flow cytometer (Bio-Plex 200; Bio-Rad, Life Sciences Research) in duplicates and assessed for predictive ability in discriminating infection status and healing status using receiver operating characteristic (ROC) curve analysis, with accuracy summarized by the area under the ROC curve (AUC). Nonparametric estimates and 95% confidence intervals for the AUC were computed for each predictor along with p-values for testing the significance of each AUC. Results: Analysis of serum immunoassay showed significant difference in three anti-S. aureus antigens titers (IsdH (p = 0.037; AUC = 0.638), ClfB (p = 0.025; AUC = 0.644), and SCIN (p = 0.005; AUC = 0.677)) between S. aureus infected versus non- S. aureus infected DFU patients at initial presentation. NSA immunoassay showed elevation of two different S. aureus specific antigens, IsdH and LukS-PV, for S. aureus infected versus non- S. aureus infected DFU patients in ratios of approximately five and four, respectively. Changes of NSA based anti-S. aureus antibody titers over 12 weeks period, as single antigen or in combination, significantly correlated with clinical resolution of infection and wound healing status. Four anti-S. aureus antigen combinations achieved the highest AUC (Figure 1). Conclusion: Our results demonstrate that both the serum and NSA immunoassay can diagnose S. aureus infected DFUs. Furthermore, changes in NSA titers over a period of time against various S. aureus specific antigens significantly correlated with clinical representation of infection and wound healing status. The novel species-specific immunoassay can serve as a promising diagnostic system, tracking tool and prognostic potential in management of S. aureus associated foot and ankle infection.