Three Potential Biomarkers for Foot and Ankle Infection
Authors: Agidigbi Taiwo , Yi Young , Oh Irvin 
Published: December 2024

Category: Basic Sciences/Biologics; Diabetes Introduction/Purpose: Deep foot and ankle infections, such as septic arthritis, periprosthetic joint infection, and infected diabetic foot, presents a diagnostic challenge due to overlapping symptoms with other conditions such as inflammatory arthritis or neurogenic arthropathy. Failed arthrocentesis or negative cultures poses additional clinical challenges. Developing blood sample-based biomarkers may facilitate diagnosis of infection. We conducted transcriptomic analysis of peripheral blood mononuclear cells (PBMC) of the foot and ankle infection patients and observed differential gene expression. Most notably, the Ribonucleotide Reductase Regulatory Submit M2 (RRM2), Myb-related protein B (MYBL2), and CD177 (a neutrophil-specific cell surface glycoprotein) were upregulated during active diabetic foot infection. We investigated if these proteins may function as PBMC biomarkers for diagnosing deep foot and ankle infection. Methods: Patient exhibiting clinical symptoms and signs of foot and ankle infections (n=20) and a normal control group (n=5) were recruited from the PI’s clinical practice. Diagnosis included diabetic foot infections (17), septic arthritis of the ankle (2) and infected total ankle replacement (1). Blood samples were collected at enrollment and processed to yield PBMC and serum. Infected tissue samples were collected during surgical debridement for standard culture. RRM2 is involved in immune modulation and activates macrophages. MYBL2 activates the RRM2 by binding to its promoter. CD177 implies intensified immune response with the proliferation of neutrophils associated with active foot and ankle infections. The transcription and proteome of RRM2, MYBL2, and CD177 in PBMC and serum were qualitatively and quantitatively assessed through RT-qPCR and ELISA analysis and compared to the control group. We employed unpaired Student’s t-test and non-parametric Mann-Whitney U test for the comparative analysis. Results: Significantly upregulated expressions of the RRM, MYBL2, and CD177 genes were noted in the PBMC of infected patients group compared to control group (p < 0.0001, p=0.028, p< 0.0001, respectively). The PBMC and serum levels of RRM2 were significantly higher in the infected patients compared to the control group (p < 0.0001 and p=0.0072, respectively). Although the MYBL2 showed a significantly increased level in PBMC (p=0.0057), the serum level did not show a significant difference (p=0.1127). The CD177 levels in PBMC (p < 0.0001) and serum (p < 0.0001) were significantly increased compared to those in the control group. Overall, the differential expressions of the three proteins were more pronounced in PBMC compared to the serum sample (Figure 1). Conclusion: RRM2, MYBL2, and CD 177 proteins levels were significantly higher in PBMC of foot and ankle infection patients compared to the control group. These biomarkers may supplement standard culture of infected specimens obtained through arthrocentesis or surgical debridement and facilitate diagnosis of foot and ankle infection. They may assist in differentiating active infection from other inflammatory conditions, such as crystal-induced arthritis, inflammatory arthritis or neurogenic arthropathy. Further exploratory research is needed to formally confirm these initial observations and their clinical significance.