IMMUNE CELL RESPONSE TO ORTHOPAEDIC IMPLANTS IS AFFECTED BY BOTH LOCAL FRACTURE CONDITIONS AND CIRCULATING CYTOKINES

INTRODUCTION & AIM Recent work has established that endogenous cell attachment to orthopaedic implants is critical for protection from bacterial adhesion and biofilm formation. Here, we utilize external fixator (ex-fix) pins as a model to investigate the effects of fracture type and circulating inflammatory cytokines on implant-adhered cell phenotype. Hypothesis: Differences in the pin implant microenvironment based on fracture type and in circulating cytokines/signaling molecules will affect cell type prevalence in implant-adhered cells. METHODS In this IRB-approved study, enrolled patients underwent external fixation for fracture or soft tissue stabilization, and the ex-fix pins and a blood sample were collected at the time of external fixation removal. Implant-adhered mononuclear cells were isolated from explanted pins and analyzed via flow cytometry. Four main cell populations were identified: fibroblasts, fibrocytes, innate lymphoid, and leukocytes (including the subtypes of monocytes, macrophages, and other leukocytes). At the time of ex-fix removal, blood was collected and serum isolated for inflammatory cytokine quantification using a 40-target antibody array. RESULTS From the current 51 patient cohort, 66 pin samples have been analyzed. From pins implanted in a fractured bone, there were significant differences in the prevalence of leukocytes and fibroblasts depending on open or closed fracture status (2-way ANOVA, p<0.0001). Significant differences were also observed based on the presence/absence of local antibiotics applied to the fractured bone (2-way ANOVA, p<0.001). No differences in leukocyte subtypes were detected in open vs. closed fractures or based on local antibiotic application. However, the serum concentrations of several inflammatory cytokines and signaling molecules (e.g. IFNg, several interleukins, G-CSF, MIP-1a) were positively correlated with a higher percentage of macrophages (Pearson's r>0.3, p<0.05). CONCLUSIONS The present study observed differences in the proportions of select cell populations adhered to orthopedic implants based on fracture type, local antibiotic application at the time of implant placement, and circulating cytokine concentrations at the time of ex-fix removal. This suggests that both the local environment at the time of implant placement and the systemic inflammatory environment play a role in immune cell-implant interactions. Thus in the “race for the surface”, endogenous cell attachment may be altered by fracture milieu conditions. Ongoing patient accrual and mechanistic studies will provide further insight and potential targets for improved host cell integration toward the goal of infection prevention via surface coatings or modifications. ACKNOWLEDGEMENTS This project was supported by the National Center for Advancing Translational Sciences, National Institutes of Health, through Grant KL2TR001421.