In chronic osteomyelitis, inflammatory markers such as CRP and full blood count can be abnormal, yet they often do not correlate clearly with ongoing bone destruction or recurrence. This raises a recurring question in fracture-related infection and implant-associated infection: is persistent disease driven by more inflammation, or by differently organised inflammation?
Summary
Peripheral immune cell distributions in chronic osteomyelitis differ from classic acute patterns.
Genetically predicted higher lymphocyte levels are associated with osteomyelitis risk (OR 1.20).
Combined inflammatory markers distinguish chronic osteomyelitis from healthy controls with high AUC (0.988).
Experimental models show that immune cell recruitment and checkpoint signalling alter bacterial burden and bone loss.
Osteolysis in infection is not explained solely by RANKL-driven osteoclast activation.
Why This Matters
Chronic osteomyelitis and fracture-related infection remain associated with substantial morbidity, repeated surgical procedures and prolonged systemic antibiotic exposure. Despite debridement, hardware removal or retention strategies, and targeted antimicrobial therapy, recurrence rates in complex bone infection remain clinically significant in long-term series.
Inflammatory markers such as CRP and ESR are widely used in diagnosis and follow-up, yet they do not consistently predict persistence or bone destruction. Understanding how immune regulation behaves in chronic infection may clarify why technically adequate treatment does not always result in biological resolution.
What the Evidence Shows
Peripheral immune cell distributions differ from acute patterns
Patients with osteomyelitis show altered immune cell distributions compared with individuals undergoing internal fixation removal. In a retrospective cohort, neutrophil counts were reduced while lymphocyte counts were increased in osteomyelitis patients. Mendelian randomisation analysis in the same study demonstrated that genetically predicted higher circulating lymphocyte levels were associated with osteomyelitis risk (odds ratio 1.20, 95% confidence interval 1.06–1.36) (Liu, 2025). These findings describe a pattern distinct from the classic neutrophil-dominant response of acute infection.
Systemic inflammatory markers remain elevated but are context-dependent
In a case–control comparison of 100 chronic osteomyelitis patients and 100 healthy controls, CRP, neutrophil-to-lymphocyte ratio, TNF alpha and IL-6 were significantly higher in patients. When combined, these markers showed high discrimination between cases and healthy controls with an area under the curve of 0.988 (Zhao, 2025). This performance reflects comparison with healthy individuals and does not directly establish diagnostic accuracy in real-world differential settings.
Genetic analyses suggest immune set-point influences susceptibility
Genetic studies provide a different perspective on risk. Two-sample Mendelian randomisation uses naturally occurring genetic variants as proxies for lifelong differences in immune traits. Rather than measuring immune cells after infection has developed, this method asks whether people who are genetically predisposed to certain immune profiles have a higher or lower likelihood of developing osteomyelitis.
In one analysis, genetically predicted higher CD27 expression on switched memory B cells and higher counts of CD62L-positive plasmacytoid dendritic cells were associated with increased risk. Higher CD127 expression on CD8-positive T cells appeared protective (Yang, 2025). These markers relate to adaptive immune regulation, suggesting that baseline immune configuration may influence susceptibility.
A separate Mendelian randomisation study screening 91 inflammatory proteins identified associations between genetically predicted osteoprotegerin and monocyte chemoattractant proteins and osteomyelitis risk (Ren, 2025). Osteoprotegerin is linked to bone remodelling, while monocyte chemoattractant proteins regulate immune cell recruitment.
These findings describe inherited susceptibility patterns rather than immune profiles measured during active infection. They generate hypotheses about host biology, not bedside diagnostic markers.
Immune recruitment and checkpoint signalling influence infection severity in models
In an implant-associated Staphylococcus aureus osteomyelitis model, researchers examined a signalling pathway responsible for guiding specific immune cells to the site of infection. The CCL20–CCR6 axis functions as a chemokine “homing” signal that attracts subsets of T cells and macrophages into infected bone. When this pathway was genetically disrupted, immune cell recruitment to the implant site was reduced and bacterial burdens were higher compared with wild-type animals (Meghwani, 2025). This suggests that not only the strength of inflammation, but the correct trafficking of immune cells into infected tissue, influences bacterial control.
A separate murine model focused on immune exhaustion. During chronic infection, T cells can upregulate inhibitory receptors such as PD-1, which dampen their antibacterial activity. In this study, blockade of the PD-1–PD-L1 pathway restored immune cell function, reduced bacterial load, and improved bone mineral density and bone volume fraction compared with control-treated animals (Li, 2023). These findings indicate that excessive immune inhibition can contribute to bacterial persistence and bone damage in experimental osteomyelitis.
Both studies derive from controlled animal models. They demonstrate that immune regulation, including cell recruitment and functional inhibition, can directly alter infection severity and bone destruction in vivo. They do not establish clinical efficacy of immunomodulatory therapy in human chronic osteomyelitis.
Osteolysis is not solely driven by RANKL signalling
Bone destruction in chronic osteomyelitis is often interpreted through the lens of osteoclast activation. RANKL signalling is a central pathway driving osteoclast differentiation and bone resorption in inflammatory bone disease. It would therefore be intuitive to assume that blocking RANKL might reduce infection-associated osteolysis.
In a translational study combining a porcine implant-associated osteomyelitis model with human fracture-related infection tissue analysis, pharmacological inhibition of RANKL signalling did not reduce osteolysis in the animal model (Peterlin, 2026). Despite pathway inhibition, radiographic and histological bone loss persisted.
When human infection tissue was examined, osteolytic cases showed significantly higher expression of MMP1, a matrix metalloproteinase involved in collagen degradation. RANKL expression did not differ between osteolytic and non-osteolytic infections. This suggests that direct enzymatic degradation of bone matrix, rather than osteoclast activation alone, contributes substantially to bone loss in chronic infection.
These findings do not exclude a role for osteoclasts. They indicate that infection-associated osteolysis is multifactorial and cannot be reduced to a single inflammatory pathway. For surgeons, this reinforces that bone destruction in chronic osteomyelitis reflects complex tissue-level biology rather than a straightforward inflammatory cascade.
Mechanisms Behind the Pattern
Bacterial immune evasion and persistence
Staphylococcus aureus employs biofilm formation, small colony variant development, invasion of the osteocyte lacuno-canalicular network and modulation of antibody responses via protein A. Reviews describe how these mechanisms alter both innate and adaptive immune responses in chronic osteomyelitis (Chen, 2023; Song, 2025). These descriptions are derived largely from experimental and translational literature.
In practical terms, the bacterium does not remain exposed on the bone surface. It can hide within biofilm, shift into slower-growing variants, and penetrate microscopic bone channels that are difficult for immune cells and antibiotics to reach. At the same time, protein A interferes with antibody function, weakening part of the adaptive immune response. Together, these strategies allow bacteria to persist even when inflammation is present.
Immune exhaustion and altered macrophage balance
Experimental data demonstrate upregulation of immune checkpoint pathways such as PD-1–PD-L1 during infection, with functional consequences for bacterial clearance in animal models (Li, 2023). Reviews further describe altered macrophage polarisation and neutrophil-mediated tissue injury in chronic infection (Chen, 2023). These mechanisms support interpretation but do not establish causality in human chronic osteomyelitis.
In simpler terms, parts of the immune system can become functionally inhibited during prolonged infection. T cells may become less effective at killing bacteria, and macrophages may shift into states that favour ongoing inflammation rather than efficient clearance. Neutrophils, while essential for bacterial control, can also contribute to collateral tissue damage when activation is prolonged. These observations help explain how infection and inflammation can coexist without resolution.
Practical implications for clinical interpretation
Peripheral blood cell counts in chronic osteomyelitis may not mirror acute neutrophil-dominant patterns (Liu, 2025).
High combined inflammatory marker performance against healthy controls does not equate to diagnostic certainty in complex fracture-related infection (Zhao, 2025).
Genetic susceptibility signals describe risk, not bedside immune profiling (Yang, 2025; Ren, 2025).
Experimental modulation of immune pathways demonstrates biological relevance but remains preclinical (Li, 2023; Meghwani, 2025).
Bone destruction may involve proteolytic mechanisms beyond osteoclast activation (Peterlin, 2026).
Common pitfalls
Assuming normal CRP excludes chronic infection. Chronic immune adaptation may blunt systemic markers.
Interpreting lymphocytosis as unrelated or viral in origin. Chronic osteomyelitis can present with elevated lymphocytes.
Assuming chronic osteomyelitis is simply prolonged acute inflammation.
Interpreting high AUC values from case–control studies as real-world diagnostic accuracy
Closing note
Chronic osteomyelitis reflects sustained inflammatory activity, but the pattern of immune coordination differs from acute infection. Recognising this distinction may clarify why recurrence and bone destruction persist despite technically adequate surgical and antimicrobial management.
References
Chen C, et al. The effect of Staphylococcus aureus on innate and adaptive immunity and potential immunotherapy for osteomyelitis. Front Immunol. 2023.
Li X, et al. PD-1/PD-L1 blockade is a potent adjuvant in treatment of Staphylococcus aureus osteomyelitis in mice. 2023.
Liu Z, et al. Trends in immune cell profiles of osteomyelitis: a clinical study supported by Mendelian randomization. Front Med. 2025.
Meghwani H, et al. CCL20-CCR6 signaling modulates disease severity during the establishment of Staphylococcus aureus implant-associated osteomyelitis. mBio. 2025.
Peterlin P, et al. Translational investigation of osteolysis mechanisms in implant-associated osteomyelitis and fracture-related infection. APMIS. 2026.
Ren Y, Guo X, et al. Causal relationship between circulating inflammatory proteins and osteomyelitis: a Mendelian randomization study. 2025.
Song Y, et al. Comprehensive review of the pathology and treatment of Staphylococcus aureus osteomyelitis. Clin Exp Med. 2025.
Yang H, et al. Effect of immune cells and plasma metabolites on osteomyelitis: a two-sample Mendelian randomization study. Arch Med Sci. 2025.
Zhao Y, et al. Diagnostic value of neutrophil-to-lymphocyte ratio and serum biomarkers in chronic osteomyelitis. Sci Rep. 2025.