In chronic osteomyelitis, CRP and peripheral blood cell counts can be abnormal, yet they often do not correlate clearly with ongoing bone destruction or recurrence. Persistent disease may therefore reflect altered organisation of the immune response rather than simply greater inflammatory activity.
Summary
Peripheral immune cell distributions in chronic osteomyelitis differ from classic acute patterns.
A lifelong tendency towards higher lymphocyte levels was 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. Persistent or recurrent infection can occur despite technically adequate surgery and antimicrobial treatment, indicating that pathogen clearance also depends on the host response within infected bone.
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. A genetic analysis in the same study linked a lifelong tendency towards higher circulating lymphocyte levels with osteomyelitis risk (odds ratio 1.20, 95% confidence interval 1.06–1.36) (Liu, 2025). The observed increase in lymphocytes and reduction in neutrophils differs from the neutrophil-dominant response associated with 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.
Inherited immune traits and susceptibility
Genetic analyses can examine whether inherited differences that influence immune traits are associated with osteomyelitis risk. Because these differences are present before infection develops, they provide information about susceptibility rather than the immune response during active disease.
Higher CD27 expression on switched memory B cells was associated with increased osteomyelitis risk, while higher CD127 expression on CD8-positive T cells was associated with lower risk (Yang, 2025). These markers relate to adaptive immune regulation, suggesting that baseline immune configuration may influence susceptibility.
A genetic analysis of 91 circulating inflammatory proteins found associations between osteomyelitis risk and osteoprotegerin, CCL4, MCP-4 and MCP-3. Osteoprotegerin, CCL4 and MCP-4 were associated with higher risk, while MCP-3 was associated with lower risk (Ren, 2025). The opposing directions of these associations indicate that susceptibility is unlikely to be explained by a single inflammatory pathway.
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, the CCL20–CCR6 axis functioned as a chemokine “homing” signal that guided CCR6-expressing immune cells, particularly T cells, into infected bone. When this pathway was disrupted, recruitment of these cells to the implant site was reduced and bacterial burdens were higher than in wild-type animals (Meghwani, 2025).
A murine implant-associated osteomyelitis model examined PD-1–PD-L1 signalling, an inhibitory pathway that can suppress immune cell function during persistent infection. Blocking this pathway alongside gentamicin improved macrophage bacterial killing, reduced bacterial load and preserved bone structure (Li, 2023). These findings indicate that excessive immune inhibition can contribute to bacterial persistence and bone damage in experimental osteomyelitis.
In these animal models, changes in immune cell recruitment and checkpoint signalling altered bacterial burden and bone loss. The findings establish biological effects in experimental osteomyelitis, but provide no evidence of clinical benefit from immunomodulatory treatment in patients.
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 can persist through biofilm formation, small colony variant development and invasion of the osteocyte lacuno-canalicular network. It also produces protein A, a cell-wall protein that binds IgG and interferes with antibody-mediated recognition and phagocytic clearance. These mechanisms can protect bacteria from both immune attack and antimicrobial exposure in chronic osteomyelitis (Chen, 2023; Song, 2025).
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.
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
Using a normal CRP result to exclude chronic osteomyelitis. Systemic inflammatory markers have limited diagnostic value when interpreted in isolation.
Treating the lymphocyte pattern reported in one cohort as a diagnostic feature. The finding requires confirmation and does not establish lymphocytosis as a marker of chronic osteomyelitis.
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 involves persistent inflammation, but the immune response is not simply an extended acute response. Experimental and genetic evidence points to changes in immune cell recruitment, functional inhibition and bone matrix degradation, although their clinical importance remains incompletely defined.
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.