Modic Change Bone Marrow Neutrophils Are Activated and Degrade Cartilage Endplates.

BACKGROUND

Vertebral endplate bone marrow lesions (Modic changes, MC) are linked to vertebrogenic pain and to structural defects of the bony and cartilaginous endplates (CEP). However, the immune mechanisms that may perpetuate CEP damage in MC remain unclear.

OBJECTIVE

To (i) characterize neutrophils in MC bone marrow and (ii) test whether activated neutrophils can degrade human CEP tissue.

METHODS

In low back pain patients undergoing lumbar fusion, paired bone marrow aspirates were collected intraoperatively from an MC level and an adjacent non-MC vertebra (intra-patient control). MC neutrophils were characterized by bulk RNA sequencing of sorted CD45+CD66b+ cells (n = 7), flow cytometry (activation (CD66b); neutrophil maturation subsets), and neutrophil elastase (NE) activity in short-term culture supernatants. To model CEP degradation, conditioned media from healthy-donor blood neutrophils incubated ± neutrophil activator phorbol 12-myristate 13-acetate (PMA) (12.5 or 25 Mio cells/mL) were applied to human CEP explants (18 h). Proteoglycan loss (sGAG) and collagen loss (hydroxyproline) were quantified as release fractions, normalized to media-only controls.

RESULTS

MC neutrophils showed an activated, pro-inflammatory transcriptomic signature, including enrichment of calcium-associated processes consistent with degranulation. Flow cytometry demonstrated higher CD66b intensity in MC neutrophils versus intra-patient controls (125.4 ± 34.2%, p = 0.022) and a trend toward increased band neutrophils (126.7 ± 33.4%, p = 0.069). NE release was higher in 4/6 patients (191.2 ± 142.7%, p = 0.178) and correlated with band neutrophil abundance (r = 0.67, p = 0.033). PMA activation markedly increased NE activity in conditioned media and was cell-number dependent. CEP exposure to activated neutrophil conditioned media induced significant sGAG release (25 Mio/mL: 380.1 ± 177.0%, p = 0.012; 12.5 Mio/mL: 123.7 ± 22.3%, p = 0.048), while hydroxyproline release was not significantly increased.

CONCLUSIONS

Neutrophils within MC bone marrow have an activated pro-inflammatory phenotype, and activated neutrophils can degrade CEP proteoglycans ex vivo. These findings support a potential immune-mediated mechanism contributing to CEP weakening and indicate that MC are not just reactive changes but themselves have degenerative effects.