Integrated histopathology and single-cell transcriptomics reveal distinct pathological features and foamy macrophage characteristics in spinal tuberculosis and brucellar spondylitis

Abstract

Background Spinal tuberculosis (STB) and brucellar spondylitis (BS) show substantial overlap in clinical and imaging manifestations, but their local immunopathological programs and corresponding cellular states have not been systematically compared. This study integrated histopathology and single-cell transcriptomics to compare lesion-level differences between STB and BS and to characterize macrophage states associated with the foamy cell reaction. Methods This retrospective case-control study included 80 STB cases and 80 BS cases treated between January 2020 and December 2024. H&E-stained lesion tissues from the full surgical cohort were reviewed for comparative histopathology. Fresh lesion nucleus pulposus tissue from six representative cases (STB = 3, BS = 3) underwent scRNA-seq re-analysis with cell-type annotation, myeloid subclustering, functional module scoring, GSVA, and CellChat analysis. Dual immunofluorescence for CD68 paired with CYP27A1, PLIN2, or PPARγ was used as tissue-level validation. The scRNA-seq workflow focused on the monocyte-macrophage lineage to evaluate lipid-remodelled states associated with the foamy cell reaction. Tissue-level immunofluorescence and supplementary statistical summaries were interpreted as orthogonal support for the transcriptomic findings rather than as stand-alone diagnostic criteria. Results Histopathological comparison showed that STB and BS were both dominated by chronic inflammation but exhibited distinct lesion patterns. STB was characterized mainly by tuberculous nodules, caseous necrosis, Langhans giant cells, and granulomatous inflammation, whereas BS more often showed plasma cell infiltration, foamy cell reaction, eosinophil infiltration, and fibrotic repair. These between-group differences were statistically significant (P < 0.05 or P < 0.01, as appropriate). scRNA-seq yielded 67,274 high-quality cells and identified nine major cell types. Re-clustering of the Mono_Macrophage_DC lineage identified 15 subclusters, among which Macro4 was relatively enriched in BS. Rather than showing the highest value for every broad foam-cell-related metric, Macro4 was supported by convergent evidence including relative enrichment in BS, coordinated expression of PPARG, APOE, APOC1, PLIN2, and CYP27A1, and higher lipid-droplet-formation and cholesterol-efflux-related programs in BS_Macro4. Dual immunofluorescence showed higher CD68-normalized overlap fractions for CYP27A1, PLIN2, and PPARγ in BS lesion tissues than in STB lesion tissues (P < 0.05). CellChat analysis further suggested that Macro4 contributed relatively more to the HGF-MET signalling context in BS, although these ligand-receptor relationships should be interpreted as transcriptome-inferred candidate interactions. Conclusions STB and BS exhibit distinct local immunopathological programs. STB is characterized by a granulomatous-necrotizing inflammatory pattern, whereas BS more often shows a chronic inflammatory and tissue-repair-associated lesion background with a more prominent foamy cell reaction. Macro4 is best interpreted as a BS-associated lipid-remodelled macrophage state supported by convergent histological and transcriptomic evidence. These findings provide a lesion-level cellular framework for further pathological and mechanistic study rather than a validated stand-alone diagnostic cutoff.