Age-Associated Transcriptional Programs in Tumor-Proximal Macrophage-Associated Cells during Bone Metastasis

Host age can profoundly remodel the metastatic microenvironment, yet the transcriptional consequences of aging within tumor-proximal myeloid cells remain incompletely defined. In this study, we analyzed a mouse bone-metastasis single-cell RNA-sequencing dataset stratified by age and biotin labeling using SAMENT, an in vivo niche-labeling approach that selectively biotin-labels cells in close proximity to disseminated cancer cells. These neighboring cells are designated biotin-positive (biotin+) cells, whereas unlabeled cells are classified as biotin-negative (biotin−) cells. Sample-level pseudobulk differential expression analysis and Gene Ontology Biological Process gene set enrichment analysis were employed to compare aged and young mice across macrophage, neutrophil, and pre-neutrophil populations. The most pronounced age-associated transcriptional pattern was observed in biotin+ macrophage-associated cells. Young samples exhibited significant enrichment for canonical glycolysis and intrinsic apoptotic signaling pathways, whereas aged samples demonstrated enrichment for bacterial-response and hemopoiesis-regulatory processes. Notably, excluding a granulocyte-like subpopulation attenuated the aged-associated pathways while preserving the young-associated transcriptional directions. These findings reveal distinct age-associated transcriptional reprogramming within tumor-proximal myeloid cells and underscore the considerable heterogeneity present in the myeloid compartment of the metastatic niche. The results highlight the importance of considering host age as a critical variable in understanding tumor-immune interactions during bone metastasis. Further phenotypic and metabolic validation studies will be essential to confirm these transcriptional observations and to elucidate the functional implications of age-dependent myeloid cell states in the metastatic microenvironment.