Background: Recent experimental approaches have unraveled essential migratory and functional differences of monocyte subpopulations in mice. In order to possibly translate these findings into human physiology and pathophysiology, human monocyte subsets need to be carefully revisited in health and disease. In analogy to murine studies, we hypothesized that human monocyte subsets dynamically change during ageing, potentially influencing their functionality and contributing to immunosenescence. Results: Circulating monocyte subsets, surface marker and chemokine receptor expression were analyzed in 181 healthy volunteers (median age 42, range 18-88). Unlike the unaffected total leukocyte or total monocyte counts, non-classical CD14⁺CD16⁺ monocytes significantly increased with age, but displayed reduced HLA-DR and CX₃CR1 surface expression in the elderly. Classical CD14⁺⁺CD16^- monocyte counts did not vary dependent on age. Serum MCP-1 (CCL2), but not MIP1α (CCL3), MIP1β (CCL4) or fractalkine (CX₃CL1) concentrations increased with age. Monocyte-derived macrophages from old or young individuals did not differ with respect to cytokine release in vitro at steady state or upon LPS stimulation. Conclusions: Our study demonstrates dynamic changes of circulating monocytes during ageing in humans. The expansion of the non-classical CD14⁺CD16⁺ subtype, alterations of surface protein and chemokine receptor expression as well as circulating monocyte-related chemokines possibly contribute to the preserved functionality of the monocyte pool throughout adulthood.

Hepatocellular carcinoma (HCC) typically arises in fibrotic or cirrhotic livers, which are characterized by pathogenic angiogenesis. Myeloid immune cells, specifically tumor-associated macrophages (TAMs), may represent potential novel therapeutic targets in HCC, complementing current ablative or immune therapies. However, the detailed functions of TAM subsets in hepatocarcinogenesis have remained obscure. TAM subsets were analyzed in-depth in human HCC samples and a combined fibrosis-HCC mouse model, established by i.p. injection with diethylnitrosamine after birth and repetitive carbon tetrachloride (CCl4) treatment for 16 weeks. Based on comprehensively phenotyping TAM subsets (fluorescence-activated cell sorter, transcriptomics) in mice, the function of CCR2+ TAM was assessed by a pharmacologic chemokine inhibitor. Angiogenesis was evaluated by contrast-enhanced micro-computed tomography and histology. We show that human CCR2+ TAM accumulate at the highly vascularized HCC border and express the inflammatory marker S100A9, whereas CD163+ immune-suppressive TAM accrue in the HCC center. In the fibrosis-cancer mouse model, we identified 3 major hepatic myeloid cell populations with distinct messenger RNA profiles, of which CCR2+ TAM particularly showed activated inflammatory and angiogenic pathways. Inhibiting CCR2+ TAM infiltration using a pharmacologic chemokine CCL2 antagonist in the fibrosis-HCC model significantly reduced pathogenic vascularization and hepatic blood volume, alongside attenuated tumor volume. The HCC microenvironment in human patients and mice is characterized by functionally distinct macrophage populations, of which the CCR2+ inflammatory TAM subset has pro-angiogenic properties. Understanding the functional differentiation of myeloid cell subsets in chronically inflamed liver may provide novel opportunities for modulating hepatic macrophages to inhibit tumor-promoting pathogenic angiogenesis.