Macrophages in lungs can be classified into two subpopulations, alveolar macrophages (AMs) and interstitial macrophages (IMs), which reside in the alveolar and interstitial spaces, respectively. Accumulating evidence indicates the involvement of IMs in lung metastasis, but the roles of AMs in lung metastasis still remain elusive. An i.v. injection of a mouse hepatocellular carcinoma (HCC) cell line, BNL, caused lung metastasis foci with infiltration of AMs and IMs. Comprehensive determination of arachidonic acid metabolite levels revealed increases in leukotrienes and PGs in lungs in this metastasis model. A 5-lipoxygenase (LOX) inhibitor but not a cyclooxygenase inhibitor reduced the numbers of metastatic foci, particularly those of a larger size. A major 5-LOX metabolite, LTB₄, augmented in vitro cell proliferation of human HCC cell lines as well as BNL cells. Moreover, in this lung metastasis course, AMs exhibited higher expression levels of the 5-LOX and LTB₄ than IMs. Consistently, 5-LOX–expressing AMs increased in the lungs of human HCC patients with lung metastasis, compared with those without lung metastasis. Furthermore, intratracheal clodronate liposome injection selectively depleted AMs but not IMs, together with reduced LTB₄ content and metastatic foci numbers in this lung metastasis process. Finally, IMs in mouse metastatic foci produced CCL2, thereby recruiting blood-borne, CCR2–expressing AMs into lungs. Thus, AMs can be recruited under the guidance of IM-derived CCL2 into metastatic lungs and can eventually contribute to the progression of lung metastasis by providing a potent arachidonic acid–derived tumor growth promoting mediator, LTB₄.

Metastasis is responsible for most of the cancer-associated deaths and proceeds through multiple steps. Several lines of evidence have established an indispensable involvement of macrophages present at the primary tumor sites in various steps of metastasis, from primary tumor growth to its intravasation into circulation. The lungs encompass a large, dense vascular area and, therefore, are vulnerable to metastasis, particularly, hematogenous ones arising from various types of neoplasms. Lung tissues constitutively contain several types of tissue-resident macrophages and circulating monocytes to counteract potentially harmful exogenous materials, which directly reach through the airway. Recent advances have provided an insight into the ontogenetic, phenotypic, and functional heterogeneity of these lung macrophage and monocyte populations, under resting and inflammatory conditions. In this review, we discuss the ontogeny, trafficking dynamics, and functions of these pulmonary macrophages and monocytes and their potential roles in lung metastasis and measures to combat lung metastasis by targeting these populations.

The therapeutic efficacy of immune checkpoint inhibitors (ICIs) in patients with hepatocellular carcinoma (HCC) is profoundly influenced by the tumor immune microenvironment (TIME), where tumor-associated macrophages (TAMs) expressing programmed death-ligand 1 (PD-L1) serve as key modulators of immune suppression and tumor progression. Although PD-L1(+) TAMs have attracted increasing attention, their precise immunological functions in patients with HCC remain incompletely understood. In this study, we conducted an integrated analysis combining single-cell transcriptomics, spatial profiling, in vitro functional assays, and in vivo therapeutic modeling to clarify the role of PD-L1(+) TAMs in HCC. Single-cell RNA sequencing of tumor samples from patients with HCC (GSE189903) revealed that intratumoral PD-L1(+) TAMs were enriched for immune-related signaling pathways and expressed chemokines including CXCL9, CXCL10, and CXCL11. In vitro, GM-CSF-induced PD-L1(+) macrophages promoted CD8(+) T cell exhaustion, characterized by increased expression of TIM3 and suppression of cytotoxic molecules such as GZMB. Spatial analysis using multiplex immunofluorescence staining of surgical specimens from 113 patients with HCC demonstrated that close proximity between PD-L1(+) TAMs and CD8(+) T cells within tumors was an independent predictor of early postoperative recurrence and poor outcome. Moreover, in a murine orthotopic liver cancer model, the combination of anti-GM-CSF and anti-PD-L1 antibodies inhibited the differentiation of PD-L1(+) TAMs, reduced their contact with CD8(+) T cells, alleviated T cell exhaustion, and potentiated antitumor immunity. These findings highlight the critical contribution of PD-L1(+) TAMs to immune evasion in patients with HCC and support their therapeutic targeting as a strategy to enhance ICI responses.

Arachidonic acid 5-lipoxygenase (5-LOX), an enzyme that synthesizes leukotrienes (LTs), is involved in cancer development including proliferation, invasion, metastasis and drug resistance. However, the functional role of 5-LOX in hepatocellular carcinoma (HCC) remains to be elucidated. In this study, we analyzed the contribution of 5-LOX in HCC progression and investigated the potential of targeted therapy. Analysis of 86 resected HCC specimens and the clinical data of 362 cases of liver cancer from The Cancer Genome Atlas Liver Hepatocellular Carcinoma dataset, showed that 5-LOX expression was associated with postoperative survival. The cancer proliferative and stem cell potential were correlated with the levels of 5-LOX in CD163(+) tumor-associated macrophages (TAMs). In an HCC mouse model, CD163(+) TAMs expressed 5-LOX and produced LTB4 and LTC/D/E4; the 5-LOX inhibitor, zileuton, suppressed HCC progression. LTB4 and LTC/D/E4 promoted cancer proliferation and stem cell capacity via phosphorylation of extracellular signal-regulated kinase 1/2 and stem cell-associated genes. Taken together, we identified a novel mechanism of HCC progression in which CD163(+) TAMs express 5-LOX and produce LTB4 and LTC/D/E4, thereby enhancing the proliferative and stem cell potential of HCC cells. Furthermore, inhibition of 5-LOX activity regulates HCC progression, suggesting it has potential as a new therapeutic target.