Background: 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.
Results: Injection of a mouse hepatocellular carcinoma (HCC) cell line, BNL, into tail vein of mice caused multiple lung metastasis foci until 21 days after the injection. Comprehensive quantitative analysis on eicosanoid contents revealed increases in leukotrienes (LTs) and prostaglandins in metastatic lungs. A 5-lipoxygenase (LOX) inhibitor but not a cyclooxygenase (COX) inhibitor reduced the numbers of metastatic foci, particularly those with a diameter larger than 2 mm. A major 5-LOX metabolite, LTB4, augmented in vitro cell proliferation of both mouse and human HCC cell lines. Moreover, AMs exhibited a higher 5-LOX mRNA expression level than IMs. Consistently, 5-LOX-expressing AMs increased in the lungs of human HCC patients with metastasis. Intratracheal clodronate liposome injection selectively depleted AMs, together with reduced LTB4 content and metastatic focus numbers, recapitulating the effects of a 5-LOX inhibitor. Finally, IMs in metastatic foci produced CCL2, and the analysis using competitive bone marrow chimeric mice demonstrated that the migration of AMs from the bloodstream to metastatic lungs was significantly retarded when CCR2−/− BM cells were used as a donor in chimeric mice.
Conclusion: We demonstrated the cooperation beween two distinct subsets of macrophages in the progression of lung metastasis, wherein AMs can be recruited under the guidance of IM-derived CCL2 into metastatic lungs and can eventually contribute to lung metastasis progression by providing LTB4.