In our study, clinicopathological factors showed that patients with a high peritumoral AM content had more aggressive tumors, such as with larger pathologic invasive tumor size; the presence of pleural invasion, lymphatic permeation, and a higher proportion of more invasive predominant subtype, than patients with a low peritumoral AM content. The multivariate analysis revealed that the number of peritumoral AMs was an independent prognostic factor. These results suggest that the number of peritumoral AMs is a strong factor for poor prognosis in patients with stage I lung adenocarcinoma. To the best of our knowledge, this is the first report showing that the number of AMs is associated with the prognosis of stage I lung adenocarcinoma.
In the analysis using the TCGA dataset, patients with a high peritumoral AM content showed shorter DFS than those with a low peritumoral AM content, indicating that the results using our hospital cohort are reproducible. The significant difference in some GO pathways, such as chemotaxis and epithelial proliferation, might be one of the reasons why the prognosis was poorer in the group with a high peritumoral AM content.
Previous reports have described the relationship between the tumor and the microenvironment around the tumor. In some types of cancer, peritumoral macrophages facilitate the progression in tumor cells (Kuang et al. 2010; Iriki et al. 2017). These reports indicate that the microenvironment around the tumor, not only within the tumor, is involved in tumor progression. Alternatively, Sharma et al. reported that AMs provide an environment where lung metastases are likely to occur by suppressing antitumor T cell responses in breast cancer (Sharma et al. 2015). Future studies are needed to examine the gene expression profiles of AMs around tumors.
We suggest two possible reasons why the number of peritumoral AMs differs depending on the patient: (1) Cancer cells themselves mobilize the peritumoral AMs, and the ability to attract AMs varies depending on the characteristics of cancer cells in each case. (2) AMs that are originally present in alveoli before tumorigenesis and the characteristics of AMs in each case may influence the development of adenocarcinoma. To verify our theory, we made the following considerations: We defined nonadjacent alveolar space as the air space 2 mm away from the tumor edge on the largest cross-sectional slide, to avoid overlapping with the peritumoral alveolar space, and counted macrophages per alveolar space (Supplementary Figs. 3A, B). There was a strong correlation between peritumoral and nonadjacent AMs (r² = 0.723; Supplementary Fig. 3C). The median number of nonadjacent AMs was 8.6 per alveolar space. In the multivariate analysis, there were no significant differences between patients with a high and low non-adjacent AM content with the cut-off values of 8.6 and 10 (p = 0.54 and 0.50, respectively) (Supplementary Tables 1, 2). For the cut-off value of 20, patients with a high nonadjacent AM content showed poorer DFS than those with a low nonadjacent AM content (HR 1.57, 95% CI: 1.21–2.04, p < 0.01) (Supplementary Table 2). This result may indicate that highly malignant tumors are likely to occur in cases with many AMs where the tumor develops. However, we cannot rule out the possibility that AMs 2mm apart may also be mobilized by cancer cells. Further studies are needed to elucidate why the number of peritumoral AMs varies for each case, further studies will be needed.
There are some limitations to our study. First, this was a retrospective study in a single institution. Second, we did not evaluate the number of AMs on all slides with tumors. Third, the number of AMs is affected by how the lungs are inflated during fixation. Therefore, we should interpret the results of this study with caution.
The prognostic influence of the number of peritumoral AMs in our study suggests that both intratumoral and the peritumoral microenvironments are important. Recent studies have investigated novel therapeutic strategies aimed at depleting TAMs and/or reprogramming their tumor-promoting effects (Mantovani et al. 2017; Anfray et al. 2020; Sarode et al. 2020). Considering the results of our study, there may be a need for therapeutic efforts targeting AMs, not only TAMs.