Lung adenocarcinoma is the most common subtype of lung cancer, which has the second highest incidence among cancers. Immunotherapy has revolutionized lung cancer treatment, yet the checkpoint blockade response rate is less than 20% in patients with lung adenocarcinoma. As lung adenocarcinoma consists of heterogeneous histologic subsets with diverse tumor invasion phenotypes and clinical outcomes, understanding the mechanisms of resistance based on the histology subset is essential. In the current issue, Jang and colleagues demonstrated that PD-1–expressing macrophages are the dominant immune cell population in the tumor-immune microenvironment (TiME) of invasive lung adenocarcinoma and are responsible for driving tumor progression from preinvasive to invasive subtypes. PD-1–expressing macrophages are protumorigenic and highly plastic, potentially promoting invasive solid tumor development. Ablation of macrophages remodels the TiME and leads to a favorable anti-PD-1 blockade response, suggesting a potential combination therapy in patients with lung adenocarcinoma resistant to monotherapy. This current work highlights the spatiotemporal dynamics of the TiME during lung adenocarcinoma progression and the critical role of PD-1–expressing macrophages in driving tumorigenesis as well as resistance to immunotherapy.

See related article by Jang et al., p. 2593

Non–small cell lung cancer (NSCLC) is the most common type of lung cancer, and lung adenocarcinoma (LUAD) represents 40% of all NSCLC diagnoses. Early-stage LUAD often develops an invasive phenotype, which leads to metastatic spread and correlates with worse prognosis. Understanding the mechanisms by which early-stage LUAD progresses into invasive histologic subtypes including lepidic predominant adenocarcinoma and solid subtype adenocarcinoma is critical to reduce lung cancer invasiveness and to improve patient outcome. Recent studies have reported that tumor cell–intrinsic differential protein expression and signaling pathway activity determine invasiveness and metastasis of LUAD (1). However, it remains unclear how tumor cell–extrinsic compartments, such as immune infiltration and polarization, contribute to tumor progression.

Immune checkpoint inhibitors (ICI) against PD-1 and PD-L1 have shown promising results in patients with NSCLC in multiple clinical trials. Nonetheless, the majority of patients with NSCLC show disease progression on immunotherapy (2) and limited response to ICI monotherapy (3, 4). Multiple mechanisms of resistance have been explored. Tumor cells can intrinsically develop resistance via upregulation of oncogenic drivers such as Wnt and Myc or downregulation of immune-modulatory molecules including PD-L1 and MHC I on the cell surface (5). A tumor microenvironment skewed toward more immune suppression also hampers the efficacy of ICI treatment (3, 5). Given the heterogeneous subtypes of LUAD, dissecting the unique resistance mechanisms based on the histologic subset is necessary for effective treatment of LUAD.

In this article, Jang and colleagues demonstrate how immune cell profiles change during tumor progression and identified PD-1hi macrophages as a critical driver of invasive tumor development as well as a potential target for improved immunotherapy (6). Addressing some of the unanswered questions described above, the authors investigated the role of tumor-immune microenvironment (TiME) changes during lung cancer progression through preinvasive lesions to invasive lepidic and solid adenocarcinoma. High-resolution immune cell profiling using a mass cytometry time of flight (CyTOF) platform in a mouse model, which replicates the progressive feature of human lung cancer, revealed striking differences in immune cell composition between preinvasive and invasive tumor stages. The most abundant immune cells in invasive LUAD were tumor-associated macrophages (TAM), which were phenotypically distinct from alveolar macrophages, indicating that they were derived from peripheral monocytes. Further phenotyping analyses identified subtypes of TAMs based on PD-1 expression level as PD-1hi and PD-1lo. These subsets showed differential spatial distribution as PD-1hi TAMs colocalized with cancer cells in solid tumor components, whereas PD-1lo TAMs were primarily present in lepidic components, suggesting a distinct role for each macrophage subset. Indeed, PD-1hi TAMs protected tumor cells from apoptosis and enhanced tumor cell-to-cell interactions as well as a cancer stem cell phenotype in a PD-1–dependent manner. Thus, these findings suggest that PD-1 expression on TAMs may play a critical role in regulating tumor progression.

To evaluate the efficacy of TAM-targeted therapy, Jang and colleagues treated the mouse model with a macrophage-depleting strategy, via trabectedin, at different stages of tumor progression (6). Trabectedin is a clinically approved chemotherapy due to its direct killing effect on cancer cells. Its immunoregulatory function has recently been discovered and is being explored in multiple preclinical models and clinical trials (7). Macrophage depletion at the early-lepidic noninvasive tumor stage dramatically decreased lepidic tumor formation and inhibited solid tumor development. The impact on the solid tumor component was milder when the drug was administered following establishment of invasive solid components, yet the lepidic component was still significantly reduced. These findings indicate that TAMs play a critical role in developing an invasive phenotype in LUAD. In addition, in vitro treatment of TAM subsets showed that PD-1hi TAMs were more susceptible to trabectedin, implying that the inhibition of invasive tumor progression was most likely due to depletion of PD-1hi TAMs.

Increased abundance of macrophages in the TiME often correlates with reduced efficacy of ICI treatment (8). The authors showed that macrophage depletion reshaped the TiME, with increased T-cell populations at both early and late time points (6). Combining macrophage depletion with anti-PD-1 therapy resulted in a significant reduction in the development of both solid and lepidic components. Interestingly, the authors pointed out that anti-PD-1 monotherapy changed the spatial distribution of PD-1–expressing TAMs from primarily in the solid component toward a more diffuse pattern within the lepidic component, which is in line with the reduction of the solid component but not the lepidic component with the monotherapy. This finding corroborates the proposed role of PD-1–expressing TAMs in promoting solid tumor progression. The authors further evaluated the translational aspects of their study on patients with human lung cancer. Consistent with their findings in mouse models, PD-1hi TAMs were found to be more enriched in solid components compared with lepidic components in human LUAD. In addition, the frequency of PD-1hi TAMs negatively correlated with clinical outcomes, as patients whose tumors contained abundant PD-1hi TAMs showed increased disease progression during anti-PD-1 blockade treatment and decreased overall survival. These data suggest a potential for PD-1hi TAMs as a predictive biomarker in immunotherapy resistance. Currently, expression of PD-L1 on tumor cells is used as an indicator for anti-PD-1 and anti-PD-L1 therapy efficacy in patients with NSCLC. However, it is reported that some patients with tumors expressing low levels of PD-L1 show a robust response to anti-PD-1 therapy (9). Therefore, it would be of interest whether PD-1 expression on TAMs can be used as a determining factor of which patients benefit from ICI therapy.

Taken together, the current study by Jang and colleagues demonstrated the dynamic alterations of the immune cell compartment during tumor progression by elegant longitudinal and spatial analysis approaches. Yet, more precise mechanisms need to be explored in the future studies. For example, which factors regulate emergence and maintenance of PD-1hi TAMs? How do these macrophages influence the tumor phenotype in situ? Does PD-1–mediated TAM–tumor interaction elicit other means of cell-cell communication, such as release of cytokines? Moreover, while lung cancer was assessed here, what is the extent to which these mechanisms are present in the TiME of other solid cancers refractory to ICI therapy, and can a similar strategy be leveraged for improved outcomes in other cancers? Unraveling the cross-talk between tumor cells and immune cells during tumor progression will help advance the efficacy of immunotherapy, ultimately improving the survival outcome of lung cancer and possibly other patients with cancer.

D. Bayik reports grants from NCI during the conduct of the study. No disclosures were reported by the other authors.

This work was supported by NIH P01 CA245705 (to J.D. Lathia), R35 NS127083 (to J.D. Lathia), and K99 CA248611 (to D. Bayik), Lerner Research Institute (to J.D. Lathia), and the Case Comprehensive Cancer Center (to J.D. Lathia).

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