Cancer Cells First “Go Silent,” Then Macrophages Build a Nest: Two-Stage Immune Evasion in Lung Metastasis

The most difficult point at which to intercept cancer metastasis may not be after a tumor has already grown in a distant organ, but when a small number of cancer cells have just arrived and have yet to form lesions visible on imaging. A study published in *Science* tracked the process by which liver cancer cells take hold in the lungs and found that immune evasion is not a single step, but a two-stage operation in which cancer cells and surrounding immune cells take over in succession.

The research team combined lineage tracing, single-cell and spatial multi-omics, mouse experiments, and human metastatic specimens to examine nine time points spanning the progression from a single disseminated cancer cell to overt lung metastasis. The results showed that most cells disappear during early innate immune clearance, while a small number of survivors briefly enter a quiescent, PHGDH-high state and cluster during the formation of micrometastases.

PHGDH is a key enzyme in the serine synthesis pathway, but the role identified in this study extends beyond nutrient supply. The team proposed that a PHGDH-related mechanism can silence the transcription of multiple chemokines through the repressive histone mark H3K27me3. Cancer cells thereby reduce signals that attract immune cells, allowing newly emerging metastatic lesions to form immune-cell-poor regions and survive the most vulnerable early stage.

Next to appear are lung interstitial macrophages with high CX3CR1 expression. The study observed that this cell population briefly increases before large-scale cancer cell expansion and recruits cells with immunosuppressive effects, further transforming what was initially a temporary concealed state into an “immune-privileged” microenvironment conducive to growth. In other words, cancer cells first turn down the distress signals, and macrophages then build a protective nest around them.

In mouse models, disrupting the PHGDH–H3K27me3 axis in cancer cells or removing the relevant interstitial macrophages restored some immune surveillance and inhibited metastatic colonization in the lungs. This makes the two transient biological windows potential points for therapeutic intervention and also suggests that preventing metastasis may require different strategies at different stages, rather than targeting only rapidly proliferating cancer cells.

However, this mechanism has so far been established mainly in preclinical models; human specimens provide consistent clues but are not yet sufficient to prove that progression in patients necessarily follows the same sequence. PHGDH and macrophages also participate in normal metabolic and immune functions, and how to precisely target the metastatic microenvironment while avoiding systemic side effects remains a question that must be answered before clinical translation. What the study first provides is a more detailed map of metastasis, not a treatment regimen ready for use in patients.