Background

Immune checkpoint inhibitors (ICIs) have become a standard treatment for non-small cell lung cancer (NSCLC), improving survival rates. However, most patients eventually develop resistance, limiting the long-term efficacy of ICIs. Understanding the precise mechanisms by which cancer cells evade immune surveillance is crucial.

The interaction between immune cells and cancer cells within the tumor microenvironment (TME) has long been known to contribute to drug resistance. However, the specific connections between metabolic changes and epigenetic regulation within cancer cells that induce resistance remain unclear. Detailed elucidation of the mechanisms by which cancer cells neutralize immune cells in the tumor microenvironment using metabolic products has been limited by existing analytical methods.

Key Findings

The researchers identified the cause of immunotherapy resistance within cancer cells using single-cell multiomics technology. Aldehyde Dehydrogenase 9 Family Member A1 (ALDH9A1) and the carnitine signaling pathway within cancer cells were identified as key factors driving immune evasion and anti-PD-1 treatment resistance. ALDH9A1 is a key enzyme in carnitine synthesis.

Activation of ALDH9A1 in cancer cells increases carnitine biosynthesis and acetyl-CoA levels. The accumulated acetyl-CoA remodels chromatin accessibility, promoting the activity of superenhancers in the Interleukin-1 Beta (IL-1β) gene, an inflammatory cytokine.

The secreted IL-1β disrupts surrounding immune cells. It promotes the differentiation of myeloid-derived suppressor cells (MDSCs), which have strong immunosuppressive functions, and induces CD8+ T cell depletion, thereby suppressing the immune response. Furthermore, the secreted IL-1β activates a feedback loop that increases ALDH9A1 expression within cancer cells via the NF-κB signaling pathway. As a result, the tumor microenvironment becomes fixed in a state where immune cells cannot attack cancer cells.

The mechanism of action of this signaling pathway was demonstrated in animal experiments. In a mouse model, the elimination of the Aldh9a1 gene inhibited tumor growth. Similar effects were observed when ALDH9A1 activity was blocked with diethylaminobenzaldehyde (DEAB) or when IL-1β was blocked with a neutralizing antibody. After treatment, MDSC infiltration decreased significantly, and the maturation of tertiary lymphoid structures (TLS) was induced. Consequently, the mouse model, which had previously been unresponsive to anti-PD-1 treatment, became sensitive to the treatment again, restoring its efficacy.

Significance and Prospects

This study is valuable because it explains the fundamental metabolic reasons why patients develop immunotherapy resistance in conjunction with genetic perspectives. It elucidates the entire process by which the activation of specific metabolic enzymes within cancer cells induces epigenetic changes and alters the characteristics of surrounding immune cells. It demonstrates that single-cell multiomics technology is a useful tool for elucidating the mechanisms of complex tumor microenvironments.

This research also holds promise for clinical application. It may be possible to use the expression level of ALDH9A1 or the carnitine concentration in lung cancer tissue as a diagnostic marker to predict the prognosis of immune checkpoint inhibitor treatment. Furthermore, the development of new drugs that combine ALDH9A1 and IL-1β signaling pathway inhibitors with immunotherapy is expected to gain momentum.

However, there are still challenges to be addressed before actual application to patients. DEAB, which was used in the experiments, is a broad-spectrum inhibitor, so the development of targeted drugs that specifically target ALDH9A1 is needed. The potential side effects of artificially regulating carnitine metabolism in the body also need to be investigated.