Although immune checkpoint inhibitors have transformed cancer treatment, many tumors remain intrinsically resistant or eventually develop acquired resistance to PD-1/PD-L1 blockade. One of the major mechanisms underlying resistance involves expansion of immunosuppressive myeloid populations, particularly polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) and pathologic neutrophils, which suppress cytotoxic T-cell activity and prevent effective antitumor immunity.
Previous studies established that chronic inflammation promotes emergency myelopoiesis, a process in which bone marrow rapidly produces suppressive myeloid cells during tumor progression. However, the signaling pathways driving this reprogramming remained incompletely understood.
In this study, investigators identified cysteinyl leukotriene receptor 1 (CysLTR1) as a central regulator of tumor-promoting myelopoiesis. Under physiologic conditions, CysLTR1 participates in asthma and allergic inflammatory responses. Tumors, however, appear capable of hijacking this pathway to generate immunosuppressive neutrophils that interfere with immunotherapy efficacy.
Mechanistic Findings
The investigators demonstrated that tumors activate inflammatory cytokines including GM-CSF and IL-6, which subsequently stimulate STAT3 signaling. Activated STAT3 then upregulates CysLTR1 expression in myeloid precursors.
Once activated, CysLTR1 signaling promotes downstream AKT and ERK pathway activation, leading to enhanced neutrophil maturation, granule synthesis, and immunosuppressive programming.
Rather than supporting antitumor immunity, these neutrophils evolve into PMN-MDSCs capable of suppressing CD8-positive T-cell infiltration and cytotoxic function within tumors. This creates an immune-excluded tumor microenvironment associated with resistance to immune checkpoint blockade.
The investigators further identified transcription factors MXD1 and NFE2 as downstream regulators involved in neutrophil granule production and suppressive differentiation.
Experimental Design
The study combined multiple experimental platforms, including:
- Murine tumor models of triple-negative breast cancer, melanoma, ovarian cancer, colorectal cancer, and prostate cancer.
- Human immune-cell analyses.
- Human tumor tissue samples.
- Public cancer transcriptomic and survival datasets.
The investigators either genetically deleted CysLTR1 or pharmacologically inhibited the pathway using existing leukotriene receptor antagonists, primarily montelukast.
Antitumor Activity and Immunotherapy Enhancement
Blocking CysLTR1 consistently suppressed tumor growth and improved survival across multiple preclinical models. Most importantly, inhibition of the pathway restored responsiveness to anti–PD-1 immunotherapy even in tumors that had previously become resistant.
Key Findings
- CysLTR1 inhibition significantly reduced tumor growth across multiple aggressive cancer models.
- Combination treatment with montelukast and anti–PD-1 therapy restored immunotherapy sensitivity in resistant tumors.
- Blocking the pathway increased CD8-positive T-cell infiltration and antitumor immune activation.
- Suppressive PMN-MDSC populations were substantially reduced and functionally reprogrammed.
- High CysLTR1 expression in human tumors correlated with poorer survival and inferior immunotherapy outcomes across several cancer types.
One of the most interesting observations was that the therapeutic effect did not simply arise from eliminating neutrophils. Instead, inhibition of CysLTR1 appeared to reprogram these cells into a more immune-supportive phenotype capable of facilitating antitumor responses.
This distinction may be biologically important because neutrophils also play critical physiologic roles in host defense, and broad neutrophil depletion strategies can produce significant toxicity.
Relevance to Triple-Negative Breast Cancer
The findings may be particularly relevant for triple-negative breast cancer (TNBC), where immune checkpoint inhibitors produce meaningful but still limited clinical benefit. TNBC frequently demonstrates high inflammatory signaling and substantial myeloid-cell infiltration, making it an attractive setting for strategies targeting PMN-MDSCs and suppressive neutrophils.
The study suggests that leukotriene pathway inhibition could potentially convert immunologically resistant tumors into more responsive immune-active microenvironments.
Triple-Negative Breast Cancer: Symptoms ,Causes, Types, Diagnosis and Treatment
Clinical Implications
One of the most clinically attractive aspects of this work is the immediate translational potential. Because montelukast and related CysLTR1 antagonists are already FDA-approved and widely used for asthma and allergic diseases, their pharmacology, toxicity profile, and dosing strategies are already well established.
This may significantly accelerate development compared with entirely novel immunotherapeutic agents.
The study therefore supports the possibility of rapid initiation of combination immunotherapy trials evaluating montelukast with PD-1/PD-L1 inhibitors in difficult-to-treat malignancies.
Potential future applications could include:
- Triple-negative breast cancer.
- Immunotherapy-resistant melanoma.
- Refractory colorectal cancer.
- Ovarian cancer.
- Prostate cancer.
- Other tumors characterized by neutrophil-rich immune suppression.
Scientific Significance
This work contributes to the growing understanding that effective immunotherapy depends not only on T-cell activation but also on reprogramming the broader tumor immune microenvironment.
The study positions CysLTR1 as a novel immunoregulatory checkpoint linking chronic inflammation, emergency myelopoiesis, neutrophil biology, and immune resistance. It also reinforces the increasingly important role of myeloid-targeted therapies in next-generation immuno-oncology strategies.
Rather than focusing exclusively on adaptive immunity, the findings highlight how modulation of innate immune cell programming may substantially enhance checkpoint inhibitor efficacy.
Although the results remain preclinical and require validation in prospective clinical trials, the ability to repurpose an inexpensive and widely available FDA-approved drug makes this approach particularly compelling for translational oncology research.
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