Coactivation of Innate Immune Suppressive Cells Induces Acquired Resistance Against Combined TLR Agonism and PD-1 Blockade

Immunology Life Sciences Oncology Medicine
innate immune suppressive cells acquired resistance TLR agonists PD-1 blockade tumor microenvironment CXCL1-CXCR2 axis polymorphonuclear myeloid-derived suppressor cells

Mechanism of Combined Immune Checkpoint Blockade Therapy

Academic Background
Immune Checkpoint Blockade (ICB) is a revolutionary cancer treatment aimed at reactivating effector T cells to combat cancer. However, more than half of patients do not respond to ICB, especially those with immunologically “cold” tumors (tumors with fewer immune cells in the tumor microenvironment). To address this, researchers have explored various strategies to enhance ICB efficacy by modulating the tumor microenvironment (TME), including activating the innate immune system. Toll-like receptor (TLR) agonists, which stimulate innate immunity, have shown limited success in clinical applications when combined with ICB. This study aims to reveal how the coactivation of innate immune suppressive cells leads to acquired resistance in combined treatment using TLR agonists (e.g., OK-432) and PD-1 blockade.

Source
This study was conducted by a collaborative team from the National Cancer Center Japan, Nagoya University, Osaka International Cancer Institute, and other institutions. The first authors are Hitomi Nishinakamura and Sayoko Shinya, and the corresponding author is Hiroyoshi Nishikawa. The research was published on February 12, 2025, in Science Translational Medicine, titled “Coactivation of innate immune suppressive cells induces acquired resistance against combined TLR agonism and PD-1 blockade.”

Research Process
1. Activation of Dendritic Cells (DCs) by OK-432
The study first validated the activating effect of OK-432 on human dendritic cells through in vitro experiments. As antigen-presenting cells, the maturity of DCs directly affects T cell activation. The results showed that OK-432 significantly increased the expression of co-stimulatory molecules (e.g., CD86, CD80, CD40) and PD-L1 on DCs, indicating that OK-432 effectively promotes DC maturation. Additionally, OK-432-treated DCs significantly enhanced the activation of antigen-specific CD8+ T cells, supporting the potential of OK-432 as an innate immune activator.

  1. Experimental Validation of Combined OK-432 and PD-1 Blockade Therapy
    The study tested the efficacy of combined OK-432 and PD-1 blockade therapy in multiple immunologically “cold” tumor models (e.g., Lewis lung carcinoma LL/2, colon cancer CT26). The results showed that neither OK-432 alone nor its combination with PD-1 blockade significantly enhanced antitumor immunity, particularly in activating CD8+ T cells. This suggests that OK-432 failed to convert “cold” tumors into “hot” tumors.

  2. Role of Immune Suppressive Cells in the Tumor Microenvironment
    Using flow cytometry and RNA sequencing (RNA-seq), the study found that the number of polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) in the tumor microenvironment significantly increased after OK-432 treatment. PMN-MDSCs are crucial immune suppressive cells that inhibit CD8+ T cell activity. Further analysis revealed that the accumulation of PMN-MDSCs is closely related to the activation of the CXCL1-CXCR2 signaling pathway. CXCL1, a chemokine, recruits PMN-MDSCs into the tumor microenvironment by binding to its receptor CXCR2.

  3. Experimental Validation of Triple Combination Therapy
    To overcome PMN-MDSC-induced resistance, the research team designed a triple combination therapy by adding a CXCR2 neutralizing antibody or LY6G antibody (to deplete PMN-MDSCs) to the OK-432 and PD-1 blockade regimen. The results showed that this triple combination therapy significantly inhibited tumor growth and prolonged survival in mice. Specifically, the triple combination therapy significantly reduced PMN-MDSCs in the tumor microenvironment while enhancing CD8+ T cell activity.

  4. Validation in Clinical Trials
    The study further validated the effects of OK-432 on CXCL1 and PMN-MDSCs in lung cancer patients. The results showed that after OK-432 treatment, CXCL1 concentrations in pleural effusions significantly increased, and PMN-MDSCs were also notably elevated. This finding aligns with the results from animal models, further supporting the critical role of PMN-MDSCs in acquired resistance.

Conclusions
This study demonstrates that OK-432, as a TLR agonist, effectively activates dendritic cells. However, when combined with PD-1 blockade, the coactivation of PMN-MDSCs suppresses antitumor immunity. By blocking the CXCL1-CXCR2 signaling pathway, the research team successfully overcame this acquired resistance, significantly enhancing the antitumor efficacy of the combined therapy. This discovery provides important guidance for designing more effective immunotherapy strategies in the future.

Highlights
1. Revealing the Mechanism of Acquired Resistance: First to reveal that the coactivation of PMN-MDSCs is a key factor in the failure of combined TLR agonist and PD-1 blockade therapy.
2. Innovative Triple Combination Therapy: Proposed and validated a triple combination therapy, offering a new approach to overcoming acquired resistance in tumors.
3. Clinical Application Potential: The findings apply not only to animal models but have also been validated in human patients, laying the groundwork for future clinical trials.

Significance
This study not only deepens the understanding of the mechanisms underlying combined TLR agonist and PD-1 blockade therapy but also provides a theoretical basis for developing more effective cancer immunotherapy strategies. In particular, the research highlights the importance of controlling immune suppressive cells while activating innate immunity, offering hope for more patients.