Myeloid-driven immunosuppression as a context-dependent therapeutic bottleneck in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) remains one of the most treatment-refractory solid malignancies despite improvements in perioperative care and multi-agent chemotherapy [1,2]. Immune checkpoint inhibitors (ICIs) provide little benefit outside the uncommon mismatch repair-deficient or microsatellite instability-high subgroup, and adding PD-1/PD-L1 or CTLA-4 blockade to chemotherapy has not produced a reproducible survival benefit in unselected PDAC [3-5]. The resulting description of PDAC as an "immune-cold" tumor is convenient but biologically imprecise. A microsatellite-stable subset has high tumor mutational burden and prominent immune infiltration [6]; neoantigen quality is associated with immunoediting and long-term survival [7]; and individualized RNA vaccination can induce high-magnitude, polyfunctional, and durable neoantigen-specific T cells after resection [8,9]. PDAC is therefore not uniformly incapable of generating antitumor immunity. The more consistent problem is the failure to convert immune recognition into sustained tumor control.We argue that suppressive myeloid programs should be treated as a context-dependent therapeutic bottleneck rather than a universal target. When such programs are demonstrably rate-limiting, durable benefit from T-cell-directed therapy is unlikely unless the barrier is directly reprogrammed or functionally bypassed. Here, functional bypass denotes restoration of antigen presentation, lymphocyte access, or T-cell effector function without direct pharmacological targeting of the myeloid population that creates the barrier. Conversely, a change in myeloid abundance or phenotype should not be regarded as therapeutic success unless it is followed by improved tumor-reactive T-cell function. This framework does not justify pan-myeloid depletion or assume that one target applies across PDAC. It calls for selective correction of the relevant defect, integrated with antigenic priming and T-cell support.The PDAC myeloid compartment includes tumor-associated macrophages (TAMs), monocytic and polymorphonuclear MDSCs, neutrophils, and conventional dendritic-cell subsets. These populations are heterogeneous and plastic; lineage markers and the binary M1/M2 macrophage model do not reliably predict function [10][11][12]. Tumor genotype, tissue injury, hypoxia, nutrient competition, microbial products, chemotherapy, and fibroblast-derived signals can alter recruitment and cell state. Immunosuppressive P2RX1-negative neutrophils, for example, accumulate in pancreatic cancer liver metastases and are associated with impaired T-cell activity [13]. Single-cell and spatial studies further show that myeloid, fibroblast, and lymphocyte states form multicellular ecosystems that vary between patients and after treatment [14,15]. Genetically and anatomically distinct PDACs can therefore use different combinations of immune suppression [16].Two implications follow. First, cell density is an inadequate therapeutic classifier: abundant macrophages may reflect suppressive scavenging and fibrosis in one tumor but retain phagocytic or antigen-presenting functions in another. Second, a peripheral-blood change does not establish that the relevant intratumoral state has changed. Trial design therefore needs to distinguish abundance, location, and function rather than treat all CD68-positive or CD163-positive cells as one population.Myeloid dysfunction can limit immunity at three connected stages (Figure 1). Reduced abundance or function of type 1 conventional dendritic cells impairs cross-presentation and tumor-reactive CD8 + T-cell priming. TAMs, MDSCs, and neutrophils can then contribute to chemokine imbalance, abnormal vascular trafficking, extracellular-matrix remodeling, and fibroblast activation, separating lymphocytes from malignant cells. After T cells enter the tumor, arginase activity, reactive oxygen species, adenosine, IL-10, TGF-β, and checkpoint-ligand expression can suppress proliferation, cytokine production, and cytotoxicity. Productive immunity is consequently determined not only by cell counts but also by spatial cooperation among dendritic cells, helper T cells, cytotoxic T cells, and myeloid populations [10][11][12]16,17].This framework also explains why pan-myeloid depletion is a poor default. Dendritic cells are indispensable for priming, and some macrophage states support phagocytosis, tissue repair, and antigen presentation. Broad depletion could remove beneficial cells or induce compensatory recruitment [10][11][12]16]. The therapeutic objective is selective inhibition of a defined suppressive function or reprogramming of a harmful state while preserving host defense and antigen presentation. Selected studies are summarized in Table 1. Randomized studies have not established an incremental benefit from empirically adding checkpoint blockade or CD40 agonism to chemotherapy in unselected PDAC, whereas single-arm studies repeatedly show that immune or stromal pharmacodynamic changes can occur without proven clinical benefit. In PA.7, adding durvalumab and tremelimumab to gemcitabine plus nab-paclitaxel did not improve overall survival [4]. The negative result is compatible with multiple unresolved immune barriers, but the trial did not determine which barrier was limiting.The randomized phase II PRINCE trial was informative but not definitive. Nivolumab plus chemotherapy met its prespecified 1-year overall-survival benchmark, whereas the sotigalimab-containing arms did not [5]. The study was not designed to establish superiority between arms. In exploratory analyses, outcomes within the nivolumab/chemotherapy arm were associated with activated, antigen-experienced T-cell features, whereas outcomes in sotigalimab-containing arms were associated with antigen-presenting-cell and CD4 + T-cell features. These arm-specific associations are hypothesis-generating rather than validated predictive biomarkers.Subsequent studies further illustrate the gap between ph