Immunosuppressive tumor microenvironment shape pancreatic cancer unresponsive to current immunotherapies

Abstract

Pancreatic ductal adenocarcinoma remains largely refractory to current immunotherapies due to a profoundly immunosuppressive tumor microenvironment dominated by regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs). These cells form a coordinated network that suppresses cytotoxic T lymphocytes and fosters tumor progression. Key mechanisms include Tregs secreting inhibitory cytokines like transforming growth factor β and interleukin-10, and upregulating immune checkpoints such as cytotoxic T-lymphocyte-associated protein 4 and programmed death 1. MDSCs deplete essential nutrients like arginine and generate reactive oxygen species, while TAMs polarized to an M2 phenotype produce chemokines including C-C motif chemokine ligand 2 and C-X-C motif chemokine ligand 12, which recruit more suppressive cells. Single-cell transcriptomic studies have uncovered prognostically relevant cellular subsets, such as caspase-4-high Tregs, highlighting this heterogeneity. Reciprocal signaling via interleukin-10 and transforming growth factor β creates a self-reinforcing immunosuppressive loop. Emerging therapeutic strategies aim to disrupt this axis by depleting Tregs (e.g., anti-CD25), blocking MDSC recruitment (e.g., CCR2 inhibitors), or reprogramming TAMs (e.g., CD40 agonists), often in combination with programmed death 1/programmed death-ligand 1 blockade. An integrated approach targeting these populations holds promise for converting pancreatic ductal adenocarcinoma into an immunologically responsive tumor.

Key Words: Pancreatic ductal adenocarcinoma; Immunosuppressive tumor microenvironment; Regulatory T cells; Myeloid-derived suppressor cells; Tumor-associated macrophages

Core Tip: Pancreatic ductal adenocarcinoma is characterized by a highly immunosuppressive tumor microenvironment dominated by regulatory T cells, myeloid-derived suppressor cells, and tumor-associated macrophages. These cells form a synergistic network that promotes immune evasion and resistance to immunotherapy. Recent advances in single-cell transcriptomics have revealed cellular heterogeneity and identified novel therapeutic targets. Emerging strategies focus on depleting or reprogramming these immunosuppressive populations, often in combination with checkpoint blockade or chemotherapy. A multi-target approach is essential to convert pancreatic ductal adenocarcinoma from an immunologically “cold” to “hot” tumor and improve patient outcomes.

INTRODUCTION

Pancreatic ductal adenocarcinoma (PDAC) is the predominant histological subtype of pancreatic cancer, accounting for approximately 90% of all cases. It is characterized by its high malignancy, rapid progression, and strong propensity for recurrence and metastasis, posing a formidable clinical challenge[1,2]. Mortality from PDAC continues to rise, with a median overall survival typically less than one year and a five-year survival rate of only around 8%. Consequently, PDAC has become the third leading cause of cancer-related death worldwide[1]. Although chemotherapy regimens such as gemcitabine and various emerging immunotherapies have extended patient survival to some extent, curative outcomes remain elusive[3,4]. Notably, unlike tumor types such as melanoma that respond favorably to immune checkpoint inhibitors (ICIs), particularly programmed death 1 (PD-1)/programmed death-ligand 1 (PD-L1) blockade, PDAC exhibits widespread primary resistance, and multiple clinical studies have failed to demonstrate significant survival benefit in this setting[5,6].

The resistance of PDAC to immunotherapy is largely attributed to its unique tumor microenvironment (TME). PDAC represents a prototypical “immune-cold tumor” with a hallmark “immune-excluded” phenotype: Tumor nests are surrounded by an exceedingly dense fibrotic stroma, infiltrated by abundant immunosuppressive cell populations including regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs). In contrast, cytotoxic CD8+ T cells are physically excluded from penetrating the tumor core, preventing effective tumor recognition and immune surveillance[7,8]. The combination of desmoplastic stroma and immunosuppressive cells establishes a dual barrier - both physical and functional - that not only impedes therapeutic drug delivery but also actively fosters a profoundly immunosuppressive ecosystem. This environment renders immunotherapies largely ineffective and may even accelerate tumor progression and adaptive resistance[9,10].

Accordingly, this review aims to systematically dissect the immunosuppressive mechanisms orchestrated by the three dominant cellular populations within the PDAC TME - Tregs, MDSCs, and TAMs. We specifically focus on their synergistic interactions, dynamic evolution across disease stages, and the therapeutic potential of targeting these cells individually or in combination. We will highlight their interconnected signaling networks, their dynamic and stage-specific evolution during tumor development, and therapeutic strategies that specifically target these populations and their crosstalk. The ultimate goal is to provide new insights and directions for overcoming the immunotherapy resistance that defines PDAC.

DOMINANT CELL POPULATIONS IN THE IMMUNOSUPPRESSIVE MICROENVIRONMENT: MECHANISMS AND HETEROGENEITY

Tregs

Tregs represent one of the predominant immunosuppressive subsets in the PDAC TME. They exert their effects both through direct interactions with tumor cells and by suppressing effector immune responses, while forming a partially redundant network with other immunosuppressive populations such as MDSCs and TAMs[11,12]. In PDAC, Treg infiltration is a dynamic process that begins as early as the stage of pancreatic intraepithelial neoplasia[11]. Their expansion is driven by pathways including transforming growth factor β (TGF-β) and C-C motif chemokine ligand (CCL) 22 signaling[13,14]. Functionally, Tregs secrete inhibitory cytokines such as interleukin (IL)-10, IL-35, and TGF-β, which suppress T-cell-mediated immunity. In addition, membrane-bound TGF-β and preferential access to antigen-presenting dendritic cells (DCs) further inhibit CD8+ effector T-cell priming[15,16].

Competition for IL-2 is another critical mechanism: Both effector T cells and Tregs depend on IL-2 for survival, but the high-affinity CD25 receptor expressed on Tregs sequesters IL-2, leading to effector T-cell metabolic stress and dysfunction. Simultaneously, Tregs promote the accumulation of cyclic AMP in the TME, further impairing effector T-cell activity[15,17]. Tregs also overexpress immune checkpoint molecules such as cytotoxic T-lymphocyte-associated protein 4 and PD-1, directly dampening effect or T-cell function[16]. Moreover, the CD39/CD73 extracellular nucleotidase axis in Tregs generates extracellular adenosine, which delivers potent inhibitory signals through A2A receptor engagement[11]. Single-cell RNA sequencing has further revealed specialized Treg subpopulations with enhanced suppressive function, such as caspase-4-high Treg2 cells, which are associated with poor prognosis in PDAC[18].

MDSCs

MDSCs are a heterogeneous population of immature myeloid cells that expand aberrantly in cancer. They are commonly divided into two main subsets: Polymorphonuclear MDSCs (PMN-MDSCs), which are phenotypically and morphologically similar to neutrophils, and monocytic MDSCs (M-MDSCs), which resemble monocytes. Within the PDAC TME, MDSCs represent a pivotal immunosuppressive component that facilitates immune evasion and tumor progression through diverse mechanisms[19]. Their recruitment and expansion are continuously fueled by tumor-derived cytokines such as granulocyte-macrophage-colony-stimulating factor (CSF) and IL-6[19].

Functionally, tumor-infiltrating MDSCs upregulate PD-L1 in response to interferon-γ, via the phosphorylated signal transducer and activator of transcription 1-interferon regulatory factor 1 signaling axis, thereby inactivating effector T cells[20]. In addition, MDSCs suppress T-cell function by metabolically depleting critical nutrients. Arginase 1 consumes arginine, while inducible nitric oxide synthase generates nitric oxide, together disrupting T-cell signaling and proliferation[21]. Reactive oxygen and nitrogen species released by MDSCs further induce T-cell apoptosis and dysfunction[22]. MDSCs also regulate tryptophan metabolism through indoleamine 2,3-dioxygenase 1, producing kynurenine that fosters Treg differentiation and T-cell exhaustion[23]. Beyond amino acid metabolism, MDSCs contribute to immunosuppression through lipid metabolic reprogramming and prostaglandin E₂ secretion[23]. Critically, the cooperative suppression between PMN-MDSCs such as C-X-C motif chemokine ligand (CXCL) 8-high subsets and M-MDSCs arises from their complementary functions, which together are instrumental in orchestrating a potent immunosuppressive state within the TME[24].

TAMs

TAMs constitute another abundant immunoregulatory cell type in PDAC. They typically originate from circulating monocytes that infiltrate tumor tissue and differentiate locally[25]. Under the influence of cytokines such as IL-4, IL-10, IL-13, and CSF-1, TAMs are skewed toward an alternatively activated M2-like phenotype, regulated by inflammatory, immunologic, and metabolic cues[26]. Evidence suggests that reprogramming TAMs from an M2-like to a pro-inflammatory M1-like phenotype can effectively suppress PDAC progression, underscoring their potential as therapeutic targets[27-29].

M2-like TAMs foster tumor progression by secreting IL-10 and TGF-β, thereby reinforcing local immunosuppression, as well as by promoting fibrosis, angiogenesis, and metastasis through multiple growth factors and enzymes[30]. TAMs also produce chemokines such as CCL2 and CXCL12, which efficiently recruit Tregs and MDSCs[31], and cooperate with cancer-associated fibroblasts (CAFs) to establish a CXCL12-mediated immune-exclusion barrier[32]. Specialized TAM subpopulations further contribute to immune evasion; for example, CCL22-secreting TAMs selectively recruit Tregs to suppress cytotoxic T lymphocyte activity[31]. Notably, IL-1β-expressing TAM subsets have been linked with poor prognosis and reduced survival in PDAC patients[33].

THE DYNAMIC INTERACTIONS OF THE IMMUNE SUPPRESSION NETWORK AND THE MECHANISM OF TREATMENT RESISTANCE

The immunosuppressive landscape of PDAC is orchestrated by a highly organized and dynamically evolving cellular network composed of Tregs, MDSCs, and TAMs. Through reciprocal signaling, these populations establish a robust positive feedback loop that represents the fundamental basis of PDAC resistance to current immunotherapies (Figure 1). Within this network, Tregs, MDSCs, and TAMs communicate intensively via soluble mediators. Among them, TGF-β serves as a pivotal cytokine, abundantly secreted by Tregs and TAMs. TGF-β not only directly suppresses the cytotoxic activity of effector T cells and natural killing (NK) cells but also induces naïve T cells to differentiate into Tregs and promotes the polarization of macrophages toward the M2 phenotype, thereby perpetually expanding the immunosuppressive pool[26,34-37]. IL-10, primarily derived from Tregs and M2-like TAMs, inhibits DC maturation and antigen presentation while simultaneously enhancing Treg survival and function[38]. In parallel, MDSCs and TAMs represent major sources of prostaglandin E₂, which directly dampens effector T-cell responses, induces the generation of FoxP3+ Tregs, and reciprocally promotes the expansion and suppressive capacity of MDSCs[39-41]. This closed signaling loop exemplifies a vicious cycle, in which Treg-derived factors facilitate the recruitment and functional activation of MDSCs and TAMs, while the latter secrete mediators that further stabilize and expand the Treg compartment. Collectively, these interactions consolidate a highly resilient state of immune suppression.

Figure 1 Immune suppression network in the tumor microenvironment of pancreatic ductal adenocarcinoma. TGF-β: Transforming growth factor β; Treg: Regulatory T cells; TAM: Tumor-associated macrophages; MDSCs: Myeloid-derived suppressor cells; IL: Interleukin; PGE₂: Prostaglandin E₂.

This immunosuppressive network undergoes dynamic evolution as the disease progresses. In the early stages of pancreatic intraepithelial neoplasia and carcinoma in situ, the predominant feature is the accumulation and expansion of Tregs, which establishes an immune-privileged environment that facilitates early tumor development[42]. As lesions advance, MDSCs and TAMs massively accumulate under the chemotactic influence of tumor-derived signals such as granulocyte-macrophage-CSF and CCL2, subsequently differentiating into mature immunosuppressive populations within the TME[43,44]. At this stage, the positive feedback loop among Tregs, MDSCs, and TAMs becomes consolidated, driving immunosuppressive activity to its peak. In the metastatic phase, these cells are not confined to the primary tumor but also migrate through the peripheral circulation to distant organs, where they shape the so-called pre-metastatic niche and create a fertile ground for tumor cell colonization and dissemination[45].

This network drives therapeutic immune resistance through multiple mechanisms, particularly against ICIs. The dense fibrotic stroma and aberrant vasculature create a physical barrier that prevents effector T-cell infiltration[44]. At the same time, CAFs and immunosuppressive cells secrete chemokines such as CXCL12 at the tumor margins, establishing a chemical barrier that traps CXCR4-expressing effector T cells within the stromal compartment and fosters immune exclusion[46,47]. Even when a minority of effector T cells succeed in entering the tumor core, they are chronically exposed to suppressive mediators (TGF-β, IL-10, lactate) released by Tregs, MDSCs, and TAMs, as well as the high surface expression of PD-L1 on tumor cells, which rapidly drives them into profound exhaustion[48,49]. Such T cells typically co-express multiple inhibitory receptors, including PD-1, cytotoxic T-lymphocyte-associated protein 4, T cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domains, and lymphocyte-activation gene 3, losing both proliferative capacity and cytotoxic function, and thus failing to execute antitumor immunity[48]. The complexity and redundancy of these signaling pathways render single blockade strategies largely ineffective; once the PD-1/PD-L1 axis is inhibited, alternative checkpoints such as TGF-β, T cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domains, or lymphocyte-activation gene 3 can rapidly compensate to maintain immune tolerance. This mechanistic plasticity explains why “silver bullet” immune monotherapies have consistently failed in PDAC.

In summary, the immunosuppression in PDAC is sustained by a dynamic ecosystem constructed through a positive feedback loop among Tregs, MDSCs, and TAMs, exhibiting temporal characteristics across distinct stages of tumor initiation and progression. The fundamental drivers of therapeutic resistance lie in physical barriers, functional exhaustion of T cells, and the high redundancy of immunosuppressive signaling pathways. To truly dismantle this immunosuppressive fortress, targeted strategies must simultaneously intervene at multiple nodes - both relieving suppression and reactivating effector immunity - thereby enabling a systemic reprogramming of the PDAC immune microenvironment.

INNOVATIVE THERAPEUTIC STRATEGIES TARGETING THE IMMUNOSUPPRESSIVE MICROENVIRONMENT

Tregs targeting strategy

PDAC is notorious for its profoundly immunosuppressive TME, in which dense infiltration of Tregs represents a central mechanism driving immune evasion and resistance to immunotherapy. Targeting Tregs has therefore emerged as a promising strategy to overcome the immunosuppressive barrier in PDAC. Current research efforts can broadly be categorized into three major approaches: Depletion strategies, functional inhibition strategies, and signaling reprogramming strategies (Table 1).

Table 1 Therapeutic strategies targeting immunosuppressive cells in pancreatic ductal adenocarcinoma.

Target	Strategy	Core mechanism	Example agents
Tregs	Depletion	Anti-CD25 depletion	Basiliximab mogamulizumab
		Anti-CCR4 blockade
	Functional inhibition	TGF-β pathway blockade	Galunisertib oleclumab/ciforadenant
		Adenosine axis blockade
	Reprogramming	Selective IL-2Rβγ activation	Bempegaldesleukin THOR-707
MDSCs	Block recruitment	CCR2 inhibition	PF-04136309 plerixafor/BL-8040
		CXCR4 inhibition
	Induce differentiation	Retinoic acid-induced maturation	ATRA
	Selective depletion	Cytotoxic targeting of proliferating cells	Gemcitabine
TAMs	Depletion	CSF1R signaling inhibition	Pexidartinib
	Repolarization	CD40 activation	Sotigalimab motolimod eganelisib
		TLR activation
		PI3Kγ inhibition

TGF-β: Transforming growth factor β; IL: Interleukin; Treg: Regulatory T cells; TAM: Tumor-associated macrophages; MDSCs: Myeloid-derived suppressor cells; CCR4: C-C motif chemokine receptor 4; CCR2: C-C motif chemokine receptor 2; CSF1R: Colony-stimulating factor 1 receptor; TLR: Toll-like receptor; PI3Kγ: Phosphoinositide 3-kinase γ; ATRA: All-trans retinoic acid.

In terms of depletion strategies, the most commonly explored targets include CD25 and CCR4 on the surface of Tregs. CD25, the α-chain of the IL-2 receptor, is highly expressed on Tregs and was among the earliest molecules tested for Treg depletion. Anti-CD25 monoclonal antibodies (e.g., daclizumab, basiliximab) as well as IL-2-toxin fusion proteins (e.g., denileukin diftitox, Ontak) have demonstrated the ability to selectively eliminate Tregs[50,51]. However, it should be noted that activated Teff cells can transiently express CD25, creating a risk of collateral depletion[52]. To minimize damage to Teff cells, current anti-CD25 antibody research focuses on differential targeting approaches, including the development of low-affinity CD25 antibodies, the use of non-depleting Fc-silent antibodies, optimized dosing schedules, and intratumoral delivery. Among these, the most promising strategy is combination therapy with biased IL-2 muteins, which selectively engage the IL-2Rβγ pathway on effector cells. This dual approach enables Treg depletion while simultaneously supporting Teff function, thereby achieving synergistic antitumor efficacy[52-54]. CCR4, on the other hand, is a key chemokine receptor mediating Treg homing to tumors. Anti-CCR4 antibodies such as mogamulizumab can effectively deplete CCR4+ Tregs through ADCC while also blocking their continuous recruitment. In the PDAC TME, which is enriched in CCL17/CCL22, CCR4-directed depletion offers a theoretical dual advantage[55,56].

Beyond direct depletion, functional inhibition strategies focus on attenuating the key suppressive signals mediated by Tregs. Among these, the TGF-β pathway is the most critical. TGF-β not only suppresses the activity of Teff and NK cells but also drives CAF activation and stromal fibrosis, thereby further hindering immune cell infiltration. Small-molecule TGF-β inhibitors such as galunisertib have shown dual potential in early clinical trials against PDAC - both relieving immunosuppression and softening the stroma to improve the delivery of drugs and immune cells[57,58]. Another important immunosuppressive axis is the adenosine metabolism pathway. Tregs highly express CD39 and CD73, which convert ATP released from necrotic tumor cells into adenosine. The latter strongly suppresses immune effector function via the A2AR receptor. Therapeutic agents targeting this pathway, including A2AR antagonists such as ciforadenant and anti-CD73 antibodies such as oleclumab, have already entered clinical evaluation and hold substantial translational promise[59-61].

Signaling reprogramming has recently emerged as a novel therapeutic concept, centered on reshaping the cytokine milieu of the TME to redirect factors such as IL-2 toward effector cells. By converting an immunosuppressive TME into an immunoactive one, this approach aims to restore the capacity of the host immune system to effectively attack tumors[62,63]. Native IL-2 has strong affinity for the high-affinity trimeric receptor (IL-2Rαβγ) expressed on Tregs, thereby preferentially expanding Tregs and limiting its immunostimulatory potential[64]. To overcome this limitation, next-generation engineered IL-2 variants - so-called non-α IL-2 - have been designed to selectively engage the intermediate-affinity IL-2Rβγ complex, preferentially stimulating CD8+ T cells and NK cells[65]. Bempegaldesleukin, an early non-α IL-2 agonist, incorporates PEG moieties to sterically hinder CD25 binding and thereby favor IL-2Rβγ-biased activation of effector T and NK cells. Early-phase studies such as the phase I/II PIVOT-02 trial in first-line renal cell carcinoma showed pharmacodynamic evidence of preferential expansion of effector lymphocytes and preliminary antitumor activity, whereas the phase III PIVOT IO 001 trial in untreated advanced melanoma failed to meet its primary efficacy end points. Taken together, these data provide important clinical proof-of-concept for IL-2 signaling reprogramming while also highlighting the limitations of this first-generation non-α IL-2 design[66,67]. Building on this foundation, THOR-707 introduces site-specific mutations to eliminate α-chain binding while adding a PEGylation site to extend half-life, and has recently completed a phase II clinical study (NCT05104567)[68,69].

MDSCs targeting strategy

MDSCs arise under the influence of aberrant hematopoiesis and chronic inflammatory signals, and they can profoundly impair the functions of effector T cells and NK cells through metabolic regulation, nutrient competition, and the release of immunosuppressive mediators, thereby promoting tumor progression and metastasis[23]. Therapeutic strategies targeting MDSCs mainly focus on three approaches: Blocking their homing to the tumor, inducing their differentiation or depletion, and reprogramming their metabolic vulnerabilities (Table 1).

The infiltration of MDSCs into tumors relies on the finely tuned regulation of chemokine-receptor axes, among which the CCL2-CCR2 and CXCL12-CXCR4 pathways are the most critical. PDAC cells and associated fibroblasts secrete large amounts of CCL2, which recruits CCR2-expressing M-MDSCs into the tumor[70,71]; while CXCL12 drives the migration of PMN-MDSCs via CXCR4[72,73]. In animal models, treatment with the CCR2 inhibitor PF-04136309 significantly reduced intratumoral M-MDSC levels, as shown by flow cytometric analysis, and restored the functional activity of CD8+ and CD4+ T cells previously suppressed by MDSCs[74]. Similarly, CXCR4 inhibition disrupts the CXCL12 barrier, reduces MDSC infiltration, enhances the abundance of tumor-infiltrating lymphocytes, and augments the efficacy of anti-PD-1 therapy. A phase II clinical trial published in 2025 evaluated the combination of Plerixafor with the PD-1 inhibitor cemiplimab in metastatic PDAC[75]. As a clinically approved hematologic drug, Plerixafor markedly increased intratumoral CD8+ T-cell infiltration in PDAC, confirming its potential to modulate the TME in solid tumors. The phase IIa COMBAT trial, initiated in 2016, similarly investigated the CXCR4 antagonist BL-8040 in combination with pembrolizumab and chemotherapy for metastatic PDAC, and reported increased infiltration of CD8+ effector T cells along with reductions in both MDSCs and circulating Tregs[76]. These findings not only validate BL-8040’s role in converting “cold” tumors into “hot” ones but also highlight the potential of targeting MDSC recruitment pathways to synergize with existing immunotherapies.

Fundamentally, MDSCs are developmentally arrested immature myeloid cells; therefore, inducing their differentiation into mature phenotypes represents an effective means of alleviating immunosuppression. All-trans retinoic acid (ATRA) promotes MDSC differentiation into DCs or macrophages by activating retinoic acid receptor signaling, thereby reducing their immunosuppressive capacity[77]. The STARPAC trial, which combined ATRA with standard chemotherapy (gemcitabine and nanoparticle albumin-bound paclitaxel), concluded that the addition of ATRA exerted a favorable therapeutic effect in PDAC[78]. A phase IIb expansion of STARPAC is currently in preparation to further evaluate the efficacy of ATRA-based regimens and to determine the real clinical benefit for PDAC patients. Low-dose chemotherapy has also demonstrated relative selectivity in depleting MDSCs while sparing effector T cells in PDAC patients. Clinical observations revealed that gemcitabine markedly reduced circulating MDSCs and Tregs without impairing effector lymphocyte expansion[79]. Moreover, gemcitabine plus nab-paclitaxel has emerged as a cornerstone first-line chemotherapy regimen in PDAC. The phase III MPACT trial showed that this doublet significantly improved objective response rate, progression-free survival, and overall survival compared with gemcitabine monotherapy in metastatic PDAC, thereby establishing it as a standard first-line option[80]. In the neoadjuvant setting, gemcitabine-based doublet therapy has also demonstrated promise. The randomized phase II SWOG S1505 trial compared perioperative mFOLFIRINOX with gemcitabine plus nab-paclitaxel, reporting comparable safety and R0 resection rates, thus providing feasibility benchmarks for doublet therapy in resectable PDAC. Several prospective single-arm and small phase II studies further suggest that doublet therapy can achieve acceptable R0 rates, though larger, stratified phase III trials remain necessary for confirmation[81]. Despite these advances, the phase III APACT trial showed that gemcitabine plus nab-paclitaxel failed to demonstrate significant disease-free survival benefit over gemcitabine alone in the adjuvant setting, indicating that the regimen may not surpass monotherapy after curative surgery and that individualized patient selection and alternative strategies must be carefully considered[82].

Equally important is the fact that the immunosuppressive function of MDSCs relies on distinct metabolic pathways, and targeting these dependencies can markedly attenuate their activity. In PDAC, clinical exploration of MDSC metabolic vulnerabilities remains at an early stage, with a primary focus on glutamine, lactate, and fatty acid oxidation pathways. The representative glutamine antagonist DRP-104 (sirpiglenastat) is currently being evaluated in a phase I/Ib-II basket trial that includes PDAC patients; preclinical studies demonstrated its capacity to impair MDSC function and enhance T-cell activity, and the trial is still ongoing[83]. In the context of lactate metabolism, the mono-carboxylate transporter 1 inhibitor AZD3965 has shown safety and target inhibition in a phase I study, though no PDAC-specific outcomes have yet been reported[84]. For fatty acid oxidation, the inhibitor perhexiline remains confined to preclinical development[85]. Overall, these investigations suggest that blockade of metabolic pathways can partially weaken MDSC-mediated immunosuppression and improve the TME. However, in PDAC, more extensive randomized controlled studies will be required to validate efficacy signals. At the present stage, advances are primarily reflected in safety confirmation, mechanistic rationale, and the exploratory deployment of predictive biomarkers.

TAMs targeting strategy

As a critical component of the immunosuppressive TME in PDAC, TAMs promote immune evasion largely through polarization toward an M2-like phenotype, which is characterized by protumorigenic, pro-angiogenic, tissue-remodeling, and immunosuppressive functions. In PDAC, tumor cells and stromal cells produce high levels of CSF1, which engages CSF1 receptor (CSF1R) on TAMs to sustain their survival and maintain their M2-like functions. Current therapeutic approaches can be broadly divided into depletion and repolarization strategies. The former primarily relies on blockade of the CSF1/CSF1R signaling axis, thereby depriving TAMs of survival cues, leading to apoptosis and depletion. Preclinical studies have shown that the CSF1R inhibitor pexidartinib markedly reduced TAM infiltration, downregulated M2 markers, and improved the immune microenvironment in murine PDAC models, thereby delaying tumor growth and metastasis[86]. However, in clinical trials involving PDAC patients, CSF1R inhibitor-based combination therapies, while generally safe and tolerable, have not yet provided compelling evidence of objective responses or survival benefit[87]. Nevertheless, CSF1R inhibitors remain a mechanistically rational and promising therapeutic avenue, warranting further validation in large-scale randomized controlled trials.

Compared with depletion approaches, repolarization strategies - such as CD40 agonists - have shown broader translational promise in PDAC. Activation of CD40 signaling not only drives macrophages and DCs toward a proinflammatory M1 phenotype but also promotes stromal remodeling and enhances T-cell infiltration, thereby increasing sensitivity to chemotherapy and immunotherapy. The PRINCE trial (NCT03214250), a three-arm randomized phase II study, evaluated gemcitabine/nab-paclitaxel in combination with nivolumab, with sotigalimab, or with both agents. Results demonstrated that only the “chemotherapy + nivolumab” arm met the prespecified statistical threshold, whereas the sotigalimab-containing arms, despite inducing remarkable tumor shrinkage in subsets of patients, failed to deliver survival benefits in the overall population[88]. These findings highlight the immune fingerprint-dependent nature of CD40 agonist activity, suggesting that the absence of precise patient stratification may obscure real efficacy signals. More recently, the OPTIMIZE-1 trial (NCT04888312) investigated mitazalimab combined with mFOLFIRINOX in metastatic PDAC. This single-arm phase Ib/II study confirmed favorable tolerability and yielded encouraging efficacy signals, with an objective response rate of about 40% and median overall survival approaching 15 months - substantially exceeding historical chemotherapy benchmarks[89]. Consequently, mitazalimab has advanced to become the first CD40 agonist under evaluation in a global phase III trial for PDAC.

Combined therapy new paradigm

The fundamental reason why PDAC remains broadly unresponsive to current single-agent immunotherapies or targeted therapies lies in its highly complex and multilayered immunosuppressive TME. Populations such as Tregs, MDSCs, and TAMs collectively establish a formidable immune barrier that limits the efficacy of ICIs. To overcome these intricate suppressive mechanisms, breakthroughs in PDAC immunotherapy must rely on rational combination strategies built upon mechanistic complementarity, temporal coordination, and patient stratification. On one hand, such strategies aim to “release the brakes” by attenuating immunosuppression - for example, through the use of CCR2, CSF1R, or CXCR4 inhibitors. On the other hand, they seek to “press the accelerator” by enhancing antigen presentation or directly activating effector lymphocytes, such as with CD40 or toll-like receptor agonists to boost antigen-presenting cell function, or with chemotherapy and radiotherapy to induce immunogenic cell death and release tumor antigens that fuel sustained immune attack. The ultimate goal of these combinations is to establish a self-reinforcing positive feedback loop within the TME: Tumor cell death liberates antigens, which are captured and presented by activated DCs s and macrophages, leading to further cytotoxic killing and antigen release, thereby converting “cold” tumors into “hot” tumors.

Multiple clinical studies have provided real-world evidence supporting this paradigm. In the phase IIa COMBAT trial (NCT02826486), the CXCR4 inhibitor BL-8040 combined with anti-PD-1 therapy showed limited efficacy in metastatic PDAC when used as a doublet, but its combination with the NAPOLI-1 chemotherapy backbone significantly improved objective response rate and disease control rate, accompanied by increased CD8+ T-cell infiltration and reduced suppressive myeloid cells[76]. These findings suggest a complementary synergy among chemotherapy-mediated antigen release, CXCR4 inhibition-driven enhancement of immune infiltration, and the sustained immune activation provided by ICIs. In the neoadjuvant setting, the selicrelumab study directly demonstrated in human PDAC tissues that CD40 agonism promoted TAM repolarization toward an M1-like phenotype, DCs maturation, and enhanced T-cell infiltration, providing solid mechanistic evidence for the “pressing the accelerator” role of CD40 agonists[90]. On the “releasing the brakes” side, CCR2 inhibition has shown regimen-dependent outcomes: In early studies combining CCR2 blockade with FOLFIRINOX, inhibition of monocyte-macrophage recruitment was associated with promising response rates and local control (objective response rate about 49%, local control rate 97%), with manageable safety[43]; however, when combined with gemcitabine/nab-paclitaxel, the regimen produced signals of synergistic pulmonary toxicity and failed to demonstrate superior efficacy over the control arm, underscoring the critical importance of matching chemotherapy backbones with immuno-myeloid modulators[91]. All the relevant clinical trial summaries are presented in Table 2.

Table 2 Selected landmark clinical trials targeting the immunosuppressive microenvironment in pancreatic ductal adenocarcinoma.

Drug	Trial name	NCT number	Strategy	Phase	Combination
Motixafortide	COMBAT	NCT02826486	CXCR4 inhibitor	Phase IIa	+ Pembrolizumab ± chemo
			Disrupt CXCL12 barrier
			Reduce MDSCs
Plerixafor	-	NCT04543071	CXCR4 inhibitor	Phase II	+ PD-1 blockade
Cemiplimab			Enhance TILs
ATRA	STARPAC	NCT03307148	Induce MDSC differentiation	Phase I	+ Gemcitabine/nab-paclitaxel
Gemcitabine	MPACT/APACT	NCT01746979	Low-dose chemo	Phase III	Doublet chemo
Nab-paclitaxel			Deplete MDSCs/Tregs
Selicrelumab	-	NCT02588443	TAM repolarization	Phase I	Monotherapy
			APC activation
Sotigalimab	PRINCE	NCT03214250	CD40 agonist	Phase II	+ Gem/nab-pac ± nivolumab
Mitazalimab	OPTIMIZE-1	NCT04888312	CD40 agonist	Phase Ib/II	+ mFOLFIRINOX

NCT: National clinical trial; CXCR4: C-X-C motif chemokine receptor 4; CXCL12: C-X-C motif chemokine ligand 12; TILs: Tumor-infiltrating lymphocytes; ATRA: All-trans retinoic acid; PD-1: Programmed cell death protein 1; MDSC: Myeloid-derived suppressor cell; Tregs: Regulatory T cells; TAM: Tumor-associated macrophage; APC: Antigen-presenting cell.

Overall, the emerging paradigm of combination therapy emphasizes the need for systemic reprogramming of the immune microenvironment in PDAC, a prototypical “cold” tumor. Negative trial results remind us that targeting a single pathway - such as depleting TAMs or enhancing immune infiltration - may reduce suppressive populations without necessarily translating into survival benefit if immune activation is lacking. Conversely, positive outcomes demonstrate that rational combinations and precise patient stratification can substantially improve response rates and long-term survival. The future challenge lies in leveraging tools such as immunogenomics and spatial transcriptomics to accurately define patient-specific immune fingerprints, and in validating the optimal combinations and sequencing of CD40, CXCR4, and CCR2/5 pathway inhibitors with chemotherapy and ICIs through platform-based, multi-arm, and multi-stage clinical trials.

CONCLUSION

PDAC remains a formidable challenge in oncology due to its complex and immunosuppressive TME. The interplay between Tregs, MDSCs, and TAMs establishes a robust barrier to effective immune surveillance and response to current immunotherapies. Understanding the temporal dynamics and molecular mechanisms of these cell populations provides critical insights into potential therapeutic vulnerabilities. While single-agent immunotherapies have largely failed in PDAC, rational combination strategies - such as targeting chemokine axes (CCR2/CXCR4), repolarizing TAMs (via CSF1R or CD40), depleting Tregs, and modulating metabolic pathways - hold promise for reprogramming the TME. Specifically, combining the blockade of the CCR2/CXCR4 axis with chemotherapy may break the physical and chemical barriers of the TME, enhance the infiltration of cytotoxic T cells, and simultaneously reduce the recruitment of MDSCs. Secondly, combining the CD40 agonist with the checkpoint inhibitors of PD-1/PD-L1, CD40 activation can promote the maturation of DCs and macrophages, thereby enhancing antigen presentation and the activation of T cells, while checkpoint blockade can reverse the exhaustion of T cells, thereby synergistically converting immunologically “cold” tumors into “hot” tumors. Future efforts must integrate multimodal approaches, patient stratification, and biomarker-driven trials to overcome immune resistance and improve survival in this devastating disease.