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Background:
Prostate cancer (PCa) is characterized by an immunologically “cold” tumor microenvironment (TME), with prominent enrichment of myeloid-derived suppressor cells (MDSCs), which compromises effective antitumor immunity and underlies the poor clinical response to immune checkpoint blockade (ICB). Although MDSCs have been extensively implicated in PCa immune evasion, the tumor-intrinsic signaling mechanisms that educate, sustain, and amplify MDSC expansion and immunosuppressive function remain incompletely understood.
Methods:
Single-cell RNA sequencing and flow cytometry were employed to quantify and characterize MDSCs within the PCa TME. Tumor cell–MDSC co-culture systems combined with flow cytometric analysis were used to interrogate the role of RelB signaling in MDSC expansion and immunosuppressive activity. RNA sequencing, chromatin immunoprecipitation (ChIP), and RT–qPCR assays were performed to define RelB-mediated transcriptional regulation of
CSF2
.LC–MS/MS screening combined with immunofluorescence and co-immunoprecipitation assays was used to define the RelB–ILF2 interaction and RelB ubiquitination, while RelB nuclear translocation was assessed under GM-CSF stimulation. Syngeneic PCa mouse models were used to evaluate the impact of the RelB/ILF2–GM-CSF axis on MDSCs, CD8⁺ T cell function, tumor growth, and PD-1 blockade response.
Results:
We identified a tumor-intrinsic signaling circuit in PCa cells whereby RelB, a key component of the noncanonical NF-κB pathway, drives MDSC expansion through transcriptional activation of
CSF2
. RelB binds to a distal NF-κB element within the
CSF2
promoter, thereby enhancing GM-CSF expression. Mechanistically, ILF2 interacts with RelB at the Rel homology domain (Arg176), inhibiting RelB ubiquitination and promoting nuclear accumulation. GM-CSF, in turn, reinforces RelB nuclear translocation, establishing a self-sustaining RelB/ILF2–GM-CSF positive feedback loop. This axis activates STAT3 signaling in MDSCs, promoting their expansion and immunosuppressive function. Disruption of the RelB/ILF2 complex abrogated GM-CSF–driven MDSC expansion, restored CD8⁺ T cell–mediated antitumor immunity, and suppressed tumor growth. Notably, pharmacological targeting of RelB using the inhibitory peptide SN52 significantly enhanced tumor sensitivity to PD-1 blockade.
Conclusions:
Our findings uncover a previously unrecognized RelB/ILF2–GM-CSF feed-forward loop that sustains MDSC-mediated immunosuppression in PCa by enabling RelB-dependent recycling of GM-CSF signaling. Targeting this axis not only disrupts tumor-driven MDSC expansion but also restores effective CD8⁺ T cell immunity and sensitizes PCa to immune checkpoint therapy, highlighting a promising therapeutic strategy for overcoming immune resistance in advanced PCa.