Distinct phenotypic and transcriptomic profiles of myeloid-derived suppressor cells induced by 4T1 tumor growth and cyclophosphamide treatment

Technical Summary: Distinct Phenotypic and Transcriptomic Profiles of MDSCs Induced by 4T1 Tumor Growth and Cyclophosphamide

Problem Statement
Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that expand under pathological conditions, such as cancer and chronic inflammation, or can be induced pharmacologically by agents like cyclophosphamide (CTX). While MDSCs are known to mediate immunosuppression through mechanisms involving arginase, nitric oxide (NO), reactive oxygen species (ROS), and cytokines, it remains unclear whether MDSCs generated by tumor growth versus those induced by CTX treatment represent phenotypically and molecularly distinct populations. Previous studies suggest that tumor-derived signals often favor polymorphonuclear MDSCs (PMN-MDSCs) with ROS-dependent activity, whereas CTX-induced bone marrow injury and subsequent recovery may favor monocytic MDSCs (M-MDSCs) with NO-dependent activity. However, a direct comparison of their subset composition, suppressive mediator profiles, and transcriptomic landscapes has been lacking.

Methodology
The study utilized a murine model involving female BALB/c mice to compare bone marrow MDSCs under four conditions: control, 4T1 tumor-bearing (Tumor), CTX-treated (CTX), and a combination of tumor-bearing plus CTX treatment (Tumor + CTX).

• Induction Models: 4T1-Luc cells were inoculated into mammary fat pads to induce tumor growth. CTX (150 mg/kg) was administered intraperitoneally, either alone or in a schedule following tumor inoculation.
• Cell Isolation: Bone marrow cells were harvested, and CD11b⁺Gr-1⁺ MDSCs were isolated using magnetic separation.
• Phenotypic Analysis: Flow cytometry was employed to quantify total MDSCs and distinguish subsets: PMN-MDSCs (CD11b⁺Ly6G⁺Ly6Clow) and M-MDSCs (CD11b⁺Ly6G⁻Ly6Chi).
• Functional Assays: Intracellular ROS levels were measured using DCFH-DA fluorescence; NO production was assessed via DAF-FM DA fluorescence; and arginase activity was quantified enzymatically.
• Molecular Profiling: Quantitative RT-PCR (qRT-PCR) analyzed the expression of key MDSC-associated genes (Cybb, Nos2, Arg1, Cxcr2, Tgfb1, Cd84, etc.).
• Transcriptomics: Bulk RNA sequencing (RNA-seq) was performed on MDSCs from all four groups (n=4 per group). Data analysis included differential expression analysis (DESeq2), Principal Component Analysis (PCA), hierarchical clustering, and pathway enrichment (KEGG and Gene Ontology).

Key Results

1. Subset Composition: All three induction conditions significantly increased the absolute number and frequency of bone marrow MDSCs. However, the subset composition differed:

   • Tumor growth preferentially expanded PMN-MDSCs.
   • CTX treatment and the Tumor + CTX combination preferentially expanded M-MDSCs.
   • Total leukocyte numbers increased in tumor-bearing groups but not in the CTX-only group, suggesting different underlying mechanisms (tumor-driven myelopoiesis vs. compensatory hematopoietic recovery).

2. Suppressive Mediator Profiles:

   • ROS: Tumor-induced MDSCs showed increased expression of Cybb and numerically higher ROS-associated fluorescence compared to other groups.
   • NO: CTX-containing groups (CTX and Tumor + CTX) exhibited increased NO-associated fluorescence, despite limited changes in Nos2 mRNA levels in qRT-PCR, suggesting potential post-transcriptional regulation.
   • Arginase: Arg1 expression was reduced in Tumor and CTX groups compared to controls, and no significant differences in total arginase activity were observed among groups.

3. Gene Expression and Transcriptomics:

   • qRT-PCR: The Tumor + CTX group showed selective upregulation of Cd84 and Tgfb1, distinct from the other groups.
   • RNA-seq: PCA and hierarchical clustering clearly separated the four groups, indicating unique transcriptional programs for each condition.
   • Differentially Expressed Genes (DEGs): The Tumor group had the highest number of DEGs (2,283), followed by CTX (1,438) and Tumor + CTX (812).
   • Pathway Enrichment:
     • Tumor: Enriched in JAK-STAT signaling, efferocytosis, and cholesterol metabolism.
     • CTX: Enriched in Fanconi anemia pathway, homologous recombination, and cell cycle pathways (reflecting DNA damage response and recovery).
     • Tumor + CTX: Enriched in JAK-STAT signaling, cytokine-cytokine receptor interactions, and complement/coagulation cascades.

Significance and Claims
The authors conclude that tumor growth, CTX treatment, and their combination generate MDSC populations that are phenotypically and transcriptionally distinct, despite sharing the same surface markers (CD11b⁺Gr-1⁺).

• Distinct Mechanisms: The study demonstrates that the induction method dictates the lineage bias (PMN vs. M-MDSC), the dominant suppressive mediator (ROS vs. NO), and the underlying transcriptional program.
• Context Dependency: The findings underscore that MDSCs are not a uniform entity; their biological characteristics are shaped by the specific environmental signals (tumor-derived inflammation vs. chemotherapy-induced stress).
• Implications for Research: The paper asserts that MDSCs induced by different experimental protocols should not be considered interchangeable. Researchers must carefully consider the induction context when interpreting MDSC biology, selecting models for tumor immunity studies, or evaluating therapy-induced immunoregulation. The study highlights that relying solely on surface markers without considering the induction history may overlook critical functional and molecular differences.