Myeloid-Derived Suppressor Cells Beyond Cancer: Expanding Roles in Health and Disease

Abstract

Myeloid-derived suppressor cells (MDSCs) have emerged as pivotal regulators of immune responses, extending well beyond their canonical involvement in cancer. Recent research highlights their significant role in chronic infections, autoimmune diseases, transplantation, and inflammatory conditions. This review synthesizes contemporary evidence on MDSCs outside the oncology sphere, focusing on their pathophysiological mechanisms, clinical significance, diagnostic challenges, and therapeutic implications. A nuanced understanding of MDSC biology is increasingly relevant for clinicians aiming to translate these insights into improved patient care across a spectrum of disorders.

Introduction

MDSCs are a heterogeneous population of immature myeloid cells recognized for their potent immunosuppressive activities. Originally described in the context of tumor-induced immune evasion, MDSCs have since been implicated in diverse pathological states, including chronic infections, autoimmune disorders, and organ transplantation. Their ability to modulate both innate and adaptive immunity renders them key players in immune homeostasis and dysregulation. This review aims to provide healthcare professionals with an evidence-based update on the roles of MDSCs beyond oncology, emphasizing clinical relevance and translational potential.

Epidemiology / Disease Burden

While the prevalence of MDSCs per se is not routinely quantified in epidemiological studies, their expansion is now recognized in a range of non-malignant conditions. Elevated MDSC levels are reported in chronic viral infections (e.g., HIV, hepatitis C), sepsis, autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus, and following organ transplantation. These conditions collectively affect millions worldwide, underscoring the broad relevance of MDSC-mediated pathways in global disease burden and the potential for targeted interventions.

Pathophysiology

MDSCs arise from disrupted myelopoiesis under pathologic conditions, resulting in the accumulation of immature myeloid precursors in peripheral blood and tissues. Two main subsets exist: monocytic (M-MDSC) and granulocytic or polymorphonuclear (PMN-MDSC), each employing distinct suppressive mechanisms. These include the production of arginase-1, inducible nitric oxide synthase (iNOS), reactive oxygen species (ROS), and immunosuppressive cytokines such as IL-10 and TGF-β. MDSCs inhibit T cell proliferation, impair NK cell cytotoxicity, and promote regulatory T cell (Treg) expansion, establishing a microenvironment conducive to chronic inflammation or immune tolerance depending on the context. In infections, MDSCs may dampen protective immunity, whereas in autoimmunity and transplantation, they can mitigate tissue damage by restraining excessive immune activation.

Risk Factors

Risk factors for MDSC expansion include persistent antigenic stimulation, chronic infection, dysregulated cytokine milieus (notably elevated GM-CSF, IL-6, and G-CSF), and ongoing inflammation. Genetic predisposition, age, and underlying comorbidities such as metabolic syndrome or chronic liver disease may also influence MDSC development. In transplantation, alloantigen exposure and immunosuppressive therapy can further drive MDSC mobilization, highlighting the interplay between host factors and environmental triggers.

Clinical Features

Unlike classical disease markers, MDSCs are not associated with specific clinical symptoms but rather influence disease trajectory by modulating immune responses. In chronic infections, increased MDSCs correlate with higher pathogen burden and impaired viral clearance. In autoimmune diseases, elevated MDSC frequencies are linked with disease severity and flare risk, while in transplantation, they are associated with reduced rejection rates but increased susceptibility to opportunistic infections. Thus, MDSC enumeration and functional profiling may have prognostic value in several clinical settings.

Diagnosis

The identification and quantification of MDSCs rely on multiparametric flow cytometry, employing markers such as CD11b, CD33, HLA-DR, CD14, and CD15, with additional functional assays to assess suppressive capacity. Standardization across laboratories remains a challenge due to phenotypic heterogeneity. Recent developments in single-cell transcriptomics and mass cytometry are refining our understanding of MDSC subpopulations and their functional states. Incorporating MDSC assessment into routine diagnostics is an area of active investigation, particularly in immunologically complex diseases.

Treatment & Management

Therapeutic strategies targeting MDSCs are context-dependent. In autoimmunity and transplantation, harnessing or expanding MDSC populations may promote tolerance and mitigate tissue damage. Approaches include adoptive transfer of ex vivo-expanded MDSCs or pharmacologic agents that enhance their suppressive function, such as low-dose GM-CSF or all-trans retinoic acid. Conversely, in chronic infections, strategies to inhibit MDSC expansion or function (e.g., using phosphodiesterase-5 inhibitors, arginase inhibitors, or STAT3 blockade) are under evaluation to restore effective immune responses. Balancing the dual-edged nature of MDSCs protective versus pathogenic remains a therapeutic challenge, necessitating precise biomarker-guided interventions.

Recent Advances / Emerging Therapies

Recent advances include the identification of novel MDSC-specific targets (e.g., S100A8/A9, LOX-1) and the development of small-molecule inhibitors to selectively deplete or reprogram these cells. Clinical trials are underway assessing MDSC-modulating agents in autoimmune diseases and transplantation. There is growing interest in combining MDSC-targeted therapies with conventional immunomodulators to enhance efficacy and minimize adverse effects. Furthermore, the application of high-dimensional immune profiling is facilitating personalized approaches to MDSC modulation, particularly in complex immune-mediated diseases.

Guideline Recommendations

While formal guidelines for MDSC-targeted interventions are limited, expert consensus supports investigating MDSC dynamics in patients with refractory autoimmune diseases, recurrent transplant rejection, or persistent infections. The integration of MDSC assessment into clinical trials and translational research is recommended to elucidate their prognostic and therapeutic potential. Ongoing efforts by immunology societies aim to standardize phenotyping protocols and foster collaborative studies to accelerate clinical translation.

Conclusion

Myeloid-derived suppressor cells are now recognized as crucial immunoregulatory players beyond cancer, with implications for a broad range of chronic and inflammatory diseases. Advances in mechanistic understanding and diagnostic technologies are paving the way for innovative therapies that modulate MDSC function according to clinical context. Continued research and multidimensional analysis will be essential to harness the full therapeutic potential of MDSCs in personalized medicine, offering new hope for patients with complex immune-mediated disorders.