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Introduction and Objectives Myeloid-derived suppressor cells (MDSCs) are immature cells of the myeloid lineage with immunosuppressive properties, divided into polymorphonuclear and monocytic subpopulations, and are commonly involved in chronic inflammatory diseases. We aimed to systematically review the literature to assess the quantitative and qualitative alterations of MDSCs and their involvement in allergic airway diseases, i.e. asthma and allergic rhinitis.Methods The systematic review protocol was registered in PROSPERO, and the PRISMA guidelines for systematic reviews were followed. The databases MEDLINE via PubMed, Embase, and Scopus were systematically searched for original research studies involving the measurement of human MDSCs in allergic airway diseases by the end of 2024. A narrative synthesis of the extracted data from the included studies was performed.Results Among a total of 429 records screened, 8 studies were considered eligible to be included in the review. The studies were run from 2011 to 2021 and cumulatively included 212 patients with asthma and 90 patients with allergic rhinitis. Healthy controls (cumulatively 203) were included in 7 studies, while 4 studies also included patients with other respiratory diseases for comparison, i.e. chronic obstructive pulmonary disease (35 patients), pneumonia (65 patients), and respiratory viral infections (27 patients). Out of 6 studies about asthma, MDSC counts were found either increased or increasing after allergen challenge in bronchoalveolar lavage (2 studies) and in peripheral blood mononuclear cells (3 studies), while the monocytic MDSC subpopulation was found downregulated in isolated white blood cells in only 1 study. The 2 studies about allergic rhinitis used diverse experimental set-ups and were not comparable.Conclusions Increased counts of MDSCs are mostly found in asthma, pointing towards the role of these immunomodulatory cells in its pathogenesis, despite the heterogeneity of the methodology among the different studies. Further research and harmonisation of the markers and techniques for the identification of MDSCs is needed, as they may be used as novel biomarkers and therapeutic targets in these disease entities.