Tehran University of Medical Sciences Iranian Journal of Allergy, Asthma and Immunology 1735-1502 0 0 2026 07 08 1 9 Sara Iranparast Department of Immunology, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran AND Cancer, Environmental and Petroleum Pollutants Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran Maryam Tahmasebi-Birgani Department of Medical Genetics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran AND Cellular and Molecular Research Center, Medical Basic Sciences Research Institute, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran Azim Motamedfar Department of Interventional Radiology, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran AND Department of Radiology, Golestan Hospital, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran Afshin Amari Department of Immunology, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 2 Cancer, Environmental and Petroleum Pollutants Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 3 Department of Medical Genetics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 4 Cellular and Molecular Research Center, Medical Basic Sciences Research Institute, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran Mehri Ghafourian Department of Immunology, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 2 Cancer, Environmental and Petroleum Pollutants Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 3 Department of Medical Genetics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 4 Cellular and Molecular Research Center, Medical Basic Sciences Research Institute, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 2025 11 30 2026 01 31 miR-155-5p is a well-established oncomiR upregulated in a variety of human malignancies, including breast cancer, where its aberrant expression has been documented in both tumor tissue and serum. Given its role as a key modulator of immune-inflammatory pathways and regulatory T cell (Treg) expansion, we aimed to investigate its expression in the peripheral blood mononuclear cells (PBMCs) of breast cancer patients. This study enrolled sixty (n=60) breast cancer patients and forty (n=40) age-matched healthy controls. The relative expression of miR-155-5p in PBMCs was quantified using qPCR, and Treg cell frequency (defined as CD4⁺CD25⁺Foxp3⁺) was assessed by flow cytometry.  The expression of miR-155 was significantly increased in PBMCs of breast cancer patients compared with that of the control group, especially in patients with high tumor grade. The number of Treg cells was also increased in PBMCs of patients than in the healthy group, however, no significant correlation was found between the expression levels of miR-155 and Treg cell count (r=-0.16).  Our findings demonstrate a significant up-regulation of miR-155-5p and a concomitant increase in Treg frequency in breast cancer patients, suggesting its potential role in shaping an immunosuppressive microenvironment. Consequently, miR-155-5p emerges as a promising non-invasive prognostic biomarker, warranting further validation in larger studies. https://ijaai.tums.ac.ir/index.php/ijaai/article/view/4685 https://ijaai.tums.ac.ir/index.php/ijaai/article/download/4685/2364