Ulcerative colitis (UC) is a chronic inflammatory bowel disease that affects the colon and rectum with an unknown cause [1]. It has a high incidence globally, with complications and late effects that can last the entire lifetime of a patient [2]. Although the precise cause of UC is unknown, studies have suggested that immune response disorders, changes in intestinal flora, genetic susceptibility, and living environment all play crucial roles in the initiation and development of the disease. Standard methods for the treatment of UC involve the use of anti-inflammatory medications, immunosuppressive drugs, biological therapy, and in severe cases, surgery to dissect the colon [3], [4], [5]. However, the side effects and complications associated with traditional treatment have made people turn their attention to cell therapy [6], [7].

Endometrial regenerative cells (ERCs) are a novel source of adult stem cells that are derived from human menstrual blood [8], [9]. In comparison to conventional mesenchymal stem/stromal cells (MSCs), ERCs have several advantages, such as the reuse of body waste, an abundant source, a noninvasively obtained method, and strong proliferation ability. Importantly, as previous studies have reported, human ERCs are not rejected in xenogeneic animal models, and they have demonstrated strong therapeutic effects on several experimental disease models, such as renal ischemia–reperfusion injury, stroke, myocardial infarction, Con A-induced acute hepatitis, and allograft rejection [10], [11], [12], [13]. Our team has recently proven that ERCs can attenuate DSS-induced experimental colitis in mice by reducing inflammatory cell infiltration in the damaged tissues and regulating humoral and cellular immunity responses [9], [14], [15]. Although ERCs have more advantages than MSCs, their survivability and effects are limited by shortened cell lifespan, loss of vitality in inappropriate ex vivo conditions, and apoptosis due to the inflammation microenvironment in vivo. Therefore, identifying a way to improve the therapeutic effects of ERCs in vivo is both urgent and necessary.

Melatonin (MT) is an endogenous antioxidant that is mainly secreted by the pineal gland in mammals. Previous studies have found MT to regulate a variety of physiological functions including sleep, neuroendocrine actions, circadian rhythms, and reproductive [16], [17], [18], [19]. Increasing evidence has recently implied that the crucial role of MT in the protection of MSCs against oxidation, inflammation, apoptosis, ischemia, and aging for the regulation of MSC differentiation and function in different organs and tissues has been verified in several experimental models, including cyclophosphamide-induced acute interstitial cystitis (AIC), small bowel ischemia–reperfusion, myocardial infarction, and chronic renal failure [20], [21], [22], [23], [24], [25], [26], [27]. Studies have shown that MT pre-treated MSCs can improve the protective function of MSCs due to the reduced release of inflammatory cytokines, downregulated expressions of ROS-generating and antioxidant genes, and upregulated levels of mitochondrial respiration and ATP production [28], [29], [30].

However, it is uncertain whether MT pretreatment exerts similar protective roles in ERCs for enhancing therapeutic efficiency in vivo. In order to attain the objective, a UC model in vivo and a cell model in vitro were established to mimic the oxidative condition and reveal the role of MT-pretreated effects on ERCs.