Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disease that primarily affects the joints and causes excessive production of pro-inflammatory cytokines [1]. Degradation of bone and articular cartilage, reduced joint space, swollen synovial tissue from cell proliferation, and inflammatory cell infiltration are the key features of RA [2]. Worldwide, between 0.5% and 1% of people have RA[3]. Physical dysfunction is a persistent condition in RA with a high rate of disability risk, which impairs the quality of life [4].

Commonly used RA activity indices are the Clinical Disease Activity Index (CDAI) and the Disease Activity Score 28 joint count with ESR (DAS28-ESR). Both indices use subjective metrics and weigh the same factors differently [5].

Relapse is documented in 30–50% of RA patients in the first two years of remission, despite apparent clinical remission. In addition, joint degeneration advances in 10–30% of patients [6]. This may be attributed to the fact that “clinical remission does not exclude the ongoing inflammatory process within the joints”. This is an important factor to consider when figuring out a potential prognostic marker(s) for RA and emphasizes that the commonly used activity indices may not accurately represent the actual RA activity. Hence, it is crucial to discover new, reliable, and objective markers for RA activity, incorporating imaging techniques for remission assessment, to get over the subjectivity of the clinical scoring systems mentioned earlier.

MicroRNAs (miRs), which are small non-protein-coding RNA (ncRNA) molecules, have been proposed as potential biomarkers for estimating RA activity. As a result of their release from tissues into plasma and easy detection in peripheral blood, several Homo sapiens-microRNAs (hsa-miRs) have been shown to be dysregulated in RA [7]. Circulating miRs can also make great candidates for biomarkers due to their stability in serum and plasma and resistance to endogenous RNase [8]. Several miRs patterns have been described across the course of an illness, suggesting that miRs could be candidates to monitor RA severity [9].

The hsa-miR-21–5p has particular relevance to arthritis because it has been shown to have a significant role in modulating bone homeostasis and joint remodeling through regulating fibroblast-like synoviocytes, chondrocytes, and osteocytes [10]. Additionally, hsa-miR-21–5p has been linked to the development and osteogenic differentiation of synoviocytes that resemble RA fibroblasts [11]. Moreover, hsa-miR-21–5p targets tumor necrosis factor-α-induced protein 8 (TNFAIP8)-like which is an anti-inflammatory protein and is crucial for preserving immunological homeostasis [12].

Based on immune cell differentiation level, hsa-miR-21–5p plays an important role in the pathogenesis of many autoimmune diseases due to the imbalance of T regulatory cells/ T helper-17 ratio (Tregs/Th17) [13] hsa-miR-21–5p promotes the proliferation of Th17 cells and the production of pro-inflammatory cytokines [14]. Also, hsa-miR-21–5p suppresses reprogramming of anti-inflammatory Tregs into pro-inflammatory IL-17-secreting cells through down-regulating Stat3 activity [15]. During the course of RA, the balance between (Treg/Th17) proved to be crucial [16].

Regarding the function and mode of hsa-miR-21–5p expression in RA, conflicting findings have been published. hsa-miR-21–5p is claimed to confer an anti-inflammatory role in RA and that its up-regulation protects against RA activity [17]. Still, hsa-miR-21–5p expression and activity markers in RA were found to be positively correlated in a different study [18]. This justifies the need to assess the role of hsa-miR-21–5p and its mode of expression in RA. Furthermore, it remains elusive whether hsa-miR-21–5p is linked to the RA imaging modality findings or not.

Musculoskeletal ultrasound (MSUS) is an imaging modality that allows joint structure visualization and is more sensitive than the current clinical examination to predict joint destruction [19]. A semi-quantitative MSUS grading system," MSUS 7 Score" (MSUS7), has been proposed to pick up persistently low disease activity, evaluate relapse, and assess radiographic progression [20], [21]. Different approaches are used for the sonographic evaluation of joint inflammation. Each joint is evaluated for synovitis and tenosynovitis/para-tenonitis scores on the Gray-Scale US (GSUS) and on the Power Doppler US (PDUS). In addition to the evaluation of the erosion score [5]. Rheumatic activity is considered, if a synovial hypertrophy score is two or higher by GSUS7 and PDUS7 score is one or higher [20]. Then, scores are combined to create the verified MSUS7 score [22], [23]. Studying new possible biomarkers like miRs reflecting the components of the MSUS7 score to monitor disease activity would be of value.

We hypothesized that due to the hsa-miR-21–5p immune-modulatory role regarding Tregs/Th17 balance; it might serve as an objective activity indicator in RA. Also, there might be a relationship between plasma hsa-miR-21–5p due to its joint remodeling role and the radiographic changes detected by the MSUS7 score in the context of RA activity.

Therefore, in the current prospective study, we aimed to evaluate plasma hsa-miR-21–5p as a noninvasive RA activity biomarker to stratify high-risk patients and to assess the relationship between hsa-miR-21–5p and MSUS7 changes in an Egyptian RA patient cohort classified according to DAS28-ESR. A preliminary step in the quest for potential targeted therapies.