**5. Conclusions**

In conclusion, our association study revealed correlations of expression levels in Treg cells obtained from RA patients between miR-26 and SOCS1, miR-31 and SMAD3, miR-155 and SMAD3, SMAD4. Moreover, we also noticed correlations in Th17 cells from RA between miR-26 and SMAD3, STAT3, SOCS1, miR-155 and STAT3. The present study also showed that RA Treg cells have a negative correlation between STAT5a and miR-26 level as well as between miR-126 and STAT5a level. Our research established that miR-26 and miR-155 can be good possible biomarkers for rheumatoid arthritis and osteoarthritis. After analyzing gene expression in a tested group of patients, we assume that SMAD2 shows the most promise to be used in the future to allow for earlier recognition of RA. Our study presented some interesting observations about connections between microRNAs' expression and the clinical characteristics of RA patients. The current study revealed that RA patients with DAS28 ≤ 5.1 have significantly higher miR-31 expression levels in Th17 cells and, on the contrary, the twice higher expression level of miR-24 in Treg cells from patients with DAS28 > 5.1. We also exposed that the expression level of miR-146a in Treg cells is much higher in RA patients with RF than those with RF-negative. Furthermore, our outcomes presented that anti-CCP positive RA patients have meaningfully lower miR-31 expression levels in Treg cells than anti-CCP negative RA patients.

We believe that our study may be useful in elucidating and describing the inflammatory processes leading to the RA development and RA course. There is a need for further studies with an enlarged sample size as well as the need for functional studies. Functional studies may provide insights into the effect of microRNAs on gene function. Further studies are essential to clarify the microRNAs' impact on Treg and Th17 cells, as well as transcriptional factors in the development and the course of rheumatoid arthritis. Moreover, we trust that this knowledge can lead to the development of new therapeutics for rheumatoid arthritis.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1422-0067/21/19/7169/s1.

**Author Contributions:** T.K. and E.R. wrote and edited the original draft of the manuscript and performed experiments. E.W., A.W., B.S., E.K.-W. performed experiments. A.W. performed data analysis. K.R.-P., D.M. provided research material, developed clinical database. P.P.J. and A.P. reviewed the manuscript. A.P.-G. invented, designed, and supervised the experiment and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Polish National Science Center, grant number 2015/19/B/NZ5/00247.

**Acknowledgments:** The technical assistance of Agnieszka Hertel and Wieslawa Frankowska is gratefully acknowledged. We are also thankful to all of the RA patients, OA patients and healthy subjects whose cooperation made this study possible.

**Conflicts of Interest:** The authors declare no conflict of interest.
