1. Mak WY, Zhao M, Ng SC, Burisch J. The epidemiology of inflammatory bowel disease: east meets west. J Gastroenterol Hepatol 2020;35:380–389.

2. Park SH, Kim YJ, Rhee KH, et al. A 30-year trend analysis in the epidemiology of inflammatory bowel disease in the Songpa-Kangdong district of Seoul, Korea in 1986-2015. J Crohns Colitis 2019;13:1410–1417.

3. Guan Q. A comprehensive review and update on the pathogenesis of inflammatory bowel disease. J Immunol Res 2019;2019:7247238.

4. Keir ME, Butte MJ, Freeman GJ, Sharpe AH. PD-1 and its ligands in tolerance and immunity. Annu Rev Immunol 2008;26:677–704.

5. Kim JY, Park M, Kim YH, et al. Tonsil-derived mesenchymal stem cells (T-MSCs) prevent Th17-mediated autoimmune response via regulation of the programmed death-1/programmed death ligand-1 (PD-1/PD-L1) pathway. J Tissue Eng Regen Med 2018;12:e1022.

6. Beswick EJ, Grim C, Singh A, et al. Expression of programmed death-ligand 1 by human colonic CD90+ stromal cells differs between ulcerative colitis and Crohn’s disease and determines their capacity to suppress Th1 cells. Front Immunol 2018;9:1125.

7. Kanai T, Totsuka T, Uraushihara K, et al. Blockade of B7-H1 suppresses the development of chronic intestinal inflammation. J Immunol 2003;171:4156–4163.

8. Scandiuzzi L, Ghosh K, Hofmeyer KA, et al. Tissue-expressed B7-H1 critically controls intestinal inflammation. Cell Rep 2014;6:625–632.

9. Park SJ, Kim JH, Song MY, Sung YC, Lee SW, Park Y. PD-1 deficiency protects experimental colitis via alteration of gut microbiota. BMB Rep 2017;50:578–583.

10. Flores AI, Gómez-Gómez GJ, Masedo-González Á, Martínez-Montiel MP. Stem cell therapy in inflammatory bowel disease: a promising therapeutic strategy? World J Stem Cells 2015;7:343–351.

11. Ciccocioppo R, Bernardo ME, Sgarella A, et al. Autologous bone marrow-derived mesenchymal stromal cells in the treatment of fistulising Crohn’s disease. Gut 2011;60:788–798.

12. Janjanin S, Djouad F, Shanti RM, et al. Human palatine tonsil: a new potential tissue source of multipotent mesenchymal progenitor cells. Arthritis Res Ther 2008;10:R83.

13. Oh SY, Choi YM, Kim HY, et al. Application of tonsil-derived mesenchymal stem cells in tissue regeneration: concise review. Stem Cells 2019;37:1252–1260.

14. Yu Y, Song EM, Lee KE, et al. Therapeutic potential of tonsil-derived mesenchymal stem cells in dextran sulfate sodium-induced experimental murine colitis. PLoS One 2017;12:e0183141.

15. Lee KE, Jung SA, Joo YH, et al. The efficacy of conditioned medium released by tonsil-derived mesenchymal stem cells in a chronic murine colitis model. PLoS One 2019;14:e0225739.

16. Dominici M, Le Blanc K, Mueller I, et al. Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement. Cytotherapy 2006;8:315–317.

17. Yu Y, Park YS, Kim HS, et al. Characterization of long-term in vitro culture-related alterations of human tonsil-derived mesenchymal stem cells: role for CCN1 in replicative senescence-associated increase in osteogenic differentiation. J Anat 2014;225:510–518.

18. Rajabian Z, Kalani F, Taghiloo S, et al. Over-expression of immunosuppressive molecules, PD-L1 and PD-L2, in ulcerative colitis patients. Iran J Immunol 2019;16:62–70.

19. Nguyen J, Finkelman BS, Escobar D, Xue Y, Wolniak K, Pezhouh M. Overexpression of programmed death ligand 1 in refractory inflammatory bowel disease. Hum Pathol 2022;126:19–27.

20. Zhou L, Liu D, Xie Y, Yao X, Li Y. Bifidobacterium infantis induces protective colonic PD-L1 and Foxp3 regulatory T cells in an acute murine experimental model of inflammatory bowel disease. Gut Liver 2019;13:430–439.

21. Zhou LY, Xie Y, Li Y. Bifidobacterium infantis regulates the programmed cell death 1 pathway and immune response in mice with inflammatory bowel disease. World J Gastroenterol 2022;28:3164–3176.

22. Song MY, Hong CP, Park SJ, et al. Protective effects of Fcfused PD-L1 on two different animal models of colitis. Gut 2015;64:260–271.

23. Silva FA, Rodrigues BL, Ayrizono ML, Leal RF. The immunological basis of inflammatory bowel disease. Gastroenterol Res Pract 2016;2016:2097274.

24. Kolls JK, Lindén A. Interleukin-17 family members and inflammation. Immunity 2004;21:467–476.