ObjectivesTo systematically review the proportion of Tregs in peripheral blood of patients with ankylosing spondylitis (AS) and its relationship with Treg's diffrent phenotypes.MethodsPubMed, EMbase, The Cochrane Library, CNKI, WanFang Data and VIP databases were electronically searched to collect case-control studies on peripheral Tregs of AS patients from inception to November 31st, 2018. Two reviewers independently screened literature, extracted data and assessed the risk of bias of included studies. Then, meta-analysis was performed by using Stata 12.0 software.ResultsA total of 61 case-control studies involving 2 466 AS patients and 1 879 controls were included. The results of meta-analysis showed that: the proportion of peripheral Tregs of patients with AS was significantly lower than that of the normal control (SMD=−0.905, 95%CI −1.294 to −0.517, P<0.000 1), and the proportion of Tregs in the disease-active group was significantly lower than that in disease-inactive group (SMD=−0.928, 95%CI −1.431 to −0.425, P<0.000 1). The proportion of CD4+CD25+FOXP3+Tregs and CD4+CD25+CD127low/−Tregs were lower in AS patients than that in control subjects (SMD=−2.547, 95%CI −3.521 to −1.573, P<0.000 1; SMD=−0.709, 95% CI −1.056 to −0.362, P<0.000 1). The proportion of Tregs defined by CD4+CD25low/−FOXP3+ was higher in AS patients (SMD=0.683, 95%CI 0.161 to 1.206, P=0.01). There was no significant difference betweew other phenotypes of Tregs groups.roups.ConclusionsThe reduction of Tregs may be one of the important reasons for the occurrence and development of AS, which may provide a new approach for the diagnosis and treatment of AS.