Genome-wide hypomethylation has been confirmed in patients with primary immune thrombocytopenia (ITP). Proteins containing methylcytosine-binding domain (MBD) are involved in promoter methylation as transcriptional repressors and promote the gene-silencing effect of DNA methylation. The purpose of this study was to investigate the methylation pattern of T cells and the relationship between genomic methylation and the expression of MBD2 and MBD4 in ITP patients. DNA deoxymethylcytosine content of CD4(+) cells from peripheral blood mononuclear cells was measured by enzyme-linked immunoassay. Real-time polymerase chain reaction was performed to quantify the transcription levels of MBD2 and MBD4 in peripheral blood mononuclear cells and CD4(+) cells. DNA dmC content in CD4(+) cells of ITP patients was significantly lower than in the controls (p=0.001). The mRNA level of MBD2 and MBD4 in CD4(+) cells of ITP patients was statistically lower than those of the controls (p<0.001). Positive correlations between methylation indexes and expression of each enzyme were observed in the control group (r(2)=0.718, p=0.004 for MBD2; r(2)=0.608, p=0.015 for MBD4). However, inverse correlations were found in ITP patients (r(2)=0.604, p=0.008 for MBD2; r(2)=0.498, p=0.027 for MBD4). Our results indicate that decreased expression of MBD2 and MBD4 might involve in the pathogenesis of ITP. (C) 2011 American Society for Histocompatibility and Immunogenetics. Published by Elsevier Inc. All rights reserved.