Imatinib mesylate (IM) has proven to be a highly effective treatment for chronic phase (CP) chronic myeloid leukaemia (CML) patients by targeting the tyrosine-kinase domain of the BCR-ABL oncoprotein. CD4+ regulatory T cells (Treg) play a crucial role in controlling autoimmunity. As a matter of fact, lower proportions or functional activities of Treg have been described in numerous autoimmune disorders. Conversely, increased proportions and/or functional activities of Treg are found in patients with solid or haematological malignancies, thereby providing strong evidence that Treg may also play a critical role in suppressing an effective antitumour immune response. However, little is known about Treg in CML patients at diagnosis or on IM.

In this study, 36 CML patients in CP were enrolled for quantitative analysis: blood samples both at diagnosis and on IM treatment were collected for eight patients, at diagnosis only for six patients and on IM for 22 patients. In five patients, IM was combined with investigational agents. Eleven peripheral blood samples from healthy donors were analysed as controls. For functional analysis, three samples from patients at diagnosis, seven from patients on IM and six from healthy volunteers were collected (Table I and Table SI). For quantitative analyses, thawed peripheral blood mononuclear cells (PBMC) were positively enriched using Miltenyi