ObjectiveTo explore the relationship between the negative co-inhibitor programmed death-1 ( PD-1 ) and the pathogenesis of myasthenia gravis ( MG), by detecting the expression of PD-1 and programmed death ligand-1 ( PD-L1 ) on peripheral blood mononuclear cells (PBMCs) and soluble PD-1 (sPD-1) in plasma from myasthenia gravis patients. MethodsPeripheral blood samples were collected from 45 MG patients and 33 healthy persons without prednisone or other immunodepressant treatment during the half year ahead of withdrawal.The expression of PD-1 and PD-L1 on PBMCs were detected using immuno-fluorescence labeling and flow cytometry, and the concentrations of sPD-1 in plasma were measured using an ELISA kit. Results(1) The proportion of CD4+ PD-1 + T cells, as well as CD14+ PD-L1 +monocytes of the MG group was higher than that of the control group. There were no significant differences in the proportion of CD4+ PD-1 + T cells or CD14+ PD-L1 + monocytes in the MG sub-groups between different genders or MG types. While the proportion of CD4+ PD-1 + T cells of the late-onset MG (age ≥40) group was higher than that of the early-onset MG group (age ＜40). And it was higher in the MG patients with thymoma or thymus hyperplasia than that from the MG patients with normal thymus. The proportion of CD14+ PD-L1 +monocytes from the MG patients with thymoma or thymus hyperplasia group decreased obviously compared with that of the patients with normal thymus group; but no difference could be found between the late-onset group and early-onset group. (2)The concentration of sPD-1 in the plasma from the group of MG patients was(6. 92 ±0. 72) ng/ml,which was higher than that of the healthy control group ( (3.28 ±0. 42) ng/ml),even more, it was significantly higher in the early-onset MG group than that of the late-onset MG group,there was a negative correlation( r =-0. 526, P =0. 000) between the age of onset and the concentration of sPD-1. ConclusionsThe increased expressions of PD-1 on CD4+ T cells and PD-L1 on CD14+ monocytes in MG patients suggested the involvement of the couple of molecules in the pathogenesis of MG.Higher concentration of soluble PD-1 in the plasma of patients with MG suggested that it might disturb the ligation of PD-1 and PD-L1 on T cells and antigen presenting cells, which might result in the abnormal transportation of the negative modulating signal, and accelerate the pathological progress of MG.