ABSTRACT
Objective To evaluate the potential value of IgG antibodies against recombinant PPE65 protein (rPPE65) of Mycobacterium tuberculosis in serodiagnosis of tuberculosis.Methods The gene encoding PPE65 protein of M.tuberculosis was cloned into the PET-28a vector and then expressed in Escherichia coli.The rPPE65 was purified with Ni-NTA affinity and ion exchange chromatography.After dialysis renaturation, the concentration of rPPE65 was determined using Lowry assay.ELISA was used to detect the levels of specific IgG against rPPE65 and recombinant PstS1 protein (rPstS1) in sera from 144 patients with pulmonary tuberculosis (PTB patients), 144 health controls, and 56 patients with non-tuberculosis pulmonary diseases.ROC curves were used to determine cut-off values with the results of IgG antibodies against rPPE65 and rPstS1 for 144 PTB patients and 97 controls with negative PPD skin test.The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy of rPPE65 and the combination of rPPE65 and rPstS1 were counted.Results The PPE65 protein of M.tuberculosis was successfully expressed in E.coli. The purity and concentration of rPPE65 were 95% and 0.5 mg/ml, respectively.ROC analysis showed that the cut-off of ELISA using rPPE65 was 0.64.The sensitivity, specificity, PPV, NPV, and accuracy of rPPE65 were 34.7%(50/144), 93.5%(187/200), 79.4%(50/63), 66.5%(187/287), and 68.9%(237/344), respectively.The sensitivity, specificity, PPV, NPV, and accuracy of the combination of rPPE65 and rPstS1 were 59.0%, 91.0%, 82.5%, 75.5%, 77.6%, respectively.Conclusions The rPPE65 of M.tuberculosis appears to be a candidate antigen for serodiagnosis of tuberculosis.Detection of IgG antibodies against the combination of rPPE65 and rPstS1 can increase the sensitivity of serological test for tuberculosis.