ИСТИНА

Background: Autoantibodies specific to cardiac troponins (TnAAbs) have been found in the blood of 6-7% of the population. It is believed that TnAAbs may negatively affect cardiac troponin I (cTnI) measurements in the blood of patients with acute myocardium infarction (AMI) by immunoassays that utilize monoclonal antibodies (mAbs) recognizing certain epitopes in the mid-part of the cTnI molecule. In the current study we investigated the epitope specificity of TnAAbs and their influence on the cTnI immunodetection in AMI samples. Methods: The presence of TnAAbs in plasma samples from healthy donors was detected according to Eriksson et al., 2004. 191 plasma samples were spiked with ternary cTnI-cTnT-TnC complex and cTnI recovery was measured using an immunoassay sensitive to the presence of TnAAbs. Twelve plasma samples showing low cTnI recovery were selected and studied for TnAAbs epitope specificity. cTnT recovery was measured in the same plasma samples. The mapping of sites on cTnT that are affected by TnAAbs was performed by using eleven anti-cTnT mAbs. The effect of TnAAbs on the measurements of cTnI in blood of AMI patients was analyzed after mixing TnAAbs-containing plasmas 1:1 with the plasma samples of AMI patients (n=35; cTnI concentrations from 2.5 to 35.1 μg/L). Results: Human cardiac troponins (cTnI, binary cTnI-TnC or ternary cTnI-cTnT-TnC complex) were spiked into twelve TnAAbs-containing plasma samples to the concentration 50 μg/L and the recoveries of cTnI were analyzed. The well-pronounced inhibitory effect of TnAAbs on cTnI measurements (mean recovery 10.3%) was observed only when cTnI was added in the form of a ternary complex. The inhibitory effect was significantly lower with spiked cTnI-TnC complex or free cTnI (mean recoveries 71.0% and 96.5%, respectively). Since cTnT appeared to be important for the manifestation of the negative interference of TnAAbs on cTnI measurements, the influence of TnAAbs on cTnT measurements was also studied. Only one epitope (223-242 aar) of cTnT was influenced by TnAAbs. The inhibitory effect of TnAAbs on cTnT detection (mean recovery 14%) was only found with spiked ternary complex, whereas it was much less pronounced with spiked free cTnT (mean recovery 73%). Since the inhibitory effects of TnAAbs on the detection of both cTnI and cTnT were observed only for the cTnI-cTnT-TnC complex, we suggest that TnAAbs are specific to structural epitopes that are formed by closely located cTnI and cTnT polypeptide chains. The negative effect of TnAAbs on the measurements of endogenous cTnI in AMI samples was significantly less pronounced compared to measurements of spiked cTnI in the form of ternary complex. The mean recovery was 61.4% vs. 10.3%, respectively. Conclusion: Unlike the common notion, our study shows that the anti-cTnI TnAAbs are not specific to cTnI per se but to the structural epitopes formed by cTnI and cTnT polypeptide chains. In our experiments, added TnAAbs have a rather limited impact on the immunodetection of cTnI in AMI samples probably because TnAAbs affect the measurements of cTnI and cTnT in ternary complex, whereas the predominant form of cTnI in the blood of patients is believed to be a cTnI-TnC binary complex.