Serological studies on Chlamydia pneumoniae infections

Chlamydia pneumoniae is a common, widespread pathogen that causes acute and chronic infections. Serological diagnosis of C. pneumoniae infection is primarily based on the microimmunofluorescence (MIF) method, but only a fourfold IgG antibody increase between paired sera and the presence of IgM antibodies have generally been accepted as markers of acute infection. At the present, no commonly accepted, reliable serological or other methods for the diagnosis of chronic C. pneumoniae infection exist. We evaluated C. pneumoniae specific serological tests in different populations, followed the kinetics of C. pneumoniae antibodies in multiple sera obtained from the same individuals, compared anti-human IgA FITC conjugates in MIF test and evaluated C. pneumoniae specific antibody tests before and after coronary events in case-control pairs matched for the time point of serum sampling, place of residence, and treatment. We showed that reinfection or reactivation is needed for the persistence of elevated IgG and IgA antibody levels. In chronic infections and upon reactivation, chronic processes may be better diagnosable based on IgA persistence than IgG levels because of the rapid disappearance of IgA levels after seroconversions. The cycle of reinfection and reactivation seems to be faster than previously thought in crowded conditions, such as in military service, since we recorded several antibody changes between the arrival and departure sera of military recruits during 6-month service. The presence of antibodies does not provide protection from reinfection. Commercial anti-human IgA conjugates act differently in MIF tests, and there is marked variation in their ability to detect IgA antibodies. The EIA test used here overestimated the prevalence and persistence of IgA antibodies when compared to MIF. The best compability between MIF and EIA antibody levels was seen in the participants with high titers. Only high IgA MIF titers to C. pneumoniae at the baseline predicted future coronary events. In the present study, seroconversions both in the participants who developed a coronary event and in the controls were detected by MIF and EIA, but mostly in different persons. Seroconversion suggesting reinfection or reactivation of persistent infection may have a role in accelerating chronic processes, because the participants with MIF seroconversion between consecutive sera had a slightly higher risk for coronary events than the controls. EIA seroconversions were more common in the controls than in the cases before the coronary event. The difference in the kinetics of EIA and MIF antibodies warrants future research and supports the use of the MIF method as a golden standard in the measurement of C. pneumoniae IgG and IgA antibody levels and seroconversions. In their diagnostic practice, laboratories should use, compare, and validate more C. pneumoniae IgA antibody tests in addition to IgG tests. Unspecific findings in C. pneumoniae EIA tests require re-estimation and a new way to interpret the results. Chlamydia experts should speak for MIF and rethink the meaning of IgA antibodies and recommendations in the diagnosis of C. pneumoniae infections.