Seroepidemiological survey of Chlamydia pneumoniae infection among Singapore university undergraduates: comparison between microimmunofluorescence and neutralization tests

Chlamydia pneumoniae causes acute human respiratory tract infections, and has been implicated in the pathogenesis of atherosclerosis. A seroepidemiological study using the microimmunofluorescence (MIF) technique was conducted to determine the prevalence of C. pneumoniae IgG antibodies (at titres of at least 1:16) among 205 apparently healthy Singapore university undergraduates. The overall seroprevalence was 35.1%, with significantly higher seropositivity rates among males than females (48.2% vs. 18.7%, P < 0.001). Out of 78 samples subjected to further neutralization tests in vitro, complement-independent neutralizing antibodies were detected in 22.2% (14/63) of MIF-positive sera, but only in 6.7% (1/15) of MIF-negative sera. The percentages of MIF-positive sera with neutralizing activity increased with the grade of MIF positivity, i.e. 0% (1+), 7.1% (2+), 18.8% (3+), and 63.6% (4+), with the latter being comparatively significant (P < 0.05). The percentages of reduction in the inclusion-forming unit (IFU) count correlated well with the MIF data, as reflected by their mean percentages of IFU reduction of 0.6% for MIF-negative sera, 15.4, 26.5, 40.1 and 51.5% for MIF 1+, 2+, 3+ and 4+ sera, respectively, with these differences being statistically significant. The relatively high and gender-biased seroprevalence of antibodies to C. pneumoniae among young adults highlights the importance of this common yet under-recognized infection in the local community. Furthermore, high grade MIF positivity may represent a useful serological marker of predictive value for neutralizing activity.