Small repeating units within the Ureaplasma urealyticum MB antigen gene encode serovar specificity and are associated with antigen size variation.

Ureaplasma urealyticum is a common commensal of the female lower urogenital tract, yet it has been shown to be an important cause of chorioamnion infection, respiratory and central nervous system disease, and death in premature infants. It has been suggested that only certain serovars are capable of producing invasive disease. However, we previously showed that many serotypes are invasive and that perhaps antigen variability and host factors are more important determinants of ureaplasma infections than are different serotypes per se. The molecular characterization in this report describes a mechanism available to ureaplasmas for producing antigen variation. That antigen, designated MB and previously identified on U. urealyticum, contains serovar-specific and cross-reactive epitopes, is produced both in vitro and in vivo, is a predominant antigen recognized during ureaplasma infections of humans, undergoes a high rate of size variation in vitro, and is size variable on invasive ureaplasma isolates. In the present study, we cloned and sequenced the gene of the MB antigen from serovar 3, the serovar most commonly isolated from humans. The 3' two-thirds of the gene was shown to contain identical 18-nucleotide tandem repeats. PCR analysis and direct sequencing of two variants indicated that alterations within this repeat region are responsible for the size variation of the MB antigen. Intact recombinant serovar 3 MB antigen and truncated products, expressed by coupled in vitro transcription and translation of the cloned gene, were immunoprecipitated by both a serovar-specific monoclonal antibody and the serum of a U. urealyticum-infected patient, and these results identified the repeat region of the MB antigen as serovar defining. Resolution of the precise amino acids responsible for specific epitopes and characterization of similar genes in the other serovars should yield reagents useful in elucidating the role of antigen size variants in disease production and the role of specific antibody in protection from ureaplasma disease.