Size variation of a major serotype-specific antigen of Ureaplasma urealyticum

The Human Ureaplasma Species as Causative Agents of Chorioamnionitis

The human Ureaplasma species are the most frequently isolated microorganisms from the amniotic fluid and placentae of women who deliver preterm and are also associated with spontaneous abortions or miscarriages, neonatal respiratory diseases, and chorioamnionitis. Despite the fact that these microorganisms have been habitually found within placentae of pregnancies with chorioamnionitis, the role of Ureaplasma species as a causative agent has not been satisfactorily explained. There is also controversy surrounding their role in disease, particularly as not all women infected with Ureaplasma spp. develop chorioamnionitis. In this review, we provide evidence that Ureaplasma spp. are associated with diseases of pregnancy and discuss recent findings which demonstrate that Ureaplasma spp. are associated with chorioamnionitis, regardless of gestational age at the time of delivery. Here, we also discuss the proposed major virulence factors of Ureaplasma spp., with a focus on the multiple-banded antigen (MBA), which may facilitate modulation/alteration of the host immune response and potentially explain why only subpopulations of infected women experience adverse pregnancy outcomes. The information presented within this review confirms that Ureaplasma spp. are not simply "innocent bystanders" in disease and highlights that these microorganisms are an often underestimated pathogen of pregnancy.

XerC-mediated DNA inversion at the inverted repeats of the UU172-phase-variable element of Ureaplasma parvum serovar 3

Phase variation of the UU172 phase-variable element of Ureaplasma parvum is governed by a DNA inversion event that takes place at short inverted repeats. The putative tyrosine recombinase XerC of Ureaplasma has been suggested as a mediator in the proposed site-specific recombination event. Here, we provide evidence that XerC mediates DNA inversion at the inverted repeats located on a synthetic locus that was introduced into the model organism Escherichia coli. Synthetic loci were created by exchanging the genes UU171 and UU172 with the two reporter genes gfp (green fluorescent protein) and mrfp1 (monomeric red fluorescent protein 1) either containing or missing the inverted repeats of the UU172 phase-variable element. E. coli was transformed with these loci and also co-transformed with the expression vector pBAD24 that contained the xerC gene behind the arabinose inducible pBAD promoter. Upon XerC expression, DNA inversion was observed only in the locus that contained the inverted repeat regions. We also demonstrate that XerC can process the recombination event with both an N-terminal maltose binding protein tag and a C-terminal 6×His tag in E. coli. A XerC mutant, where the proposed catalytic tyrosine residue 228 was exchanged with an alanine, did not process the recombination event.

Ureaplasma parvum undergoes selection in utero resulting in genetically diverse isolates colonizing the chorioamnion of fetal sheep

Ureaplasmas are the microorganisms most frequently isolated from the amniotic fluid of pregnant women and can cause chronic intrauterine infections. These tiny bacteria are thought to undergo rapid evolution and exhibit a hypermutatable phenotype; however, little is known about how ureaplasmas respond to selective pressures in utero. Using an ovine model of chronic intraamniotic infection, we investigated if exposure of ureaplasmas to subinhibitory concentrations of erythromycin could induce phenotypic or genetic indicators of macrolide resistance. At 55 days gestation, 12 pregnant ewes received an intraamniotic injection of a nonclonal, clinical Ureaplasma parvum strain followed by (i) erythromycin treatment (intramuscularly, 30 mg/kg/day, n = 6) or (ii) saline (intramuscularly, n = 6) at 100 days gestation. Fetuses were then delivered surgically at 125 days gestation. Despite injecting the same inoculum into all the ewes, significant differences between amniotic fluid and chorioamnion ureaplasmas were detected following chronic intraamniotic infection. Numerous polymorphisms were observed in domain V of the 23S rRNA gene of ureaplasmas isolated from the chorioamnion (but not the amniotic fluid), resulting in a mosaiclike sequence. Chorioamnion isolates also harbored the macrolide resistance genes erm(B) and msr(D) and were associated with variable roxithromycin minimum inhibitory concentrations. Remarkably, this variability occurred independently of exposure of ureaplasmas to erythromycin, suggesting that low-level erythromycin exposure does not induce ureaplasmal macrolide resistance in utero. Rather, the significant differences observed between amniotic fluid and chorioamnion ureaplasmas suggest that different anatomical sites may select for ureaplasma subtypes within nonclonal, clinical strains. This may have implications for the treatment of intrauterine ureaplasma infections.

Synergic activation of toll-like receptor (TLR) 2/6 and 9 in response to Ureaplasma parvum & urealyticum in human amniotic epithelial cells

Ureaplasma species are the most frequently isolated microorganisms inside the amniotic cavity and have been associated with spontaneous abortion, chorioamnionitis, premature rupture of the membranes (PROM), preterm labour (PL) pneumonia in neonates and bronchopulmonary dysplasia in neonates. The mechanisms by which Ureaplasmas cause such diseases remain unclear, but it is believed that inappropriate induction of inflammatory responses is involved, triggered by the innate immune system. As part of its mechanism of activation, the innate immune system employs germ-lined encoded receptors, called pattern recognition receptors (PRRs) in order to “sense” pathogens. One such family of PRRs are the Toll like receptor family (TLR). In the current study we aimed to elucidate the role of TLRs in Ureaplasma-induced inflammation in human amniotic epithelial cells. Using silencing, as well as human embryonic kidney (HEK) transfected cell lines, we demonstrate that TLR2, TLR6 and TLR9 are involved in the inflammatory responses against Ureaplasma parvum and urealyticum serovars. Ureaplasma lipoproteins, such as Multiple Banded antigen (MBA), trigger responses via TLR2/TLR6, whereas the whole bacterium is required for TLR9 activation. No major differences were observed between the different serovars. Cell activation by Ureaplasma parvum and urealyticum seem to require lipid raft function and formation of heterotypic receptor complexes comprising of TLR2 and TLR6 on the cell surface and TLR9 intracellularly.

Interaction of the putative tyrosine recombinases RipX (UU145), XerC (UU222), and CodV (UU529) of Ureaplasma parvum serovar 3 with specific DNA

Phase variation of two loci (‘mba locus’ and ‘UU172 phase-variable element’) in Ureaplasma parvum serovar 3 has been suggested as result of site-specific DNA inversion occurring at short inverted repeats. Three potential tyrosine recombinases (RipX, XerC, and CodV encoded by the genes UU145, UU222, and UU529) have been annotated in the genome of U. parvum serovar 3, which could be mediators in the proposed recombination event. We document that only orthologs of the gene xerC are present in all strains that show phase variation in the two loci. We demonstrate in vitro binding of recombinant maltose-binding protein fusions of XerC to the inverted repeats of the phase-variable loci, of RipX to a direct repeat that flanks a 20-kbp region, which has been proposed as putative pathogenicity island, and of CodV to a putative dif site. Co-transformation of the model organism Mycoplasma pneumoniae M129 with both the ‘mba locus’ and the recombinase gene xerC behind an active promoter region resulted in DNA inversion in the ‘mba locus’. Results suggest that XerC of U. parvum serovar 3 is a mediator in the proposed DNA inversion event of the two phase-variable loci.

The role of the multiple banded antigen of Ureaplasma parvum in intra-amniotic infection: major virulence factor or decoy?

The multiple banded antigen (MBA) is a predicted virulence factor of Ureaplasma species. Antigenic variation of the MBA is a potential mechanism by which ureaplasmas avoid immune recognition and cause chronic infections of the upper genital tract of pregnant women. We tested whether the MBA is involved in the pathogenesis of intra-amniotic infection and chorioamnionitis by injecting virulent or avirulent-derived ureaplasma clones (expressing single MBA variants) into the amniotic fluid of pregnant sheep. At 55 days of gestation pregnant ewes (n = 20) received intra-amniotic injections of virulent-derived or avirulent-derived U. parvum serovar 6 strains (2×10⁴ CFU), or 10B medium (n = 5). Amniotic fluid was collected every two weeks post-infection and fetal tissues were collected at the time of surgical delivery of the fetus (140 days of gestation). Whilst chronic colonisation was established in the amniotic fluid of animals infected with avirulent-derived and virulent-derived ureaplasmas, the severity of chorioamnionitis and fetal inflammation was not different between these groups (p>0.05). MBA size variants (32–170 kDa) were generated in vivo in amniotic fluid samples from both the avirulent and virulent groups, whereas in vitro antibody selection experiments led to the emergence of MBA-negative escape variants in both strains. Anti-ureaplasma IgG antibodies were detected in the maternal serum of animals from the avirulent (40%) and virulent (55%) groups, and these antibodies correlated with increased IL-1β, IL-6 and IL-8 expression in chorioamnion tissue (p<0.05). We demonstrate that ureaplasmas are capable of MBA phase variation in vitro; however, ureaplasmas undergo MBA size variation in vivo, to potentially prevent eradication by the immune response. Size variation of the MBA did not correlate with the severity of chorioamnionitis. Nonetheless, the correlation between a maternal humoral response and the expression of chorioamnion cytokines is a novel finding. This host response may be important in the pathogenesis of inflammation-mediated adverse pregnancy outcomes.

Ureaplasma antigenic variation beyond MBA phase variation: DNA inversions generating chimeric structures and switching in expression of the MBA N-terminal paralogue UU172

Phase variation of the major ureaplasma surface membrane protein, the multiple-banded antigen (MBA), with its counterpart, the UU376 protein, was recently discussed as a result of DNA inversion occurring at specific inverted repeats. Two similar inverted repeats to the ones within the mba locus were found in the genome of Ureaplasma parvum serovar 3; one within the MBA N-terminal paralogue UU172 and another in the adjacent intergenic spacer region. In this report, we demonstrate on both genomic and protein level that DNA inversion at these inverted repeats leads to alternating expression between UU172 and the neighbouring conserved hypothetical ORF UU171. Sequence analysis of this phase-variable ‘UU172 element’ from both U. parvum and U. urealyticum strains revealed that it is highly conserved among both species and that it also includes the orthologue of UU144. A third inverted repeat region in UU144 is proposed to serve as an additional potential inversion site from which chimeric genes can evolve. Our results indicate that site-specific recombination events in the genome of U. parvum serovar 3 are dynamic and frequent, leading to a broad spectrum of antigenic variation by which the organism may evade host immune responses.

Alternate phase variation in expression of two major surface membrane proteins (MBA and UU376) of Ureaplasma parvum serovar 3

Ureaplasma urealyticum and Ureaplasma parvum are commensals and pathogens of the human urogenital tract and of newborn infants. There are four distinct U. parvum serovars and 10 distinct U. urealyticum serovars. Both species possess a distinct immunodominant variable surface protein, the multiple banded antigen (MBA), which shows size variability among isolates as a result of changes in the number of C-terminal repeating units. Adjacent to the MBA gene (UU375) lies UU376, which was annotated as 'Ureaplasma-specific conserved hypothetical gene'. In four different strains of U. parvum serovar 3, we demonstrated expression of UU376 by Western blot analysis and phase variation between UU376, here designated Upvmp376 (Ureaplasma phase-variable membrane protein 376), and MBA after application of selective pressure with hyperimmune antisera directed against either protein. By Southern blot analysis, we found that the switch between MBA and Upvmp376 expression is associated with a DNA inversion event in which the nonrepetitive region of the MBA gene and its putative promoter region are opposed to either the repetitive region of MBA or UU376. We propose that in U. parvum serovar 3, and presumably in all U. parvum and U. urealyticum, an inversion event at specific sites effects an alternate ON/OFF switching of the genes UU375 and UU376.

Phase variation of the multiple banded protein in Ureaplasma urealyticum and Ureaplasma parvum

Ureaplasma urealyticum and U. parvum are common commensals and, possibly, pathogens of the human urogenital tract. Like other Mycoplasmatales they possess variable surface proteins. The multiple banded (MB) protein shows a striking variability of its molecular weight. This is caused by changes of the number of C-terminal repeating units. In this study, selective pressure was imposed against cytadherence of U. urealyticum and U. parvum. Ureaplasmas were co-incubated with either erythrocytes or HeLa cells and the cell-bound fraction was removed. Additionally, U. urealyticum populations were transferred serially through broth containing specific polyclonal antibodies. Both approaches led to the emergence of escape variants in which no MB protein was detectable. PCR studies with several primers on different parts of the mba gene indicated major differences between wild-type strains and MB-negative escape variants. In experiments with clonal lineages, however, the loss of the MB protein was shown to be reversible. Therefore, it is proposed that the multiple banded proteins of U. urealyticum and U. parvum are subjected to a phase-switching mechanism as it has already been described for several other Mycoplasmatales.