Continuing on their previous finding that the insertion of IS1301 in the intergenic region (IGR) between siaA and ctrA operons in the MenC isolates from Spain was responsible for resistance to serum bactericidal antibodies, the authors in the present work have investigated whether isolates with IS1301 in IGR are emerging among vaccine failure strains in UK.
1. IS1301 insertion in the IGR was not associated with MenC conjugate vaccine failure in UK: MenC isolates from individuals from UK with meningococcal disease who had completed a course of the MenC conjugate vaccine were screened. 79% of the isolates were ST-11. IS1301 was not present in the IGR of any of the 33 MenC isolates, even though most of these had IS1301 insertions elsewhere in the genome. The investigators also examined MenC isolates in the UK before the introduction of conjugate vaccine. Insertion of IS1301 in the IGR region was observed in only one out of 104 isolates. In this isolate IS1301 was inserted in the same location but in opposite direction to isolates from Spain. Thus, the results suggest that IS1301 insertion in IGR is not responsible for resistance to immunity induced by vaccine, in UK.
2. Presence of IS1301 in the IGR of UK meningococcal disease and carriage isolates: The authors further examined 741 carriage and 345 disease isolates of different serogroups, collected from 1998 to 2000 in the South East of England. Insertion of IS1301 in the IGR was observed to be significantly more frequent in the disease isolates than in the carriage isolates. In the study, insertion of IS1301 was most frequently observed in the MenB isolates belonging to clonal complex ST-269 (cc269). In the collection cc269 was more commonly found among the disease isolates compared to the carriage isolates.
3. IGRs with IS1301 in MenB isolates are associated with polymorphisms: Of the 44 isolates in which IS1301 insertion was observed in the IGR in the UK, one isolate showed exactly same insertion to that of Spanish isolates. Most of the UK isolates showed 8bp deletion upstream of siaA in the IS1301. In other cases IGR contained inverted IS1301 with an internal 69 bp duplications.
4. Polymorphic sia/ctr IGRs with IS1301 lead to increased transcription of the genes for capsule transport but not capsule biosynthesis: To investigate the effect of insertion of IS1301 with associated polymorphisms in the IGR, the investigators constructed isogenic strains by introducing two common types of IGR with IS1301 with IS-8 and IS1301 with iIS+69 into a MenB cc269 strain. Transformants were selected with and without changes in IGR. That is those containing IS1301 with IS-8+ and iIS+69+ and those with IS1301 with IS-8- and IS+69-. Both IS-8+ and iIS+69+ led to a two fold increase in the expression of ctrA, which was comparable to that observed in MenC isolates. However, no upregulation in siaA expression was observed.
5. Increased transcription of capsule transport genes does not change the amount of capsules or resistance against complement mediated killing: The previous study conducted in Spain showed that increased transcription of siaA and ctrA leads to increase capsule expression but does the increased transcription of only ctrA has the same effect on capsule expression? To understand this, FACS analysis of the amount of capsule expressed by IS-8+, IS-8-, IS-69-, IS-69+ and D-157 (MenB cc269 disease isolate without insertion in the IGR) was done. It was observed that polymorphic IGRs with IS1301 do not express increased amounts of capsules. The SBA titres were also same for strains with and without the polymorphic IGRs with IS1301.
6. Presence of IS1301 in MenB cc269 diseases and carriage isolates: The investigators further examined a large number of MenB cc269 isolates (241 carriage and 421 disease isolates). Consistent with the earlier finding the insertion of IS1301 was more frequent in disease versus carriage isolates. All the insertions were polymorphic and were either IS-8 or iIS+69. Next, the authors studied genetic relatedness between STs within cc269. It was observed that the STs that are distant from the ancestral ST-269 and share only four out of seven MLST loci with ST-269, were clustered together around the subgroup founder ST-275. IS1301 insertion was observed in only one of these 217 isolates. These were significantly more prevalent in the carriage isolates. In contrast, 50% of the isolates that shared 5 or more loci with ST269 had IS1301 in the IGR. These isolates showed a similar prevalence in both the disease and carriage isolates.
Conclusions and Further investigations:
1. MenC isolates from people immunized with the MenC vaccine in the UK had no evidence of IS1301 in the IGR.
2. In UK isolates, IS1301 insertion inn IGR was restricted to MenB isolates (cc269).
3. There was a significant association of IS1301 in IGR in disease compared to carriage isolates.
4. Novel polymorphisms were observed in the IS1301 in IGR,
5. The mechanisms by which insertion of IS1301 leads to upregulation of sia (in MenC) and ctr (in MenB and MenC isolates) is not known.
6. Sia expression was not upregulated in MenB isolates in either the presence of IS-8 or iIS+69. The reason is not known.
7. Studies are being carried out to determine virulence factors by comparing the frequency of genetic traits between disease and carriage isolates.
No comments:
Post a Comment