Antigenic and phase variation of the adhesins of the gonococcus Flashcards
what are Neisseria gonorrhœae – the gonococcus?
- Gram-negative diplococcus
* Causative agent of gonorrhoea, sexually transmitted infection of lower urinary tract
what is Neisseria meningitides?
• Gram-negative diplococcus
• Causative agent of septicaemia and meningitis
– bacterial attachment to mucosal surface – invasion of epithelial cells
– intracellular multiplication
– passage into sub-epithelial space
– replication in blood
– invasion of meninges
Virulence factors of Neisseria
• Adhesion Pili (fimbriae)
- Porin (PorB)
- Lipopolysaccharide
- IgA proteases
- Iron binding proteins
describe the Pili of Neisseria
- Outer membrane structures ~10,000 subunits of 18-kDa pilin protein
- Clinical isolates piliated, but repeated laboratory subculture results in non-piliated variants
- Early attempts to use pili as vaccines for gonococcus unsuccessful
- Later studies show that pilin proteins from different isolates differ significantly in: size and pI, AA composition, ability to bind epithelial cells and immunological specificity
Functions of Pili
- Mediate attachment to host tissues
- Mediate formation of bacterial aggregates
- Twitching motility
- Required for DNA uptake during transformation
describe Antigenic variation of Neisserial pili
- Single cell gives rise to daughter cells expressing structurally and antigenically different pili
- Genetic capacity to make thousands of different pilin variants
- Daughter cells not recognised by antibodies directed against pili produced by the parent
- BUT still able to bind to same host tissues and to cause the same disease symptoms
Clinical consequences of antigenic variation
- Bacteria isolated from an individual patient at various times may express different pili
- Serologically different strains are frequently identified in epidemics
- Cured patients can be re-infected despite a strong antibody response
Structure of pilin (PilE)
- Variation localised to 6 mini-cassettes
- Constant - subunit linkage
- Semi-variable - adhesion
- Hyper-variable - immunodominan
Pilus gene organisation in Gonococcus
- Many copies of pilin gene throughout chromosome
- Only one is expressed, pilE - has promoter, ribosome binding site and the constant domain
- All other copies are silent, pilS - 4-6 pilS loci, each containing 1-6 pilS genes - no promoter, no ribosome binding site and no constant region
- Antigenic variation occurs due to recombination (between mini-cassettes) between pilS and pilE
PilE Recombination Events
- standard recombination events with reciprocal exchange of DNA fragments
- phase variation events occur due to insertion of ‘stop’ codons or disruption of the pilE reading frame
PII or opacity protein (Opa)
- 30-kDa, involved in close attachment, subject to phase and antigenic variation
- Strains produce no PII or several variant proteins that differ in immunogenicity and function
- Bind to CEACAMs (carcinoembryonic antigen- related cell adhesion molecules) and heparin sulphate proteoglycans (HSPGs)
- Main target is CEACAM1, ubiquitous on epithelial cells and present on the apical surface
- HSPGs are present on basolateral side of polarised cells (re-emergence?)
Opa-Mediated Invasion and Spread
• Mediate invasion of host cells but this is inefficient as most epithelial cells express low levels of CEACAM1
• Inflammation (by prior infections) increases levels
of CEACAM1 on surface and enhances invasion
even of capsulated meningococci (IFNg gamma induces CEACAM1 in epithelial cells and TNFa in endothelial cells)
• Opa-CEACAM1 interaction can suppress TLR-2 mediated inflammatory response
• Opa-CEACAM1 interaction promotes adhesion of host cells to ECM and prevents exfoliation
Opa-Mediated Killing
- Neutrophils express CEACAM molecules on their surface and these mediate non-opsonic uptake of neisserial cells
- CEACAM3 is only present on neutrophils and may act as a ‘DECOY RECEPTOR’
- CEACAM3 has ITAM cytoplasmic tail and Opa binding activates a Src-family tyrosine phosphorylase and a cascade of signalling events
- Opa-CEACAM interactions are inhibited/reduced by sialylation of bacterial cells (role for LPS and capsule)
Number of opa genes
- Meningococci have 3-4 Opa genes
- Gonocococci have 11-12 Opa genes
- Opa genes are not clustered but distributed around the genome
- All opa genes have similar structure
- All opa genes contain a pentanucleotide repeat within the reading frame and undergo phase variation between ON and OFF
PII or opacity protein (Opa) structure
- Opa variants encoded by up to 12 separate genes
* Similar structures but two hyper-variable regions
