Antigenic and phase variation of the adhesins of the gonococcus Flashcards Preview

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Flashcards in Antigenic and phase variation of the adhesins of the gonococcus Deck (21):
1

what are Neisseria gonorrhœae – the gonococcus?

• Gram-negative diplococcus
• Causative agent of gonorrhoea, sexually transmitted infection of lower urinary tract

2

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

3

Virulence factors of Neisseria

• Adhesion Pili (fimbriae)
- Porin (PorB)
- Lipopolysaccharide
- IgA proteases
- Iron binding proteins

4

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

5

Functions of Pili

• Mediate attachment to host tissues
• Mediate formation of bacterial aggregates
• Twitching motility
• Required for DNA uptake during transformation

6

describe Antigenic variation of Neisserial pili

1. Single cell gives rise to daughter cells expressing structurally and antigenically different pili
2. Genetic capacity to make thousands of different pilin variants
3. Daughter cells not recognised by antibodies directed against pili produced by the parent
4. BUT still able to bind to same host tissues and to cause the same disease symptoms

7

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

8

Structure of pilin (PilE)

• Variation localised to 6 mini-cassettes
• Constant - subunit linkage
• Semi-variable - adhesion
• Hyper-variable - immunodominan

9

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

10

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

11

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?)

12

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

13

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)

14

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

15

PII or opacity protein (Opa) structure

• Opa variants encoded by up to 12 separate genes
• Similar structures but two hyper-variable regions

16

Genetics of PII

• Genes constantly transcribed, but not all
transcripts produce functional proteins
• Expression depends on 7 to 27 copies of repeat sequence CTCTT that differentially affect the reading frame
• Variation occurs by slippage during DNA replication

17

Consequences of Phase Variation of Opa Genes

ON = 12 CTCTT repeats
Mutation from 12 to 11 repeats = ‘on’ to ‘off’ switch
No Opa proteins = no Opa-mediated attachment and invasion
12 genes switching ‘on’ and ‘off’ gives 212 combinations (4096 genotypes)

18

Antigenic variation of Opa

Intra- or inter-chromosomal recombination produces antigenic variation

19

what does the gonorrhoea infection involve?


– bacterial attachment to mucosal surface
– invasion of epithelial cells
– intracellular multiplication
– passage into sub-epithelial space
– multiplication and shedding
• Invasion of sub-mucosa provokes inflammatory response with infiltration of phagocytes

20

what is ANTIGENIC VARIATION?

alteration of antigenic characteristics while retaining biological function

21

what does a meningitis infection involve?

– bacterial attachment to mucosal surface
– invasion of epithelial cells
– intracellular multiplication
– passage into sub-epithelial space
– replication in blood
– invasion of meninges