Bacterial evasion of host immunity
Gunnar Lindahl
Lund University, Sweden and Copenhagen University, Denmark
A pathogen must evade attack
by the immune system of the host
• innate immunity
• adaptive immunity
A pathogen must evade attack
by the immune system of the host
• innate immunity
• adaptive immunity
• extracellular pathogens
• intracellular pathogens
A) Innate immunity
# phagocytosis
(recognition; killing)
# complement
# anti-bacterial peptides
# TLRs/NLRs
(proinflammatory cytokines)
# apoptosis
# autophagy
Phagocytosis is a major defense mechanism,
in particular against extracellular Gram-positive bacteria
Phagocyte
(neutrophil)
Pathogens may inhibit the function of neutrophils
by blocking their recruitment of by killing them
# Streptococci secrete enzymes that degrade C5a or IL-8, thereby
interfering with the recruitment of neutrophils
# Staphylococci secrete small peptides (PSMs) that lyze neutrophils
Phagocytosis may be inhibited by a polysaccharide capsule
Phagocyte
Even if the bacteria encounter a phagocyte and lack a capsule,
they must be opsonized for efficient phagocytosis to occur
Phagocyte
Opsonization
is promoted by:
Complement
Immunoglobulin (Ig)
activation
C3b
iC3b
Fab
(Innate immunity)
Fc
(Adaptive immunity)
Opsonization for phagocytosis: innate immunity
Phagocyte
Complement
receptor (CR)
iC3b
Opsonization for phagocytosis: innate immunity
Phagocyte
Complement
receptor (CR)
A bacterium may evade complement-mediated phagocytosis
by blocking complement deposition
The complement system
Classical pathway (CP)
Lectin-pathway
- specific antibodies
- nonimmune activation
- MBL
Alternative pathway (AP)
- spontaneous activation
-
- amplification loop
C3
C4b2a
C3bBb
AP C3-convertase
CP C3-convertase
C3b
Opsonin
Amplification
loop
MAC
Membrane Attack
Complex
Human complement regulators in the RCA family
Lindahl et al, Curr Opin Immunol (2000)
The complement system
Classical pathway (CP)
Lectin-pathway
- specific antibodies
- nonimmune activation
- MBL
Alternative pathway (AP)
- spontaneous activation
-
- amplification loop
C3
C4b2a
factor I
C4BP
C3bBb
AP C3-convertase
CP C3-convertase
C3b
Opsonin
factor H
Amplification
loop
MAC
Membrane Attack
Complex
factor I
Low-level continuous activation and the presence of
specific inhibitors allows the complement system
to distinguish between self and non-self
Many pathogens inhibit complement deposition by ‘hi-jacking’
C4BP or factor H
C4BP
(C4b-binding protein)
A human plasma protein that inhibits
the classical pathway C3 convertase
β
α
MW ~ 570,000 Da
Conc. in plasma ~200 µg/ml
= SCR/CCP module
Many streptococcal M proteins hijack human C4BP,
for protection against phagocytosis
Complement activation
via classical pathway
C4BP
Inhib.
C3b
Carlsson et al, J Exp Med (2003)
M22
Examples of bacterial pathogens that hijack human C4BP or factor H,
thereby blocking complement deposition
Bacterium
Ligand
• Streptococcus pyogenes (GAS)
C4BP
• Neisseria meningitidis
C4BP, FH
• Moraxella catharralis
C4BP
• Haemophilus influenzae
C4BP, FH
• Streptococcus pneumoniae
FH
• Neisseria gonorrhoeae
C4BP
• Borrelia burgdorferi
FH
B) Adaptive immunity
antigen
uptake
antigen
presentation
specific
T cells
adaptive immunity
(cellular or
humoral)
antigen
uptake
antigen
presentation
inhibition of
antigen
presentation
specific
T cells
inhibition
(via T-reg)
adaptive immunity
(cellular or
humoral)
evasion through
antigenic variation
Hansen and Bouvier, Nat Rev Immunol (2009)
Baena and Porcelli, Tissue antigens (2009)
antigen
uptake
antigen
presentation
inhibition of
antigen
presentation
specific
T cells
inhibition
(via T-reg)
adaptive immunity
(cellular or
humoral)
evasion through
antigenic variation
Antigenic variation
Immunological change in a surface protein of a pathogen,
allowing evasion of protective immunity
Two strategies:
a) Appearance of new variants during an infection
b) Circulation of many stable variants (types) in the population
Antigenic variation: some well-known examples
Pathogen
Variable surface protein
Plasmodium falciparum
PfEmp1
Trypanosoma brucei
VSG
Neisseria gonorrhoeae
Pilin
Streptococcus pyogenes
M protein
HIV-1
gp120
Influenza virus
Hemagglutinin
Aspects on antigenic variation
# genetic mechanisms: shift and drift
# the immunology of escape
Antigenic variation in Neisseria pili:
antigenic shift
Davidsen & Tønjum, Nature Rev Microbiol (2006)
Antigenic variation in three bacterial pathogens:
antigenic shift
Palmer et al., Cell. Microbiol. (2009)
Antigenic variation by epigenetic mechanisms in Giardia lamblia:
role of RNA interference
Prucca et al., Nature (2008)
Ankarklev et al., Nat Rev Microbiol (2010)
Antigenic drift in HIV-1:
sequence variability in the V3 loop of gp120
LaRosa et al., Science (1990)
Antigenic variation:
genetic drift and shift (in a virus)
Malim & Emerman, Cell (2001)
Antigenic variation and Alice in wonderland
The most curious part of the thing was, that the trees and the other things
round them never changed their places at all: however fast they went, they never
seemed to pass anything. “I wonder if all the things move along with us?”
thought poor puzzled Alice.
(Lewis Carroll, 1872)
Hedrick, Immunity (2004)
A classical problem
• How does immune pressure select for sequence variants?
• This is simple to understand for shift, much less clear for drift
Antigenic drift in HIV-1:
sequence variability in the V3 loop of gp120
LaRosa et al., Science (1990)
Antigenic drift
host
receptor
microbial
adhesin
immunity
mutations
mutation
immune escape
Alternative model
host
receptor
microbial
adhesin
immunity
mutation
immune escape
Science 326, 734 (2009)
Antigenic sites in influenza virus hemagglutinin
Xu et al., Science 2010
New model
host
receptor
1
microbial
adhesin
mutation 1 causes
very strong
binding!
immunity
immune host
2
mutation 1
1
in new naïve host:
compensatory
mutation 2 results
in normal binding
strength
New model, continued
mutation 3 causes
very strong binding even in immune host
Infection of new host:
2
1
3
immunity
in new naïve host:
compensatory
mutation 4
1
2 4
mutation 3
3
Science 326, 734 (2009)
C) Streptococcal M protein and evasion of host immunity
Two common diseases caused by Streptococcus pyogenes
(Group A Streptococcus, GAS)
Acute pharyngitis
Impetigo
Global burden of S. pyogenes disease
Superficial infections
Pharyngitis and skin infections: > 700 million cases/year
Severe infections
> 500,000 deaths per year
Carapetis et al., Lancet Infect Dis (2005)
The M protein of Streptococcus pyogenes
M pos.
Swanson et al., J Exp Med (1969)
M neg.
J Exp Med 47, 91 (1928)
Rebecca Lancefield (1895-1981)
Streptococcal M protein:
some members of the family
Smeesters et al, Trends Microbiol (2010)
Many M proteins hijack
the human complement inhibitor C4BP:
evasion of innate immunity
C4BP
inhibition
complement
deposition
M protein
C
S. pyogenes
contributes to
phagocytosis
resistance
Adaptive immunity: antibodies that
inhibit C4BP-binding promote phagocytosis
C4BP
M protein
S. pyogenes
Opsonization
by antibodies
and complement
phagocytosis
The pathogen’s response: selection of mutants
that still bind C4BP, but escape antibodies
C4BP
Hypervariable
region
(HVR)
M protein
bacterial survival
S. pyogenes
Darwinian evolution!
Extreme sequence variation
in hypervariable regions (HVRs) that bind C4BP
M2
M4
M4.1
M22
M60
M114
prtH
----------NSKNPVPVKKEAKLSEAELHDKIKNLEEEKAELFEKLDKVEEEHKKVE---------------AEIKKPQADSAWNWPKEYNALLKENEELKVEREKYLSYADDKEKD--------------------AEIKKPQADSAWNWPKEYNALLKENEEFKVEREKYLSYADDKEKD--------------------ESSNNAESSNISQESKLINTLTDENEKLREELQQYYALSDAKEEEPRYKALR-----ESSTVKAESSTVKAESSTISKERELINTLVDENNKLMEERARHLDLIDNIREKDPQYRALRGENQD
--NSKNPAPAPASAVPVKKEATKLSEAELYNKIQELEEGKAELFDKLEKVEEENK-----------EGAKIDWQEEYKKLDEDNAKLVEVVETTSLENEKLKSENEENKKNLDKLSKD-------------
No residue identities!
Persson et al, PLoS Pathogens (2006)
HVR
Y
Y YY Y
Y
Y
Y Y
Y YY Y
Y
Y
Selection of protein variants with new HVRs
Y
Y YY Y
Y
Y
Y Y
Y YY Y
Y
Y
)
Y
Y
(Y
Y
Y
Y
The hypothesis was analyzed for the M5 and M1 proteins
The HVR is a major target for protective antibodies
Immunization of mice with M5 or M1
elicits a very weak anti-HVR response
Weak anti-HVR response in humans:
invasive infections caused by M1-expressing S. pyogenes
Conclusion:
The HVR of streptococcal M protein elicits a weak antibody response
Working hypothesis:
The C-terminal region actively interferes with the antibody response to
the HVR:
HVR
(Y)
inhib.
Y Y YYYYY Y
Streptococcal escape from antibodies
Healthy host
Antigenic variation allows escape from
preexisting antibodies and allows
establishment of infection
Infected host
Weak anti-HVR response
prolongs infection
Conclusion of M protein work
• Our data contradict the common assumption that the HVR of a
virulence factor is immunodominant
• Two independent mechanisms allow S. pyogenes to evade antibodies
against the HVR: antigenic variation and weak antibody response
• The two escape mechanisms work at different stages of an infection
• Weak antibody responses might be as important as antigenic variation
in promoting bacterial virulence
Acknowledgements
Jonas Lannergård
Mattias Gustafsson
Johan Waldemarsson
Margaretha Stålhammar-Carlemalm
Anna Norrby-Teglund (Stockholm)