Lecture 2 Bacterial strategies to overcome the host immune response Concepts:

1. Lecture 2 Bacterial strategies to overcome the host immune response Concepts: PAMPS: LPS, Teichoic acids, Peptidoglycan, Capsules Nod-like and TLR receptors Complement killing Antimicrobial peptides Molecular mimicry

2. Bacteria don’t have a second chance to give a first impression….. Gram Negative Cell Wall Gram Positive Cell Wall

3. The Gram positive cell wall Famous Gram positive Bacteria -Listeria -Streptococcus -Staphylococcus -Bacillus antracis, B. subtilis

4. Peptidoglycan structure and function -Essential for bacteria (present in almost all bacteria) -Main functions: provide stability against osmotic imbalance, permeable barrier -Its synthesis is the target of penicillin and other antibiotics

5. Famous Gram negative Bacteria -Salmonella, E. coli -Campylobacter, Helicobacter -Shigella -Yersinia, Legionella -Pseudomonas

6. The structure of the peptide is variable

7. TLR recognize different PAMPS in bacteria and activate different cascades

9. Nod-like receptors (NLR) (nucleotide-binding domain, Leucine-rich repeat–containing receptors) Sensors of intracellular PAMPS

11. Peptidoglycan structures recognized by NLRs. McDonald, C. et al. J. Biol. Chem. 2005;280:20177-20180 Schematic of PGN structure. Shown are two examples of the general types of PGN, Lys-PGN (from the Gram-positive bacterium, S. aureus, strain Copenhagen) and DAP-PGN (from Gram-negative bacteria such as Escherichia coli). Ligands for Nod1 (red box), Nod2, and cryopyrin (blue boxes) are shown. The putative binding sites for PGRP-S (green box) and PGRP-L (orange boxes) are indicated. MTP, muramyl tripeptide; sPGN, soluble PGN; GlcNAc, N-acetylglucosamine; MurNAc, N-acetylmuramic acid.

12. Activation of NLRs

14. Bacterial countermeasures to NLR-mediated response: modification of peptidoglycan to avoid recognition by NLR The structure of Listeria monocytogenes (Lm) peptidoglycan (PG) has been recently characterized. It was found that Lm peptidoglycan contains unusual modifications. Approximately 50% of its muropeptides featured a glucosamine residue instead of the canonical N-acetylglucosamine (GlcNAc). A gene named pgdA encoding for a deacetylase was identified and mutated. The pgdA mutant contains the normal content of GlcNAc. The pgdA mutant strain was rapidly destroyed and induced a massive induction of an inflammatory response in wild type human macrophages. However, the pgdA mutant cells could resist longer to Nod-1 or TLR2 deficient macrophages. Muropeptide This acetyl group is eliminated in Lm cells by the PgdA enzyme

15. Mutations in Nod-like receptor can cause diseases! Alberta is among the places with highest incidence of ulcerative colitis and Crohn's disease cases per capita in the world (one in 330 people) An average Crohn's disease patient costs the Canadian health-care system $50,000 a year for drugs, doctor's visits, surgeries and hospitalizations

16. TLR4 detects LPS

17. non-essential essential Tremendous variability: Only in E. coli, about 180 different strain having each one a different serotype have been described LPS: “the basics”

18. Structure of Lipid A, the part of the LPS that interacts with TLR4

19. How does LPS interact with TLRs?

20. Some bacterial can modify their lipid A to avoid recognition by TLRs Some bacteria can modify their LPS to avoid its interaction with TLR4

21. Antimicrobial peptides kill bacteria by making pores

22. Bacterial countermeasures to antimicrobial peptides Specific mechanisms that confer increased bacterial resistance to defensins have been identified: -Salmonella typhimurium and many other species modify their LPS decreasing their affinity for cationic peptides and also expresses membrane proteases that degrade cationic peptides. I -Staphylococcus aureus, uses the Dlt protein for covalent modification of cell wall teichoic acid by alanine, and the MprF for the covalent modification of membrane phosphatidylglycerol with L-lysine. These modifications probably act by decreasing the negative charge of the cell wall and bacterial membrane, respectively, and diminish their attraction for the cationic defensins. -Homologues of these resistance genes have been identified in many bacterial species, indicating that these mechanisms might be widespread.

23. Humoral immunity Wow, that is a Big Mac! wikipedia.org

24. Bacterial countermeasures to humoral immunity

25. Bacterial countermeasures to humoral immunity

26. Campylobacter jejuni: a example of molecular mimicry Molecular mimicry responsible for immune responses in post-infectious autoimmune diseases. Guillain–Barre´syndrome, the most frequent cause of acute neuromuscular paralysis, sometimes occurs after Campylobacter jejuni enteritis. Recent studies have revealed that carbohydrate mimicry of the human ganglioside by the bacterial lipo-oligosaccharide is an important cause of the syndrome. This new concept that carbohydrate mimicry can cause an autoimmune disease provides a clue to inducing the resolution of pathogenesis of other immune-mediated diseases.

27. Conclusion: Yes, the immune system is great (and this is why we don’t get sick all the time)…but bacteria have developed (and will keep developing) new and sophisticated tools to defend and fight against it…more examples to come…