Lecture 13 Immunology and disease: parasite antigenic diversity

1. Lecture 13Immunology and disease: parasite antigenic diversity

2. Today: • Benefits and mechanisms of antigenic variation • Antigenic variation that allows pathogens to persist in the individual host they’ve infected • Antigenic variation that allows pathogens to infect hosts with prior exposure

3. Benefits of antigenic variation • Persist in infected host Let’s look at some experimental results…

4. Experimental evolution • Manipulates the environment of a population and then looks at the resulting patterns of evolutionary change • Allows for the direct study of the selective forces that shape antigenic diversity • We’ll focus on CTL escape, which gets us down to the level of single amino acids changes that can mean life or death for both hosts and parasites

5. Figure 1-27 Review • The two main classes of MHC molecules present antigen from cytosol (MHC class I) and vesicles (MHC class II)

6. Figure 3-23 MHC class I molecule presenting an epitope

7. Figure 1-30

8. CTL escape • CTL pressure favors “escape mutants”, pathogens with amino acid substitutions in their epitopes that make them escape recognition. Substitutions can lead to escape in three ways. • They can interfere with processing and transport of peptides. • They can reduce binding to MHC molecules. • And they can reduce the affinity of TCR receptor binding.

10. CTL escape: interfering with processing/transport • A study of murine leukemia virus showed that a single amino acid substitution in a viral peptide can alter the cleavage pattern, and hence epitope presentation, and hence CTL response • MuLV is an oncogenic retrovirus • There are two main types (MCF and FMR) • Both types are controlled in large part by CTL responses, but with different immunodominant epitopes • The immunodominant CTL epitope for MCF is KSPWFTTL

11. CTL escape: interfering with processing/transport mcf fmr

12. CTL escape: interfering with processing/transport • Proteasomes are hollow multiprotein complexes. They are like meat-grinders for pathogen proteins found in the cytosol • Cellular proteasomes continuously chop up proteins into smaller peptides, for presentation by MHC • Proteasomal cleavage patterns determine which bits of pathogen peptides get to the cell surface

13. CTL escape: interfering with processing/transport • Changing KSPWFTTL to RSPWFTTL introduces a new cleavage site (the proteasome likes to chop after R) • Viruses with RSPWFTTL are cleaved right within what would otherwise be a great epitope, leading to a huge reduction in the abundance of the R-containing epitope available for MHC presentation • Inspection of the nucleotides reveals that this escape is mediated by a single point mutation! • End result: that epitope is unavailable to MHC and the CTL response to FMR type is weak

14. CTL escape: reducing MHC binding • Several studies report mutations that reduce peptide-MHC binding • This can either prevent MHC from dragging the peptide successfully to the cell surface, or from holding on to it once there

16. CTL escape: reducing MHC binding • Lymphocytic choriomeningitis virus (LCMV) is an RNA virus that naturally infects mice • Infection can be controlled or eliminated by a strong CTL response • Puglielli et al. used an LCMV system with transgenic mice that expressed an MHC molecule that binds a particular epitope of LCMV (GP33-43) • After infection, an initial viremia was beaten down by CTL pressure

17. CTL escape: reducing MHC binding • Later, virus titers increased. Were escape mutants to blame? • The late viruses indeed had a V to A substitution at the 3rd site of the epitope. • This substitution nearly abolished binding to the MHC molecule expressed by the mice

19. CTL escape: reducing MHC binding • SIV/macaques is used as a model system for HIV since you can’t experimentally infect humans to study the arms race between HIV and humans • Escape from CTLs appears to be a key component of the dynamics and persistence of infection within hosts • Allen et al. (2000) studied 18 rhesus macaques infected with SIV

20. CTL escape: reducing MHC binding • Ten of the monkeys expressed a particular MHC, and these all made CTLs to an epitope in the Tat protein in the acute phase of infection • Shortly after, the frequency of these Tat-specific CTLs dropped off • Sequencing showed that a majority of these animals had mutations in the Tat viral epitope that destroyed binding to the MHC • There was little variation outside of the epitope • End result: positive selection to block MHC binding

21. CTL escape: reducing TCR binding • The LCMV system also shows examples of single amino acid changes that can lead to a decline in affinity for the TCR • Tissot et al (2000) showed that a Y to F substitution in one immunodominant epitope obtained during experimental evolution in vivo caused a 100-fold reduction in affinity for the TCR • End result: escape mutation that destroys the immune system’s ability to see that epitope

22. Benefits of antigenic variation 2. Infect hosts with prior exposure • Hosts often maintain memory against prior infections, generating a selective pressure for parasites to vary • Cross-reaction occurs when the host can use its specific recognition from a prior exposure to fight against a later, slightly different antigenic variant • Good vaccines are ones that have excellent cross-reactivity (e.g. measles virus)

23. Figure 11-1 part 1 of 3 In the simplest case, each antigenic variant acts like a separate parasite that doesn’t cross-react with other variants

24. Figure 11-1 part 2 of 3

25. Figure 11-1 part 3 of 3

26. Benefits of antigenic variation 2. Infect hosts with prior exposure • A more dynamic mechanism of antigenic variation is seen in influenza virus • Antigenic drift is caused by point mutations in the genes encoding surface proteins • Antigenic shift is caused by reassortments leading to novel surface proteins

27. Figure 11-2 part 1 of 2

28. Figure 11-2 part 2 of 2

29. Benefits of antigenic variation 2. Infect hosts with prior exposure • Antigenic drift is caused by point mutations in the hemagglutinin and neuraminidase genes, which code for surface proteins • Every 2-3 years a variant arises that can evade neutralization by antibodies in the population • Previously immune individuals become susceptible • Most individuals still have some cross-reactivity and the ensuing epidemic tends to be relatively mild (but still kills 100s of thousands per year!)

30. Benefits of antigenic variation 2. Infect hosts with prior exposure • Antigenic shift brings in an all-new hemagglutinin or neuraminidase gene to a naïve population • Can lead to severe infections and massive pandemics like the Spanish flu of 1918.

31. Benefits of antigenic variation Why, fundamentally, is it of benefit to a parasite to extend the length of infection or re-infect hosts with prior exposure?