Lecture 10 - Evolution of Allorecognition What gave rise to our system of allo-recognition??

1. Lecture 10 - Evolution of Allorecognition What gave rise to our system of allo-recognition?? Xeno-recognition - distinguishing other species from self species Pattern Recognition Receptors, NK cells & post-education B- & T-cells Allo-recognition - distinguishing others of same species from self/ kin Common in plants & invertebrates - has known functions Is there a real function for allo-recognition in us (vertebrates)? Is our allorecognition system derived from invertebrate system?

2. Allorecognition - recognition of non-self in us is based on x-reactivity self MHC + non-self Ag looks like non-self MHC + Ag TcR “cross-reactivity” not a requirement - just strong binding of non-self MHC + Ag

3. Theories as to how TcR-MHC recognition system evolved: 1) To combat unanticipated pathogens & allorecognition is by-product 2) Allorecognition evolved 1st for surveillance against cancers 3) MHC evolved for some other function - neural patterning or repair? 4) TcR & MHC evolved from a relict allorecognition system (options 2, 3 & 4 are not necessarily mutually exclusive)

4. 1) Allorecognition as a coincidental by-product of TcR-MHC immunity This assumes that TcR & MHC presentation preceded allorecognition & that allorecognition has no biological function This creates a problem of immediate complexity (lacking parsimony): A highly variable receptor (TcR) that can induce cytotoxicity must tolerate self - requires immediate evolution of thymic selection process. What’s a more parsimonious explanation?

5. Alternative (linked) hypotheses: 1) TcR evolved for some other purpose and interaction with presented Ag is a derived trait The progenitor TcR/Ig may have been a PRR that began with limited diversity that it allowed it to recognize species variants of a PAMP Tolerance to self evolves as the diversified PRR sometimes x-reacts with self 2) MHC evolves for allorecognition and later acquires Ag presentation Ag presentation may also first have been for allorecognition

6. allorecognition before allorecognition???? what?? why??

7. We have 2 types of allorecognition – one not requiring Ag presentation ‘accidental’ TcR based x-reactivity & allorecognition absence of self-MHC based activation & killing by NK-cells Both are premised on a highly polymorphic MHC but the NK-cell system has less complexity

8. If NK-like allorecognition evolved first, why did it evolve? • It has its drawbacks: • it’s a barrier to organ transplants - but this is not natural biology • it’s a barrier to vivipary (carrying offspring internally) • but vivipary evolved after peptide presentation & this can • be resolved by immuno-privilege & immuno-suppression • Positive selective pressure: • Kin recognition for mate selection & communal lifestyle • Prevents parasitism by cells of members of the same species • imagine if you could ‘catch’ cancer from someone!

9. If allorecognition evolved first what drove the evolution of the system? • It has its’ drawbacks: • it’s a barrier to organ transplants - but this is not natural biology • it’s a barrier to vivipary (carrying offspring internally) • but vivipary evolved after peptide presenation & this can • be resolved by immuno-privilege & immuno-suppression • Positive selective pressures: • Kin recognition for mate selection & communal lifestyle • Prevents parasitism by cells of members of the same species • imagine if you could ‘catch’ cancer from someone!

10. An NK-cell based system that attacks non-self could be the basis for an early allorecognition system it’s consistent with theories of early lymphocyte evolution it would require a polymorphic target - such as MHC it would require ‘education’ - as exists for NK receptors it would not require peptide presentation though this might facilitate kin recognition (back to this) Pleiotropic functions of an early MHC may have driven, or maintained polymorphism: Kin recognition, Mate Selection, Allorecognition & organogenesis …..

11. Allorecognition to combat cancers: 1) Burnett’s cancer surveillance hypothesis: allorecognition evolved to combat cancer problem is that cancers rarely have ‘foreign’ peptides or loss of MHC 2) Allorecognition prevents cancer as a contagion as in the case of the tumours killing the Tasmanian Devils

12. 2) did allorecognition evolve to prevent cancer spread? Tasmanian Devil Facial-Tumour is spread by devil to devil contact Healthy --> Diseased Is this the result of being too genetically alike? i.e. lacking allorecognition

13. 3) Did MHC arise to control neural development? Mouse brain showing expression of classical H-2D (blue) and non-classical class I - Qa-1 (green) and T22 (red) genes - or protect immuno-privileged sites Nature Reviews Neuroscience5, 521-531 (2004)

14. MHC-1 expression is somehow involved in establishing or retaining • synaptic connections to injured axons & neuro-muscular junctions b-2M or TAP-1 knockouts have even greater loss of connections Oliveira, Alexandre L. R. et al. (2004) Proc. Natl. Acad. Sci. USA 101, 17843-17848

15. Neuron 64: 463-70, 2009 Knockouts of PirB & specific MHC class I alleles lead to identical changes in synapse networks during development of the retina …. PIRs are paired immunoglobulin receptors encoded in the same leukocyte receptor complex (LRC) as KIRs Curiously, complement component C1q also is involved in synapsis

16. MHC and neural development though there is differential distribution of different MHC genes expressed in the brain it’s not clear how or if MHC polymorphism is necessary (i.e. it seems to be achieved through a few MHC genes)

17. 4) Did gnathostomes just retain/develop a relict allorecognition system? A highly polymorphic & coevolving pre-MHC & NK-like receptor?? Invert immunity already includes xeno-recognition Some colonial inverts also have non-immune (?) allorecognition Easier to develop TcR-MHC system from non-lethal allorecognition How would the ancestral vertebrate maintain pre-MHC polymorphism? perhaps through pleiotropic functions of a pre-MHC!?

18. Allorecognition systems are not uncommon among invertebrates Observed in: Plants, Fungi, Slime molds (protists), Sponges & anenomoes (early metazoans), & tunicates (protochordates) Appear to operate differently in each - analogous systems These systems are used for: Kin recognition and/or selection Prevention of intraspecific cell parasitism Prevention of self-fertilization (distinct from somatic system!?) Prominent among colonial organisms that may share resources

19. Anemones compete for substrate to grow & feed on. Non-kin are attacked Larva may preferentially settle near kin

20. Some colonial organisms will fuse colonies - only if they are related Fusion allows: size advantages for sexual maturation, or if damaged Each colony of this tunicate Botyrllus, contains about 70 individuals all connected by a common vascular network Risks: Disease transmission Germ cell parasitism. More later…

21. In some cases a stage in the lifecycle can only be completed by cooperative fusion. This slime mold fruiting body is made up of thousands of individual amoeba. Head Stalk Base A slime mold grex Spores are 'shot' out of the head - towards potential new nutrient sources. Grex Amoebocyte aggregate - a forming grex

22. Forming a slime mold fruiting body (Strassmann et al 2000 Nature, 408: 965-967) fruiting body (grex) differentiation amoeba aggregate When the going gets tough… Fruiting body formation Grex

23. Stalk & base forming amoebocytes are committing genetic suicide if the spores they help to disperse aren't from related amoeba. It's supposed to be random distribution to grex areas - but the cheater phenotype will not contribute to the base or stalk - and prevents other amoeba from forming the fruiting body. Cheater is a germ cell parasite - but at least they're kin ;) (Strassmann et al 2000 Nature, 408: 965-967) so why don't these cheaters prosper?

24. rejection Colonial tunicates (and other inverts) face similar 'parasitism' problems So fusion to non-kin leads to a rejection response. First a primer on tunicate biology…… they're small

25. Free swimming tunicate larva has a notochord - so they are euchordates The larva settle to form sessile filter feeding adults. This solitary tunicate has in & out current siphons that pass water thru a filter feeding basket. It is encased in a gelatinous tunic. In colonial tunicates individuals share a common tunic and vascular network

26. In colonial tunicates the tunic & vascular network is shared. The vascular network carries stem & 'blood' cells. Stem cells can stop circulating and form testes/eggs (hermaphroditic). Water Out Water In filter feeding gut ≈ a zooid Tunic Sperm/Egg ‘blood’ vessel & cells including some stem cells

27. Morphology of a colonial tunicate - Botryllus schlosseri Ampulla (vessel ends) line colony margin

28. rejection When colonies grow into each other the vessels start to fuse. If colonies are not related tissue necrosis (cytotoxicity??) begins, a tunic barrier forms & colonies continue growth in other directions.

29. If fusing colonies share one allele of a polymorphic gene then vessels remain fused and 'blood' & stem cells circulate freely between colonies fusion

30. The Fu/HC gene regulates allorecognition - colonies must share an Fu/HC gene allele in order to fuse. Schematic shows 3 'colonies'. Stem cells can circulate amongst fused colonies Allelic type The Fu/HC gene is highly polymorphic - is it the pre-MHC??? Magor et al. Immunol. Rev. 1999 167:69 cf. Buss 1999 PNAS 96: 8801-3

31. Why confine fusion to kin? To limit stem cell parasitism to kin Circulating stem cells of one individual may be better at settling in the egg/testes niche than those of another individual. If blue and orange colonies fuse it may be that orange stem cells are better at forming gonads than the blue stem cells, therefore all gonads carry the orange genome If you're going to be parasitized it's better to have it done by kin!

32. The Botryllus Fu/HC receptor is called "Fester" It regulates outgrowth of ampulla and fusion of colonies It's polymorphic, a bit Polygenic, and can be somatically diversified by alternative splicing of exons. administration of anti-fester MAb causes rejecting colonies to fuse Knockdown of Fester causes colonies to be unresponsive to each other So Fester has a dual function - it can initiate fusion it also acts as an inhibitor of rejection if it engages a FuHC Science 303: 997, 2004; Nature 438: 454, 2005; Immunity 25: 163, 2006

33. Though Fu/HC has Ig domains, neither FuHC nor the Festers appear to be precursors to MHC or any known NK cell receptors EGF & SCR are conserved & widespread protein domains arrows indicate alternative splice sites C2 type Ig domains Seminars in Immunol. 22: 34-38, 2010

34. some Botryllus granulocytes express cell surface CD94 - an NK marker Khalturin K et al. PNAS 2003;100:622-627 3 types of granulocytes -CD94 Dapi auto-fluorescence (CD94/NKG2 on NK cells interacts with non-classical MHC HLA-E in humans)

35. cells expressing the CD94 are found at sites of colony rejection Whole-mount immunostaining of rejecting colonies. white arrows here corresp to black arrows above anti-BsCD94 staining Khalturin K et al. PNAS 2003;100:622-627

36. What are the implications of an NK like system in proto-chordates? that the common ancestor to agnathans/gnathostomes inherited systems for self non-self discrimination along with the regulatory and signaling systems These were likely developed into the VLR and BCR/TcR systems

37. What is the sum of our evidence to date? Forms of allo-recognition based on polymorphic receptors exist in proto-chordate evolution (Botryllus) protochordateallorecognition may have involved NK cell homologues MHC does play a role in neural patterning - but how ancient it this? Clearance of altered self cells (cancers) would be an easy leap from allo-recognition of a different type (remember that SIRP - our candidate pre-Ig/TcR is used for altered self recognition & homeostasis) Parsimony would put peptide presentation & classical MHC later - consistent with data from gene duplications

38. And how might peptide presentation have been used first for allorecognition – independent of CTLs and TcRs?