lecture 21 Flashcards
xenotransplantation
How many cadaveric donors/solid organ transplants do we have compared to patients on the waiting list?
- 1998 - 2004: always much high numbers on the waiting list than transplants that occur
~ 1800 patients waiting for a transplant - 700 patients get transplants
- 200 cadaveric donors (this doesn’t match up)
- before about 1978 there wasn’t really a waiting list, ~ 500 translplants
- 1978 waiting list appears and increass exponentially
- number of transplants rises but plateaus
- 1978 was when the technique for immunosuppression was approved (cyclosporin?)
What are transplantation statistics?
- UK:
- ‘92 - ‘95 1700 transplants/year
- waiting list grew from 4,000 to 5,000
- USA:
- 45,000 patients waiting for transplants
- <6,000 cadaveric donors = 20,000 organs
no way for supply to meet the demand
~10-15% chance of dying while waiting for an organ, not unique to australia
Why do transplants?
costs
- dialysis $40,000
- kidney transplant $30,000 in first year
- $8,000 per year
improved quality of life
Why is there a shortage of donors?
- decrease in road accidents
- family permission less
- not suitable (age, disease, etc)
- time factor
- greater demand
What are possible solutions to this shortage?
- increase donors
- e.g. donor games
- getting older so not necessarily the solution
- artificial organs
- stem cells –> organs/tissue
- animal as donors: xenografts
- first transplantations were chimpanzee kidneys to human
What were patient attitudes towards xenografts?
131 renal patients:
- 51% would accept xenograft
- 39% unsure, wanted more information
- 10% unacceptable
Why did we revisit xenografts?
- molecular biology
- transgenesis/animal cloning
What are problems with xenografts?
- physiological
- ethical
- biological
- infection
- immunological
What are physiological problems of xenografts?
- size e.g. can’t use a mouse (too small) or a horse (too big), maybe a kangaroo? but we use sheep/pig
- complexity of function
e. g. sheep regulate insulin by glucose AND fatty acids –> not the same
What are ethical considerations of xenografts?
- emotional
- conservation
e.g. chimps etc are probably the best immunologically speaking, but people have issues with taking organs from this animal, cf pig
considerations for use of pigs:
- food (as long as people are willing to slaughter pigs for food, they should be able to be used for donating organs)
- insulin (major source of insulin for diabetics)
- heparin (anticoagulant)
- heart valves (used to replace broken valves in humans)
- breeding (a forerunner for genetic manipulation)
What are biological considerations of xenografts?
- adaptability to captivity
- pigs breed well in captivity
- primates breed poorly in captivity
- litter size
- pigs: 10
- primates: 1
- pig husbandry relatively easy
- gnotobiotic pigs available (germ designed/germ free)
What are infectious considerations of xenografts?
- primates: potential source of infection, especially viruses
- pigs: lower potential source for infection
- human viruses could infect primates, but not pigs
What are immunological problems associated with xenografts?
- hyperacute rejection: antibody causes immediate graft loss
- delayed xenograft rejection: macrophages/NK cells
- cell mediated rejection: T cells
What is hyperacute rejection?
- antibody
- antigen
- complement
- immediate
- clots
- loss of vascular endothelium
- haemmhorage and oedema
- not unique to xenografts
- e.g. mis-match blood transfusion
Do we have an ‘anti-pig’ antibody?
- yes: normal, present in all humans
- IgM, IgG, IgA
- 1-2% of all Ig
- > 95% to Gal-alpha (1,3) Gal
What is the xenoantigen?
- Gal-alpha (1,3) Gal
- present on many different molecules
- expressed in most tissues
- alpha 1,3, galactosyltransferase
- GKO: mice, pigs
How has transgenesis played a role in xenotransplantation?
CD46 transgenic pigs:
- ubiquitously expressed human complement regulator
- renal transplants to non-immunosuppressed baboons
- > 72 hr (normal <1hr)
- histologically normal
pancreatic islets
- destroyed by anti-pig antibodies
- protected by expression of CD46
How can you use ES cells?
- get rid of the antigen in mice
- can’t use ES cells of pigs
What are Gal o/o pigs?
- cloned pigs
- reported by 4 groups
- Gal- survive >150 days
- HAR/DXR solved
What pig transplantations have occured?
pig heart transplants
- baboons (n=8)
- anti-CD154 mAb, mycophenolate mofetil, and methylprednisolone
- no HAR
- graft survival extended to between 2 and 6 mo
- thrombotic microangiopathy
pig kidney transplantation
- baboons (n=11)
- tolerance induction, including thymectomy, splenectomy, T-cell depletion, anti-CD154 monoclonal antibody and mycophenolate mofetil, with or without low-dose steroids
- no HAR
- graft survival extended to 83 days (now more than 160)
- thrombotic microangiopathy
pig kidney transplantation 2
- baboons (n=3)
- no immunosuppression
- no HAR
- graft survival extended to 4 days
- antibody and C’ deposition, platelet deposition, perivascular infiltrate
- intravascular coagulopathy
pancreatic islets: more than a year
looking a blocking costimulation
What are some factors to consider in transgenesis?
- microinjection –> 1%
- xeno transgenes
– DAF 1992
– DAF + CD59 1998
– CD46 2000
ie. 1-2 genes in 10 years
= 100 year programme unless SMGT - nuclear transfer 0.5% : Gal knock out (some groups claim up to 10% in pigs… more in mice)
- retrovirus: Lenti virus in mice
What is sperm mediated gene transfer?
- collection of ejaculate from male, mixed with DNA (and appropriate buffers)
- artificially inseminated females
- could use this to create multi-transgenic pigs
What are unsolved problems of xenografts?
- organ compatibility
- delayed xenograft rejection
- retrovirus infection
- cellular immunity/immunosuppression
- tolerance induction
- other carbohydrate antigens
- further genetic engineering
What are the important points re: xenografts?
- the pig is the most suitable animal for donor
- hyperacute rejection entirely due to Ab, Ag and C’
- antigen negative pigs cloned
- more genetic engineering required
