Transplantation & Immunization Flashcards
What does MHC stand for an why?
Major Histocompatibility Complex → tissue histocompatibility allows for transfer without rejection
*MHC match required for graft
MHC-II = 2 letters → HLA-DP/DQ/DR for humans and H-2 IA/IE for mice
MHC-I = 1 letter → HLA-A/B/C for humans and H-2K/D/L
What is transplantation?
What is the major difficulty to a successful transplantation?
The act of transferring cells, tissues, or organs from one site to another or from one individual to another is now an important medical therapy
Adaptive immune responses to the grafted tissues are the major difficulty to successful transplantation
Rejection is caused by immune responses to alloantigens on the graft
What is the definition of Autograft?
Self tissue transferred from one body site to another
ex: skin graft for severe burns
What is the definition of Isograft?
Tissue transferred between genetically identical individuals Ex: twins, inbred mice
- Donor & recipient are syngeneic
What is the definition of Allograft?
Tissue transferred between genetically different members of the same species
- different human individuals, between mouse strains = are allogeneic
What is the definition of Xenograft?
Tissue transferred between different species
What are the different types of unsuccessful transplantation?
- Graft rejection: recognition of graft as foreign
- Hyperacute
- Acute
- Chronic
*Recipient makes immune response against donor - Graft vs host disease (GVHD): graft recipient as foreign
*Donor graft makes response against recipient
- Only in bone marrow transplants
What is hyperacute transplant rejection?
- Most intense graft rejection ~24 hrs
- Pre-existing antibodies → Ag-Ab complexes → complement activation → will result in rapid Ab-mediated lysis of incompatible donor cells (+ neutrophil attraction)
Often seen in ABO blood graft in incompatible groups (match ABO groups)
Other case:
Pre-formed antibodies against non-self HLA can lead to rapid rejection
- Check for anti-donor HLA antibodies: cross- matching
- In the case of prior unsuccessful graft → Abs where made and memory cells remain against the graft MHC or other proteins → 2nd graft leads to hyperacute transplant rejection
What is acute transplant rejection?
- Begins 7-10 days after graft transplantation, full rejection by 10-14 days (slower kinetics because have to generate T cells by clonal expansion)
- Mediated by T cells → both CD4 and CD8 T cells involved
Similar to infection:
Graft → Re-vascularization → drainage of Ags to local dLN → priming of T cells → Cellular infiltration + effector T cell response → thrombosis and necrosis
What is seen in T cell depletions (by injection of monoclonal Abs) in context of acute transplant rejection?
Anti-CD8 → no difference with control (still get rejection ~ 14 days)
Anti-CD4 → Significant prolongation of the graft
*Need CD4 T cells to give CD8 T cells help and CD4 T cells can also be cytotoxic in that context (removing CD4 dampens a lot the CD8 T cell response as well)
Anti-CD8 + Anti-CD4 → survival of the graft up to 60 days
What happens when you generate allogeneic T cells to MHC-a in MHC-b mouse and then transfer the T cells to a naïve MHC-b mouse given graft from MHC-a?
Graft 1: MHC-a → MHC-b
Transfer allogeneic T cells to MHC-a into naïve MHC-b
Graft 2: MHC-a → naïve MHC-b
Will you get skin graft rejection in a H2b nude mouse?
T cells derived from an allograft-primed mouse can transfer second-set allograft rejection to an unprimed syngeneic recipient
Graft 1: MHC-a → MHC-b → acute transplant rejection (necrosis)
Transfer allogeneic T cells to MHC-a into naïve MHC-b
Graft 2: MHC-a → naïve MHC-b → hyperacute transplant rejection (necrosis)
Nude mice have no thymus → no T cells → graft would be accepted because nothing to mediate the rejection response
What is acute transplant rejection in the case of formation of MINOR histocompatibility antigens?
MHC-a → MHC-a, but still get slow (~60 days) graft rejection
- Same MHC allele, but other minor proteins might be seen as foreign
- Even complete MHC matching does not ensure graft survival
- Polymorphic proteins will be digested by proteasomes within the cell and displayed by MHC-I on cell surface. If the peptide differs from ‘self’ then can generate a T cell response
- Donor & recipient differ at other (non MHC) loci
- Takes longer to reject the graft, but will ultimately still destroy graft (takes time for the protein to be digested and presented on MHC-I)
What is an example of non-MHC situation that can lead to slow acute transplant rejection
Male graft → Woman even if MHC matches, women are not tolerized to proteins encoded for on the Y chromosome
Woman graft → Man is fine because men have an X chromosome
What are 2 ways to get antigen presentation in the case of acute transplant rejection?
Direct allorecognition: the “passenger leukocytes” in the transplanted organ includes DCs which can present antigen to T cells of the recipient
– supporting this idea: depleting DC from grafted tissue can prolong graft survival
– often involved in acute graft rejection
Indirect allorecognition: uptake of allogeneic proteins by recipient antigen presenting cells and presentation to T cells by recipient’s MHC
- peptides from the foreign MHC taken up and presented
- minor histocompatibility antigens
- often these responses lead to antibody responses being generated: alloantibodies
- drainage of antigen to local dLN → antigen uptake by lymph nodes resident DCs → digest antigen and present to T cells → germinal center, follicular T cells, etc.
What are different targets to prevent graft rejection?
- Anti-CD25 antibody preventing IL2 signaling (basiliximab)
- Activation of mTOR to block effector T cell differentiation (sirolimus)
- Depletion of T cells via anti-CD52 antibody (alemtuzumab)
- Prevention of NFAT translocation to the nucleus (cyclosporin A, tacrolimus)
- Inhibition of T cell replication (mycophenolate, azathioprine)
- Blocking co-stimulation (belatacept: CTLA4-Fc fusion)
*drugs ending with mab = monoclonal Ab
- What is a potential off-target effect of blocking anti-CD25 antibody to prevent graft rejection?
- What is a potential off-target effect of depleting T cells via anti-CD52 Ab?
- It would also block regulatory t cells → break tolerance at some sites
- Immuno-deficiency because no Tregs
Why might blocking co-stimulation (belatacept: CTLA-Fc fusion) be better than using the anti-CD25 or cyclosporin A?
Cyclosporin A → prevention of NFAT translocation to the nucleus → general immunosuppression
anti-CD25 → preventing IL2 signaling
Blocking co-stimulation leads to T cells not being able to be fully activated and becoming anergic which prevents graft rejection
*belatacept block CD28-mediated co-stimulation
It allows to preserve the Treg repertoire, a bit more selective
Which grafts are accepted vs rejected?
Donor is always Balb/c
Recipients:
a) C3H
b) Rat
c) Nude mouse
d) C3H, had previous Balb/c graft
e) C3H, had previous C57Bl/6 graft
f) Balb/c
*Assume different strains have different MHC haplotypes
Donor is always Balb/c
a) C3H → rejection, First set rejection (12-14 days)
b) Rat → rejection (MHC and other genomic mismatches, FSR)
c) Nude mouse → Acceptance (no T cells)
d) C3H, had previous Balb/c graft → rejection, SSR (6 days)
e) C3H, had previous C57Bl/6 graft → rejection, FSR
f) Balb/c → acceptance
Which grafts are accepted vs rejected?
a) D = Balb/c, R = (Balb/c x C3H)F1
b) D = Balb/c, R = (C3H x C57Bl/6)F1
c) D = (Balb/c x C3H)F1, R = Balb/c
d) D = (Balb/c x C3H)F1, R = Balb/c, had previous F1 graft
*Assume different strains have different MHC haplotypes
a) D = Balb/c, R = (Balb/c x C3H)F1 → acceptance (MHC codominance)
b) D = Balb/c, R = (C3H x C57Bl/6)F1 → rejection, FSR (12-14 days)
c) D = (Balb/c x C3H)F1, R = Balb/c → rejection (MHC mismatch of C3H, FSR)
d) D = (Balb/c x C3H)F1, R = Balb/c, had previous F1 graft → rejection (SSR, 6 days)
True or False?
a) Second set rejection is a manifestation of immunologic memory
b) Acute rejection is mediated by preexisting host antibodies specific for antigens on the grafted tissue
c) Host dendritic cells can migrate into grafted tissue and act as APCs
d) All allografts between individuals with identical HLA haplotypes will be accepted
e) Cytokines produced by host T helper cells activated in response to alloantigens play a major role in graft rejection
a) Second set rejection is a manifestation of immunologic memory → TRUE
b) Acute rejection is mediated by preexisting host antibodies specific for antigens on the grafted tissue → FALSE (hyperacute)
c) Host dendritic cells can migrate into grafted tissue and act as APCs → TRUE (direct presentation)
d) All allografts between individuals with identical HLA haplotypes will be accepted → FALSE (minor histocompatibility antigens/other proteins)
e) Cytokines produced by host T helper cells activated in response to alloantigens play a major role in graft rejection → TRUE (blocking some cytokines will cause slower graft rejection)
What is chronic transplant rejection?
- Late failure of a transplanted organ – years later
- Not well understood what the cause is: specific immune alloreactivity, non-immune injury or both
- Irreversible & progressive, leading to failure of organ
Often major component is chronic allograft vasculopathy → concentric arteriosclerosis of graft blood vessels:
- Vasculature starts becoming dammaged overtime, specifically in heart and kidney grafts → die off of tissue
- Can get recurring subclinical acute rejection events more or less controlling it (growing dammage overtime)
- Can get recurrence of the pathology that initially lead to the transplant dammaging the tissue
What is graft-vs-host rejection?
Converse of graft rejection can occur when the grafted tissue is bone marrow
Mature donor T cells in the hematopoietic stem cell transplant recognize the recipient tissue as foreign, resulting in inflammatory response in multiple tissues (multi-organ failure)
- Espacially in the case of HLA mismatch (very important for BM transplants)
- Memory T cells are depleted from bone marrow before transplant
What explains feto-maternal tolerance?
physiological “transplant” with no rejection because lots of tolerance mechanisms are in place
- Lots of Tregs
- Lots of control even if exchange between bloods
- Could lead to death of the fetus
How frequent are alloreactive T cells in our repertoire?
Why is this % so high?
Expose T cells to MHC alleles from someone else → Mixed lymphcyte reaction:
Measures the immune response of T cells when they encounter lymphocytes from a different person
~ 1-10% of all T cells will respond to stimulation by cells from another unrelated member of the same species (alloreaction) MHC mismatch
Very high % of alloreactive T cells:
- Positive selection for T cells binding MHC + high cross-reactivity
- TCRs have evolved to bind MHC
What are the 2 types of immunity following Antigen exposure?
Sterilizing immunity → neutralizing Abs when 2ndary infection (prevents re-infection)
Disease prevention → so the infection doesn’t become a pathology (doesn’t prevent re-infection, but prevents it from becoming a disease)
- Even if a disease enters the body, the immune system controls preventing any symptoms
*Vaccine mimics natural infection, but bypasses the pathology and goes directly to immunization (Active or passive immunization)
- Contributes to development of memory cells
What are the causative agent of measles?
How is it transmitted?
What are syptoms?
- Causative agent: measles virus in genus of morbillivirus, family Paramyxoviridae (-ssRNA genome)
- Transmitted through aerosols, surfaces, coughing
- Symptoms:
- high fever for 4-7 days, runny nose, watery red eyes, rash covering face & then spreading over whole body
- 1/10 children have ear infections which can result
in hearing loss
- 1/20 children develop pneumonia (most common cause of death)
- 1/1000 children develop encephalitis (brain swelling) leaving child deaf or with permanent intellectual disabilities
- 1-2/10,000 children measles can reactivate several years later causing a progressive, fatal brain inflammation (SSPE: subacute sclerosing panencephalitis)
NO treatment
What are the causative agent of measles?
How is it transmitted?
What are syptoms?
- Causative agent: family Picorna virus (+ssRNA genome)
- Transmitted by fecal-oral route
- Symptoms:
- 75% of cases asymptomatic or not recognized as polio
- 25% of cases: fever, sore throat, headache, GI tract symptoms
- 1% of cases: irreversible paralysis (poliomyelitis) – polio invades the nervous system, death in some cases (paralysis of the heart can lead to death, paralysis of lungs requires breathing machines)
Which are were the 2 first vaccines developped for polio?
Salk (1955) → inactivated poli vaccine (IPV)
Sabin (1961) → oral polio vaccine (OPV, it is active)
1988 = launch of eradication program; currently still cases in Pakistant, Afghanistant (wild poliovirus)
What are important requirements for an effective vaccine?
- Safe → no side effects
- Protective → must protect from exposure to live pathogen
- Gives sustained protection → for several years
- Induces neutralizing antibody → prevent infection of cells that can not be replaced ex: neurons
- Induces protective T cells → intracellular pathogens are more effictively dealt with by cell-mediated responses (vs humoral responses)
- Practical considerations → low cost/dose, biological stability, ease of administration, few side-effects
Vaccines must be given to a large number of people so
- Side-effects have to be very rare (given to healthy population so strict rules)
- Efficacy (protective protection) has to be high
- Induce long-lived memory (avoid too many boosters)
- Low cost/dose ratio
- Ideally primes both B and T cells
What is Herd Immunity?
What is R0?
Herd Immunity → resistance to the spread of an infectious disease within a population that is based on pre-existing immunity
- Lowering the number of susceptible people (through vaccination) in the population reduces the likelihood of transmission even for unimmunized individuals
R0 = # of secondary cases from a single primary case in a completely susceptible population
- Disease-dependent, population-dependent (depending on how contagious, the route of transmission)
- The larger R0, the more people you have to vaccinate in order to protect a population from an epidemic
What is the critical immunization threshold? (qc)
Based on the R0 and the concept of herd immunity
qc = 1 - 1/R0
Which of these disease have the lowest vs highest critical immuniziation thresholds?
Measles, SmallPox, Polio, COVID
Measles → R0 = 15, Pv = 94%
Smallpox → R0 = 5, Pv = 80%
Polio → R0 = 5, Pv = 80%
COVID is similar to polio and smallpox, a bit lower
what are the main childhood vaccines?
16 recommended vaccines:
- HepB, Polio, tetanus, pneumonia, Flu, Menigococcus C, measles, mumps, rubella, chicken pox, HPV
Others are also available:
- Vibrio cholerae, Yersinia pestis, salmonella typhi, Bacillus anthracis, yellow fever, hepatitis A, varicella zoster, tick- borne encephalitis, small pox
What are some general impacts of childhood vaccination?
2-3 million deaths of children prevented per year (but ~19.5 million children globally are still missing out on basic vaccines)
Investing in child health has economic benefits, raises child IQ, improves parent productivity
Every $1 spent on vaccines yields an ~$44 economic return
What are some features HPV and impacts the vaccine has had?
human papilloma virus (HPV): in US 14 million infections/year, causes 30,700 cancers in men & women annually
- HPV 16 & 18 strains are associated with 70% of cervical cancers
- HPV 6 & 11 strains are associated with 90% of genital warts
Gardasil (Merck) vaccine against all 4 strains
- Recommended in US from 2006, age 11/12 boys & girls
- recent report by CDC: precancer incidence in women aged 20-24yrs has decreased by ~80% (2008-2022)
Why are there no vaccines for some pathogens?
Specifically, why do we not have a vaccine for HIV?
- natural infection does not generate protective immunity/clear infection = correlates of protection undefined
Why do we not have an HIV vaccine?
- huge diversity in strains circulating in the
population (if your vaccine targets 1 strain, it is not very efficient)
- correlates of protective immunity are unclear
- virus rapidly escapes antigen specific immunity (antibody & T cell)
- establishment of latent viral reservoir
- no such thing as safe “attenuated” HIV
What are the 2 main types of vaccines?
PASSIVE
Transfer of preformed antibodies from an immune individual to a non-immune recipient
ACTIVE
Inoculation of people with microbial pathogens and/or Ag(s) that induce adaptive immunity without causing disease
What are natural and therapeutics examples of passive vaccines?
Natural:
Transfer of maternal Abs across placenta to developing fetus or during breastfeeding to infant
Therapeutic:
- antiserum treatment to toxins/venoms
with immediate threat to life (generated on other animals)
- babies with immunodeficiency
- unvaccinated individuals to infections
(tetanus, diptheria, measles..) are given Abs if they get infected
- ZMapp: combination of 3 anti-ebola
monoclonal antibodies → mouse neutralizing Abs chimerized to have human Fc component, came before the Ebola vaccine
What are the different vaccine testing phases?
Why is the testing process so long?
Phase I → small number of volunteers, testing for safety
Phase II → effectiveness against pathogen evaluated (efficacy) through correlates of protection (ex: Ab titres measured)
- You can’t expose the people to the actual virus for safety reasons so have to find another way to measure the efficacy of the vaccine
Phase III → large # of people assessed for protection against pathogen
The testing process is so long because vaccines belong to the preventive branch of health care → high safety standards
- All vaccines must go through rigourous testing in animals and humans → expensive process and many vaccines never progress to use in humans
What are different approaches to vaccine design?
- Whole organisms
- Purified macromolecules
- Recombinant vectors
- DNA vaccines
- mRNA vaccines
What are 2 types of vaccines generated through whole organisms? What are examples of diseases associated with each?
- Live attenuated
Take pathogenic virus from someone infected → culture it in cells from a different species → the virus adapts (mutations) to better grow in that other specie which attenuates its ability to infect human cells
- measles, mumps, polio (Sabin), rota, yellow fever
- inactivated or killed
Inactivated through heat or chemical treatment (ex: formaldehyde)
- hep A, cholera, influenza, plague, polio (Salk), rabies
What are pros and cons of live attenuated vs inactivated or killed whole organism vaccines?
How are they done?
Live attenuated:
Take pathogenic virus from someone infected → culture it in cells from a different species → the virus adapts (mutations) to better grow in that other specie which attenuates its ability to infect human cells
Pros → Very effective
Cons → Refrigerated storage, may mutate back to virulent form
Inactivated or killed:
- Inactivated through heat or chemical treatment (ex: formaldehyde)
- Grow very large amounts → purify → inactivate
Pros → Very safe, stable without refrigeration
Cons → Less durable immune responses (boosters required)
- Not alive so can’t revert to virulent form, but doesn’t mount as strong of an immune response
What are purified macrocolecules vaccines ?
What are the 2 types of purified macromolecules?
They are vaccines that use specific components of a pathogen rather than the whole organism
Toxoids → Use inactivated bacterial toxins to induce immunity
Ex: diphtheria, tetanus
Subunits → Contain only specific antigenic parts of a pathogen
Ex: Hepatitis B, pertussis, S. penumonia
Pro → very low chance of adverse reactions
From the live pathogen, what are 2 types of processing to purify macromolecules for vaccines?
- Break up pathogen to small pieces → Extract Ags from pathogen → NATIVE
- Clone gene encoding for Ag → Express and purify recombinant Ag → RECOMBINANT
What are features of tetanus?
- Causative agent: Clostridium tetani
- Found in soil/animal intestines
- Produces an exotoxin which causes rigidity and convulsive spasms of skeletal muscles
- 500-600 case per year prior to vaccine (USA)
- 1948 vaccine became available
- now ~ 20 cases/year (USA)
- renew every 10 years
What are adjuvants?
Vaccines based on purified proteins require the addition of adjuvants → enhance immunogenicity of antigens, as not very potent on their own
- Are not approved on their own – only in context of specific vaccine formulation
What are 4 different adjuvants that are approved for human use?
- Alum: inorganic aluminium salts, results in slow release of Ag at injection site and recruits APCs
- Contains PAMPs → activtaes innate immune response - MF59: oil in water emulsion to aid in slow Ag deliver
- AS04: alum plus TLR4 agonist
- CpG 1018 (heb B vaccine): TLR9 agonist
- CpG: DNA analogs containing unmethylated cytosine/guanine motifs that mimic bacterial DNA
What are conjugate vaccines?
Conjugate = polysaccharide capsule linked to a protein carrier
Polysaccharides alone typically trigger a T-cell-independent immune response
Example:
1. B cell binds bacterial polysaccharide epitope linked to tetanus toxoid protein
2. Antigen is internalized and processed
3. Peptides from protein component are presented to the T cells
4. Activated B cell produces Ab against polysaccharide antigen on the surface of the bacterium
Other definition: Conjugate vaccines are a type of purified macromolecule vaccine where a polysaccharide capsule (found on the surface of certain bacteria) is chemically linked to a protein carrier. This linkage enhances the immune response, especially in young children, whose immune systems do not respond well to plain polysaccharide antigens.
What are some conjugate purified molecule vaccines?
Haemophilus influenzae type B
S. penumonia
What type of vaccin was AstraZeneca?
Recombinant vector vaccines:
- live attenuated vectors that express gene products of pathogens to be vaccinated against
Pro: cannot revert to virulence
What are DNA vaccines?
What are pros?
- Inject DNA encoding antigenic proteins directly into muscle of recipient
- Host cells take up DNA and produce immunogenic protein, generating immune response
pro: inexpensive to make, very stable (no refrigeration needed)
What are mRNA vaccines?
What are 2 examples of mRNA vaccines?
- lipid nanoparticles with mRNA → easy to scale, produces harmless piece of pathogen
Moderna and Pfizer-BioNTech against COVID19
mRNA encodes for spike proteins → uptaken by local APCs or muscle cells
what type of vaccine was used for pancreatic cancer?
mRNA vaccine
Pancreatic cancer: kills 467,000 people world wide/year; 90% die within 5 years of diagnosis = ineffectiveness of current treatments (few mutations so hard to target with checkpoint blockade)
Phase I trial, bespoke mRNA vaccines with cancer-specific mutant proteins, 16 patients vaccinated
Outcome: 8 of 16 people had vaccine-elicited CD8 T cell responses (responders) and 6 remained cancer free at 3.2 years, while non-responders had cancer recurrence by 13.4 months
