Immunology of the gut mucosa #2

• LO:


  • Understand the concept of mucosal immunity and its importance in vaccine development.
  • Explain the differences between mucosal vaccines and traditional injectable vaccines in terms of administration, mechanism of action, and advantages.
  • Describe the challenges and barriers associated with developing effective mucosal vaccines.
  • Discuss the various mucosal vaccine delivery routes and their advantages and limitations.
•                 Traditional Vaccines


  • Elicit systemic immune responses

  1. Intramuscular injection (e.g., Influenza, Diphtheria, Tetanus)
  2. Subcutaneous injection (e.g., MMR and IPV)
  3. Intradermal injection (e.g., BCG)

  • Live attenuated viruses (Rubella, Mumps)
  • Killed pathogens or toxins (Diptheria, pertussis toxin)
  • New platforms:

  1. RNA vaccines
  2. Viral vectors

  But these traditional approaches have not been efficient at generating immunity for some of the world’s biggest killers (malaria, HIV, TB etc)
• Systemic vs Mucosal Immunization


  • Systemic

  1. systemic immunity only (serum IgG, systemic T-cells)
  2. resolves infection once virus has spread from a mucosal site
  3. can allow shedding from replication in a mucosal site
  4. may be sufficient to control both if mucosal inflammation recruits systemic effectors

  • Mucosal

  1. mucosal and systemic immunity (local and serum IgA, IgG, mucosal and systemic T-cells)
  2. prevents uptake of virus into mucosal tissues
  3. reduces local replication
  4. shed virus may be complexed with IgA and non-infectious
• Polio


  • WT Polio trasmits by fecal-oral route. Oral Polio Vaccine (OPV) is delivered by 2 drops orally
  • Active replication of OPV in epithelial cells.
  • Stimulation of mucosal immune responses – protects against WT infection and transmission.
  • Vaccine-associated paralytic polio (VAPP) occurs in approximately 1/2.7 million doses of OPV administered.
  • Inactivated polio vaccine (IPV) injected into the leg or arm
  • IPV is very protective against disease but elicits a weaker mucosal immune response (only 20-30% have mucosal immunity) than OPV and does not avert transmission. Virus replicates in the gut

  
• Smallpox


  • mortality rate ~ 35%
  • Records indicate that variolation was first practiced by the Chinese as early as the 15th century. They practiced variolation by nasal insufflation
  • Edward Jenner heard a milkmaid brag that having cowpox made her immune to smallpox. And years later, as a doctor, he drew matter from a cowpox pustule on the arm of a milkmaid to vaccinate a young test subject
• Vaccine uptake at mucosal sites.

  
• Functions of IgA: Multiple Neutralising Properties of sIgA at Gut Mucosal Surfaces


  • Inhibition of adherence:

  1. SIgA antibodies to microbes shown to prevent adherence to mucosal epithelia

  • Mucus trapping:

  1. SIgA antibodies may associate with mucins, thereby trapping SIgA-bound microbes in the mucin layer, preventing adherence

  • Pathogen / toxin neutralisation:

  1. Binding to attachment receptors and toxins, preventing internalisation / infection and toxin entry

  Transport:

  sIgA can bind pathogens and mediate their transport back out of the cell / mucosa. It will do this by binding to the pIgR.

  
•  The mucosal immune system must distinguish between harmless antigens (associated with food and commensal microflora) and those associated with pathogens and respond appropriately.
• Tolerance


  • Specific acquired mechanism - whereby prior feeding reduces an individual's ability to respond to subsequent presentation of that antigen.
  • Believed to protect from allergy induction (eczema, hay fever, asthma, food allergy)
  • In ‘adults’: Probably most of the mucosal immune system is geared towards tolerance – that is, to not respond to foreign antigens unless necessary.
• Implications for Vaccine Design


  Animals recovered from mucosal infections tend to be solidly immune to homologous re-challenge.

  Animals fed simple proteins tend to become immunologically tolerant.

  • ‘danger signals’ / ‘cell activation’  (e.g. TLR ligands, C-type lectins)?
  • live vaccines?
  • novel adjuvants and delivery vehicles?

  1. Attenuated Salmonella sp
  2. Cholera like enterotoxins
  3. ISCOMS

  • Is diff. between tolerance and active immunity dependent on site of antigen uptake?

  1. Peyer's patch immunisation results in active immune responses
  2. Antigen uptake across villus epithelium results in tolerance
• Antigen delivery systems:living


  Bacteria as vaccine vectors: Mycobacteria, Vibrio cholera, Listeria, Shigella, Salmonella

  Viruses as vaccine vectors: Adenoviruses, Poxviruses (e.g. MVA), VSV
• Salmonella as a vector


  Properties:

  • rod shaped gram -ve facultative bacterium
  • shown promis for mucosal immunization

  Pros:

  • mimic natural infection of most muscosal pathogens, infecting through mucosal surface
  • intracellular pathogens; capable of surviving & replicating within antigen presenting cells; facilitate continual processing & presentation of foreign antigen to immune system
  • inexpensive
  • Irreversibly attentuated; safe for human
  • If adverse rxn, antibiotics
  • delivered orally
  • hold large amounts of foreign DNA, more than 1 foreign antigen
• Adenoviruses as a vector


  Properties

  • double stranded DNA virus w/ small genome size; easy manipulation

  Pros:

  • infect wide range of mammalian cells
• Adenoviruses as a vector

  Rabies.

  • Attenuated virus developed and used to vaccinate foxes, BUT attenuation not stable. The vaccine was placed into bait.  Recombinant vaccines now based on poxvirus and adenovirus vectors.

  Influenza virus.

  • Live attenuated influenza vaccine (LAIV) is a type of influenza vaccine in the form of a nasal spray (FluMist Quadrivalent).

  Poliovirus

  • mutations of capsid protein gene, and non-coding regions that prevent the translation of viral RNAs in the CNS.
• Antigen delivery systems:non-living

  • Micro and nano-particles

  (polylactides & polylactides-co-glycolides)

  • Liposomes (phospholipid vesicles)
  • DNA Vaccines (“gene gun”)
  • Transgenic Plants (e.g. ‘Mucorice’)
• Mucosal Adjuvants


  • The identification of safe and effective mucosal adjuvants allied to innovative delivery strategies is key to advancing mucosal vaccines.
  • An adjuvant: is a substance that increases or modulates the immune response to a vaccine antigen.
  • Adjuvant = Delivery system and/or immune stimulating  agent.
• Licensed vaccine adjuvants for human use.

  
• Immune stimulating complexes (ISCOMs)


  1. Antigens: components from the pathogen that are intended to stimulate the immune response.
  2. Lipids: outer layer of the ISCOM structure; help stabilize the complex and interacte with immune cells.
  3. Quillaja Saponins: natural compounds derived from the bark of the Quillaja saponaria tree. Immunostimulatory properties and responsible for the "immunostimulating" part of ISCOMs. Activate various immune cells and enhance the overall immune response.
  4. Cholesterol: Cholesterol is often included in ISCOMs to provide structural stability to the complexes.
• ISCOMS as mucosal antigen delivery systems


  • Induction of potent antibody responses, including neutralising antibody.
  • Induction of potent T-cell responses (MHC class II restricted CD4+ and MHC class I restricted CD8 + CTL)
  • Induction of immunity upon local application - mucosal immunity and systemic immunity
  • Absence of toxicity
  • Accepted for animal vaccines
  • Acceptable for human vaccines. Suitable for large-scale vaccine production. Well-defined production technology
  • Very stable vaccines (lyophilization possible)
• Mucosal Vaccines


  • highly prevalent
  • elicit mucosal immune response in gut: inhaling, reproductive: swallowing
  • route of immunization

  1. Inhalation: respiratory
  2. Orally: gastrointestinal
  3. Injection: specific mucosal areas

  • highly desirable for pathogens entering via mucosal surfaces; systemic immunity covers whole body and needed for overall defence

  Benefits:

  1. protection at the entry points
  2. induce broad immune responses (IgA production), pivotal for mucosal immunity

  Cost justification:

  1. high efficacy in preventing diseases that enter through mucosal surfaces
  2. non-invasive administrative route
• Case study in delivering vaccines mucosally - HIV


  • Retrovirus with RNA genome.
  • spread via sexual intercourse & contaminated blood.
  • replicates in CD4 T cells, macrophages and dendritic cells. 
  • 90% of CD4+ T cells in gut die in acute infection. Bystander activation and death of T cells – probably caused by leakage of gut bacterial products.
  • Decline in CD4 T cells renders individuals susceptible to opportunistic infections.
  • Antiretrovirals are used to treat HIV, these drugs inhibit various stages of the virus replication cycle.
  • No vaccine available – numbers infected increase
• HIV Vaccine?


  • HIV is a rapidly mutating virus
  • 1011 virions are produced daily within infected individuals
  • Individuals that become infected through mucosal transmission are exposed to a viral quasi-species (swarm of genetically distinct virus).
  • The variation in a single HIV-infected individual 6 years after infection is broadly equivalent to the global variation in the HA gene of seasonal influenza A H3N2 in 1 year.
  • How can we develop a vaccine against a target that is so vast and diverse?

  
• HIV Vaccine (pt. 2)


  • During the early stages of HIV infection, a single viral variant (called the transmitted/founder virus) traverses the mucosal barrier and established infection (~80% of cases).
  • This means a genetic bottle neck occurs - presenting a “Window of Opportunity” for a vaccine, as it  would only need to neutralise the one T/F virus (not the whole quasi-species).

  
• Neutralising antibody at mucosal surfaces protects against HIV infection.

  
• Mucosal vaccination using a vaginal ring generates humoral immunity in sheep.