Avian Immunity and Maternal Transfer

Innate Immunity

• Rapid response
• Recognition of traits shared by broad ranges of pathogens, using a small number of receptors
• Barrier defences (skin, mucous membranes, secretions)
• Internal defences (phagocytic cells, antimicrobial cells, inflammatory response NK cells)

Acquired Immunity

• Slower response
• Recognition of traits specific to particular pathogens, using a range of different receptors
• Humoral response: antibodies defend against infection in body fluids
• Cell mediated response: cytotoxic lymphocytes defend against infection in body cells

• Offers a protective layer and acts as an immunological barrier
• 2 layers - one develops from the foetus and the other from the uterus
• Baby is connected to the placenta by the umbilical cord - site of nutrient transfer and gas exchange
• Mother's blood doesn't pass directly into the baby - arteries pass over the placenta and bathe the network of capillaries (this is where the processes mentioned above occur)
• Secretes Neurokinin B and Phosphocholine 
• Express specific set of MHC classes:
  • Does express HLA-E and HLA-G (unusual isotopes)
  • Doesn't express HLA-A and HLA-B - these would not match the mother's and therefore the baby would be attacked by cytotoxic T cells
• In some species maternal IgG can cross over - true for humans and mice but not for cows or horses

• Foetus was not present when mother's immune system was 'educated' to distinguish it's own tissues from others:
  • Foetus may be considered as an allograft (tissue/organ transplant)
  • Semi-allograft; half DNA from father
  • Some spontaneous abortions (miscarriages in humans) resemble transplant rejections
• The mother's immune system is down-regulated as a whole throughout pregnancy
• The foetus doesn't attack the mother as the placenta is an immunological barrier
• The foetus/placenta modulates local immune components to facilitate tolerance

Process

1. Mother ingests pathogen

2. Antigen handling by the digestive system makes the pathogen safe

3. Antigen passes into milk which is passed on to the offspring

4. Offspring has oral tolerance to antigen

• Contains free antigens, antigens complexed to IgA and IgG, tolerogenic immune mediators (eg. Vitamin A), microbiota modulating factors (eg. prebiotics, antimicrobials) and gut growth factors
• Colostrum (breast milk) provides IgA:
  • Infant gains beneficial Ig's from infected mothers
  • Gains Ig's from vaccinated mothers
• Passively acquired maternal Ig's have various effects:
  • Coats pathogens, reduces infectivity and tags for destruction
  • Antigen presentation to T cells is improved
  • Maternal Ig's are trapped by dendritic cells
  • Help to prime B cells
• However, B cell epitopes can be masked by maternal Ig - this can interefere with B cell priming

Some mammals, such as puppies, need to be vaccinated twice. This is to ensure they always have antibodies and aren't left exposed (their acquired immunity only lasts between 8-12 weeks.

• Neutrophils
• Pattern Recognition Molecules
• Antibodies - IgG (sometimes referred to as IgY - same thing though), IgM and IgA
• T cells
• NK cells
• Cytokines
• Thymus/MHC/lymphoid tissues
• Have a reduced amount of bone marrow due to the replacement of air sacs within bones

• Unique to birds
• B cells are named after this as they were first found in birds (however they are now known to be in all mammals)
• Epithelial/lymphoid organ
• Active in young birds
• Humoral equivalent to thymus
• B cell education education and maturation:
  • Gene rearrangement 
  • Gene conversion - increased diversity

• Immature T cells develop CD8 and CD4 (DP)
• DP cells seek interaction with self MHC
• Selected for survival or death
• Become single positive (CD4 or CD8)
• Populate organs before hatching
• Self-tolerant, diverse T cell population
• Clonal expansion of T cells follows exposure to antigen

• HALT
• B cells populate the blood, spleen, bone marrow, thymus and Hardrian gland (another name for HALT)
• Primary site for antibody production
• Still have GALT, but HALT is specific to birds
• Found behind eye

Immunity Through the Egg

• Short duration - 1-2 weeks and generally less than 4 weeks
• Function is to protect young chicks during their first few weeks when their immune system is not fully developed to properly react to an early challenge
• The transfer of antibodies to the embryo occurs in 2 steps:
  • First the antibodies are deposited in the egg yolk and the albumin
  • It is then transferred to the embryo

Immunity in the Yolk

• IgG is secreted by the avian ovarian into the developing ova (egg yolk)
• Regulated by the follicular. This epithelium becomes flatter and thinner in a larger ovum allowing the passage of a large amount of IgY
• Max IgY exchange is 3-4 days prior to ovulation and decreases to the development of the viteline membrane between the ovum and the follicular epithelium of the ovary preparation for ovulation
• As a single bird has several ova in different stages of development the amount of IgY transferred to each one is not the same
• The membrane forms in the isthmus and the calcium egg shell is formed in the uterus

Immunity from the Yolk to the Foetus

• IgY is transferred from the egg yolk to the offspring via the embryonic circulation
• The transfer starts from day 7 of embryonic development and reaches its maximum rate 3-4 days before hatching
• The amount of IgY transferred to the egg yolk and from the egg yolk to the embryohas been reported to be proportional to maternal serum IgY concentrations

Immunity in the Albumin

• The IgA and IgM are mainly found in the albumin
• They are transferred to the albumin as a result of mucosal secretion in the oviduct, more specifically the magnum

Immunity from the Albumin to the Foetus

• IgA and IgM are transferred to the embryo by absorption of the albumin directly through the embryonic gut
• Major function in the newly hatched chick - protective and as an additional source of protein
• IgM is the first Ig isotope to be synthesised by the newly hatched chick, followed by IgA and IgY

• Birds don't have mammary glands
• In some species both males and females can produce (eg. pigeons and flamingos)
• In some species only males produce (some penguins)
• High in protein and fat - more than mammalian milk
• Contains IgA and in some species leukocytes
• Produced in crop (muscular) - regurgitate it and feed it to their offspring

• Proteins on surface cells:
  • MHC class I - all cells
  • MHC class II - antigen presenting cells
• Enables recognition of self and non-self:
  • MHC class I - intracellular threats
  • MHC class II - extracellular threats