A lot can happen in 310 million years. That’s the length of time, the phylogenetic distance, which separates humans and chickens from our most recent common ancestor [1].

In that time our immune systems, and proteomes in general, have diverged considerably and it is precisely because of this that chicken antibodies—IgY specifically—are so useful; a fact increasingly recognised and utilized by the research community.

The reason for this is that the phylogenetic distance magnifies the subtle differences in homologous protein sequences more than is seen between different mammals. For example, there is only a single amino acid different between human and porcine insulin, but seven different amino acids between human and chicken insulin.

When a chicken is immunised with a mammalian antigen these differences result in a broader range of epitopes being recognised than would be by an immunised mammal; the antigen is more ‘foreign’. The result is increased immunogenicity and specificity of response.

This was exploited in a recent proof of concept study published in the Journal of Proteome Research [2]. In the search for disease biomarkers, of prognostic and diagnostic value, the large number of high and medium abundance proteins in human plasma can ‘mask’ potential biomarkers in low abundance proteins. This usually requires time consuming, costly sample preparation and analysis methods. Due to the high sequence homology between mammals immune-depletion has typically been seen to be of limited use.

To overcome this limitation, chickens were immunised with fractions of human plasma from an ion exchange column. The resulting ‘anti-human plasma’ polyclonal IgY was used to prepare an affinity-purified antibody column. Using this column they ‘ultra-depleted’ human plasma samples, allowing the team to identify 39 low abundance proteins not seen using other techniques; they could see ‘see far more deeply’ into the human plasma proteome.

But it isn’t just increased immunogenicity to conserved mammalian antigens that makes IgY uniquely useful. The structural differences of chicken IgY also offer advantages over traditional antibody production models. IgY still exhibits the same Y-shape as most mammalian antibodies, but with a larger heavy chain region, but it lacks an ‘Fc domain’.

As such they are not able to activate human compliment factors and there is no interaction with mammalian, or bacterial, Fc receptors or endogenous anti-Fc antibodies [3]; the most commonly known being rheumatoid factor (RF). Depending on the intended use of an assay such as ELISA or flow cytometry based assays—particularly for assays in a clinical setting—RF can confound results. IgY can allow this limitation to be overcome [4].

The unique characteristics of IgY, while decreasing background signals, can also increase the potential sensitivity and specificity of many immunological assay types. A recent study published in the Journal of Clinical Chemistry [5] compared an ELISA utilizing IgY to an assay using a rabbit polyclonal antibody to measure KLK- 6—implicated in diseases such as cancer and multiple sclerosis.

They found that the sensitivity of the assay could be improved by an order of magnitude over the more commonly used rabbit polyclonal, down to 30 fmol. In addition to this they found that, in contrast to the rabbit antibody, the IgY showed no cross reactivity to other cellular proteins or recombinant KLK’s; even though they were polyclonal, they were still more specific.

Though this post doesn’t cover all of the unique features that make chicken a desirable host option for antibody production, it can be said that IgY, with its unique potential for high specificity against mammalian proteins and resistance to the limitations of more traditional antibody host species, while not new, is increasingly gaining traction in the research community, and with good reason.

References

 

  1. International Chicken Genome Sequencing Consortium. Sequence and comparative analysis of the chicken genome provide unique perspectives on vertebrate evolution. Nature. 2004 Dec 9;432(7018):695-716. Erratum in: Nature. 2005 Feb 17;433(7027):777. PubMed PMID: 15592404.
  2. Tan SH, Mohamedali A, Kapur A, Baker MS. Ultradepletion of Human Plasma using Chicken Antibodies: A Proof of Concept study. J Proteome Res. 2012 Oct 22. [Epub ahead of print] PubMed PMID: 23082986.
  3. Larsson A, Sjöquist J. Chicken IgY: utilizing the evolutionary difference. Comp Immunol Microbiol Infect Dis. 1990;13(4):199-201. PubMed PMID: 2076606.
  4. Larsson A, Karlsson-Parra A, Sjöquist J. Use of chicken antibodies in enzyme immunoassays to avoid interference by rheumatoid factors. Clin Chem. 1991 Mar;37(3):411-4. PubMed PMID: 2004449.
  5. Sotiropoulou G, Pampalakis G, Prosnikli E, Evangelatos GP, Livaniou E. Development and immunochemical evaluation of a novel chicken IgY antibody specific for KLK6. Chem Cent J. 2012 Dec 5;6(1):148. PubMed PMID: 23216878.