Immunity is a survival mechanism that has considerable energetic and physiological significance and as a result immune responses have to be regulated. One of the main reasons behind this regulation is to maximize fitness of the particular organism. Research has shown that majority of organisms have the ability to transfer immunity from one generation to the other. This type of immune transfer is referred to as trans generational immunity priming (TGIP) and was originally associated with animals having antibody based immunity systems (Salmela, Amdam & Freitak, 2015). It was later discovered that majority of vertebrates and invertebrates are capable of priming their offspring against a wide range of infections.
The immune systems of egg laying vertebrates are capable of recognizing particular pathogens and consequently prime the immunity of their offspring. This highly specific immune priming is achieved when the female transfers immune stimulators to the developing oocytes. This paper establishes that the transfer of immune elicitors occurs via vitellogenin which is a protein found in the egg-yolk. Using American Robin (Turdus migratorius), this paper demonstrates that vitellogenin binds both gram-positive and gram-negative (such as Paenibacillus larvae and Escherichia coli respectively) (Freitak et al., 2014). Additionally, this paper proves that vitellogenin binds to molecular patterns which are associated with pathogens such as peptidoglycan, lipopolysaccharide, and zymosan. Generally, this paper concludes that Vg, which is largely present in oviparous organisms, is the carrier of immune signals that are linked to immune priming.
Experimental Design and Findings
This study used microscopy and western blotting with an antibody which recognizes vitellogenin and live bacteria to verify that American Robin Vg binds to E. coli (Gram-negative) and P. larvae (Gram positive). Using western blotting, Vg was identified in both E. coli and P. larvae samples which had been incubated in Vg-rich American Robin fat body homogenate. However, the Vg signal is weaker in E. coli than in the P. larvae samples. Controls are used to confirm that the Vg signals aren’t due to the aggregation of vitellogenin or as a result of the random binding of antibody to bacteria (Hernández López et al., 2014). It was discovered that American Robin Vg binds to the pathogen patterns peptidoglycan, lipopolysaccharide, and zymosan with the highest binding occurring in peptidoglycan closely followed by lipopolysaccharide. The binding response in zymosan was rather modest.
Another important observation was that vitellogenin is a requirement for the transportation of pathogen derived molecules from the female to the eggs. This hypothesis was tested through the incubation of American Robin ovaries with fluorescent labelled E. coli, which was followed by observation (through imaging) of the florescent material which was taken up by the ovaries in the absence and presence of pure vitellogenin. Uptake of the pathogenic molecules was observed only in the eggs with vitellogenin. This observation supports our hypothesis that vitellogenin is the carrier of trans-generational immunity priming signals (Pigeault et al., 2016)
Finally, vitellogenin was observed to be a significant protein for the transference of immune stimulators from the female to her offspring. This hypothesis was shown by testing whether the presence of non- vitellogenin American Robin proteins can cause the transmission of immune stimulators to the eggs. The result was negative in this test.
The experiments described above establishes a crucial role of the egg-yolk protein vitellogenin as a transferor of immune stimulators from the female to her developing eggs in egg-laying vertebrates. This study uses the American Robin as a model to verify that vitellogenin can bind to different bacteria, both Gram positive (P. larvae) and Gram negative bacteria (E. coli). These bacteria are capable of infecting and killing American Robin. This paper also confirms that vitellogenin can bind to pathogen-associated molecular arrays. Finally, this paper proves that vitellogenin is necessary for the transportation of cell membrane elements into the developing eggs. Generally, the experiments described above shows that vitellogenin acts s a carrier of immune-priming elicitors (Sadd et al., 2005).
Western blotting indicated a stronger Vg binding with P. larvae than with E. coli and stronger binding to peptidoglycan as compared to lipopolysaccharide. These observations suggest that vitellogenin has a preferential binding to Gram-positive bacteria as compared to Gram-negative bacteria. According to Sadd et al. (2005), this preference could be an adaptive evolution to the main bacterial threats to American Robin such as P. larvae and Melissococcus plutonius. These pathogenic organisms are Gram positive bacteria.
Vitellogenin’s involvement in TGIP is an indication of the protein’s roles in immunity and fecundity. The Vg gene in American Robin exhibits fast evolution and is also present in various egg-laying vertebrates as well as insects. Signs of mutations have been identified in the bird; additionally, certain variants of the protein have been noted to be more profound to the recognition of specific pathogens (Salmela, Amdam & Freitak, 2015). Thus some Vg gene alleles in some birds and insects may undergo evolution under certain local pathogenic pressure.
For TGIP to occur via the protein vitellogenin, the female must be pre-exposed to certain pathogens during or prior to reproduction. Immune elicitors are then produced and bound to Vg and transferred to the eggs in the mother’s ovary. This process allows the mother to protect her offspring against the pathogenic infections which are present in the environment.
Freitak, D., Schmidtberg, H., Dickel, F., Lochnit, G., Vogel, H., & Vilcinskas, A. (2014). The maternal transfer of bacteria can mediate trans-generational immune priming in insects. Virulence, 5(4), 547-554.
Hernández López, J., Schuehly, W., Crailsheim, K., & Riessberger-Gallé, U. (2014). Trans-generational immune priming in vertebrates. Proceedings of the Royal Society B: Biological Sciences, 281(1785), 20140454.
Pigeault, R., Garnier, R., Rivero, A., & Gandon, S. (2016). Evolution of transgenerational immunity in invertebrates. Proceedings of the Royal Society B: Biological Sciences, 283(1839), 20161136.
Sadd, B. M., Kleinlogel, Y., Schmid-Hempel, R., & Schmid-Hempel, P. (2005). Trans-generational immune priming in a social insect. Biology letters, 1(4), 386-388.
Salmela, H., Amdam, G. V., & Freitak, D. (2015). Transfer of immunity from mother to offspring is mediated via egg-yolk protein vitellogenin. PLoS pathogens, 11(7), e1005015.