copyright StockVault

copyright StockVault

One of the greater uncertanties about Avian Influenza is why it is not efficiently transmitted among humans. Thus, it is clear how likely it is that the virus is able to cross this barrier and a more efficient line appears. For example, until now the H5N1 was transmitted mainly to breeders and people in very close and extended contact with poultry.

The entrance door of Influenza in mammals is the respiratory tract, primarily ciliated cells of the respiratory epithelium, where inhaled saliva droplets come in. One of the reasons already approached for the inefficient transmission of avian viruses in humans is the difference of type of sialic acid.

In our respiratory tract, the acid α2,6 prevails in the upper part, region of nasal cavities and pharynx, while the α2,3 occurs mainly in the lower respiratory tract, alveoli, bronchi and other pulmonary cells. Human viruses have hemagglutinin (HA) with more affinity with sugars which terminate in α2,6-sialic acid and thus they can be replicated in the upper tract and better dispersed through cough and sneezes. In turn, the avian viruses have more affinity with sugars terminated in α2,3, prevailing in the digestive system of birds.

Another reason is the metabolic difference. Humans have an average body temperature of 37°C and birds between 40°C and 41°C. The temperature has a great influence in the protein structure and the virus needs several changes before getting bound to mammal receptors below 40°C. This difference is more stressed by the infection spots. While in birds the influenza virus infects the digestive system, which is at the same temperature of the body, 40-41°C, in mammals the respiratory system is cooled by the inhaled air and in the nasal region it reaches temperatures until 32°C, producing a gradient which ends in 37°C in the lungs. A recent work tested a way of adaptation.

Growing human epithelial cells, a group of American and British researchers was able to test several conditions of infection by avian and human viruses in temperatures of 32 °C and 37 °C, simulating both upper and lower respiratory tract. It was already demonstrated in avian viruses that a mutation in the amino acid 627 in the PB2 subunit of the virus polymerase (the enzyme responsible for coping its genome) has an important role in the sensitivity to temperature. The same mutation was able to make the H5N1 more infective in mice. For this reason, it was induced in human viruses in experiments in order to “avianize” the lines and test its influence in infectivity.

While the model of human virus – A/Victoria/3/75 (H3N2) – replicated both at 32°C and 37°C, the avian model – A/Dk/Eng/62 (H4N6) – replicated between one thousand and ten thousand folds less at 32°C than 37°C, where it produced almost the same number of particles than the human virus. In other words, although the avian virus grows well at a temperature equivalent to that of the lungs, the same does not occurs in the upper airway so the virus cannot be transmitted efficiently.

Given the amazing result, new human lines – A/Eng/26/99 (H3N2) and A/Udorn/307/72 (H3N2) – and avian – A/Dk/Sing/97 (H5N3) – were tested and the differences were confirmed. Human lines grew well in both temperatures and avian lines grew less in lower temperatures. Even the avian H5N1 isolated from a fatal human case (A/VN/1203/04) was tested and behaved in the same way.

When the human viruses were “avianezed” through mutation of the amino acid 627 of humans (Lysine) to the most common amino acid in birds (Glutamic Acid), in order to confirm if the mutation in PB2 would be responsible for the better growth at 32 °C, the difference was not confirmed. It was necessary to add to human viruses more characteristics of avian viruses, Hemagglutinin (HA) and Neuraminidase (NA), inducing the virus to use more the α2,3-sialic acid in order to reduce the growth in lower temperatures.

That indicates that the amino acid 627 can be an important region in the adaptation of avian viruses for humans and it should be followed during the emergence of pandemic viruses such as H5N1. However, other changes also have an important role, and a rearrangement with human viruses which could transmit HA and NA able to bind more easily to the upper ciliated epithelium could cause a big trouble.

Scull MA, Gillim-Ross L, Santos C, Roberts KL, Bordonali E, Subbarao K, Barclay WS, & Pickles RJ (2009). Avian Influenza virus glycoproteins restrict virus replication and spread through human airway epithelium at temperatures of the proximal airways. PLoS pathogens, 5 (5) PMID: 19436701