Early immune responses and development of pathogenesis of avian infectious bronchitis viruses with different virulence profiles

<div><p>Avian infectious bronchitis virus (IBV) primarily replicates in epithelial cells of the upper respiratory tract of chickens, inducing both morphological and immune modulatory changes. However, the association between the local immune responses induced by IBV and the mechanisms of pathogenesis has not yet been completely elucidated. This study compared the expression profile of genes related to immune responses in tracheal samples after challenge with two Brazilian field isolates (A and B) of IBV from the same genotype, associating these responses with viral replication and with pathological changes in trachea and kidney. We detected a suppressive effect on the early activation of TLR7 pathway, followed by lower expression levels of inflammatory related genes induced by challenge with the IBV B isolate when compared to the challenge with to the IBV A isolate. Cell-mediated immune (CMI) related genes presented also lower levels of expression in tracheal samples from birds challenged with B isolate at 1dpi. Increased viral load and a higher percentage of birds with relevant lesions were observed in both tracheal and renal samples from chickens exposed to challenge with IBV B isolate. This differential pattern of early immune responses developed after challenge with IBV B isolate, related to the downregulation of TLR7, leading to insufficient pro-inflammatory response and lower CMI responses, seem to have an association with a most severe renal lesion and an enhanced capability of replication of this isolate in chicken.</p></div

Log10 of IBV RNA copies in tracheas and kidneys from experimentally infected chickens or mock infected.

<p>(A) Log10 of IBV RNA copies in tracheas from chickens challenged with IBV A isolate and IBV B isolate, collected at 1dpi, 5dpi and 8dpi. (B) Log10 of IBV RNA copies in kidneys from chickens challenged with IBV A isolate and IBV B isolate, collected at 5dpi and 8dpi. Medians followed by different letters differ significantly (p≤0.05).</p

Relative expression of cell-mediated immune response related genes in tracheal samples from chickens experimentally infected with IBV or mock infected.

<p>Relative expression of cell-mediated immune response related genes measured by RT-qPCR: CD3 (A), CD4 (B), CD8β (C), IFNγ (D) and Granzyme homolog A (E), in tracheal samples collected 1 day-post infection (dpi), 5dpi and 8dpi from chickens experimentally challenged at 28 days of age with Brazilian field isolates A or B of infectious bronchitis virus, or mock infected (NC). Medians followed by different letters differ significantly (p≤0.05).</p

Percentage of chickens experimentally infected with IBV or mock infected, with relevant tracheal and renal microscopic lesions, at 1dpi, 5dpi and 8dpi.

<p>Percentage of chickens experimentally infected with IBV or mock infected, with relevant tracheal and renal microscopic lesions, at 1dpi, 5dpi and 8dpi.</p

Percentage of positivity for the presence of IBV genome in tracheal and renal samples from chickens experimentally infected with IBV A isolate, or IBV B isolate, or mock infected, at 1dpi (trachea), 5dpi and 8dpi (trachea and kidneys).

<p>Percentage of positivity for the presence of IBV genome in tracheal and renal samples from chickens experimentally infected with IBV A isolate, or IBV B isolate, or mock infected, at 1dpi (trachea), 5dpi and 8dpi (trachea and kidneys).</p

Micrographs showing kidney histopathology from chickens experimentally infected with IBV or mock infected.

<p>(A) Distal fragment of right kidney from the group challenged with IBV B strain, at 8dpi, presenting (arrow) marked lymphoplasmacytic intersticial nephritis (score 3). (B) Distal fragment of right kidney from the negative control group.</p

Micrographs showing tracheal histopathology from chickens challenged with IBV or mock infected.

<p>(A) Proximal third of trachea from the group challenged with IBV A strain, at 1dpi, showing acute tracheitis with epithelial cells losses [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172275#pone.0172275.ref001" target="_blank">1</a>], congestion [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172275#pone.0172275.ref002" target="_blank">2</a>], heterophilic cell infiltrate [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172275#pone.0172275.ref003" target="_blank">3</a>]. (B) Proximal third of trachea from the group challenged with IBV A strain, at 5dpi, showing ciliary loss [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172275#pone.0172275.ref001" target="_blank">1</a>], degeneration and necrosis of some epithelial cells [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172275#pone.0172275.ref002" target="_blank">2</a>], lymphoid cell infiltrate [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172275#pone.0172275.ref003" target="_blank">3</a>] and epithelial hyperplasia [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172275#pone.0172275.ref004" target="_blank">4</a>]. (C) Proximal third of trachea from NC group.</p