Temperature-dependent growth of different serotypes of influenza viruses in HAE.

<p>Multi-step growth kinetics of (A) human influenza virus A/Eng/26/99 or (C) avian influenza virus A/Dk/Sing/97 (MOI∼0.1) at 32°C (<i>open circles</i>, <i>dashed line</i>) or 37°C (<i>closed circles</i>, <i>solid line</i>) in HAE +/−SE (n = 3 cultures). Multi-step growth kinetics in HAE inoculated with an MOI∼0.03 of (B) A/Udorn/307/72 (H3N2) or (D) A/VN/1203/04 (H5N1) at 33°C (<i>open circles</i>, <i>dashed line</i>) or 37°C (<i>closed circles</i>, <i>solid line</i>). Data represents mean titer across two different donors, each performed in duplicate +/−SE. Viral titers were determined by plaque assay in (A) and (B) and by TCID₅₀ assay for (C) and (D). No significant differences in growth between temperatures were found for either A/Eng/26/99 or A/Udorn/307/72. A/VN/1203/04 was significantly restricted for growth at 24, 48 and 72 hrs pi (*p<0.05). (E) Representative histological cross-sections of HAE infected for 72 hrs at 37°C with A/Udorn/307/72 or A/VN/1203/04 and compared to mock-inoculated HAE. H&E counterstain. Scale bar equals 20 µm.</p

Infection of HAE by avian, but not human, influenza viruses is restricted at temperatures of the proximal airways.

<p>(A) Comparison of multi-cycle virus growth in HAE inoculated with either A/Victoria/3/75 at 32°C (<i>closed triangles</i>) or 37°C (<i>open triangles</i>) and A/Dk/Eng/62 at 32°C (<i>closed circles</i>) or 37°C (<i>open circles</i>) both at MOI∼0.01. Apical viral titers at times shown were determined by standard plaque assay on MDCK cells. Data shown represents the mean titer +/−standard error (SE; n = 3–10 cultures). (B) Adenylate kinase activity released into the apical compartment of HAE over time after inoculation with A/Victoria/3/75 or A/Dk/Eng/62 at 32°C and 37°C as a measure of viral-induced CPE. Data shown represents the mean fold change over adenylate kinase activity derived from mock-inoculated HAE +/−SE (n = 3–8). Significance is noted (*p<0.05) where viral titers or AK levels obtained for A/Dk/Eng/62 at 32°C were statistically different from all other titers/AK measurements (Dk/37°C, Vic/32°C and Vic/37°C) at that particular time point. Significance is noted (^†p<0.05) where AK levels obtained for A/Dk/Eng/62 at 32°C and 37°C were statistically different.</p

Temperature restriction of avian influenza viruses at 32°C can be mimicked by inserting avian envelope glycoproteins into human influenza viruses.

<p>(A) Multi-step growth kinetics initiated in HAE over time with PR8+Vic HA/NA at 32°C (<i>closed triangles</i>) or 37°C (<i>open triangles</i>) and PR8+Chick HA/NA at 32°C (<i>closed circles</i>) or 37°C (<i>open circle</i>s) in HAE. Apical viral titers were determined at the times shown by standard plaque assay. Data shown represents mean titer across 4–8 cultures +/−SE. (B) Adenylate kinase activity in apical washes of virus-infected HAE expressed as fold-change over adenylate kinase activity in mock-inoculated HAE +/−SE (n = 4–8). Significance is noted (*p<0.05) where viral titers or AK levels obtained for PR8+Chick HA/NA at 32°C were statistically different from all other titers/AK measurements (Chick/37°C, Vic/32°C and Vic/37°C) at that particular time point. Significance is noted (^†p<0.05) where AK levels obtained for PR8+Chick HA/NA at 32°C and 37°C were statistically different. (C,D) Representative <i>en face</i> photomicrographs of viral nucleoprotein immunoreactivity (<i>green</i>) in HAE inoculated with (C) PR8+Vic HA/NA or (D) PR8+Chick HA/NA, at 24, 48 and 72 hrs pi at 32°C (lower rows) or 37°C (upper rows).</p

Cell tropism of human, avian and avianized viruses in HAE.

<p>Representative cross-sections of inoculated HAE, fixed 24 hrs pi, were probed for viral antigen (NP; green) and α−acetylated tubulin, a marker for ciliated cells (red). Notably, the staining pattern for wild-type A/Victoria/3/75 was identical to that of PR8+Vic HA/NA. Arrows mark ciliated cells infected with either wild-type A/Victoria/3/75 or PR8+Vic HA/NA; arrow-head denotes non-ciliated cells infected by these viruses. These data indicate that viruses with Victoria glycoproteins were able to infect both cell types previously shown to express α2,6 SA <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000424#ppat.1000424-Thompson1" target="_blank">[13]</a>. Viral antigen was detected only in ciliated cells in cultures inoculated with Vic-226-228HA (in the Victoria background with either endogenous N2 or avian N1 or PR8+Chick HA/NA). Scale bar equals 20 µm.</p

Spread and histopathology of avian and human influenza viruses in HAE at temperatures of the proximal and distal airway.

<p>(A) Representative <i>en face</i> photomicrographs of HAE inoculated with either A/Victoria/3/75 or A/Dk/Eng/62 at 32°C or 37°C, fixed at 6, 24, 48 and 72 hrs pi and stained for viral nucleoprotein (<i>green</i>) to determine numbers of cells infected. Scale bar equals 100 µm. (B) Representative histological cross-sections of HAE at 24, 72 and 120 hrs after inoculation with A/Victoria/3/75 or A/Dk/Eng/62 at 32°C or 37°C. H&E counterstain. Scale bar equals 20 µm.</p