Fig. 6: Detailed histopathological analysis of the lung.

a–c Representative hematoxylin and eosin (HE)-stained lung sections to illustrate the scoring system of pathology parameters. Scores of the pathology parameters that go along with the images (a.1-c3) are indicated in the table. All individual scores are presented in supplementary Fig. 1. Bronchioli and blood vessels are marked in blue and green, respectively. Enlargements: ×10 objective, bar represents 100 µm. a images a.1–3 are representative sections for the placebo groups and illustrate necrotizing denudation of bronchioli with luminal plugs of inflammatory exudate (a.1,*), accompanied by a large peribronchiolar rim of lymphocytes and some polymorphonuclear (PMN) cells (a.1,+). Inflammation is spread into the interstitium (a.1, ↖) and alveolar space (a.3, ↖). Hyperemia is strongly present and alveolar space is filled with edema, fibrin, and debris and PMN are regularly present (a.2 and A.3, ○). Inflamed small arteries with perivascular inflammation are also observed (a.2, +). b images b.1–3 are typical for a few placebo- and split-vaccinated animals that were either less or more affected than the general trend, respectively. Bronchioli with moderate peribronchial inflammation (b1, +) and hyperplasia of epithelium (b.1, ↖). Alveolar septa with mild or slight (b2, ↖) inflammation and hyperemia and alveoli filled with erythrocytes (b3, ○). c images c.1–3 are representative for the split-vaccinated ferrets. Observable are a bronchiolus with minimal peribronchiolar inflammation and hyperplasia of epithelium (c.1, +), alveolar septa with minimal inflammation and hyperemia (c.1, c2 ↖) and minimal inflammation of the alveoli (c.2). The small vene minimally inflamed (c.2, +). c3 shows a normal histology of the lung parenchyma. Presence of normal bronchiolar epithelia and a blood vessel, regular alveoli with thin septa and absence of enhanced cellularity.