Intraepithelial T-cell cytotoxicity, induced bronchus-associated lymphoid tissue, and proliferation of pneumocytes in experimental mouse models of influenza.

Immunopathologic examination of the lungs of mice with experimental influenza virus infection reveals three prominent findings. (i) There is rapidly developing perivascular (arterial) and peribronchial infiltration with T-cells and invasion of T-cells into the bronchiolar epithelium, separation of epithelial cells from each other and from the basement membrane, leading to defoliation of the bronchial epithelium. This reaction is analogous to a viral exanthema of the skin, such as measles and smallpox. This previously described but unappreciated reaction most likely is an effective way to eliminate virus-infected cells, but may contribute to acute toxicity and mortality. (ii) After this, there is formation of dense collections of lymphocytes adjacent to bronchi consisting mainly of B-cells, with a scattering of T-cells and macrophages. This is known as induced bronchial-associated lymphoid tissue (iBALT) and correlates with increased interleukin (IL)-17 in the lung. iBALT provides sites for a local immune reaction in the lung to both the original infection and related viral infections (heterologous immunity). (iii) Within the first 2-3 weeks, there is proliferation of type II pneumocytes and/or terminal bronchial epithelial cells extending from the terminal bronchioles into the adjacent alveoli, eventually leading to large zones of the lung filled with tumor-like epithelial cells with squamous metaplasia. The proliferation correlates with IL-17 and IL-22 expression, and the extent of this reaction appears to be determined by the availability of T-regulatory cells.