Figure 2.

The immunohistochemical appearance of HNE in the lungs of the deceased COVID-19 patient. Eosinophilic (HE) liquid of edema in the alveoli (asterisks) shows abundant HNE content (HNE-His giving brown-colored immunopositive reaction of DAB staining). However, inflammatory mononuclear cells in the alveoli and in their septa (black arrows) are mostly negative for HNE (only blue-colored hemotoxylin contrast staining is present), with some macrophages are positive for HNE (yellow arrow). On the contrary, blood vessels (both their walls and the blood content, indicated by the black stars) are HNE-positive, as are the hyaline membranes on the surface of the alveolar septa (red arrows). Conclusion—HNE might be responsible for the fatal outcome, but more likely due to the systemic, vascular oxidative stress (sepsis-like, originating from the blood), not due to the pulmonary inflammatory cells, which are mostly negative for HNE.