Sputum characteristics of patients with severe COVID-19: report of two cases with immunocytochemical detection of SARS-CoV-2 spike protein

Patients with SARS-CoV-2 infection and with severe COVID-19 often have multiple coinfections, and their treatment is challenging. Here, we performed cytology analysis on sputum samples from two patients with severe COVID-19. The specimens were prepared using the rubbing method and stained with Papanicolaou stain. In both cases, several cells with frosted nuclei were observed, and the cytological findings per 100 cells were evaluated. The infected cells were mononuclear to multinuclear, showing chromatin aggregation at the nuclear margins, intranuclear inclusion bodies, eosinophilic cytoplasmic inclusion bodies, and mutual pressure exclusion of the nuclei. Immunocytochemical staining revealed that the cells were positive for AE1/AE3 and negative for CD68 expression, indicating their epithelial origin. Furthermore, infected cells with frosted nuclei were positive for surfactant protein A (SP-A) in Case 2, suggesting infection of type II alveolar pneumocytes or Clara cells. Moreover, in Case 2, the infected cells were positive for herpes simplex virus (HSV) I + II and SARS-CoV-2 spike protein, confirming double infection in these cells. In conclusion, sputum cytology is an important tool for determining the diversity of viral infection, and additional immunocytochemistry can be used for definitive diagnosis.