Ultrastructural study confirms the formation of single and heterotypic syncytial cells in bronchoalveolar fluids of COVID-19 patients.

BACKGROUND: SARS-CoV-2 was reported to induce cell fusions to form multinuclear syncytia that might facilitate viral replication, dissemination, immune evasion, and inflammatory responses. In this study, we have reported the types of cells involved in syncytia formation at different stages of COVID-19 disease through electron microscopy. METHODS: Bronchoalveolar fluids from the mild (n = 8, SpO2 > 95%, no hypoxia, within 2-8 days of infection), moderate (n = 8, SpO2 90% to ≤ 93% on room air, respiratory rate ≥ 24/min, breathlessness, within 9-16 days of infection), and severe (n = 8, SpO2 < 90%, respiratory rate > 30/min, external oxygen support, after 17th days of infection) COVID-19 patients were examined by PAP (cell type identification), immunofluorescence (for the level of viral infection), scanning (SEM), and transmission (TEM) electron microscopy to identify the syncytia. RESULTS: Immunofluorescence studies (S protein-specific antibodies) from each syncytium indicate a very high infection level. We could not find any syncytial cells in mildly infected patients. However, identical (neutrophils or type 2 pneumocytes) and heterotypic (neutrophils-monocytes) plasma membrane initial fusion (indicating initiation of fusion) was observed under TEM in moderately infected patients. Fully matured large-size (20-100 µm) syncytial cells were found in severe acute respiratory distress syndrome (ARDS-like) patients of neutrophils, monocytes, and macrophage origin under SEM. CONCLUSIONS: This ultrastructural study on the syncytial cells from COVID-19 patients sheds light on the disease's stages and types of cells involved in the syncytia formations. Syncytia formation was first induced in type II pneumocytes by homotypic fusion and later with haematopoetic cells (monocyte and neutrophils) by heterotypic fusion in the moderate stage (9-16 days) of the disease. Matured syncytia were reported in the late phase of the disease and formed large giant cells of 20 to 100 µm.

Modulation of Host Immune Response Is an Alternative Strategy to Combat SARS-CoV-2 Pathogenesis.

The novel SARS-CoV-2virus that caused the disease COVID-19 is currently a pandemic worldwide. The virus requires an alveolar type-2 pneumocyte in the host to initiate its life cycle. The viral S1 spike protein helps in the attachment of the virus on toACE-2 receptors present on type-2 pneumocytes, and the S2 spike protein helps in the fusion of the viral membrane with the host membrane. Fusion of the SARS-CoV-2virus and host membrane is followed by entry of viral RNA into the host cells which is directly translated into the replicase-transcriptase complex (RTC) following viral RNA and structural protein syntheses. As the virus replicates within type-2 pneumocytes, the host immune system is activated and alveolar macrophages start secreting cytokines and chemokines, acting as an inflammatory mediator, and chemotactic neutrophils, monocytes, natural NK cells, and CD8+ T cells initiate the local phagocytosis of infected cells. It is not the virus that kills COVID-19 patients; instead, the aberrant host immune response kills them. Modifying the response from the host immune system could reduce the high mortality due to SARS-CoV-2 infection. The present study examines the viral life cycle intype-2 pneumocytes and resultant host immune response along with possible therapeutic targets.