Using EM data to understand COVID-19 pathophysiology

The pathophysiology of multisystem inflammatory syndrome in children is not completely understood, but it is a field in COVID-19 under extensive investigation. Evidence of the effects of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on extrapulmonary tissues is essential for understanding the disease's course and treatment.

We read with concern Carsten Dittmayer and colleagues' Correspondence,¹ in which they which questioned the evidence of a viral particle shown in our recent Case Report² of a child with COVID-19-related multisystem inflammatory syndrome.

The ultrastructural evidence of SARS-CoV-2 in cardiac tissue was undisputed (a cardiomyocyte was shown in figure 3A, an endothelial cell was shown in figure 3C, and a neutrophil in figure 3D).² This finding was further corroborated by the detection of SARS-CoV-2 RNA by RT-PCR, and by immunohistochemistry.

We share Dittmayer and colleagues'¹ opinion that electron microscopy (EM) is the gold standard to prove the presence of an infectious unit and requires specialised staff. For this reason, the EM in our report was done by a professor with more than 30 years of experience in ultrastructural analysis.3, 4 Figure 3B in our case report² does show a rough endoplasmic reticulum, as suggested by Dittmayer and colleagues, but it also shows a particular aspect of viral particle assembly (a section through a spherical cluster of viral nucleocapsids apposed on the membrane of the rough endoplasmic reticulum), which is probably a viral translation centre.⁵

The presence of SARS-CoV-2 particles within membrane compartments, as shown by Dittmayer and colleagues,¹ is typical of preserved non-necrotic cells in which there is viral replication. In our Case Report, cells were undergoing necrotic degeneration (corroborated by C4d staining on figure 2D),² which led to cardiac failure and death. In this situation, viral particles might not appear in clusters within membranes but free in the cytosol, intermingled with organelle membranes undergoing fragmentation—much harder to recognise.

Although the criticism of one of the figures in our report does not affect the main message—that SARS-CoV-2 infection of cardiac tissue was probably a major contributor to the child's myocarditis and heart failure²—in our opinion, criticism of peer-reviewed published data should be more careful and preferentially addressed directly to the authors to avoid the spread of misleading information, clouding the scientific literature.