Appearances Can Be Deceiving - Viral-like Inclusions in COVID-19 Negative Renal Biopsies by Electron Microscopy

"Since the discovery of the causative agent for the novel severe acute respiratory syndrome (SARS)–like pneumonia syndrome pandemic that started in China in 2019 (1,2), a coronavirus named SARS coronavirus 2 (SARS-CoV-2), electron microscopy images have populated the medical literature (2) and media outlets alike displaying the characteristic 60–140 nm round particles surrounded by a “corona” of 9–12 nm distinctive spikes (2). Although many of these images were obtained after “in vitro” infection of cultured cells with SARS-CoV-2 (2) and are thus likely a true representation of viral particles, we have observed morphologically indistinguishable inclusions within podocytes and tubular epithelial cells both in patients negative for coronavirus disease 2019 (COVID-19) as well as in renal biopsies from the pre–COVID-19 era. Although direct infection of the kidney is theoretically possible, given the presence of angiotensin-converting enzyme 2 (the receptor used by SARS-CoV-2 to gain access to cells) within proximal tubular epithelium (3) and podocytes (4), the virus has not been detected by real-time RT-PCR in urine samples from patients with COVID-19 (5⇓–7). Additionally, for the virus to have access to kidney parenchyma, viremia should occur, and this has only been detected in a minority of patients (6⇓–8).

We would, therefore, like to issue a note of caution for inferring viral tissue infection by morphology alone using electron microscopy images from tissues obtained from biopsies or autopsy material in patients with COVID-19. Moreover, caution should be used when interpreting immunohistochemical results, especially within proximal tubules, which are prone to nonspecific staining by a variety of antibodies due to their intense reabsorptive capacity. Additionally, more specific techniques such as immunoelectron microscopy using specific viral antigens (9), or in situhybridization for viral RNA, are likely necessary to undoubtedly confirm tissue infection in these cases.

Indeed, in two recent reports of collapsing glomerulopathy in patients with COVID-19, viral RNA was not detected in the kidney by in situ hybridization (10,11). Additionally, immunohistochemical analysis using a SARS-CoV-2 nucleoprotein antibody previously shown to have positive staining in the kidney of patients with COVID-19 showed nonspecific positive staining in the renal parenchyma of all kidneys in our laboratory (10).

We postulated that endogenous mimickers could be present that are morphologically indistinguishable from SARS-CoV-2 virions ultrastructurally."

"Recognition of this pitfall of “viral-like particles” actually dates back to the 1970s, when the potential for mistakenly assuming that normal cellular components, such as phagocytic vacuoles, microvesicular bodies, or extracellular breakdown products, could represent viral particles was emphasized after a proliferation of studies claiming to have found ultrastructural viral particles within different types of cancer cells and fluids (23). Thus, we would like to echo the CDC (9) and earlier authors’ observations (15,23) and issue a note of caution regarding the use of ultrastructural images as evidence of SARS-CoV-2 tissue infection without confirmatory evidence of viral proteins or RNA in the tissue through immunoelectron microscopy or in situ hybridization."

Source:https://kidney360.asnjournals.org/content/1/8/824

https://medicine.uams.edu/wp-content/uploads/2021/05/COVID-19-negative-renal-biopsies-by-EM.pdf