Eosinophil responses during COVID-19 infections and coronavirus vaccination

Abstract

Eosinophils are circulating and tissue-resident leukocytes that have potent proinflammatory effects in a number of diseases. Recently, eosinophils have been shown to have various other functions, including immunoregulation and antiviral activity. Eosinophil levels vary dramatically in a number of clinical settings, especially following eosinophil-targeted therapy, which is now available to selectively deplete these cells. There are key coronavirus disease 2019 (COVID-19)-related questions concerning eosinophils whose answers affect recommended prevention and care. First, do patients with eosinophilia-associated diseases have an altered course of COVID-19? Second, do patients with eosinopenia (now intentionally induced by biological drugs) have unique COVID-19 susceptibility and/or disease course? This is a particularly relevant question because eosinopenia is associated with acute respiratory deterioration during infection with the severe acute respiratory syndrome coronavirus 2, the causative agent of COVID-19. Third, do eosinophils contribute to the lung pathology induced during COVID-19 and will they contribute to immunopotentiation potentially associated with emerging COVID-19 vaccines? Herein, we address these timely questions and project considerations during the emerging COVID-19 pandemic.

Keywords: COVID-19; Coronavirus; SARS; eosinophils; immunopathology; immunopotentiation; vaccines.

Fig 1

SARS-CoV immunity. The structural proteins of the SARS-CoV viral particle are shown and putative T_H1- vs T_H2-mediated immune responses detailed. The Spike (S) glycoprotein mediates binding of the virus to the angiotensin-converting enzyme-2 protein and subsequent fusion/entry into host cells. Sera from convalescing patients have revealed that anti–nucleocapsid protein and anti–S protein antibodies predominate the humoral immune response to SARS-CoV-1 but that only anti–S protein antibodies (especially those targeting the receptor-binding domain region) are neutralizing and protective. Beneficial antiviral responses appear to be linked to T_H1-skewed immunity, whereas T_H2 immunopotentiation in multiple animal model systems is associated with vaccination-enhanced disease, leading to pulmonary eosinophilia. To date, these potentially adverse consequences have been observed only in animal model systems following virus challenge with certain vaccine formulations (see Table I). Various SARS-CoV-2 vaccine candidates are currently under development (see box), which should be scrutinized for safety before widespread deployment. CTL, Cytotoxic T lymphocyte; ssRNA, single-stranded RNA.