RNA Sensors as a Mechanism of Innate Immune Evasion among SARSCoV2, HIV and Nipah Viruses

A novel hypothesis for COVID-19 pathogenesis: Retinol depletion and retinoid signaling disorder

The SARS-CoV-2 virus has caused a worldwide COVID-19 pandemic. In less than a year and a half, more than 200 million people have been infected and more than four million have died. Despite some improvement in the treatment strategies, no definitive treatment protocol has been developed. The pathogenesis of the disease has not been clearly elucidated yet. A clear understanding of its pathogenesis will help develop effective vaccines and drugs. The immunopathogenesis of COVID-19 is characteristic with acute respiratory distress syndrome and multiorgan involvement with impaired Type I interferon response and hyperinflammation. The destructive systemic effects of COVID-19 cannot be explained simply by the viral tropism through the ACE2 and TMPRSS2 receptors. In addition, the recently identified mutations cannot fully explain the defect in all cases of Type I interferon synthesis. We hypothesize that retinol depletion and resulting impaired retinoid signaling play a central role in the COVID-19 pathogenesis that is characteristic for dysregulated immune system, defect in Type I interferon synthesis, severe inflammatory process, and destructive systemic multiorgan involvement. Viral RNA recognition mechanism through RIG-I receptors can quickly consume a large amount of the body's retinoid reserve, which causes the retinol levels to fall below the normal serum levels. This causes retinoid insufficiency and impaired retinoid signaling, which leads to interruption in Type I interferon synthesis and an excessive inflammation. Therefore, reconstitution of the retinoid signaling may prove to be a valid strategy for management of COVID-19 as well for some other chronic, degenerative, inflammatory, and autoimmune diseases.

HIV and SARS-CoV-2 Co-Infection: What are the Risks?

The dramatic increase of the global pandemic of SARS-CoV-2 infection represents a critical issue that needs to be investigated to evaluate the associated risk factors for acquisition and worse outcome. The interplay between immune activation and immune depression during SARS-CoV-2 infection is an intriguing topic that still needs to be clarified. The role of HIV in SARS-CoV-2 infection is not well defined. Chronic inflammation linked to HIV infection could be a driver for a worse prognosis in people living with HIV (PLWH). We explored the role of HIV as a risk factor for SARS-CoV-2 infection and severity and which factors contributed to a worse prognosis when HIV infection was present. PubMed/MEDLINE was searched for "COVID-19" or "SARS-CoV2" and "HIV" or "AIDS" and ("hospitalization" or "intensive care" or "mechanical ventilation" or "death" OR "mortality"), both in MeSH and as free text in all fields. Our review focused on 21 studies that enrolled at least 40 PLWH. In most studies, HIV infection did not represent a risk factor for SARS-CoV-2 infection. On the contrary, the risk of severe COVID-19 and hospitalization was higher in PLWH. Low CD4 cell count consistently emerged as a risk factor for severe COVID-19. Comorbidities, either in people with or without HIV diagnosis, played a key role, especially because of their early development in PLWH.

Cellular and Molecular Effects of SARS-CoV-2 Linking Lung Infection to the Brain

In December 2019, a new viral disease emerged and quickly spread all around the world. In March 2020, the COVID-19 outbreak was classified as a global pandemic and by June 2021, the number of infected people grew to over 170 million. Along with the patients' mild-to-severe respiratory symptoms, reports on probable central nervous system (CNS) effects appeared shortly, raising concerns about the possible long-term detrimental effects on human cognition. It remains unresolved whether the neurological symptoms are caused directly by the SARS-CoV-2 infiltration in the brain, indirectly by secondary immune effects of a cytokine storm and antibody overproduction, or as a consequence of systemic hypoxia-mediated microglia activation. In severe COVID-19 cases with impaired lung capacity, hypoxia is an anticipated subsidiary event that can cause progressive and irreversible damage to neurons. To resolve this problem, intensive research is currently ongoing, which seeks to evaluate the SARS-CoV-2 virus' neuroinvasive potential and the examination of the antibody and autoantibody generation upon infection, as well as the effects of prolonged systemic hypoxia on the CNS. In this review, we summarize the current research on the possible interplay of the SARS-CoV-2 effects on the lung, especially on alveolar macrophages and direct and indirect effects on the brain, with special emphasis on microglia, as a possible culprit of neurological manifestation during COVID-19.