1. Defining COVID-19
2. Explaining the subclassification of COVID-19 (i.e mild, moderate, severe and critical COVID-19)
3. Understanding the virus(SARS CoV-2) causing COVID-19 including its molecular structure, the mechanism of gaining access into the host cell, and mode of transmission
4. The relationship of SARS CoV-2, the causative agent of COVID-19, with other coronaviruses.
5. Understanding how the virus causes lung injury and distant vascular injury and thrombosis and the cytokine storm in the minor subset of patients with severe, critical and fatal COVID-19
6. The relationship of SARS CoV-2, the causative agent of COVID-19, with other coronaviruses.
7. Understanding how the virus causes lung injury and distant vascular injury and thrombosis and the cytokine storm in the minor subset of patients with severe, critical and fatal COVID-19
8. The importance of the interferon driven immune response in reducing viral replication and therefore as a determinant of clinical outcome especially as it applies to older adults with severe and critical COVID versus children with mild disease
9. The basic principles behind the vaccine against the novel coronavirus
COVID-19: A 21st Century Pandemic
1. Fatal COVID-19 pneumonitis targeted the small vessels of the terminal lung parenchyma(i.e. the septal capillaries). These are the key vessels involved in the exchange of oxygen. The lining cells of the small microvessels (i.e. capillaries) called endothelium were severely damaged resulting in the deposition of fibrin, a product of coagulation cascade activation, in the wall and lumens of the septal microvessels. Substantial inflammation and viral cytopathic changes were not identified.
2. The septal capillary injury was caused by complement pathway activation whereby the membranolytic attack complex C5b-9 was the effector mechanism of endothelial injury.
3. The complement pathway involved was the mannan binding lectin pathway as revealed by the presence of certain components of mannan binding lectin activation in the septal microvessels including MASP2, C4d, and C5b-9.
4. Patients with COVID-19 associated severe pneumonitis developed a distinctive skin rash designated thrombotic retiform purpura. The histology showed a small vessel endothelial cell injury pattern that mirrored the vascular changes in the lung including evidence of mannan binding lectin pathway complement activation. Furthermore significant complement deposition was observed in microvessels of normal skin indicative that severe COVID-19 is a form of systemic complement activation.
5. The components of complement activation were localized with SARS CoV-2 capsid protein in both the skin and lung microvessels including the part of the virus that binds to ACE-2 namely spike glycoprotein implying that spike glycoprotein on endothelium directly engaged with circulating mannan binding lectin to result in complement activation and vascular injury.
Cynthia Magro, M.D of WCM Pathology and Lab Medicine explains new paper on severe COVID-19 infection.
1. There are two distinctive acral manifestations of COVID-19 embodying disparate clinical phenotypes: one is perniosis occurring in mildly symptomatic patients, typically children and younger adults; the second is the thrombotic retiform purpura of critically ill COVID19 adults with acute respiratory distress syndrome.
2. COVID-19-associated perniosis was defined by a highly inflammatory process characterized by intense vasocentric and eccrinotropic T-cell and monocyte-derived CD11c, CD14, and CD123+ dendritic cell infiltrates.
3. Both COVID associated and idiopathic perniosis showed striking expression of the type I interferon-inducible myxovirus resistance protein-A (MXA), an established marker for type I interferon signaling in tissue. In COVID-19 perniosis, SARS-CoV-2 RNA, IL- 6 and caspase 3 were minimally expressed and confined to mononuclear inflammatory cells.
4. The biopsies from livedo/retiform purpura demonstrated pauci-inflammatory(no inflammation) vascular thrombosis and endothelial cell injury without any MXA decoration indicative of a profoundly blunted/absent type I interferon response. Blood vessels exhibited extensive complement deposition with endothelial cell localization of SARS-CoV-2 protein, IL6, and caspase 3; SARS CoV-2 RNA was not seen and hence the skin samples although showing docked SARS CoV-2 protein had no evidence of an active viral infection i.e. a true infectious endothelialitis.
SUMMARY STATEMENT: We hypothesize that in the thrombotic retiform purpura of critically ill patients with COVID-19, the vascular thrombosis in the skin and other organ systems are associated with a minimal interferon response. Type I interferon signaling is key in viral defence. The consequence of this markedly blunted interferon signaling is excessive viral replication in the lung with subsequent release of viral proteins(i.e. pseudovirions) that localize to extrapulmonary endothelium and trigger extensive complement activation. In contrast the COVID-19-associated perniosis there is striking significant type-I interferon signaling leading to rapid SARS-CoV-2 eradication -19 perniosis. Hence the polarizing paradigm of COVID-19 is reflected by this two distinctive but pathogenetically incongruous eruptions.
Dr. Cynthia Magro explains her recent British Journal of Dermatology paper.
The importance of complement activation in the pathogenesis of severe COVID-19 was established earlier in our original Translational Research paper and is now explored further in the deltoid skin study. In the Translational Research study we showed evidence of systemic complement activation based on significant complement deposition in normal skin procured from the deltoid area. A biopsy of normal skin overlying the deltoid is used to assess for evidence of systemic complement pathway activation in patients with suspect complement mediated thrombogenic microangiopathy most notably atypical hemolytic uremic syndrome. We examined normal skin procured from the deltoid area in patients with severe COVID-19 since this biopsy procedure is an established one for documenting evidence of systemic complement pathway activation. The main discoveries were:
1. Systemic complement activation was present in patients with severe COVID-19 based on the extent of complement deposition in the normal deltoid skin in patients with severe COVID-19.
2. There was colocalization of docked SARS CoV-2 capsule protein and complement within the dermal and subcutaneous vessels reflecting the engagement of the SARS COV-2 spike glycoprotein with circulating mannan binding lectin, one of the complement pathways.
3. The pseudovirions were able to dock to the skin and fat microvessels due to the expression within the microvessels of the SARS CoV-2 spike glycoprotein receptor called ACE2.
4. The greatest extent of ACE2 expression in vessels occurred in the subcutaneous fat and therefore was also the greatest extent of SARS CoV-2 surface protein and complement localization.
5. The complement pathway activation was associated with microvascular injury typical for complement mediated vascular injury such as vascular thrombosis and basement membrane zone reduplication recognizing that the changes were very focal even though the skin that was biopsied was apparently normal.
6. The extensive complement pathway activation in these microvessels of the skin and fat could serve as a fuel for distant and generalized alternative complement pathway activation and coagulation pathway activation due to the established synergy and cross talk between the complement pathways and the coagulation pathway. In addition, the extent of ACE-2 expression in subcutaneous vessels and therefore SARS CoV-2 protein localization with subsequent complement activation may provide an interest link between obesity and severe COVID-19.
Cynthia Magro, M.D explains her Human Pathology paper.
This study further explored the pathophysiology that underlies severe COVID-19 by assessing the histopathology and the in situ detection of infectious SARS-CoV-2 and viral capsid proteins along with the cellular target(s) and host response from 26 cases represented by 12 autopsies and 14 premortem specimens which were skin samples in 13 and an open lung biopsy sample in one.
There were four key findings:
1. high copy infectious virus was limited mostly to the alveolar macrophages and endothelial cells of the septal capillaries
2. viral spike protein without viral RNA localized to ACE2+ endothelial cells in microvessels that were most abundant in the subcutaneous fat and brain 3.
3. although both infectious virus and docked viral spike protein were associated with complement activation, only the endocytosed pseudovirions induced a marked up-regulation of the key COVID-19 associated proteins IL6, TNF alpha, IL1 beta, p38, IL8, and caspase 3 within endothelium. The so called cytokine storm was purely attributable to an endothelial based origin for the cytokines.
4. Microvascular changes associated with SARS CoV-2 protein(capsid, membrane and spike glycoprotein) and complement localization within the vessels included endothelial cell injury and thrombosis most apparent in the lung, skin, and brain and to a lesser extent the kidney and heart. In addition, there was larger vessel thrombosis reflective of a generalized procoagulant state. The pattern of SARS CoV-2 protein expression mirrored the expression of ACE-2 in select microvascular beds. Larger vessel thrombosis was without the microvascular pattern of ACE2, SARS CoV-2 and complement localization in endothelium and simply reflected a generalized procoagulant state.
It is concluded that serious COVID-19 infection has two distinct mechanisms:
1) a microangiopathy of pulmonary capillaries associated with a high infectious viral load where endothelial cell death releases pseudovirions into the circulation
2) the pseudovirions docking on ACE2+ endothelial cells most prevalent in the skin/subcutaneous fat and brain activates the mannan binding lectin complement pathway which in turn could promote alternative complement pathway and coagulation pathway activation. In addition mannan binding lectin activation likely accounts for the expression of certain key cytokines such as IL-6 from endothelium contributing to the cytokine storm but also further exacerbating the tendency toward blood clot formation given the known effects of enhanced expression of these cytokines in endothelium especially IL-6 in causing platelet aggregation.
3) the lack of inflammation corresponds to the cytokine storm not being derived from inflammatory cells but rather endothelial cells.
Dr. Cynthia Magro explains her new paper in The Annals of Diagnostic Pathology.