5. Pathophysiology of Kidney Damage Induced by SARS-CoV-2
The expression of ACE-2 has been shown not only in the lung but also in the liver, stomach, ileum, colon, esophagus and kidney [47]. These data associated with the evidence that AKI (7%), myocardial dysfunction with acute cardiovascular events (12%) and gastrointestinal disorders are among the most frequent clinical manifestations of COVID-19 [23], suggesting that SARS-CoV-2 can infect these organs. However, whether SARS-CoV-2 replication occurs in these organs causing functional damage and contributing to the systemic spread of the virus is not yet clear.
Zou et al. [47], stratifying the human organs in high and low risk according to the level of ACE-2 expression, have shown that the kidney is very vulnerable to SARS-CoV-2infection.
Qi et al. [12], through single cell RNA sequencing studies, have shown that ACE-2 in the kidney is expressed primarily by renal proximal tubular cells (~82%) and also to a lesser extent by cells of the intercalated duct, main cells of the collecting duct, renal distal tubular cells, glomerular parietal epithelium cells and immune cells (8%) [12].
Pan et al. [63], using single cell transcriptome analysis, confirmed the cellular co-expression of ACE-2and TMPRRS genes in proximal tubule cells and podocytes.
Batlle et al. have also suggested whether the expression of other cellular TMPRSSs other than TMPRSS2 (such as TMPRSS 4, 5 or 9) may also play a role in the priming step [64]. Studies showed that the co-expression of the ACE-2 receptor and TMPRSS genes in kidney epithelial cells was as significant as in the lung, esophagus, small intestine and colon, suggesting that the kidney might also be an important target organ for SARS-CoV-2 [63]. In particular, a high co-expression of ACE-2 and TMPRSS genes was found in podocytes and proximal rectangular tubular cells [63].
Other noteworthy observations include the presence ofSARS-CoV-2 nucleocapsid protein in renal tubular structures and virus-like particles in podocytes and renal tubular epithelial cells, as observed by electron microscopy [65,66]. Together these observations suggest the virus may cause AKI through a direct cytopathic effect on kidney cells. In particular, it is conceivable that the virus may enter the kidney by invading the podocytes first, thereby gaining access to the tubular fluid and thence to the proximal tubule cells where it may bind to ACE-2 [64]. The viral replication in podocytes and the resulting damage could explain the proteinuria and hematuria reported in a high percentage of COVID-19 patients [8,27]. However, if the renal dysfunction is caused only by direct damage of SARS-CoV-2 or is secondary also to other systemic processes triggered by the virus, it has not been well defined yet.
Diao et al. [65] observed that kidney damage associated with COVID-19 is an acute tubular necrosis induced directly by SARS-CoV-2 during infection and replication, but also indirectly through the complex immune mechanisms triggered by cellular damage. In fact, the histopathological examination performed on kidney specimens, obtained from autopsy of COVID-19 patients with renal function impairment, showed viral antigens in the cytoplasm of the tubular cells, but also a strong presence of CD68+ macrophages in the tubulo-interstitium and strong C5b-9 depositions on the apical brush border of tubular epithelial cells (TECs). This suggested that proinflammatory cytokines derived from macrophages in the tubulo-interstitium and complement-mediated mechanisms resulting from cell damage participate in the pathogenesis of tubulo-interstitial damage. In fact, despite the infiltration of infected tissue by host immune cells in order to contain viral replication, the hyperactivation of these immune cells may lead to fibrosis, epithelial cell apoptosis and cause microvasculature damage [67,68,69].
Studies performed on SARS-CoV suggested that AKI in SARS patients was the result of specific pathogenic conditions, such as the cytokine release syndrome [70], rather than active viral replication in the kidney. In consideration of the analogy between SARS-CoV and SARS-CoV-2, flow cytometry was used to study the immune phenotype and the function of peripheral blood mononuclear cells in COVID-19 patients [71]. Studies showed that patients infected by SARS-CoV-2 showed lymphopenia, mainly related to the significant reduction in absolute T cell counts, particularly cytotoxic T lymphocytes (CD8+), increased neutrophil counts and elevated levels of proinflammatory cytokines. In particular, high levels of interleukin (IL)-2, IL-6, IL-10 and interferon (IFN)-γ were observed. Therefore, it has been speculated that a loss of T cells during the viral infection may result in enhanced inflammatory responses [72]. In fact, it is known that T cells are important for dampening overactive innate immune responses during viral infection [73]. In accordance with this hypothesis, it has been observed that when the T cell count drops the serum levels of IL-2, IL-4, IL-10, tumour necrosis factor (TNF)-α and interferon (IFN)-γ reach their peaks [72].
Another important finding is that COVID-19 patients with more severe clinical manifestations have higher serum concentrations of IL-6 and lower IFN-γ than mild forms. This is mainly due to the decrease in CD4+, CD8+ and NK lymphocytes [74]. Interferons (IFN) are a family of cytokines that play a central role in innate immunity to viruses and other microbial pathogens. The IFN-receptor binding induces a cascade of signals with activation of genes coding for proteins with antiviral, antiproliferative or immunomodulatory properties [70,75]. Normally the interaction between the IFN-γ and IL-6/sIL-6R signals contributes to the recruitment and subsequent clearance of neutrophils, thereby controlling infection and resolution of acute inflammation as well as influencing the transition between innate and adaptive immunity [76]. In patients infected by SARS-CoV-2, a higher IL-6/IFN-γ ratio may be related to an enhanced cytokine storm [74].
These observations suggest that in both SARS patients, and in patients with COVID-19, AKI may have an inflammatory etiology mediated by a cytokine storm.
The cytokine storm is associated with an inflammatory process that originates in a local site and spreads via the systemic circulation. The inflammatory process can cause dysfunction in organs, particularly when tissue edema causes an increase in extravascular pressures and a consequent decrease in tissue perfusion. Compensatory repair processes arise soon after the beginning of inflammation, and in a lot of cases they can completely re-establish tissue and organ function. However, when a severe inflammation condition injures local tissue structures, healing occurs with fibrosis, which can cause permanent organ dysfunction [70]. In fact, when a cytokine storm occurs, the immune system may not be able to kill SARS-CoV-2, but it can kill large numbers of normal cells and damage organs [63]. In support of the hypothesis that AKI in COVID-19 patients may be the consequence of inflammatory damage, a cohort study found that the CT scan of kidneys showed a reduced density, indicative of inflammation and edema [8].
In addition to being frequently associated with the cytokine storm, severe lung infections often require prolonged ventilatory support. This predisposes to the development of sepsis, classically defined by marked hypotension which requires treatment with inotropic drugs. Therefore, in patients with COVID-19 or acute respiratory distress syndrome (ARDS), it is plausible that persistent hypotension and vasoconstriction induced by inotropics can participate in the fall of the glomerular filtrate and consequent acute tubular necrosis [77].
In the most recent studies, the hypothesis of a multifactorial etiology of renal damage in COVID-19 is confirmed. Su et al. [66] in a study performed by analyzing autopsy kidney samples showed pigmented casts with high levels of creatine phosphokinase, attributable to rhabdomyolysis. In rhabdomyolysis the massive release of myoglobin due to muscle damage can cause kidney dysfunction. Myoglobin, in fact, shows its renal toxicity through various mechanisms: renal vasoconstriction related to the hyperactivation of RASS by hypoperfusion and the reduction of nitric oxide levels; intratubular cast formation; direct toxicity on renal tubular cells. These processes result in acute tubular necrosis [78]. Rhabdomyolysis in COVID-19 patients is hypothetically multifactorial. In fact, it may be secondary to a direct cytotoxic effect of SARS-CoV-2 on the muscle, tissue hypoxia due to hyperventilation or also to drug-induced damage [66]. Presumably, in COVID-19 patients who develop rhabdomyolysis, it may participate in the pathogenesis of AKI.
Su et al. [66] demonstrated the presence of erythrocyte aggregates, without platelets or fibrinoid fragments, which obstructs the lumen of the peritubular and glomerular capillaries in COVID-19 patients. Erythrocyte aggregation, presumably induced by inflammation (reflected by a high rate of erythrocyte sedimentation) and hypotension, can potentiate oxidative stress, inflammation and complement activation, aggravating microvascular damage [64]. Furthermore, occlusion of microvascular lumens by erythrocytes has been associated with a variety of endothelial lesions [66]. Normally, in endothelial cells of the kidney, only ACE is expressed without detectable ACE-2 [79]. Therefore, the renal endothelium cannot be infected directly by SARS-CoV-2. However, this cannot be totally excluded, since ACE-2expressioncan be changed in pathological states or by drugs [66]. Varga et al. recently concluded that SARS-CoV-2 infection induces endothelitis in various organs, directly and indirectly, and that could explain the systemic impairment of microcirculation [80]. Further studies are necessary to better understand the genesis of renal endothelial lesions. Of note, a recent study has proposed a new route for SARS-CoV-2 invasion of host cells via an alternative cell receptor known as CD147 (and also called basigin), which is a transmembrane glycoprotein and is expressed on all endothelial cells [81].
Recently, the high incidence of thromboembolic events in COVID-19 patients suggests that SARS-CoV-2 may play an important role in inducing coagulopathy.
Analyzing the hematological profile of COVID-19 patients, a state of hypercoagulability emerged. In fact, high plasma levels of reactive protein C, fibrinogen, D-dimer and ferritin, associated with thrombocytopenia, were found in these patients [71,82]. Recently, clinical and autopsy reports from China and the U.S. confirm the development of disseminated intravascular coagulation following SARS-CoV-2 infection, with evidence of microangiopathy in several organs. In fact, the activation of macrophages associated with COVID-19, the storm of cytokines and the molecular proteins associated with the damage can cause both tissue factors’ release and the activation of coagulation factors predisposing to hypercoagulability.
In some cases, this state of hypercoagulability could favor the evolution of acute tubular necrosis into cortical necrosis and, therefore, the development of irreversible renal damage. These observations suggest that low back pain and microhematuria observed in some positive COVID-19 patients may be manifestations of renal infarction [64].
The SARS-CoV-2 contribution to the development of CKD could involve pathways similar to those described for the acute kidney injury. Indeed, it has been observed that a non-trivial portion of patients develop signs of tubular or glomerular damage during the infection. The direct tubule-glomerular cellular injury, due to the virus, often manifests with proteinuria and hematuria that, in turn, could start a chronic, non-reversible, process [83]. It has been shown that proteinuria exerts a direct toxic effect on renal tubular cells and promotes renal fibrosis over time [84,85].
In conclusion, the renal damage observed in COVID-19 patients is the result of complex mechanisms induced directly and indirectly by SARS-CoV-2 that predispose to the development of renal dysfunction (Figure 1).
Further studies are needed to better understand the pathophysiological mechanisms of kidney injury, to develop new therapeutic strategies able to limit and/or prevent kidney damage, and to improve the prognosis of COVID-19 patients.