Causes of Hemolytic Uremic Syndrome
Hemolytic uremic syndrome occurs in children and adults. In infancy and early age, it has clinical and pathogenetic features, which allows us to talk about its nosological independence as a disease. In adults, it is rarely observed and is considered a syndrome. A connection was established between hemolytic-uremic syndrome with the use of oral contraceptives, cocaine, certain drugs (cyclosporin A, mitomycin C, quinine), pregnancy, tumors, bone marrow transplantation, systemic lupus erythematosus (antiphospholipid syndrome), AIDS.
In childhood, the most common causes of hemolytic uremic syndrome are diarrhea (90%) and upper respiratory tract infection (10%). The etiological factor must have the ability to damage endothelial cells. These properties are endowed with a shiga-like toxin or verotoxin produced by Escherichia coli O157: H7, Shigella dysenteriae type I, Aeromonas hydrophilia, as well as neuraminidase Streptococcus pneumoniae. In addition, the development of hemolytic-uremic syndrome is caused by non-verotoxin-forming bacteria – Salmonella, Campylobacter, Yersinia, Clostridium difficile, chickenpox viruses, ECHO, Coxsackie A and B. Some researchers suggest that hemolytic-uremic syndrome is also associated with immune complexes. Family cases of hemolytic uremic syndrome, both a dominant and a recessive type of autosomal inheritance, are described. In these patients, a decrease in the production of prostacyclin (an endogenous platelet aggregation inhibitor) by endothelial cells was detected.
In children, in 70-85% of cases, the cause of hemolytic uremic syndrome is E. coli O157: H7 infection, leading to the development of diarrhea. Pathogenic for humans E.coli O157: H7 is found in the intestines of healthy cats and cattle, the transmission of which occurs upon contact with animals, eating food (ground beef and other meat products) that has not undergone sufficient heat treatment. Bacteria can be in unpasteurized dairy products and fruit juices, non-chlorinated water. It should be noted that food contaminated with E. coli does not have an unpleasant odor or taste. Infection of a child can occur when visiting a zoo and contact with a patient with diarrhea.
Approximately 10-15% of children infected with E. coli O157: H7 develop hemolytic-uremic syndrome, the risk of developing which increases if diarrhea is treated with antiperistaltic drugs, amoxicillin, or a combination of it with gentamicin, trimethoprim-sulfamethaxazole. The course and prognosis of hemolytic uremic syndrome is associated with an etiological factor. There are 2 forms of hemolytic uremic syndrome: typical hemolytic uremic syndrome – diarrhea plus hemolytic uremic syndrome (D + HUS), atypical hemolytic uremic syndrome – diarrhea minus hemolytic uremic syndrome (D + HUS). The latter form is noted in 10% of cases.
Causes of hemolytic uremic syndrome
- Verotoxin-producing E. coli
- Schigella dysenteriae
- Microorganisms secreting neuraminidase (Str. Pneumoniae and others)
- HIV infection
- idiopathic hemolytic uremic syndrome
- hereditary hemolytic uremic syndrome: autosomal recessive, autosomal dominant
- drug hemolytic-uremic syndrome: cyclosporin A, mitomycin C, bleomycin, duanorubicin, cytosine arabinoside, cyclophosphamide, carboplatin, doxorubicin, chlorozotocin, oral contraceptives, etc.
- Hemolytic uremic syndrome associated with pregnancy
- Hemolytic uremic syndrome associated with organ transplantation
- Hemolytic uremic syndrome associated with systemic lupus erythematosus
- Hemolytic uremic syndrome associated with tumors
- Hemolytic uremic syndrome associated with scleroderma
- Hemolytic uremic syndrome associated with malignant hypertension
- Hemolytic uremic syndrome, overlapping glomerulonephritis
In hemolytic-uremic syndrome caused by Streptococcus pneumoniae, a neuraminidase secreted by the pathogen removes sialic acid residues from the surface of red blood cells, platelets, and glomerular endothelial cells, exposing the Thomsen-Friedenreich T-cryptantigen. The presence of IgM antibodies to this antigen in the circulating blood leads to agglutination, which, in turn, leads to hemolysis, thrombocytopenia, intravascular thrombosis and further enhancement of vascular damage. Production of antibodies to T-cryptantigen may be induced by previous sensitization, or antibodies may reach a patient with donated plasma.
Hemolytic-uremic syndrome – (D-) – not associated with diarrhea (sporadic or atypical) hemolytic-uremic syndrome is more often observed in older children and adults. The disease preceding its development is not accompanied by a diarrheal prodrome, and, as a rule, manifests itself in the form of a respiratory tract infection in more than 40% of patients. This type of hemolytic-uremic syndrome is not related to the seasonal factor, it is clinically often combined with severe arterial hypertension, cardiomyopathy and seizures, it is characterized by a relapsing course, and in most cases, the final result of the disease is residual renal dysfunction with outcome in terminal chronic renal failure.
In adult patients, predisposing to the development of hemolytic uremic syndrome conditions include pregnancy, systemic diseases, family history, malignant hypertension, HIV infection, malignant neoplasms and anticancer therapy. These conditions are the cause of more than 50% of all cases of hemolytic uremic syndrome in adults.
A type of hemolytic-uremic syndrome, combined with pregnancy, is usually regarded as a complication of pregnancy (preeclampsia, eclampsia, infected miscarriage) and, in many cases, is completely cured after delivery. Postpartum hemolytic-uremic syndrome, caused by complications in childbirth and the postpartum period (placental abruption, amniotic fluid embolism, postpartum sepsis) is often associated with severe kidney damage (up to the development of cortical necrosis), severe arterial hypertension and has a poor prognosis.
Hemolytic uremic syndrome associated with HIV infection is considered one of the most common forms of microangiopathy in adults. Its outcome directly depends on the course of the underlying disease and with a detailed picture of AIDS has a poor prognosis. An unfavorable course is also characterized by hemolytic-uremic syndrome, observed with tumors and treatment with antitumor drugs.
There is evidence that hemolytic uremic syndrome may recur after kidney transplantation (in 13% of cases), and with a related donation, the risk of relapse can increase to 30%, but after treatment with cyclosporine A, the risk of such relapse after transplantation is reduced.
Very rare forms of hemolytic uremic syndrome – (D-) – autosomal recessive and autosomal dominant. The causes of hereditary forms of hemolytic uremic syndrome are unknown. It is assumed that it may be due to a congenital defect in the complement system, congenital collagen glomerulopathy (type III), a defect in antithrombin III, impaired prostacyclin metabolism, and a congenital anomaly in vitamin B12 metabolism with methylmalonic aciduria and homocystinuria. Recently, in a number of patients with hereditary hemolytic-uremic syndrome, the gene of the 1st chromosome was identified, designated as factor N.
The diagnosis of hereditary hemolytic uremic syndrome cannot be made in the first case of the disease in the family. When diagnosing this form of hemolytic-uremic syndrome, the presence of family members who have been sick, the atypical prodrome without diarrhea or its absence, a progressively recurring course, the predominance of arteriolar changes in the kidneys and / or relapses after kidney transplantation is taken into account.
Thus, hemolytic uremic syndrome – (D-) is a heterogeneous subgroup that differs from a typical hemolytic uremic syndrome in terms of epidemiological, clinical, histopathological characteristics and prognosis associated with high mortality.
Pathogenesis during the Hemolytic Uremic Syndrome
After eating E. coli-infected food or water, the pathogen binds to specific colon receptors, multiplies and causes cell death, which is usually accompanied by diarrhea, and in case of infection with strains producing verotoxin, damage to the vessels of the intestinal mucosa with the development of hemorrhagic colitis occurs. Verotoxin released in the intestine enters the liver, where it is metabolized. Its penetration into the systemic circulation is possible through port-caval anastomoses, through which normally up to 6% of the blood flowing from the intestines is discharged. The entry of verotoxin into the systemic circulation leads to microcirculatory disorders in the target organs, forming the clinical picture of hemolytic uremic syndrome or, less commonly, thrombotic thrombocytopenic purpura (TTP). The first target organ on the way of verotoxin penetrating into the bloodstream are the lungs, in which there are zones of leukocyte infiltration and dose-dependent sequestration of activated granulocytes in the vessels of the microvasculature. With an increase in the degree of endotoxemia, the spread of the damaging effect of granulocytes on the lungs leads to the formation of respiratory distress syndrome. Damage to other organs, in particular, the kidneys, is also preceded by sequestration of activated granulocytes in the microcirculatory system and interstitium of the organ. It is assumed that the involvement of various organs in the pathological process in hemolytic-uremic syndrome may reflect a different, possibly age-dependent distribution of verotoxin receptors in children and adults.
In recent years, it has been found that about 90% of children with hemolytic uremic syndrome – (D +) have signs of infection with Verotoxin-producing E. coli (VTEC). In approximately 70% of these patients, serotype 0157: H7 was isolated. This pathogen distinguishes two types of verotoxins: BT-1 and BT-2, which are also called shiga-like because of their similarity with the toxin Schigella dysenteriae. Verotoxins are a family of structurally similar, consisting of 2 endotoxin subunits. Subunit A is responsible for cytotoxic effects, while subunit B has a high degree of affinity for membrane-bound glycosphingolipids: globotriosylceramide (Gb3) and globotetraosylceramide (Gb4), as well as halabiosylceramide (Ga2) and pentosylceramide (P1). After binding and penetration of BT into the cell, subunit A is separated and transferred from the Golgi apparatus to the endoplasmic reticulum, where it splits into subunits A1 and A2. The toxic effect of verotoxin is due to its A1 subunit, which inhibits protein biosynthesis by inactivation of ribosomal subunits. The binding of verotoxin, its penetration into the cell, the activation and inhibition of the protein synthesis of the host cell occurs within about 2 hours.
It was established that the product of the vital activity of bacteria – lipopolysaccharide (LPS), is a dose-dependent synergist with verotoxin, determining the degree of its cytotoxicity. The synergism of verotoxin and LPS initiates an inflammatory reaction in the target organ, contributing to the local production of inflammatory mediators: tumor necrosis factor (TNF-α) and interleukins (IL).
Damage to the endothelium is the central pathogenetic mechanism of the hemolytic-uremic syndrome and is accompanied by the activation of platelets with their subsequent adhesion in the damage zone, where there may be a danger to subendothelial structures. A factor determining the susceptibility of endothelial cells to verotoxin is the presence on their surface of receptors with a high degree of affinity for the toxin. Cells are not sensitive to its toxic effects as long as their expression of the Gb3 receptor for verotoxin is limited. Actively dividing endothelial cells are more sensitive than non-dividing ones, since the expression of the Gb3 receptor occurs in the early S-phase of the cell cycle. Vero cells with Gb3 receptors can change their susceptibility to verotoxin 10 times during the cell cycle. The number of Gb3 receptors in animals is probably limited, since they have not obtained an adequate model of hemolytic uremic syndrome. When comparing human endothelial cells from different tissues, it was found that kidney endothelial cells are 1000 times more sensitive to verotoxin than umbilical vein endothelial cells. Moreover, their expression of Gb3 was 50 times higher, although no further induction was observed when exposed to LPS, TNF-α, or IL-1.
Verotoxin binds in the kidneys in proportion to the amount of globotriosylceramide (Gb3) available. BT-1 is associated with renal glomerular endothelium in children, but not in adults, and this relationship can be eliminated by prior administration of α-galactosidase. In addition, mesangial cells that also secrete large amounts of Gb3 can be an independent goal of hemolytic uremic syndrome – (D +). These cells are capable of phagocytosis, as a result of which verotoxin accumulates in excess in the mesangy with its subsequent damage – mesangiolysis, which manifests itself as cell dystrophy and necrosis with a kind of “dilution” of the mesangial matrix.
Cytokines have numerous effects on the endothelium, but their main effect is the stimulation of thrombosis and adhesion of neutrophils on the walls of blood vessels with the subsequent release of reactive oxygen species from them. It has been found that verotoxin and other bacterial toxins can synergistically induce the production of TNF-α in the kidneys. This to some extent explains the almost constant involvement of the kidneys in the pathological process in the hemolytic uremic syndrome caused by VTEC.
BT-1 stimulates the synthesis of IL-1β, IL-6, IL-8 and TNF-α by monocytes according to the principle of time and concentration dependence. Monocytes secrete receptors for verotoxin, and this effect after preliminary expression of LPS can increase by 30 times.
Activated polymorphonuclear leukocytes (PNL) cause endothelial damage due to the release of toxic forms of oxygen and lysosomal enzymes, for example, elastase. The severity of hemolytic-uremic syndrome due to Schigella dysenteriae or VTEC depends on the amount of PNE in the peripheral blood. Accordingly, with a high number of PNLs, serum concentrations of elastase and α1-antitrypsin increase. An additional factor in endothelial damage in hemolytic-uremic syndrome is lipid peroxidation of cell membranes, leading to damage not only to the endothelium, but also to red blood cells.
An important factor in hemolytic-uremic syndrome is probably a decrease in vitamin E levels, especially since its deficiency in newborns is described as hemolytic-uremic-like syndrome.
Endothelial damage can develop under the influence of latent (hidden) endothelial antigens. Hemolytic-uremic syndrome can develop with pneumococcal sepsis due to endothelial damage by neuraminidase, which, as mentioned above, “tears” sialic acid from cell membranes, exposing the Thomsen-Friedenreich antigen (T-antigen) in the glomeruli of the kidneys, red blood cells, and platelets. Then, in the presence of IgM antibodies to this antigen, which are contained in the plasma of most people, platelets and red blood cells agglutinate.
Many factors are involved in this process, among which the most studied are the unusually large multimers of von Willebrand factor (VF). They are deposited in platelet α-granules and in endothelial cells, mainly in Wiebel-Palade bodies. These giant PV polymers are formed by combining its monomers via disulfide bonds and more efficiently than small plasma forms bind to the glycoprotein receptors GPIb-IX and GPIIb-IIIa of platelets in circulating blood.
In damaged microvessels, in particular, in the kidneys, an increase in shear stress (“shearing force” resulting from the movement of blood layers at different speeds: with a higher speed in the parietal layer and with a lower speed closer to the center of the vessel) may be responsible von Willebrand factor proteolysis. Abnormal fragmentation of PV in the acute period of hemolytic-uremic syndrome or TTP due to an increase in “shearing force” may support platelet activation and thrombosis in microvessels. It is assumed that damage to the endothelium may be due to the excessive release of unusually large multimers of PV exceeding the ability of the blood to process them.
In addition, an increase in shear stress leads to irritation of endothelial mechanoreceptors, thereby stimulating an increase in endothelial production of nitric oxide (NO), which, in turn, induces the secretion of IL-1 and TNF-α from leukocytes with their subsequent activation. NO can also interact with the oxygen radical released from activated neutrophils, with the formation of other highly toxic radicals, which leads to the maintenance of the inflammatory reaction and subsequent morphological damage.
The endothelium-produced radical nitric oxide (NO) and endothelin peptide (ET-1) are the main paracrine and autocrine mediators that regulate local blood flow, and NO can also modulate platelet adhesion, aggregation and degranulation.
Verotoxin affects the production of endothelial mediators (ET-1 and NO) and their key regulatory enzymes: endothelin-converting enzyme (ECE) and endothelial constitutional NO synthetase (ecNOS). The ability of VT-1 and VT-2 to persistently increase the level of prepro-ET-1-matrix RNA in the vascular endothelium was also revealed, and this increase was induced by concentrations of verotoxin, which have minimal effect on protein biosynthesis and, therefore, do not exhibit ribosomal blockade. Verotoxin was found to induce the expression of prepro-ET-1 in the absence of endogenous cytokines, i.e. activation of endothelial cells with verotoxin can be carried out directly. An increase in the level of prepro-ET-1 directly or indirectly contributes to the development of vasculopathy associated with verotoxin. The pathophysiological features of hemorrhagic colitis and hemolytic-uremic syndrome, such as severe arterial hypertension, focal ischemia of the renal cortex, intestinal mucosa and central nervous system, are correlated with the production of this powerful vasoconstrictor mediated by verotoxin. The vessels of the kidneys are extremely sensitive to ET-1. It is capable of powerfully activating a cascade of reactions in glomerular mesangial cells, as well as inducing its own synthesis in these cells.
In general, ET-1 is involved in various aspects of arresters. For example, studies using ETA and ETV receptor antagonists have shown the important role of ET-1 in ischemic / reperfusion damage to the kidneys and in the manifestation of acute cephalosporin nephrotoxicity.
Vasoconstriction caused by endothelin can cause a decrease in cerebral blood flow, contributing to damage to neurons and neurotoxicosis. In the intestine, ET-1 can induce ischemia and mucosal damage, disrupt ion transport and cause smooth muscle spasm, especially in the wall of the large intestine. Under certain conditions, ET-1 may also have a prothrombotic effect.
There is much discussion about the role of prostacyclin deficiency (PgI2) in the development of hemolytic uremic syndrome. It has been established that in some patients with hemolytic uremic syndrome and thrombotic thrombocytopenic purpura (TTP, Moscovitz disease), vascular tissue produces an extremely small amount of PgI2. Moreover, the plasma of patients is unable to stimulate the normal synthesis of prostacyclin by endothelial cells. However, with hemolytic uremic syndrome – (D +), a decrease in the level of a prostacyclin-stimulating factor was not detected.
In patients with hemolytic-uremic syndrome – (D-), an increase in the mitogenic activity of plasma and serum against fibroblasts was found, but with hemolytic-uremic syndrome – (D +) this increase is absent due to the presence of an inhibitor of cell growth. The serum of patients with acute TTP in the acute phase contains an increased concentration of biologically active transforming growth factor (TGF-β1) and has an inhibitory effect on cultures of immature hematopoietic progenitor cells, and some inhibitory activity persists even with remission of the disease.
Thrombocytopenia in hemolytic uremic syndrome is the result of activation and consumption of platelets in the area of endothelial damage, and in some cases, “damage” can be no more than the loss of a normal negative charge on the surface of the endothelial cell due to exposure to bacterial or viral neuraminidase. Platelets help maintain normal blood circulation, ensuring the integrity and control of hemostasis after damage to the vessel wall. Activated platelets can disaggregate and circulate in the blood in a degranulated (“depleted”) state. At the same time, it is reported that activated platelets were not detected in TTP. Platelet changes are considered secondary to endothelial damage.
Platelet activating plasma factors are not well understood. In TTP, they cause aggregation of both platelets of the patient and normal platelets. A protein with a molecular weight of 37 kDa, detected only during relapse, is able to bind to glycoprotein IV on the platelet membrane. This aggregation is inhibited by a protein with a molecular weight of 150 kDa contained in normal plasma. A similar factor causing platelet aggregation has been described in children with epidemic HUS (HUS- (D +). The platelet aggregation factor found in TTP is likely to be related to abnormal multimers of von Willebrand factor.
A characteristic feature of hemolytic-uremic syndrome are deposits of fibrin in the glomeruli of the kidneys, which are partially removed by an internal mechanism involving tissue plasminogen activator. The insufficiency of this mechanism leads to persistent deposition of fibrin in the glomerular capillaries and glomerular necrosis. In children with hemolytic uremic syndrome, a plasma inhibitor of glomerular fibrinolysis was detected, the level of which in plasma correlates with the outcome of the disease. This inhibitor is currently known as a plasminogen-1 activation inhibitor (PAI-1). It has acid resistance, does not lose its activity during denaturation and has a powerful inhibitory effect on the tissue plasminogen activator. The neutralization of this inhibitor occurs under the influence of specific anti-PAI-1 antibodies. In acute hemolytic uremic syndrome, the effectiveness of plasminogen activator is low, which is due to a higher concentration of PAI-1 in plasma compared to acute renal failure of another etiology. Normalization of plasma PAI-1 levels (for example, using peritoneal dialysis) correlates with improved renal function. At the same time, van-Geet et al, studying the coagulation system and fibrinolysis in children with hemolytic-uremic syndrome, found that the level of PAI-1 did not differ much in patients with various causes of acute renal failure, and the levels of tissue plasminogen activator and urokinase plasminogen activator of the type were significantly higher with hemolytic uremic syndrome than with other causes of acute renal failure; while hemodialysis caused an increase in the level of tissue plasminogen activator and a decrease in the level of PAI-1. According to the authors, in children with hemolytic uremic syndrome – (D +) there is a restriction of intravascular coagulation, and there is no evidence to support the deterioration of fibrinolysis. In our opinion, these contradictions reflect the entire complexity of disorders in the hemostatic system, which are manifested by the phase nature of coagulation and fibrinolysis processes.
The development of microangiopathic hemolytic anemia in hemolytic-uremic syndrome is explained by mechanical damage to red blood cells by fibrin threads during the passage of blood cells through partially clogged microvessels. Concomitant oxidative damage to erythrocyte membranes due to the activation of lipid peroxidation worsens their deformability and decreases resistance to mechanical damage, thereby contributing to increased hemolysis.