Understand the pathogenesis and pathology of HCV-associated nephritis in three minutes
The mainland is not only a major country for hepatitis B virus infection, but also an area with a high incidence of hepatitis C. The prevalence of hepatitis C in mainland China is 3.2%, with about 40 million people infected with hepatitis C virus (HCV). It is thought that the pathogenesis of hepatitis C-associated nephritis is not a direct effect of HCV itself, but may be related to immune complex-mediated or due to cryoglobulinemia due to HCV [1].
1. Epidemiology
According to the World Health Organization, the global prevalence of HCV infection is 2.8%, with an estimated 185 million people infected with HCV and about 350,000 deaths due to HCV infection each year. However, because HCV infection is insidious, most infected people are unaware of HCV infection. Therefore, the exact global incidence of chronic hepatitis C is unknown. The 2004~2013 notifiable infectious disease epidemic released by the National Health Commission shows that the reported incidence of hepatitis C has increased year by year (3.03% in 2004 and 14.93% in 2013), and the number of cases in 2013 has increased to 5.16 times that of 2004, with an average annual incidence rate of 9.68/100,000, accounting for 9.25% of the total reported cases of viral hepatitis.
According to the 2006 national seroepidemiological survey, the anti-HCV prevalence rate of the population aged 1~59 years old in mainland China is 0.43%, which is a low prevalence area of HCV in the whole country, and it is estimated that there are about 5.6 million HCV infections in the general population in the mainland, and about 10 million cases of HCV infection in high-risk groups (including blood transfusions, dialysis patients and mothers HCV-positive patients, etc.) and HCV infections in high-incidence areas. There are certain differences in the positive rate of anti-HCV in different regions, with the Yangtze River as the boundary, and the north (0.53%) is higher than the south (0.29%). The positive rate of anti-HCV gradually increased with age, which was 0.09% in the 1~4 years old group and 0.77% in the 50~59 years old group. There were no significant differences between men and women. The most common extrahepatic manifestation of HCV infection is mixed cryoglobulin, which has been suggested in approximately 50 percent of patients infected with HCV, but there are no definitive data on the incidence of HCV-GN [2].
2. Pathogenesis[3,4]
(1) Immune mechanisms
HCV-GN can be divided into mixed cryoglobulinemia glomerulonephritis (CGN) and non-cryoglobulinemic glomerulonephritis (nCGN).
1. Mixed cryoglobulinemia glomerulonephritis
In 1993, Jonson et al. reported for the first time 8 cases of HCV infection complicated by membranous proliferative glomerulonephritis (MPGN) in patients with mixed cryoglobulinemia, and believed that MPGN was closely related to HCV infection, which attracted attention to CCN (HCV-CGN) caused by HCV infection.
Cryoglobulin refers to an immunoglobulin that agglutinates and precipitates at a low temperature (4°C) and dissolves at a temperature of 37°C. According to its composition, it can be divided into three types: type I is composed of monoclonal immunoglobulins, which can be IgG, IgM and IgA, which are common in multiple myeloma, Waldenstrom macroglobulinemia and primary monoclonal gammopathy; type II and III. are mixed cryoglobulinemia (MC), type II is composed of monoclonal IgM with rheumatoid factor activity combined with polyclonal IgG; type III refers to the combination of polyclonal IgG and polyclonal IgM antibodies.
After HCV infection, cryoglobulinemia occurs through a complex, multi-step mechanism. Mainly type II and III., the production of IgM with autoantibody activity and condensation is the key to the occurrence of the disease. The detection rate of HCV antibody and HCV-RNA in patients with MPGN caused by type II MC was more than 90%, and 30%~50% in type III, suggesting that the pathogenic mechanism of MPGN caused by cryoglobulinemia with HCV infection may be similar to that of MC. Glomerular damage is mainly due to the deposition of immune complexes formed by cryoglobulins (composed of HCV antigens, anti-HCV-IgG, and monoclonal or polyclonal IgM) in the subendothelial and mesangial regions.
From the perspective of the pathogenic mechanism of MC, the immune response produced by HCV after invading the human body is the result of the joint action of cellular and humoral immunity. The main mechanism is B cell-mediated humoral immunity, because HCV envelope E2 protein is the key protein that mediates HCV cell invasion, and has a high affinity for hepatocytes to express CD81 receptor, and B cells also express this receptor, so B cells are the target cells of HCV extrahepatic infection, after HCV infection, the activation threshold of B cells is reduced, and HCV-dependent gene translocation can protect B cells from apoptosis, and a large number of monoclonal and polyclonal autoantibody IgM with rheumatoid factor activity are produced, resulting in the occurrence of MC。
Promote the formation of rheumatic factors such as immunoglobulins. And due to the cross-immunoreactivity between some antigen components of HCV and rheumatoid factor (IgMK), the rheumatoid factor produced by B cells is combined with anti-HCV-IgG to form a megaimmune complex, which does not bind to the cell transport system, escapes the clearance of macrophages, and remains in the blood circulation for a long time, the aggregation of monocytes and neutrophils, oxidation, the release of proteases and cytokines, cause damage to renal vascular glomers and basal membrane cells and changes in the permeability of vascular glopeins, leading to the occurrence of MPGN.
2、非冷球蛋白血症肾小球肾炎
HCV infection can induce a specific humoral immune response, and the body produces anti-HCV neutralizing antibodies after infection, and then forms circulating immune complexes (CICs), mainly IgG-type CICs, which can deposit glomeruli and activate complement, resulting in glomerulonephritis. Immunohistochemical methods showed HCV core antigen expression at the site of glomerular injury in membranous nephropathy.
CIC is deposited in the glomerular capillary subendothelial and mesangial regions, inducing cell proliferation and inflammatory cell infiltration, causing MPGN. At present, it is believed that the subepithelial immune complex is mainly formed in situ, and only substances with a molecular weight of less than 1x106Da can cross the glomerular basal membrane and localize under the epithelium. The molecular diameter of HCV core antigen is close to that of hepatitis B e antigen, and it may be implanted under the epithelium through the glomerular capillary basal membrane, where it binds to circulating antibodies against HCV core antigen to form in situ immune complexes and lead to the occurrence of membranous nephropathy. Of course, the location and amount of deposition are related to the molecular size, affinity, and ability of the body to clear the immune complex. Activation of complement, production of proinflammatory and chemokines, monocyte/macrophage exudation, release of proproteases and oxidants, disrupted glomerular filtration barrier after in situ formation or deposition of circulating immune complexes, causing proteinuria and renal insufficiency.
(2) Viral factors
HCV-RNA and related proteins have been found in mesangial cells, tubular epithelial cells, and endothelial cells, suggesting that direct infection of kidney cells by HCV is a possible cause of nephritis. It has been demonstrated that the renal parenchyma expresses CD81 and SR-BI receptors, allowing HCV to bind to cell surface receptors. The presence of HCV-associated protein in the mesangial region is associated with more significant proteinuria, which also provides evidence that HCV directly causes disease in the kidney.
In addition, viral infections can cause cytopathic effects, including pro-apoptosis, growth, or degeneration. In HCV-infected B lymphocytes, the core protein of HCV has been observed to promote apoptosis, while the membrane protein can promote cell growth and degeneration. Therefore, the equilibrium between these viral proteins can determine the consequences of HCV infection. In recent years, it has also been discovered that HCV does not need to enter cells and replicate, but can cause cytopathic effects as long as it attaches to cells through certain receptors on the cell surface. For example, some researchers have found that the expression of Toll-like receptor 3 (TLR3) was enhanced in the mesangial cells of HCV-MPGN patients, accompanied by an increase in viral load, an increase in cytokines and chemokines such as interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-8 (IL-8), monocyte chemoattractant protein-1 (MCP-1) and RANTES, and decreased renal function. This observation provides some clues that cytopathic effects may be involved in nephritis pathogenesis.
(3) Genetic factors
Some researchers have studied the genetic background of HCV-GN and found that the frequency of DRB1*11 gene increased significantly and decreased significantly in patients with nephritis in this disease, while the frequency of DRB1815 gene decreased significantly, suggesting that the former may be related to the pathogenesis, while the latter has a protective effect.
III. Pathology
The most common pathological changes of HCV-GN were type I MPGN, followed by type III MPGN and MN. Other pathological types are less common, such as focal segmental glomerulosclerosis (FSGS), mesangial proliferative glomerulonephritis, IgA nephropathy, fibrous and immune tentacle glomerulopathy, thrombotic microangiopathy, etc., which are occasionally reported.
MPGN immunofluorescence showed that immunoglobulins and complement were deposited in the vascular loops and mesangial regions in a granular manner, and IgG was the most common immunoglobulin, followed by IgM and IgA. Complement C3 deposition is present in almost all cases, and C4 and C1q deposition can be seen in some cases. The composition of glomerular capillary loop deposits in patients with cryoglobulinemia is often consistent with the composition of cryoglobulin. Usually IgM and IgG are deposited in glomerular vascular loops, and IgM is often accompanied by the deposition of kappa light chains, glomerular capillary loops and lumen may be accompanied by complement C3, C4, C1q and cellulose deposition, and IgA deposition is rare.
Glomerular mesangial cells and stromal hyperplasia by type I MPGN light microscopy, glomerular lobulation or mesangial nodular lesions. Diffuse intracapillary hyperplasia leads to narrowing of the capillary lumen, basal membrane thickening, and silver staining shows mesangial insertion of peripheral capillary loops in the glomeruli, forming a "dual-track sign".
The differences between HCV-GN-related MPGN and idiopathic MPGN are as follows: (1) the inflammatory cell infiltration in the glomeruli is more prominent, and a large number of monocytes, macrophages and a few polymorphonuclear leukocytes are retained in the glomerular capillary lumen, which significantly increases the number of glomerular cells, and a large number of mononuclear macrophages in the capillary lumen can be seen in CD68 staining. (2) A large number of deposits can be seen under the endothelium, of different sizes, some are segmental, some are globular, which can occupy the entire glomerular capillary loop, resulting in glomerular capillary lumen such as "thrombosis"-like substances, which are formed by the deposition of immune complexes containing cryoglobulin, HE staining is eosinophilic, PAS is strongly positive, Masson staining is red, non-silverophilic, Congo red staining is negative, and mononuclear cells are common in the sediments. (3) The "dual-track" distribution is relatively limited, and some cases are accompanied by renal vasculitis, cell infiltration and fibrinoid necrosis of small and medium-sized arteriolar wallitis, and crescent formation in some cases.
A large number of cases of intraluminal "thrombosis" and/or vasculitis can be seen pathologically, and acute nephritic syndrome and rapid decline of renal function are prone to clinical practice. (4) HCV-GN-related type III. MPGN changes are the addition of repetitelial erythrophilic deposition on the epithelial side on the basis of type I, and some patients have the formation of "nail process" (silver staining) [4].
The pathological changes of HCV-GN-related MN were similar to those of primary MN, and immunofluorescence showed that a variety of immunoglobulins and complement were deposited in the glomerular capillary loops in granular form in the glomerular capillary loops, and some patients could be deposited in the mesangial region at the same time. The type of immunoglobulin and complement deposited is similar to that of HCV-GN-associated MPGN. Under light microscopy, in addition to the changes in the basal membrane, there are also changes in atypical membranous nephropathy such as mesangial hyperplasia, and some patients have severe chronic lesions of the tubulointerstitium. Other rare pathological changes under light microscopy include capillary hyperplasia, capillary loop collapse, loop necrosis, crescent formation, and intraglomerular inflammatory cell infiltration.
HCV-GN-related MPGN electron microscopy can show glomerular basal membrane stratification, bi-track, between the bilayer basal membrane, different numbers of mesangial cells and mesangial matrix components, dense matter and cytosolic components of endothelial cells, occasionally single nucleus and neutrophils, some patients can have subendothelial electron density, podocytes are often swollen, foot process fusion. Patients with mixed cryoglobulinemia had relatively unique changes in the kidney pathology electron microscopy. Crystalline substances in various forms (fibers, microtubules, lattices, globules, etc.) are seen in subendothelial and intraluminal "thrombosis" deposits, suggesting cryoglobulin deposition. Many monocytes containing large phagolysosomes appear in the capillaries.
Bibliography:
[1] Desbois AC, Cacoub P, Saadoun D. Cryoglobulinemia: An update in 2019. Joint Bone Spine. 2019; 86(6):707-713.
[3] Tang SC, Lai KN. Hepatitis C virus-associated glomerulonephritis. Contrib Nephrol. 2013;181:194-206.
[4] Ozkok A, Yildiz A. Hepatitis C virus associated glomerulopathies. World J Gastroenterol. 2014; 20(24):7544-7554.
[5] Ozkok A, Yildiz A. Hepatitis C virus associated glomerulopathies. World J Gastroenterol. 2014; 20(24):7544-7554.