Lupus nephritis : Update on aetiopathogenesis and controversies in classification

Correspondence: Dr. Anil Dev Pant MBBS, MD Department of Pathology Tribhuvan University Teaching Hosptial, Kathmandu, Nepal. Email: dradpa@gmail.com Abstract: Systemic Lupus Erythematosus; a chronic autoimmune disease; is characterized by loss of tolerance against its own antigens and leads to production of autoantibodies and causes formation and deposition of immune complexes in different organs. Recent articles have been trying to unravel the mysteries of SLE. Different theories that have been proposed for the aetiopathogenesis of SLE are a)The circulating immune complex theory, b) The direct binding to endogenous renal antigens theory, and c) binding of antibody to antigens that were previously ‘planted’ into the kidney. ABSTRACT


Systemic lupus erythematosus; kidney;
Immune complex 676 3) Another method is by binding of antibody to antigens that were previously 'planted' into the kidney.These antigens were probably derived from dead cells, and mostly nucleosomes are implicated.
It is the latter 2 mechanisms where most of the current evidence points towards.The antigens are usually of the IgG type, fix complement, and are highly crossreactive.Deposition of antibody alone is not sufficient for development of disease.Complement components have been found in renal biopsies along with complement activation products observed in sera as well.Deficiencies in early complements of classical pathway, i.e.C1q, C4 and C2 in patients have also been implicated in the development of lupus. 3e planted nucleosome antigen hypothesis has been gaining favour as the mechanism by which lupus nephritis occurs.The nucleosomes may originate from circulating or intraglomerular apoptotic cells and are associated with glomerular basement membrane or mesangial matrix.It has been suggested that nucleosomes are trapped in by glomerular components such as type IV collagen, heparan sulfate and other negatively charged particles.[6][7][8][9] Because anti-nucleosome antibodies may be positive when anti-dsDNA (anti double stranded DNA) is negative, these antibodies are especially important in those cases which clinically resemble lupus but are anti-dsDNA. 10

Extra-renal pathologic mechanisms
How do nuclear antigens get exposed during apoptosis?Under normal circumstances, these antigens are rapidly cleared, inhibiting interactions with T cells.However, in selected patients, there may be inherited defects in mechanisms that ensure low levels of chromatin in extracellular compartment and removal of these dead cells by apoptosis.This leads to degeneration of its components which help the body to differentiate it from viral nucleic acids. 10,11ere may be persistent activation of dendritic cells and B-cells; this can even overcome the anergy of auto-reactive B cells.Lymphocytic mitogens like B-cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) are secreted.They promote B-cell maturation and plasma cell survival (a potential therapeutic target).
Finally, environmental triggers include viral and bacterial infections.Ultraviolet light causes death of keratinocytes and causes increase in extracellular material.Drug induced  Hormones stimulate immune-regulatory pathways leading to enhanced autoimmunity.
Thus, there is an initial clonal expansion of lymphocytes which causes production of auto-antibodies and immune complex deposition.Environmental factors can further aggravate the process.

Intra-renal pathologic mechanisms
The traditional concept that circulating immune complexes passively deposit in the kidney has been challenged by the recent evidence that they may actually be formed by binding to nucleosomes from renal cells.
Cytotoxic T cells, TH17 T cells and B cells are present in the kidney in lupus nephritis.Macrophages may also contribute to the progression of the disease.
Limitations in understanding the mechanism of lupus have led to limited treatment modalities.It is also critical to understand the role of B lymphocytes in immune system.B lymphocytes are part of the adaptive immune system and have roles of secreting immunoglobulins and also act as antigen presenting cells to T cells.Bhat and Radhakrishnan focused their study on the role of B lymphocytes in lupus nephritis.With better understanding of B lymphocytes dysfunction, they studied newer treatment methods that target Bcells.The therapies act by causing B cell depletion, blockade of T cell co-stimulation or blockade of B cell stimulation and are showing promise but need to be evaluated further. 12

Site of deposition of immune complexes:
The sites of deposition of immune complexes in different parts of glomeruli depend on different factors: 1) circulating levels of antigen/antibody, 2) specificity, 3) avidity, 4) size, and 5) charge.Location of immune deposits also may influence the main effector mechanism. 13perimental models have shown that small cationic immune complexes deposit in the sub-epithelial space, where they can cause proteinuria by causing injury to podocytes.If deposits are in sub-epithelial space, the membrane attack complex which is generated by complement activation injures the endothelial cells -this leads to alteration in glomerular barrier function and significant proteinuria (inflammatory component is absent because the basement membrane prevents inflammatory cells from reaching the deposits).
Intermediate sized complexes may be deposited in the mesangium, where initial clearance may take place; later, if deposition continues, it may cause glomerular disease.Larger immune complexes may deposit in the sub-endothelial space where they come in contact with inflammatory mediators through the endothelial pores, leading to recruitment and activation of leukocytes.If the immune deposit is located near vasculature, i.e. in the sub-endothelial or mesangial space, inflammatory cells are recruited and cause injury. 14[15][16][17][18] Class I: Minimal mesangial lupus nephritis

ISN/RPS 2003 CLASSIFICATION OF LUPUS NEPHRITIS
The normal category has been removed and this term is used to represent lupus nephritis with normal appearing glomeruli but presence of mesangial deposits by immunofluorescence.

Class II: mesangial proliferative lupus nephritis
This category is characterised by mesangial proliferation seen in light microscopy and presence of mesangial deposits in immunofluorescence.Even rare sub-endothelial or sub-epithelial deposits by immunofluorescence or electron microscopy are permissible.However, presence of sub-endothelial deposits by light microscopy warrants upgrading to a higher class, i.e III or IV.(fig. 1)

Class III and IV: focal and diffuse lupus nephritis
Both these classes are actually a continuum of the disease, with the differences being only in the severity and distribution.Class III lesions are typically segmental and the endocapillary proliferation involves less than 50% of the glomeruli.Class IV lesions show endocapillary proliferation involving 50% or more of the glomeruli.(fig. 2) Class IV lesions have been subcategorised as IV-S (if proliferation is only segmental) and IV-G (if it is global and involves equal to or more than 50% of the glomerular tuft).
Based on activity and chronicity indices, they can be further subdivided into purely active (A), purely chronic (C), or mixed type (A/C).
The term proliferative has been removed because not all III and IV lesions will show proliferative features.Other lesions that lack endocapillary proliferation which can be within this group include extra capillary proliferations/ crescents, membranoproliferative features, and in cases where there is no endocapillary proliferation but presence of sub-endothelial wire loop deposits.Even global or segmental sclerosis has been included in this category as chronic lesions and they are thought to be the sequelae of previous active lupus nephritis.
A controversy exists about the sub-classification into IV-S and IV-G.IV-S patients were initially thought to have more fibrinoid necrosis, less immune deposits and worse prognosis.This lead to a hypothesis that there were different mechanisms between the two sub-classes.Supporting this hypothesis, initial studies showed difference in survival in the two sub-classes.However, other more recent Pant AD et al.
studies have challenged this theory because there were no differences shown in survival.[21]

Class V: membranous lupus nephritis
There is presence of segmental or global sub epithelial immune deposits.There may also be mesangial proliferation and mesangial deposits seen on immunofluorescence.(fig.3 & 4) If features of class III or IV lupus are present, at least 50% sub epithelial deposits in at least 50% of the glomeruli are required in order to give the additional diagnosis of class V (i.e.III + V, or IV + V).

Class VI: advanced sclerosing lupus nephritis
The is the late stage in which more than 90% of the glomeruli are globally sclerotic.There should also be no evidence of any ongoing activity.

TUBULOINTERSTITIAL LESIONS
During reporting, tubulointerstitial lesions should be mentioned as well.These include tubular atrophy, interstitial fibrosis and interstitial inflammation.Immune deposits have been seen by light microscopy and/or electron microscopy in peritubular regions.The degree of inflammation and fibrosis corresponds to the impairment of renal function and progression of lupus nephritis.

VASCULAR LESIONS
Immune deposits are seen around vessel walls, especially in class IV lupus nephritis.These deposits are detected by immunofluorescence or electron microscopy.In few patients, large deposits cause intimal expansion and may cause narrowing or even blockage of the lumina of blood vessels.Because an inflammatory component is lacking, the term lupus vasculopathy is used.
When there is association with fibrinoid necrosis with leukocyte infiltration of the vessel wall, this entity is termed lupus vasculitis -a true picture of vasculitis is present.This entity is even rarer than vasculopathy.
Both these aforementioned lesions are associated with a worse prognosis. 22,23

LUPUS PODOCYTOPATHY
These are cases of SLE presenting with nephrotic syndrome and have minimal change disease or focal segmental glomerulosclerosis like features.Only minimal mesangial immune deposits are seen in these cases, and the extensive foot process effacement is not explained.Kraft et al concluded that nephrotic syndrome can occur in SLE without features of class V glomerulonephritis (peripheral capillary wall deposits), and these cases should be referred to as 'lupus podocytopathy'.These patients also undergo rapid remission after treatment with steroids, similar to typical minimal change disease patients. 24,25

CONCLUSION
In closing, it is quite evident that lupus nephritis is a complex disease with unexplained questions in aetiopathogenesis, despite such extensive research.More studies need to be conducted in order to shed more light on the process, so that more effective treatment modalities can be developed.
In addition, the ISN/RPS classification, though much more mature and reproducible than the previous ones formulated by WHO, still has some controversies, especially in class IV-G or IV-S which need to be resolved.This will probably be done in a future classification, which is now due for an update.

Figure 1 :
Figure 1: Mesangial Lupus Nephritis, Class II showing mesangial cell proliferation and increased mesangial matrix

Figure 2 :Figure 4 :Figure 3 :
Figure 2: Lupus Nephritis Class IV with endocapillary and mesangial proliferation.Note a crescent almost surrounding the whole glomerulus