Systemic lupus erythematosus pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Raviteja Guddeti, M.B.B.S. [2]
Overview
Pathophysiology
Lupus is an example of pathophysiology, a disturbance of the normal functioning of the body. One manifestation of lupus is abnormalities in apoptosis, a type of programmed cell death in which aging or damaged cells are neatly disposed of as a part of normal growth or functioning.
Genetics
The first mechanism may arise genetically. Research indicates that SLE may have a genetic link. Lupus does run in families, but no single "lupus gene" has yet been identified. Instead, multiple genes appear to influence a person's chance of lupus developing when triggered by environmental factors. The most important genes are located on chromosome 6, where mutations may occur randomly (de novo) or be inherited. Additionally, people with SLE have an altered RUNX-1 binding site, which may be either cause or contributor (or both) to the condition. Altered binding sites for RUNX-1 have also been found in people with psoriasis and rheumatoid arthritis.
Environmental triggers
The second mechanism may be due to environmental factors. These factors may not only exacerbate existing lupus conditions, but also trigger the initial onset. They include certain medications (such as some antidepressants and antibiotics), extreme stress, exposure to sunlight, hormones, and infections. Some researchers have sought to find a connection between certain infectious agents (viruses and bacteria), but no pathogen can be consistently linked to the disease. UV radiation has been shown to trigger the photosensitive lupus rash, but some evidence also suggests that UV light is capable of altering the structure of the DNA, leading to the creation of autoantibodies. Some researchers have found that women with silicone gel-filled breast implants have produced antibodies to their own collagen, but it is not known how often these antibodies occur in the general population and there is no data that show these antibodies cause connective tissue diseases such as lupus.
Drug reactions
Drug-induced lupus erythematosus is a reversible condition that usually occurs in patients being treated for a long-term illness. Drug-induced lupus mimics systemic lupus. However, symptoms of drug-induced lupus generally disappear once a patient is taken off the medication which triggered the episode. There are about 400 medications currently in use that can cause this condition, though the most common drugs are procainamide, hydralazine and quinidine.
Non-SLE forms of lupus
Discoid (cutaneous) lupus is limited to skin symptoms and is diagnosed via biopsy of skin rash on the face, neck or scalp. Often an anti-nuclear antibody (ANA) test for discoid patients is negative or a low-titre positive. About 10% of discoid lupus patients eventually develop SLE.
Transmission
In SLE, the body's immune system produces antibodies against itself, particularly against proteins in the cell nucleus. SLE is triggered by environmental factors that are unknown.
"All the key components of the immune system are involved in the underlying mechanisms" of SLE, according to Rahman, and SLE is the prototypical autoimmune disease. The immune system must have a balance (homeostasis) between being sensitive enough to protect against infection, and being too sensitive and attacking the body's own proteins (autoimmunity). From an evolutionary perspective, according to Crow, the population must have enough genetic diversity to protect itself against a wide range of possible infection; some genetic combination's result in autoimmunity. The likely environmental triggers include ultraviolet light, drugs, and viruses. These stimuli cause the destruction of cells and expose their DNA, histones, and other proteins, particularly parts of the cell nucleus. Because of genetic variations in different components of the immune system, in some people the immune system attacks these nuclear-related proteins and produces antibodies against them. In the end, these antibody complexes damage blood vessels in critical areas of the body, such as the glomeruli of the kidney; these antibody attacks are the cause of SLE. Researchers are now identifying the individual genes, the proteins they produce, and their role in the immune system. Each protein is a link on the autoimmune chain, and researchers are trying to find drugs to break each of those links. [1][2][3]
SLE is a chronic inflammatory disease believed to be a type III hypersensitivity response with potential type II involvement.[4]
Abnormalities in apoptosis
- Apoptosis is increased in monocytes and keratinocytes
- Expression of Fas by B cells and T cells is increased
- There are correlations between the apoptotic rates of lymphocytes and disease activity
Tangible body macrophages (TBMs) are large phagocytic cells in the germinal centers of secondary lymph nodes. They express CD68 protein. These cells normally engulf B cells which have undergone apoptosis after somatic hypermutation. In some patients with SLE, significantly fewer TBMs can be found, and these cells rarely contain material from apoptotic B cells. Also, uningested apoptotic nuclei can be found outside of TBMs. This material may present a threat to the tolerization of B cells and T cells. Dendritic cells in the germinal center may endocytose such antigenic material and present it to T cells, activating them. Also, apoptotic chromatin and nuclei may attach to the surfaces of follicular dendritic cells and make this material available for activating other B cells which may have randomly acquired self-specificity through somatic hypermutation.[5]
Clearance deficiency
The exact mechanisms for the development of systemic lupus erythematosus (SLE) are still unclear since the pathogenesis is a multifactorial event. Beside discussed causations, impaired clearance of dying cells is a potential pathway for the development of this systemic autoimmune disease. This includes deficient phagocytic activity, scant serum components in addition to increased apoptosis.
Monocytes isolated from whole blood of SLE patients show reduced expression of CD44 surface molecules involved in the uptake of apoptotic cells. Most of the monocytes and tingible body macrophages (TBM), which are found in the germinal centres of lymph nodes, even show a definitely different morphology in patients with SLE. They are smaller or scarce and die earlier. Serum components like complement factors, CRP and some glycoproteins are furthermore decisively important for an efficiently operating phagocytosis. In patients these components are often missing, diminished or inefficient.
The clearance of early apoptotic cells is an important function in multicellular organisms. It leads to a progression of the apoptosis process and finally to secondary necrosis of the cells, if this ability is disturbed. Necrotic cells release nuclear fragments as potential autoantigens as well as internal danger signals, inducing maturation of dendritic cells (DC), since they have lost their membranes integrity. Increased appearance of apoptotic cells also is simulating inefficient clearance. That leads to maturation of DC and also to the presentation of intracellular antigens of late apoptotic or secondary necrotic cells, via MHC molecules.
Autoimmunity possibly results by the extended exposure to nuclear and intracellular autoantigens derived from late apoptotic and secondary necrotic cells. B and T cell tolerance for apoptotic cells is abrogated and the lymphocytes get activated by these autoantigens; inflammation and the production of autoantibodies by plasma cells is initiated. A clearance deficiency in the skin for apoptotic cells has also been observed in patients with cutaneous lupus erythematosus (CLE).
Accumulation in germinal centres (GC)
In healthy conditions apoptotic lymphocytes are removed in germinal centres by specialised phagocytes, the tingible body macrophages (TBM); that’s why no free apoptotic and potential autoantigenic material can bee seen.
In some patients with SLE accumulation of apoptotic debris can be observed in GC, because of an ineffective clearance of apoptotic cells.
In close proximity to TBM, follicular dendritic cells (FDC) are localized in GC, which attach antigen material to their surface and in contrast to bone marrow-derived DC, neither take it up nor present it via MHC molecules.
Autoreactive B cells can accidentally emerge during somatic hypermutation and migrate into the GC light zone. Autoreactive B cells, maturated coincidently, normally don’t receive survival signals by antigen planted on follicular dendritic cells and perish by apoptosis.
In the case of clearance deficiency apoptotic nuclear debris accumulates in the light zone of GC and gets attached to FDC. This serves as a germinal centre survival signal for autoreactive B-cells.
After migration into the mantle zone autoreactive B cells require further survival signals from autoreactive helper T cells, which promote the maturation of autoantibody producing plasma cells and B memory cells.
In the presence of autoreactive T cells a chronic autoimmune disease may be the consequence.
Anti-nRNP autoimmunity
Autoantibodies to nRNP A and nRNP C initially targeted restricted, proline-rich motifs. Antibody binding subsequently spread to other epitopes. The similarity and cross-reactivity between the initial targets of nRNP and Sm autoantibodies identifies a likely commonality in cause and a focal point for intermolecular epitope spreading.[6]
Others
Elevated expression of HMGB1 was found in the sera of patients and mice with systemic lupus erythematosus, High Mobility Group Box 1 (HMGB1) is a nuclear protein participating in chromatin architecture and transcriptional regulation. Recently, there is increasing evidence that HMGB1 contributes to the pathogenesis of chronic inflammatory and autoimmune diseases due to its pro-inflammatory and immunostimulatory properties.[7]
References
- ↑ Anisur Rahman and David A. Isenberg (February 28, 2008). "Review Article: Systemic Lupus Erythematosus". N Engl J Med. 358 (9): 929–939. doi:10.1056/NEJMra071297. PMID 18305268.
- ↑ Mary K. Crow (February 28, 2008). "Collaboration, Genetic Associations, and Lupus Erythematosus". N Engl J Med. 358 (9): 956–961. doi:10.1056/NEJMe0800096. PMID 18204099.
- ↑ Geoffrey Hom, Robert R. Graham, Barmak Modrek; et al. (February 28, 2008). "Association of Systemic Lupus Erythematosus with C8orf13–BLK and ITGAM–ITGAX". N Engl J Med. 358 (9): 900–909. doi:10.1056/NEJMoa0707865. PMID 18204098.
- ↑ University of South Carolina School of Medicine lecture notes, Immunology, Hypersensitivity reactions. General discussion of hypersensitivity, not specific to SLE.
- ↑ Gaipl, U S; Kuhn, A; Sheriff, A; Munoz, L E; Franz, S; Voll, R E; Kalden, J R; Herrmann, M (2006). "Clearance of apoptotic cells in human SLE". Current directions in autoimmunity. 9: 173–87. PMID: 1639466 Abstract (full text requires registration).
- ↑ Poole BD, Schneider RI, Guthridge JM; et al. (2009). "Early targets of nuclear RNP humoral autoimmunity in human systemic lupus erythematosus". Arthritis Rheum. 60 (3): 848–859. doi:10.1002/art.24306. PMID 19248110. Unknown parameter
|month=ignored (help) - ↑ Pan HF, Wu GC, Li WP, Li XP, Ye DQ (2009). "High Mobility Group Box 1: a potential therapeutic target for systemic lupus erythematosus". Mol. Biol. Rep. doi:10.1007/s11033-009-9485-7. PMID 19247800. Unknown parameter
|month=ignored (help)