Targeting BAFF and APRIL in Systemic Lupus Erythemathosis A comparison of trials of three BAFF/APRIL inhibitors Figure 1: B-cell targeting therapies to treat systemic lupus erythemathosis (1) Abstract: Systemic Lupus Erythemathosis (SLE) is a complex autoimmune disease. Diagnosis and prognosis are difficult and have a lot of confounders. Progress of the disease is thus followed by a number of measures, such as damage indices like SELENA/SLEDAI and BILAG, B-cell numbers, immunoglobulin levels, in particular autoantibodies such as anti-dsDNA antibodies, and complement C3 and C4 levels. B-cell proliferation and activation are overstimulated in SLE, particularly through the BAFF/APRIL stimulated pathways. A number of BAFF/APRIL targeting biologicals have been approved for use in treatment of SLE or are currently in clinical trials. Three of these are highlighted here: Belimumab, Blisibimod and Atacicept. Belimumab, a fully humanized anti-BAFF antibody, is already approved by the USFDA. In clinical studies it has been show to lower the damage index, B-cell numbers and immunoglobulin levels, it increases complement levels and may reduce the need for steroids. Blisibimod, a synthetic peptibody, is in stage II trials. Like Belimumab, Blisbimod lowers the damage index and B-cell numbers, increases complement levels and may reduce the need for steroids. Ataticept, a fusion protein, is in stage III trials. Atacicept also lowers the damage index, B-cell numbers and immunoglobulin levels, increases complement and may reduce the need for steroids. The differences are relatively small and to determine the most promising of these treatments close attention has to be paid to the data from trials. So far, the fully human antibody Belimumab seems to retain the advantage over biologicals like Blisibimod and Atacicept. Introduction: Lupus: Systemic Lupus Erythemathosis (SLE) is a complex and heterogeneous autoimmune disease. The pathology is so complex that it can be seen as a complex of immune diseases. The symptoms such as general malaise, fever, lethargy, renal and pulmonary problems as well as cardiac, nervous and hematopoietic manifestations are common. Because of the combination of symptoms patients have it is often hard to recognize lupus (2). Prevalence of SLE is 20-150 per 100.000 persons. Women with lupus outnumber men by ten to one, and the disease is more prevalent in Africa and Asia than it is in Europe. The reason for the differences between these groups is unclear. Diagnosis of SLE is difficult, and a large number of tests need to be done to ensure proper diagnosis. In addition to broad symptoms like rashes, arthritis, seizures, periods of lethargy and nephritis, the diagnosis has to be confirmed with laboratory tests. As has been popularized in the American hit TV series House, the main manifestations of lupus are not unique to SLE, in fact SLE shares manifestations with more common afflictions such as depression, fibromyalgia, infection, and many connective tissue diseases. From a list of eleven symptoms, a patient needs to present at least four, before laboratory tests can add to the conclusions. Immunological measures for SLE includes high levels of Anti-Nuclear Antibodies (ANA), Anti double-stranded DNA antibodies (Anti-dsDNA), low C3 and C4 complement levels, anti-smith antibodies and antiphospholipid antibodies (3). Progress of disease is followed using a number of indexes that score damage to organs, such as the SLICC/ACR damage index (SDI) (4), the SELENA/SLEDAI and British Isles Lupus Assessment Group (BILAG). These indexes measure progress of disease by scoring a number of symptoms, with each type of symptom weighted to indicate how important the symptom is for the prognosis. Examples of symptoms include a specific headache, evidence of proteinuria indicating renal damage , and evidence of chronic inflammation like rashes, arthritis and vasculitis (3). Measurements taken for the damage index include, among others, platelet count, complement count and leukocyte count in combination with assessment of pain, psychoses and sensory abnormalities. These measurements for damage indexes measure damage already present. While types of damage can be indicative for prognosis, the measurements do not detect what is happening with the immune system in a direct way. Immunological measurement, such as the complement levels, can show activity of the disease. Activity of SLE can be important for monitoring medication. Highly active disease sometimes requires suppression of acute inflammations. A burst of particularly active SLE is called a ‘flare’, though how exactly a flare is defined is still a matter of debate (5), though many adhere to a temporary increase of the SLEDAI score by four or more points. Treatment: In the short term patients with SLE can be treated with non-steroidal anti-inflammatory drugs (NSAIDs). These drugs only counter acute inflammation and have no long term immunosuppressive properties. NSAIDs need to be used only for short periods of time for reduction of symptoms, as prolonged use strongly increases risk of strokes, ulcers and renal failure (6). Main therapy in treating SLE is relatively short courses of corticosteroids which are immunosuppressant but cause osteoporotic fractures, avascular necrosis and cataract formation when used in high doses (7). In addition to being immunosuppressive in the long term, they are anti- inflammatory in the short term, making them efficient in heading off minor flares in serologically active disease. Corticosteroid therapy can be supplemented with antimalarials such as hydroxychloroquine (6,8). Other antimalarials are used but they have strong side effects. Hydroxychloroquine is immunosuppressive and additionally is known to have cardioprotective properties. In acute situations, corticosteroid use can be reduced or supplemented by lymphocytes inhibitors such as azathioprine and cyclophosphamide, both of which come with severe side effects. Supplementary drugs and alternatives are aimed at reducing immediate anti-inflammatory drugs, and the risk anti-inflammatory drugs come with. So far all therapies come with their own risks and require low doses. Pathogenesis Over 200 genes are involved with the aetiology of SLE, and the disease is thought to be triggered by a combination of hereditary factors and environmental triggers such as UV exposure and exposure to certain drugs, such as penicillamine. The nature of the genes involved in SLE and their contribution to pathogenesis of SLE is varied (9). Among the list are genes involved in nucleic acid sensing and interferon production, T-cell activation and B-cell activation. During normal inflammation, debris remains from apoptotic lymphocytes. In SLE patients this debris is insufficiently cleared. Faulty nucleic acid sensing may lead to exaggerated activation of the adaptive immune system, sometimes exacerbated by high interferon production. Activation of T and B-cells through antigen receptors is altered, leading to increased activation and longevity of active T and B-cells (10). T-cells are aberrantly activated when the intracellular domain of CD3 binds the spleen tyrosine kinase instead of the canonical CD3-ζ associated protein ZAP70 (11), leading to increased activation of B-cells, depleting the population of naïve B-cells and increasing the number of plasma cells (12). The resulting glut of peripheral blood mononuclear cells goes into apoptosis, and the debris is insufficiently cleared (13). This debris, including nuclear components, as well as the contribution of B- cells in general, may trigger production of auto-antibodies. The auto-antibodies then activate the immune system to attack healthy tissue. As a further consequence, complement is depleted by continuous activation. Tissue damage is mediated by combinations of auto-antibody-antigen immune complex deposition and apoptosis of tissue cells induced by cytokines. Large amounts of auto-antibodies and other inflammatory molecules cause plaques and chronic inflammation. B-cells: One of the major hallmarks in autoimmunity is presence of autoantibodies, such as antibodies directed against double stranded DNA. These antibodies are produced in mature B cells, thus mature B cells are a target for novel therapies to partially replace or supplement corticosteroid treatment. During inflammation, inactive CD19+/CD20+ B-cells (immature, figure 2) are activated and express CD27 and IgD. These CD27+/IgD+ B-cells mature (Mature, figure 2) and one subset becomes IgD+/CD27+ memory B-cells, while other subsets switch class to IgD+/CD27- or IgD-/CG27- (plasma, figure 2) and secrete immunoglobulins (14). In SLE, the population of B-cells that is affected most is CD19/CD20+ naïve B-cells. They are activated and subsequently undergo apoptosis and aberrantly undergo necroptosis, leaving debris in their wake (12,15). Levels of immunoglobulins can be used as a measure of B-cell activity, but not as a measure of SLE disease activity. Lymphoid Pro-B Pre-B Immature Mature Plasma CD10 CD19 CD20 CD21 CD22 CD23 CD27 CD38 CD138 Figure 2: Maturation of B-cells with associated cell markers. Immature B-cells are recognized by expression of CD19 and CD20 and lack CD27. B-cells gain CD27 when maturing and becoming memory cells. Plasma cells have lost their CD27 (1). Several promising new therapies aim to modulate the function of B-cells. Antibodies like Rituximab and Epratuzumab target receptors like CD20 and CD22 to affect B-cells. Targeting of CD20 results in activation of apoptotic pathways as well as attraction of effector T-cells which induce apoptosis (16). CD22 has a more complicated web of interactions and is involved in B-cell function and CD19 mediated B-cell survival (17). Trials with rituximab targeting CD20 however showed little benefit to patients. Targeting of CD22 with epratuzumab showed more promise. The most promising target for the treatment of SLE through manipulating B cells is the BAFF/APRIL axis. Rather than targeting total B-cell populations, activation of B-cells is target. B-cell Activating Factor (BAFF) and A PRoliferation Inducing Ligand (APRIL) are essential factors for B-cell survival and maturation (18). The BAFF/APRIL axis is part of a cascade of immune cell activation.