INTRODUCTION

Immunosuppressive therapy, the main treatment modality for severe lupus, has been effective in reducing some of the sequelae of systemic lupus erythematosus (SLE), such as end-stage renal disease, but has not proven uniformly effective in reducing SLE-associated morbidity and mortality. Furthermore, the medications used currently are universally immunosuppressive, thus increasing the risk of serious infections, and at the same time have a variety of toxicities in tissues and organs outside the immune system.¹^–³

When designing a therapeutic regimen that will be both effective and safe, key pathophysiologic steps in the development of autoimmunity, perpetuation of the abnormal immune response and immune-mediated tissue damage need to be recognized and targeted in a highly specific manner. In the case of SLE, the precise sequence of events that lead to the break in self-tolerance and the development of autoimmunity remains elusive, but recent studies have shed light on pathophysiologic processes that are important in the continuation of the abnormal immune response and ensuing tissue damage. Researchers have capitalized on these findings and designed novel therapeutic approaches using a variety of antibodies, fusion molecules, small chemical compounds, and gene therapy techniques. These novel therapies discussed herein (Table 48.1), have been tried in ex vivo experiments, and in murine models of lupus as well as in patients with SLE with encouraging albeit preliminary results.

TABLE 48.1 NEW TREATMENTS FOR SYSTEMIC LUPUS ERYTHEMATOSUS

TARGETING SOLUBLE MEDIATORS

The successful use of tumor necrosis factor a (TNF-a) inhibitors in rheumatoid arthritis ⁴ and Crohn’s disease speaks to the fact that the disruption of signaling among immune cells can lead to significant clinical improvement of autoimmune diseases without necessarily re-establishing tolerance. The recognition of such pivotal cytokines in SLE may lead to the design of medications that can induce remission and prevent flares.

B-Lymphocyte Stimulator

B-lymphocyte stimulator (BLyS) is a soluble molecule that belongs to the TNF ligand superfamily,⁵ and is secreted primarily by myeloid lineage cells such as monocytes and neutrophils. It binds exclusively to B cells via one of three surface receptors (BAFFR, BCDMA, or TACI),⁶ leading to activation of the nuclear factor (NF)-κB pathway, and by up-regulating survival molecules leads to increased B-cell survival.^7,⁸

Several studies have linked BLyS to autoimmunity, and specifically to overactivation of humoral immunity with the production of autoantibodies. Initially, it was shown that mice genetically engineered to overproduce BLyS ⁹ developed B-cell hyperplasia, hypergammaglobulinemia, and an array of autoantibodies including anti–double-stranded DNA antibodies, and had immunoglobulin deposition in their kidneys. Thus, these BLyS-transgenic mice developed a condition reminiscent of human SLE. It was thereafter shown that the levels of BLyS in lupus-prone mice (both NZB/NZW F1 and MRL-lpr/lpr) are elevated at the onset of disease,¹⁰ establishing BLyS as a potential therapeutic target.

Indeed, a fusion molecule of TACI (the BLyS receptor) with the Fc portion of the immunoglobulin blocked the effects of BLyS in NZB/NZW F1 mice, resulting in a decrease in proteinuria and increased survival.¹⁰ Similar clinical improvement was also shown in a rheumatoid arthritis murine model with the use of TACI-Ig,¹¹ pointing to the fact that BLyS plays a significant but not disease-specific role in autoimmunity.

Carrying these observations to humans, the levels of BLyS were shown to be persistently increased in 25% of patients with SLE while another 25% of the patients had intermittent elevations ^12,¹³ of BLyS in their peripheral blood. In addition, BLyS levels showed a strong correlation with anti-dsDNA levels,^14,¹⁵ further underscoring the potential importance of BLyS in the pathophysiology of human SLE.

Both murine and human trials led to the proposal that blocking BLyS can produce clinical improvement of patients with SLE and may decrease the need for corticosteroids and other immunosuppressive drugs. A phase I trial using a monoclonal antibody against BLyS (BLySmAb) was successfully completed in patients with SLE. BLySmAb was no different than the placebo in both effectiveness and side effects, but given the limited time (one or two infusions of BLySmAb vs. placebo) of treatment, the results of the larger and longer phase II trial that is underway are expected to be more informative.¹² This trial recruited patients with mild to moderately active SLE and at least history of anti-dsDNA antibody positivity; if positive, this trial may prove BLySmAb to be an important medication for maintenance therapy and prevention of flares in SLE.

In addition to BLySmAb, fusion molecules of the receptors TACI and BAFFR with immunoglobulin (TACI-Ig and BAFFR-Ig) are being assessed in normal individuals and primates, respectively. Furthermore, in a study using gene therapy techniques, MRL lpr/lpr mice were transfected with an adenovirus encoding TACI-Fc (a fusion molecule between TACI and the Fc portion of Ig). The transfected mice had milder nephritis and increased survival when compared to controls.¹⁶ All of these approaches to BLyS antagonism can help us to elucidate the role of BLyS in the pathophysiology of SLE and ultimately to provide a useful medication in the management of SLE.

Interleukin-6 (IL-6)

IL-6 is a pleiotropic proinflammatory cytokine that is mainly secreted by monocytes.¹⁷ IL-6 binds to the IL-6 receptor (IL-6R) on the surface of cells. In addition, IL-6 can bind to soluble IL-6R, and then the IL-6:IL-6R complex may directly activate cells. IL-6 promotes B-cell maturation and T-cell differentiation, while at the same time synergizes with TNF-a and IL-1 to promote systemic inflammatory response.¹⁸ In murine models of SLE, IL-6 has been shown to be increased and to contribute to autoantibody production. IL-6 was effectively blocked in NZB/NZW F1 mice using either an anti–IL-6 antibody¹⁹ or an anti–IL-6R antibody.²⁰ Blocking IL-6 led to decreased anti-dsDNA antibodies, decreased proteinuria, and improved survival.

This evidence from murine lupus led to the hypothesis that IL-6 neutralization may prove to be an important therapeutic option for patients with SLE. IL-6 levels are elevated in patients with SLE,^21,²² and may contribute to immunoglobulin production. Neutralization of IL-6 in vitro decreases the spontaneous secretion of immunoglobulin by lupus B cells while exogenous IL-6 increases it.²³ IL-6 may also play a significant role locally in the kidneys in lupus nephritis, as exhibited by increased IL-6 levels in urine of patients with active lupus nephritis²⁴ and in situ expression of IL-6 in the glomeruli of patients with lupus nephritis.²⁵

The anti–IL-6R monoclonal antibody, MRA, is currently being tested in phase I clinical trials in patients with SLE. MRA has been proven to be efficacious and in the lymphoproliferative Castleman’s disease and rheumatoid arthritis,^26,²⁷ making it an attractive therapeutic option for SLE as well.

Interferon

One of the earliest molecules to be targeted in the MRL lpr/lpr mouse was interferon (IFN)-α, a cytokine produced in large amounts by activated T cells. IFN-levels are high in the areas of inflammation in the MRL lpr/lpr mice, and play a significant role in the pathophysiology of murine lupus.²⁸ For these reasons, IFN-α was specifically targeted using, among other techniques, gene therapy. A plasmid that encodes the receptor of IFN- (IFN-R) fused to the Fc portion of IgG1 was injected in mice leading to the production of a molecule that effectively blocks IFN-.^29,³⁰ MRL lpr/lpr mice transfected with this construct before disease onset were protected against early death, and had lower levels of autoantibodies and milder renal disease. Interestingly, initiation of treatment as late as 4 months after the onset of disease led to significant disease improvement.

In addition to the important findings in murine lupus models, IFNs received more attention recently after the finding that IFN-α-inducible genes were “turned on” in peripheral blood mononuclear cells (PBMCs) from patients with SLE.²⁴ This “interferon signature” in gene expression in SLE was primarily seen in active lupus patients who also tended to have more severe disease manifestations such as brain and kidney involvement. In contrast to the mice models though, IFN-α and not IFN-α²⁵ was found to be the instigating factor for the “interferon signature” in SLE. More specifically, the levels of mRNA transcribed from genes that are inducible by IFN-α but not IFN were significantly higher in SLE compared to control PBMC. Furthermore plasma from patients with SLE up-regulates IFN-α inducible genes in normal PBMCs. Despite these very intriguing findings, direct measurement of IFN-α in the serum of patients with SLE did not show any difference between patients with SLE and controls, pointing to the fact that other factors maybe mimicking the IFN effect in SLE.

If indeed IFN-α is the culprit of these aberrations, it may prove important in the perpetuation of the autoimmune response in SLE because IFN-α is produced by antigen-presenting cells upon activation by immune complexes containing autoantibodies and apoptotic material.³¹ In turn, IFN-α further enhances the antigen-presenting function of monocytes,³² and activates T and B cells, and thus helps the vicious cycle of immune dysregulation in SLE.³³

Further studies are clearly needed to establish the precise role of IFN-α in lupus, and then whether it can be specifically targeted directly or indirectly via its intracellular signaling pathway (e.g., STAT-1).

Tumor Necrosis Factor-α

The use of TNF-α inhibitors has revolutionized the treatment of rheumatoid arthritis,⁴ but their use in SLE remains controversial. Patients without history of SLE have developed anti-dsDNA antibodies and even clinical lupus upon treatment with TNF-α inhibitors, and so it would seem counterintuitive to use such drugs in SLE. Furthermore, low overall production of TNF-α may be contributing to the decrease in the activation-induced cell death ³⁴ that is characteristic of SLE, while local production of TNF-α due to environmental factors (infection, UV light) may be contributing to triggering disease exacerbations. In particular, UVB—a known cause of disease exacerbation—is shown to trigger TNF-α production in the skin.³⁵ In turn, TNF-α has been shown to up-regulate the expression of the 52kd Ro/SSA protein and mRNA in keratinocytes.³⁶ Since 52kd Ro/SSA protein is expressed on apoptotic bodies, and anti-Ro antibodies have been associated with cutaneous lupus, these observations provide a link between UV-induced apoptosis and TNF-α production with anti-Ro–associated cutaneous lupus. Further strengthening this argument, TNF-α has been found up-regulated in the skin of patients with refractory subacute cutaneous lupus.³⁷ Taking these studies together, TNF-α neutralization may prove of benefit in the treatment of patients with certain manifestations of SLE such as severe cutaneous lupus.

Complement System

The immune dysregulation in SLE leads to the activation of the complement system, cleavage of C5, and production of the potent proinflammatory mediators C5a and C5b-9.^38,³⁹ The terminal complement components are found deposited in various tissues such as the kidney and the skin, where complement can cause direct damage and also attract neutrophils through the anaphylatoxic action of C5a.

A monoclonal antibody that prevents the cleavage of C5 and used in NZBxNZW F1 mice resulted in delayed onset of proteinuria and improved survival.⁴⁰ Furthermore, a similar anti-C5 monoclonal antibody (eculizumab) was proven safe and effective for the treatment of paroxysmal nocturnal hemoglobinemia.⁴¹ A clinical trial of eculizumab in patients with SLE, especially patients with active nephritis, is thus needed to assess its usefulness in the management of SLE.

TARGETING CELL–SURFACE MOLECULES AND CELL–CELL INTERACTION

Both B and T cells are targeted in SLE in an attempt to disrupt the cycle of immune-cell activation via up-regulated or missing cell-surface signaling molecules.

Anti-CD20

It is almost intuitive to target B cells in SLE given the several lines of evidence that B cells play a pivotal role in the pathophysiology of SLE. Not only do hyperactive B cells produce a variety of autoantibodies, but they also produce proinflammatory cytokines and serve as antigen-presenting cells.⁴² A medication that can efficiently deplete B cells can therefore prove effective in the treatment of SLE. Rituximab, an anti-CD20 chimeric antibody that targets and depletes B cells but not plasma cells, and has a proven efficacy against lymphoma⁴³ and rheumatoid arthritis,⁴⁴ was used in patients with difficult-to-manage SLE. Several small phase I/II studies and case series have shown promising results with as many as 80% of patients with lupus nephritis achieving at least partial remission.⁴⁵^–⁵⁰ The use of concomitant cyclophosphamide, various corticosteroid-tapering schedules, and various rituximab doses in these studies makes it difficult to ascertain the exact role that rituximab may play in the treatment of SLE. Furthermore, the high percentage of human antichimeric antibodies (HACA) reported in SLE,⁵⁰ as compared to those reported in patients with lymphoma or rheumatoid arthritis, may lower the efficacy of the drug over time or increase infusion reactions. There are also concerns as to what the effect of unpredictable and prolonged B-cell depletion will have on the ability of the already immunocompromised lupus patients to fight infectious agents. Currently underway is a large, double-blind, placebo control phase II/III trial, focused specifically on the effectiveness of rituximab in moderate to severe SLE (patients with active glomerulonephritis are excluded).