Emerging new pathways of pathogenesis and targets for treatment in systemic lupus erythematosus and Sjogren’s syndrome

Abstract

Purpose of review

Systemic lupus erythematosus (SLE) and Sjogren’s syndrome are chronic inflammatory diseases characterized by the dysfunction of T cells, B cells, and dendritic cells and the production of antinuclear autoantibodies. Here, we evaluate newly discovered molecular and cellular targets for the treatment of SLE and Sjogren’s syndrome.

Recent findings

The mammalian target of rapamycin in T and B cells has been successfully targeted for treatment of SLE with rapamycin or sirolimus both in patients and animal models. Inhibition of oxidative stress, nitric oxide production, interferon alpha, toll-like receptors 7 and 9, histone deacetylase, spleen tyrosine kinase, proteasome function, lysosome function, endosome recycling, and the nuclear factor kappa B pathway showed efficacy in animal models of lupus. B-cell depletion and blockade of anti-DNA antibodies and T–B cell interaction have shown success in animal models, whereas human studies have so far failed to accomplish clinical endpoints, possibly due to inadequacies in study design.

Summary

Discovery of novel genes and signaling pathways in lupus pathogenesis offers novel biomarker-targeted approaches for treatment of SLE and Sjogren’s syndrome.

Introduction

Systemic lupus erythematosus (SLE) and Sjogren’s syndrome (SjS) are autoimmune inflammatory diseases attributed to genetic and environmental factors causing the dysfunction of T cells, B cells, and dendritic cells and production of antinuclear autoantibodies [1–3]. Although the inflammatory process of SLE typically involves multiple organ systems, SjS is confined to exocrine glands, particularly salivary glands [3]. The cause of SLE and SjS is incompletely understood, and current therapies largely relying on the use of corticosteroids and cytotoxic antiproliferative drugs have limited efficacy and carry significant risks of toxicity. A more rational approach for therapeutic design requires a detailed understanding of disease pathogenesis. Independent lines of evidence have implicated environmental factors and genetic determinants of the host in the causation of the disease [4]. Concordance rates for SLE are approximately 25% in monozygotic twins [5]. Alternatively, the discordance rate may be as high as 70% among monozygotic twins [6], suggesting a significant role for exogenous agents [7,8]. Genome-wide association studies (GWAS) have identified numerous chromosomal loci that may harbor susceptibility genes [9]; however, the functional significance of these GWAS-derived polymorphisms is currently unknown. Therefore, a systematic characterization of the molecular and cellular basis of signaling abnormalities within the immune system that lead to autoreactivity and inflammation and their relationship with regulation of gene expression remain critical for understanding of disease pathogenesis [10].

Identification of new genes and pathways of pathogenesis that can be targeted for treatment in systemic lupus erythematosus and Sjogren’s syndrome

In the first study of this special issue of SLE and SjS, Scofield [11] provides an update of novel genes with a focus on signal transducer and activator of transcription 4 (STAT4) and interferon regulatory factor (IRF5), which are involved in cytokine signaling. The associations with these novel genetic loci remain less robust than the impact of the human leukocyte antigen (HLA) locus [5]. Another interesting polymorphism that has been linked to lupus results in a nonconserved R77H substitution of the integrin, alpha M (ITGAM) gene that encodes the alpha chain of CD11b [9], which is expressed on macrophages and may contribute to the dysfunction of these cells in SLE. Additionally, a polymorphism of interleukin-1 receptor-associated kinase-1 (IRAK1) has been identified as an X chromosome-encoded risk factor for SLE [12^••]. Importantly, deficiency of IRAK1 protects against the development of autoreactivity and nephritis in lupus-prone mice, suggesting that the increased activity of this gene may also be relevant for disease pathogenesis in patients with SLE (Table 1 [9,12^••,13^•,14–18,19^•,20,21^•,22]).

Table 1.

New genetic factors associated with systemic lupus erythematosus and Sjogren’s syndrome

Gene	Pathway	Reference
ITGAM/CD11b	Macrophage activation	[9]
IRAK1	NF-κB activation	[12••]
TREX1	IFNα production	[13•,14]
STAT4	Cytokine signaling	[15]
IRF-5	Cytokine signaling	[16]
PTPN22	Sustained TCR/BCR signaling	[17,18]
BANK1	Sustained BCR signaling	[19•]
HRES-1/Rab4	Receptor recycling	[20,21•]
mtDNA	Electron transport	[22]

BANK1, B-cell scaffold protein with ankyrin repeats 1; BCR, B-cell receptor; HRES-1, HTLV-1-related endogenous sequence; HTLV, human T-cell leukemia virus; IFN, interferon; IL, interleukin; IRAK1, IL-1 receptor-associated kinase-1; IRF-5, IFN regulatory factor 5; ITGAM, integrin alpha M; mtDNA, mitochondrial DNA; NF-κB, nuclear factor kappa B; PTPN22, protein tyrosine phosphatase, nonreceptor type 22; TCR, T-cell receptor; TREX1, 3′–5′ repair exonuclease 1.

Endogenous retroviruses have long been implicated in triggering autoimmunity through structural and functional molecular mimicry with viral proteins [23–26]. A polymorphic single-nucleotide polymorphism (SNP) of human T-cell leukemia virus-related endogenous sequence (HRES-1) endogenous retrovirus was earlier associated with the development of SLE [27,28]. Recently, polymorphic haplotypes of the HRES-1 long terminal repeat (LTR) have been associated with SLE in case–control and family studies [20]. The HRES-1 LTR harbors an enhancer that upregulates the expression of the HRES-1/Rab4 gene product, encoding a small guanosine triphosphate (GTP)ase that regulates receptor recycling through endosome traffic [29]. Glutathione S-transferase (GST) pull-down studies revealed a direct interaction of HRES-1/Rab4 with CD4, CD2AP, and the T-cell receptor (TCR) ζ chain [21^•]. Both the knockdown of HRES-1/Rab4 expression by small interfering RNA (siRNA) and the inhibition of lysosomal function increased TCRζ levels in lupus T-cells. These observations identified HRES-1/Rab4-dependent lysosomal degradation as a novel mechanism contributing to the critical loss of TCRζ in lupus T cells [30]. Thus, HRES-1/Rab4 may constitute the susceptibility gene at the 1q42 chromosomal locus previously linked to SLE by multiple laboratories [31–35].

Induction of type I interferons (IFNs) by viral DNA is a principal element of antiviral defense but can cause autoimmunity if misregulated. Cytosolic DNA detection activates a potent, cell-intrinsic antiviral response through a poorly defined pathway. A screen for proteins relevant to this IFN-stimulatory DNA (ISD) response identified the 3′–5′ repair exonuclease 1 (Trex1). Mutations in the human Trex1 gene are associated with Aicardi–Goutieres syndrome (AGS) and chilblain lupus [14]. Trex1 metabolizes single-stranded DNA reverse transcribed from endogenous retroelements. Single-stranded DNA accumulates and stimulates IFN alpha (IFNα) production in Trex1-deficient cells. Thus, TREX-1 deficiency is identified as a novel cell-intrinsic mechanism for initiation of autoimmunity by endogenous retroviral elements [13^•]. Lupus-linked polymorphisms of protein tyrosine phosphatase, nonreceptor type 22 (PTPN22) [17] lead to sustained signaling through the TCR and B-cell receptor [18]. STAT4 and IRF5 polymorphisms facilitate IFN signaling in lupus [36].

Nikolov and Illei [37] identified the activation of IFNα, B-cell-activating factor (BAFF), and antibodies to muscarinic acetylcholine receptor as critical new steps on the road to the development of SjS. Increased production of IFNα [38] and BAFF represents common pathways of SLE and SjS pathogenesis [39]. As described by Ronnblom et al. [40], plasmacytoid dendritic cells (pDCs) are a major source of increased IFNα production in SLE. Activation of pDC is attributed to stimulation of toll-like receptor (TLR)-7 and TLR-9 by endocytosed immune complexes [40]. A specific inhibitor of TLR-7 and TLR-9, called immunoregulatory sequence (IRS) 954, can block IFNα production by human pDC in response to DNA and RNA viruses and immune complexes from lupus patients. IRS 954 also prevented the production of autoantibodies, decreased proteinuria, glomerulonephritis, and end-organ damage and increased survival in the lupus prone [New Zealand black × New Zealand white (NZB × NZW)] F1 mice [41].

Alternatively, activation of pDC may be caused by the release of necrotic materials from T cells [42]. In particular, high mobility group B1 (HMGB1) protein, an abundant DNA-binding protein, remains immobilized on chromatin of apoptotic bodies; however, it is released from necrotic cells [42]. Necrotic but not apoptotic cells also release heat shock proteins (HSPs), HSPgp96, HSP90, HSP70, and calreticulin [43]. pDCs are also important sources of increased interleukin (IL)-10 [44] and promote plasma cell differentiation through production of IL-6 [45]. IFN-matured myeloid dendritic cells (mDCs) activate autoreactive T cells [46]. Clinical trials with type I IFN-neutralizing antibodies are currently ongoing in patients with SLE, which will ultimately determine the safety of IFN blockade and further clarify the relative importance of type I IFN and dendritic cell activation in SLE.

Perl et al. [47] reviewed the mechanism and consequences of the activation of mammalian target of rapamycin (mTOR) that plays a central role in dysfunction of T cells and B cells in patients with SLE. Rapamycin improves disease activity and normalizes enhanced T-cell activation-induced calcium fluxing in lupus T cells [48]. mTOR promotes the endosomal recycling of TCRζ and targets this protein for lysosomal degradation via activation of HRES-1/Rab4 [21^•]. Beyond its effect on endosomal traffic, the role of mTOR is likely to be more complex and cell type dependent. Indeed, mTOR also controls the expression of forkhead box P3 (Foxp3) and development of regulatory T cells (Tregs) [49^•,50^•], which are deficient in patients with SLE [51,52]. The function of dendritic cells is also dependent on mTOR [53]. Therefore, the inhibition of T-cell, B-cell, and dendritic cell activation and expansion of Tregs may all contribute to the therapeutic efficacy of rapamycin in murine [54] and human SLE [48]. A prospective, open-label, phase II study is currently underway to further assess the efficacy of rapamycin in SLE (http://clinicaltrials.gov/ct2/show/NCT00779194). mTOR is a sensor of the mitochondrial transmembrane potential, and its activation is a result of mitochondrial hyperpolarization caused by depletion of glutathione and increased nitric oxide production [21^•]. A prospective, double-blind, placebo-controlled phase I/II study is currently underway to evaluate the safety and efficacy of N-acetylcysteine, an antioxidant and precursor of glutathione in SLE (http://clinicaltrials.gov/ct2/show/NCT00775476). Blockade of nitric oxide production improved the outcome of nephritis in lupus-prone mice [55]; however, the safety of such an approach for long-term treatment in humans remains to be established. Activation of spleen tyrosine kinase (Syk) is mapped downstream of mTOR in lupus T cells [21^•]. R788, an orally bioavailable Syk inhibitor, was recently found to prevent the development of renal disease and to treat established nephritis in NZB/NZW mice [56]. R788 minimally affected autoantibody titers, whereas its dose-dependent effect reduced the numbers of CD4⁺-activated T cells, suggesting that T cells might be the effective targets of Syk inhibition [56].

Coca and Sanz [57] reviewed the current state of B-cell depletion (BCD) in SLE. Although the largest trials employing BCD with rituximab in patients with lupus have not achieved any of the targeted clinical endpoints, the failure may be due to problematic study design. BCD remains efficacious in pediatric patients and in a subset of adult lupus patients with antibody-mediated cytopenias [57]. The proteasome inhibitor bortezomib depletes plasma cells, possibly due to their high rate of protein synthesis, and blocks nephritis in Murphy Roths Large (MRL)/lpr and (NZB/NZW) F1 mice [58]. Selective removal of plasma cells may be a particularly worthwhile target in patients with SLE.

Mevorach et al. [59] reviewed exciting new developments in the detection of heart conduction defects in fetuses exposed to maternal anti-SjS A (SSA)/Ro or anti-SjS B (SSB)/La, or both antibodies neonatal lupus using tissue velocity fetal kinetocardiogram (FKCG). In comparison to traditional Doppler, FKCG has superior sensitivity to detect first-degree atrioventricular block (AVB). This allows timely and effective treatment with fluorinated steroids, which prevents third-degree AVB and obviates the need for permanent pacemaker.

Conclusion

Both GWAS and hypothesis-driven studies identified a series of novel genes and signaling pathways that contribute to the pathogenesis of lupus and SjS. Despite the setback with the BCD, there are increasing numbers of promising molecular targets for human clinical trials (Table 2 [21^•,48,58,60–68]). FKCG needs to be independently validated, and perhaps simplified, which could then become a useful diagnostic tool for detecting first-degree AVB and preventing irreversible third-degree AVB in neonatal lupus.

Table 2.

New molecular targets for treatment of systemic lupus erythematosus and Sjogren’s syndrome

Drug	Molecular/cellular target	Reference
Rapamycin/sirolimus	mTOR/T and B cell	[21•,48,60]
N-acetylcysteine	Glutathione/T cell	[61]
Atacicept	Blys/B-cell activation	[62,63]
Belimumab	Blys/B cell	[64]
Epratuzumab	CD22/B cell	[65]
CTLA4-Ig	CD28/T-cell costimulation	[62,66]
Tocilizumab	IL-6/Th17 cells	[67]
Medi-545/IFNα antibody	IFNα/T and B cells and dendritic cells	[68]
Bortezumib	Proteasome/plasma cell	[58]

CTLA4-Ig, cytotoxic T-lymphocyte antigen 4-immunoglobulin; IFN, interferon; IL-6, interleukin 6; mTOR, mammalian target of rapamycin; Th17, T helper cell 17.