Microparticles - culprits in the pathogenesis of systemic lupus erythematosus?

KEYWORDS:

• Systemic lupus erythematosus
• lupus nephritis
• pathogenesis
• microparticles
• extracellular vesicles
• immune complexes
• proteomics
• galectin-3 binding protein
• neutrophil extracellular traps

Membrane-derived extracellular vesicles include exosomes, microvesicles, and apoptotic bodies. The two latter vesicle subsets are produced during cell activation and apoptosis, respectively. Microvesicles are typically defined by sizes within the range of 0.1–1 µm in diameter [1,2]. Apoptotic bodies may be included in the microvesicle size range but do also exceed this depending on stage of apoptosis [3]. Despite differences in genesis and size, microvesicles and apoptotic bodies are often collectively referred to as microparticles (MPs). First described as ‘platelet dust’ by Peter Wolf in 1967, these tiny circulators were initially dismissed as inert cell artifacts. However, in recent years, such outlook has subsided in favor of evidence of great diversity, enrolling MPs in many physiological processes, including coagulation, inflammation, and intercellular communication [4,5]. This recognition has doubtlessly nurtured the growing interest of MPs as potential pathogenic players not least in the context of the systemic autoimmune disease, systemic lupus erythematosus (SLE). SLE is characterized by hyperproduction of autoantibodies against especially nuclear antigens and immune complex (IC) deposition in various parts of the body, leading to systemic inflammation and tissue damage. Furthermore, increased type I interferon (IFN) activity is frequent and suspected as a major disease contributor [6,7]. With defective removal of apoptotic material being recognized as a central pathoetiologic element of SLE [6], the suspicion of apoptotically derived MPs to have a role in this matter is not unforeseen.

Apoptotic MPs that derive from nucleated cells may contain chromatin including dsDNA and RNA [8]. It is, however, also of great interest that MPs derived from activated platelets may contain mitochondrial DNA [9] and partake in the formation of MP-ICs [10]. Further, oxidized mitochondrial nucleoids released by neutrophils have been shown to activate type I IFN production in SLE [11]. It could even be speculated that apoptotically derived chromatin in SLE patients may have increased immunogenic potential due to posttranslational modifications occurring during disturbed apoptosis or delayed clearing. In this respect, it is of particular interest that DNase 1L3 seems to restrict the immunogenicity of apoptotic cell-derived MPs in DNASE1L3-deficient mice [12]. SLE-MPs are also characterized by the carriage of immunoglobulins (Igs) [13,14] that in part bind to dsDNA [15]. MP-ICs in SLE could thus be regarded as large ICs with capacity for tissue deposition as well as sources of complexed nucleic acids, which may stimulate type I IFN production by interaction with internal nucleic acid sensors [7]. Clinically, MP-ICs have been associated with disease activity and vascular damage in SLE patients [10,14].

An obvious question to address is whether the MP-profile in SLE patients differs from that of healthy individuals. Several studies have quantitated MPs in plasma but with conflicting results. Whereas some have reported increased occurrence of MPs among SLE patients [16,17], others found it to be similar or even decreased relative to healthy subjects [5,18,19]. These variations may reflect a high degree of dynamism and/or pleiotropic effects of individual disease courses but also strongly depend on pre-analytical parameters and analytical methodologies. Particularly in SLE, the formation of ICs may perturb analysis of MPs due to overlapping biophysical properties [20]. Not only quantitative aberrations are of interest in disclosing potential pathogenic roles of MPs in SLE but also qualitative changes. The increased proportion of MPs not binding phosphatidylserine-binding probes such as annexin V and lactadherin [17] could be explained by reduced clearing of such MPs as externalized phosphatidylserine and the phagocytic phosphatidylserine receptor mediate effective phagocytosis of apoptotic cells by macrophages [21]. However, the safe clearance of apoptotic cells and remnants also implicates a wider range of scavenger receptors and serum opsonins and the redundancy in this system indicates its vital significance. Besides the rare state of C1q deficiency and lupus-like disease [22], the finding of reduced amounts of C3b/iC3b on SLE-MPs molecules supports the notion of diminished opsonization of MPs in SLE patients [19]. Albeit this would at least partly explain an increased load of peripheral MPs in SLE, the investigation did not reveal any quantitative differences in such regard. This could be explained by an increased tissue deposition of MPs in SLE and one may speculate if numeric aberrations are mainly visible in specific subsets of MPs rather than the collective.

Proteomic studies show that plasma MPs from SLE patients hold a characteristic signature that deviates markedly from normal [23,24]. These MPs tentatively designated ‘luposomes’ are characterized by lack of or reduced mitochondrial function, indications of disrupted cytoskeletons, and/or an excess of glycolytic proteins, immunoglobulins, ficolin 2, and galectin-3 binding protein (G3BP). Complementary to these findings are flow cytometric analyses, in which circulating MPs with surface-bound Ig [25] or G3BP [18] were quantitated and found to be highly increased among SLE patients relative to healthy individuals. G3BP is a glycoprotein with properties that may have bearings on the pathogenesis of SLE. It belongs to the scavenger receptor cysteine-rich domain family and is widely expressed in extracellular fluids [26]. Furthermore, it is type I IFN-inducible [18] and exhibits a high binding capacity toward components of the glomerular basement membrane (GBM) including collagen IV and nidogen [27]. Considering the association between G3BP and SLE, the mentioned qualities of G3BP permit an intriguing scenario. Do G3BP-positive MPs have a strong affinity for the GBM? And if so, do these particles possess a particular pathogenic potential? Although apoptotic MPs are known to carry DNA/chromatin that is accessible to autoantibodies [8,15], the antigenic availability of activation-derived MPs is less explored. However, a study on murine macrophages showed that MPs released from cells stimulated with Toll-like receptor (TLR)-3 and TLR-4 ligands possessed several similarities with apoptotic MPs, including binding of anti-DNA and anti-nucleosome antibodies [28]. Since G3BP is a type 1 IFN-inducible protein, MPs deriving from type 1 IFN activated cells may have the potential of expressing G3BP. To this end, it is of interest that lupus nephritis (LN) patients exhibit a glomerular G3BP/IgG colocalization pattern specifically in the GBM – a pattern that seems to be absent in non-inflammatory kidney tissue [18]. This finding is somewhat supportive of two circumstances, namely the presence of G3BP-positive MPs in the glomeruli of LN patients and the containment of ICs on these particles. Hence, MPs rich in G3BP might act as IC-carrying vehicles that mediate the deposition of ICs in the kidneys of SLE patients [29]. Yet, the source of such ‘luposomes’ remains to be identified.

MPs may have other sinister roles in SLE besides being perceived as mere ‘passive’ IC carriers. Dieker et al. demonstrated that MPs from SLE patients have strong proinflammatory effects on pDCs and myeloid dendritic cells [5]. Moreover, a comparable response has been observed on granulocytes where SLE MPs had the ability to initiate degranulation and production of reactive oxygen species [30]. The formation of neutrophil extracellular traps is considered to partake in the pathogenesis of SLE and lupus nephritis [31], and this phenomenon has been shown to be triggered by SLE-MPs independently of the formation of reactive oxygen species [32]. It was even found that this mechanism was dependent on the acetylation levels of histones in the SLE MPs, which brings us back to the notion of apoptosis-related modifications of MP constituents.

Besides potential effector roles of MPs in SLE, it seems plausible that some MPs are immunogenic while others are IC formators or both. Further, MPs may also carry significant information regarding the apoptotic process of the parental cells, their potential for clearance, their immunogenic potential, and they may even reflect the metabolic state of their parental cells [24]. This may hopefully leave us with many clues to help us solve the complex mystery of SLE pathogenesis in which there certainly may be several culprits. Based on current evidence, future studies will hopefully establish if and to what extent MPs are one of them.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This work has been in part been funded by The Danish Rheumatism Association (A3865) and the Lundbeck Foundation (R208-2015-4018).