Systemic lupus erythematosus and dysregulated apoptosis—what is the evidence?

N. J. McHugh

Royal National Hospital for Rheumatic Diseases, Upper Borough Walls, Bath BA1 1RL, UK

The recent surge of interest in apoptosis is not without reason, least of all for those involved in the care of patients with systemic lupus erythematosus (SLE). Although it has long been suspected that a problem in the waste-disposal mechanism of the body may account for phenomena such as the LE cell and haematoxylin bodies associated with SLE, insights into the tightly controlled process of apoptosis have provided a more unified understanding of how the protean manifestations of lupus may occur. Animal models of lupus have been linked directly to genetic abnormalities in apoptosis genes, apoptotic cells provide the fuel for unwanted and potentially pathogenic autoimmune responses, and defects in specific clearance of apoptotic debris suggest a rationale for current and future approaches to treatment.

Apoptosis is a genetically controlled sequence of events that culminates in the death and efficient disposal of a cell. Characteristic features include condensation of chromatin, DNA fragmentation, membrane blebbing and externalization of phosphatidylserine. Both extrinsic (e.g. receptor activation via Fas ligation) and intrinsic (e.g. DNA damage) events may commit a cell to a death pathway [1]. An intricate series of receptor mechanisms on reticuloendothelial cells are important for sequestering material released from apoptotic cells so as to prevent an inflammatory response [2]. A number of the key molecules regulating checkpoints in the apoptotic process have been implicated in lupus and are the subject of ongoing study.

The first piece of support for SLE as a primary disorder of dysregulated apoptosis comes from genetic studies of relevant murine models, some of which have been very instructive. For instance, the MRL lpr and MRL gld mouse models of lupus are caused by single point mutations in either Fas (CD95) or the Fas ligand respectively, leading to massive lymphoproliferation and autoimmune nephritis [3]. On the other hand, mice that have had the DNAase 1 gene deleted show classical symptoms of lupus, including the presence of antinuclear antibodies and immune complex-positive nephritis [4]. DNase I is a major nuclease responsible for the removal of DNA from nuclear antigens at sites of high cell turnover. Similar phenotypes arise in mice deficient in either C1q [5] or serum amyloid P [6], molecules thought to be involved in clearance of apoptotic material. Therefore, there is persuasive evidence that impairment of the waste-disposal mechanism is a major predisposition to lupus, at least in mice.

So do these genetic defects in animal models have any counterparts in human lupus? Intriguingly, they do, although very uncommonly—only one patient with SLE who has a point mutation in FasL has been described [7]. With respect to mutations of Fas, the association is with a rather uncommon familial condition that has come to be termed ‘autoimmune lymphoproliferative syndrome’ (ALPS) [8]. Sixty or so index cases of ALPS have been reported, the main clinical features being non-malignant lymphoma, splenomegaly, lymphadenopathy, pancytopenia and skin rashes, although other complications have been described, such as nephritis, Guillain–Barré syndrome, autoimmune hepatitis, uveitis and vasculitis. The most common mutation reported in patients with ALPS is in the death domain of Fas. A mutation in the death domain is associated with a higher penetrance of ALPS within families. In families, the clinical features can be highly variable and occasionally include lymphoma and other haematological malignancies. However, generally speaking, defects in Fas itself have not been found in human SLE. Rather, the reverse seems to apply, in that serum levels of soluble Fas, when studied, are increased in SLE [9], probably reflecting primed T-cell responsiveness. It would also seem that, rather than being reduced, the rate of apoptosis is increased in SLE lymphocytes when cultured [10].

So if not Fas or FasL, what other apoptotic genes may be associated with lupus? The most striking observation is the remarkably high incidence of SLE in individuals with deficiency in early components of complement, which reaches more than 90% in individuals with C1q deficiency [11]. The structurally related mannose-binding protein, which is thought also to regulate immune complex clearance, has functional polymorphisms associated with lupus [12]. Furthermore, polymorphisms affecting the affinity of type 2 and 3 Fc{gamma} receptors (Fc{gamma}RII and Fc{gamma}RIII) on macrophages have been the focus of many recent investigations. For instance, a recent study found an association between the low-affinity Fc{gamma}RIIIA-158F allele and the risk of nephritis among Caucasians, but not among non-Caucasians [13]. Cytokines that influence Fas levels and lymphocyte activation, such as interleukin (IL)-10, have functional polymorphisms in promoter regions that may be associated with lupus [14]. And large linkage studies using genome scans have implicated another effector molecule for apoptosis: poly-ADP-ribose polymerase, located on chromosome 1 [15]. Therefore, there is an increasing amount of evidence to suggest that genes regulating apoptosis, especially those concerned in clearing apoptotic debris, contribute to the genetic causation of lupus.

Aside from genetic factors, it is highly likely that environmental triggers exert their influence via apoptosis. Thus, ultraviolet light, a potent inducer of apoptosis, is an important provoking factor in cutaneous lupus. It is hardly coincidental that a common site to find apoptotic cells is at the dermo-epidermal junction, the primary lesional site in cutaneous lupus. Lupus-inducing drugs, such as sulphasalazine [16] and chlorpromazine [17], have been shown capable of inducing apoptosis, as have other environmental toxins, such as silica dust [18]. These and other extrinsic factors may contribute to the increased amounts of apoptotic material found specifically in lupus, such as circulating nucleosomes [19].

As well as circulating nucleosomes, apoptosis provides a mechanism for the release of other intracellular antigens that are often the target for autoimmune responses in lupus. Very elegant work from Rosen's group convincingly demonstrated that keratinocytes, when irradiated with UV light, develop small surface blebs containing lupus autoantigens such as the 52-kDa Ro particle [20]. Together with later-forming apoptotic bodies, these small surface blebs, which are induced early on in apoptosis, contain the major autoantigens recognized by lupus sera. Also, the entire membrane of the apoptotic cell provides a surface to which phospholipid-binding proteins such as ß2 GP-1 and C1q can attach. Therefore, apoptosis provides the mechanism for how intracellular proteins become accessible targets for autoantibodies.

Cytotoxic T cells have an important apoptosis-inducing role, including that in relation to killing cells infected with virus. Several pathways are used by cytotoxic T cells, including Fas-ligation and granule exocytosis. One of the effector molecules of cell-mediated pathways is granzyme B, a serine protease released by cytotoxic T cells and NK cells. Granzyme B promotes apoptosis via caspase-dependent and -independent pathways. Therefore, it is of considerable interest that an exclusive property of autoantigens is the capacity to be cleaved by granzyme in a manner that releases unique autoantigen fragments [21]. Rosen's group hypothesize that these autoantigen fragments may behave as non-tolerized neoantigens that, in circumstances favouring an inflammatory reaction, propagate autoimmune disease.

An important determinant of whether an immune response proceeds is the type and relative maturity of the antigen-presenting cell that a T lymphocyte comes in contact with [22]. Normally, macrophages provide appropriate down-regulatory cytokine signals to nearby cells on antigen ingestion, such as those given by transforming growth factor (TGF) ß and IL-10. However, under certain circumstances, such as in a proinflammatory environment driven by cytokines such as tumour necrosis factor {alpha} and IL-1, mature dendritic cells gain the necessary signalling machinery to drive an immune response (Fig. 1Go). Autoantibodies may alter this natural homeostasis and turn macrophages from pacifiers to activators. For instance, it has been shown that antiphospholipid antibodies are able to opsonize apoptotic cells, which promotes their uptake by macrophages; this mechanism (rather than sending down-regulatory signals) enables antigens to be presented in a proinflammatory manner to T cells [23, 24]. Such evidence suggests that autoantibodies may play an active role in accelerating autoimmune disease, and provides further rationale for modes of treatment centred on autoantibody depletion.



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FIG. 1.  Following heightened exposure to certain exogenous triggers, and under circumstances favouring delayed clearance, fragmented autoantigens become targets for opsonizing autoantibodies that help release proinflammatory cytokines and so perpetuate a T-cell-driven autoimmune response. RIP, apoptotic cell; C1q def, C1q deficiency; aPL, antiphospholipid antibody; M{phi}, macrophage; APC, antigen-presenting cell; Th, T helper cell; B, B cell; TNF, tumour necrosis factor.

 
So how can these advances in knowledge concerning apoptosis influence treatment strategies? First, it may be possible to explain steroid-resistant cases on the basis of levels of critical molecules regulating apoptosis [25]. Secondly, treatments that help the waste-disposal system, such as immunoadsorption with C1q columns, offer promise [26]. Thirdly, the targeted deletion of specific autoantibodies with newer technologies, such as antigen/tetramers, may be desirable. Finally, it seems clear that any form of treatment that down-regulates the presentation of apoptotic material to a primed immune system may help the devastating and ultimately wasteful condition known as SLE.

References

  1. Hengartner MO. The biochemistry of apoptosis. Nature2000;407:770–6.[ISI][Medline]
  2. Savill J, Fadok V. Corpse clearance defines the meaning of cell death. Nature2000;407:784–8.[ISI][Medline]
  3. Nagata S, Suda T. Fas and Fas ligand: lpr and gld mutations. Immunol Today1995;6:39–3.
  4. Napirei M, Karsunky H, Zevnik B, Stephan H, Mannherz HG, Moroy T. Features of systemic lupus erythematosus in Dnase1-deficient mice. Nat Genet2000;25:177–81.[ISI][Medline]
  5. Botto M, Dell'Agnola C, Bygrave AE et al. Homozygous C1q deficiency causes glomerulonephritis associated with multiple apoptotic bodies. Nat Genet1998;19:56–9.[ISI][Medline]
  6. Bickerstaff MC, Botto M, Hutchinson WL et al. Serum amyloid P component controls chromatin degradation and prevents antinuclear autoimmunity. Nat Med1999;5:694–7.[ISI][Medline]
  7. Wu J, Wilson J, He J, Xiang L, Schur PH, Mountz JD. Fas ligand mutation in a patient with systemic lupus erythematosus and lymphoproliferative disease. J Clin Invest1996;98:1107–13.[Abstract/Free Full Text]
  8. Jackson CE, Puck JM. Autoimmune lymphoproliferative syndrome, a disorder of apoptosis. Curr Opin Pediatr1999;11:521–7.[Medline]
  9. Cheng J, Zhou T, Liu C et al. Protection from Fas-mediated apoptosis by a soluble form of the Fas molecule. Science1994;263:1759–62.[ISI][Medline]
  10. Emlen W, Niebur J, Kadera R. Accelerated in vitro apoptosis of lymphocytes from patients with systemic lupus erythematosus. J Immunol1994;152:3685–92.[Abstract/Free Full Text]
  11. Walport MJ, Davies KA, Botto M. C1q and systemic lupus erythematosus. Immunobiology1998;199:265–85.[ISI][Medline]
  12. Davies EJ, Teh LS, Ordi-Ros J et al. A dysfunctional allele of the mannose binding protein gene associates with systemic lupus erythematosus in a Spanish population. J Rheumatol1997;24:485–8.[ISI][Medline]
  13. Seligman VA, Suarez C, Lum R et al. The Fcgamma receptor IIIA-158F allele is a major risk factor for the development of lupus nephritis among Caucasians but not non-Caucasians. Arthritis Rheum2001;44:618–25.[ISI][Medline]
  14. Mehrian R, Quismorio FP Jr, Strassmann G et al. Synergistic effect between IL-10 and bcl-2 genotypes in determining susceptibility to systemic lupus erythematosus. Arthritis Rheum1998;41:596–602.[ISI][Medline]
  15. Tsao BP, Cantor RM, Grossman JM et al. PARP alleles within the linked chromosomal region are associated with systemic lupus erythematosus. J Clin Invest1999;103:1135–40.[Abstract/Free Full Text]
  16. Rodenburg RJ, Ganga A, van Lent PL, Van de Putte LB, Van Venrooij WJ. The antiinflammatory drug sulfasalazine inhibits tumor necrosis factor alpha expression in macrophages by inducing apoptosis. Arthritis Rheum2000;43:1941–50.[ISI][Medline]
  17. Hieronymus T, Grotsch P, Blank N et al. Chlorpromazine induces apoptosis in activated human lymphoblasts: a mechanism supporting the induction of drug-induced lupus erythematosus? Arthritis Rheum2000;43:1994–2004.[ISI][Medline]
  18. Iyer R, Hamilton RF, Li L, Holian A. Silica-induced apoptosis mediated via scavenger receptor in human alveolar macrophages. Toxicol Appl Pharmacol1996;141:84–92.[ISI][Medline]
  19. Rumore PM, Steinman CR. Endogenous circulating DNA in systemic lupus erythematosus. Occurrence as multimeric complexes bound to histone. J Clin Invest1990;86:69–74.[ISI][Medline]
  20. Casciola-Rosen LA, Anhalt G, Rosen A. Autoantigens targeted in systemic lupus erythematosus are clustered in two populations of surface structures on apoptotic keratinocytes. J Exp Med1994;179:1317–30.[Abstract]
  21. Casciola-Rosen L, Andrade F, Ulanet D, Wong WB, Rosen A. Cleavage by granzyme B is strongly predictive of autoantigen status: implications for initiation of autoimmunity. J Exp Med1999;190:815–25.[Abstract/Free Full Text]
  22. Rovere P, Sabbadini MG, Fazzini F et al. Remnants of suicidal cells fostering systemic autoaggression. Apoptosis in the origin and maintenance of autoimmunity. Arthritis Rheum2000;43:1663–72.[ISI][Medline]
  23. Manfredi AA, Rovere P, Heltai S et al. Apoptotic cell clearance in systemic lupus erythematosus. II. Role of beta2-glycoprotein I. Arthritis Rheum1998;41:215–23.[ISI][Medline]
  24. Manfredi AA, Rovere P, Galati G et al. Apoptotic cell clearance in systemic lupus erythematosus. I. Opsonization by antiphospholipid antibodies. Arthritis Rheum1998;41:205–14.[ISI][Medline]
  25. Seki M, Ushiyama C, Seta N et al. Apoptosis of lymphocytes induced by glucocorticoids and relationship to therapeutic efficacy in patients with systemic lupus erythematosus. Arthritis Rheum1998;l41:823–30.[ISI][Medline]
  26. Pfueller B, Wolbart K, Bruns A, Burmester GR, Hiepe F. Successful treatment of patients with systemic lupus erythematosus by immunoadsorption with a C1q column: a pilot study. Arthritis Rheum2001;44:1962–3.[ISI][Medline]