The Phases of Sle: Evolution of Disease in Susceptible Persons

As shown in Figure 3-1, the development of SLE occurs in a series of steps. There is a long period of predisposition to autoimmunity, conferred by genetic susceptibility, gender, and environmental exposures, and then (in a small proportion of those predisposed) development of autoantibodies, which usually precede clinical symptoms by months to years. A proportion of individuals with autoantibodies demonstrate clinical SLE, often starting with involvement of a small number of organ systems or abnormal laboratory values, and then evolving into enough clinical and laboratory abnormalities to be classified as SLE. Finally, over a period of many years, most individuals with clinical SLE experience intermittent disease flares and improvements (usually not complete remission), and compile organ damage and comorbidities related to genetic predisposition, chronic inflammation, activation of pathways that damage organs (such as renal tubules), and/or induce fibrosis, to therapies, and to aging.

FIGURE 3-1 Overview of the pathogenesis of SLE.

In a process that probably takes decades, SLE develops in an individual. At birth the individual is predisposed by multiple genes/gene copies/epigenetic changes and by a permissive gender (usually female). Exposure to environmental stimuli such as ultraviolet B (UVB) light and silica and infections such as Epstein-Barr virus (EBV) stimulate immune responses and probably additional epigenetic changes. Over time, persistent autoantibodies appear; they are usually present for several years before the first symptom of disease. In some autoantibody-positive individuals, clinical SLE develops, shown here as polyarthritis. Within that group, some have chronic irreversible damage; end-stage renal disease with sclerotic glomeruli is shown here.

Overview: The Major Immune Pathways Favoring Autoantibody Production

These pathways are summarized in Figure 3-2.

FIGURE 3-2 Interactions between innate and acquired immune systems.

Antigen/cell interactions that drive autoimmune responses in SLE. Antigens containing nucleosomal DNA, RNA/protein, phospholipids presented by apoptotic cells, neoantigens generated from necrotic cells and inflammatory cell debris, and RNA/protein; DNA/protein in the neutrophil extracellular traps (NET) like structures of polymorphonuclear neutrophils (PMNs) and immune complexes set up immune responses that characterize human SLE. Plasmacytoid dendritic cells and B lymphocytes are activated upon engagement of these antigens by their Toll-like receptors (TLRs); plasmacytoid dendritic cells (pDCs) generate interferon alpha (IFN-α), and B cells produce autoantibodies and cytokines. The IFN-α activates PMNs to die by NETosis; the NETs they secrete contain DNA and DNA-binding proteins that further engage TLRs in B cells, with more B-cell activation. Both pDC and myeloid DC (mDC) subsets present autoantigens and cytokines to T lymphocytes, resulting in T-cell activation with pushing of T cells to helper/effector subsets that include IFN-γ–producing T helper 1 (Th1) and tissue-damaging Th17 cells (Teffectors). SLE T and B cells are intrinsically abnormal and hyperrespond to stimuli. Multiple “hits” drive B cells, which at this level of maturation are prone to hyperactivation. The hits include T-cell help, exposure to increased quantities of apoptotic materials and neoantigens recognized by their B cell receptors, and exposure to activated DCs and pools of activating cytokines. In the figure, green indicates molecules, antigens, and pathways that promote the hyperimmune responses of SLE. Green diamonds indicate cytokine receptors on cell surfaces. Black bars indicate TLRs in pDCs and B cells. Red circles or crescents indicate B-cell receptors or T-cell receptors, respectively. Pink ovoids are B cell receptors. B, B lymphocyte; M/M, monocyte/macrophage; NUC, DNA-containing nucleosome; PS, phosphatidylserine, the phospholipid presented to the immune system on the outer surface of cells undergoing apoptosis; RNAp, RNA bound to a protein that in complex can be recognized by the immune system; Teff, effector (helper) which can be CD4⁺ Th1 or Th2, or Th17, or follicular T cell helper (TFH) that secretes IL-17.

Stimulation of Innate and Adaptive Immune Responses by Autoantigens

The autoantigen stimulation of the innate and adaptive immune responses is provided by cells undergoing apoptosis (which present autoantigens such as nucleosome and Ro in surface blebs, and phosphatidyl serine on outer surfaces of membranes), by cells undergoing necrosis and releasing cell components which can form neoantigens under the influence of oxidation, phosphorylation, and cleavage, and by microorganisms that have antigenic sequences that cross-react with human autoantigens. Antigen-presenting cells—dendritic cells (DCs), monocytes/macrophages (M/Ms), and B lymphocytes process and present such antigens (Ags). In addition, cells of innate immunity (DC, M/M) are activated via internal Toll-like receptors (TLRs) by DNA/protein and RNA/protein that can be provided by dying cells, particularly polymorphonuclear neutrophils (PMNs) undergoing NETosis, by SLE immune complexes (ICs), and by infectious agents. The net result of activation of DCs from tolerogenic to proinflammatory cells secreting inflammatory cytokines (including the lupus-promoting interferon alpha [IFN-α]), and of M/Ms to proinflammatory cells secreting tumor necrosis factor alpha (TNF-α), and interleukins IL-1, IL-12, and IL-23, is activation of effector T cells that help B cells make immunoglobulin (Ig) G autoantibodies, infiltrate tissues, and be cytotoxic for some tissue cells such as podocytes in the kidney. B lymphocytes, activated directly by DNA/protein and RNA/protein via their TLRs and by IFN-α, can also be helped in their secretion of autoantibodies by T cells, and in their survival and maturation to plasmablasts by BLyS (B-lymphocyte stimulator)/BAFF (B cell–activating factor), IL-6, and other cytokines. In patients with SLE these processes escape normal regulatory mechanisms, which are listed in Box 3-1. Thus, autoantibodies induce the first phase of clinical disease (organ inflammation of joints, skin, glomeruli, destruction of platelets, etc.) because (1) the autoantibodies and the ICs they form persist, (2) they are quantitatively high, (3) they contain subsets that bind target tissues, (4) they form immune complexes that are trapped in basement membranes or bound on cell surfaces, (5) charges on antibodies or ICs favor nonspecific binding to tissues, and (6) their complexes activate complement. And yet, in spite of this deluge of autoantibodies and ICs attacking tissue, mouse models suggest that susceptibility to clinical disease requires more—there are several examples of autoantibody formation, abundant Ig deposition in glomeruli, and complement fixation without development of clinical nephritis.

Box 3-1

Mechanisms of Downregulation of the Immune Response That Are Defective in SLE

1. Disposal of immune complexes (ICs) and apoptotic cells (ACs): Defective phagocytosis, transport by complement receptors, and binding by Fcγ receptors. Can be due to macrophage defects intrinsic to SLE, low levels of complement-binding CR1 receptors—or occupied receptors, FcγRs that are occupied, downregulated, or genetically low-binding of the immunoglobulin (Ig) in ICs. Early components of complement or mannose-binding lectin/ protein (MBL) also participate in solubilizing and transporting IC. They may be missing or defective.

2. Defective idiotypic networks: due to low production of anti-idiotypic antibodies, defective regulation of T helper cells by T-regulator cells that recognize idiotypes in their T-cell receptors (TCRs).

3. Inadequate production and/or function of regulatory cells that kill or suppress autoreactive B cells, T helper cells, other effector cells. This includes CD8⁺ cytotoxic cells that kill autoreactive B, regulatory CD4⁺CD25⁺Foxp3⁺ T cells that normally target both T helper cells and autoreactive B cells, inhibitory CD8⁺Foxp3⁺ T cells that suppress both T helper and B cells, regulatory B cells, and tolerogenic dendritic cells (DCs). Possibly natural killer (NK) cell defects.

4. Low production of interleukin-2 (IL-2) by T cells. Survival of regulatory T cells requires IL-2, and effector T cells in SLE make decreased quantities of IL-2. IL-2 is also required for activation-induced death in lymphocytes.

5. Defects in apoptosis that permit survival of effector T and autoreactive B cells, usually genetically determined.

Autoantibodies and Immune Complexes of SLE

Autoantibodies are the main effectors of the onset of disease in SLE. In humans, they are probably necessary for disease, but not sufficient. That is, their deposition must be followed by activation of complement and/or other mediators of inflammation, and a series of events that include chemotaxis for lymphocytes and phagocytic mononuclear cells, and release of cytokines, chemokines, and proteolytic enzymes, as well as oxidative damage, must occur for organ inflammation and damage to be severe. In nearly 85% of patients with SLE, autoantibodies precede the first symptom of disease by an average of 2 to 3 years—sometimes as long as 9 years. The autoantibodies appear in a temporal hierarchy, with antinuclear antibodies (ANAs) first, then anti-DNA and antiphospholipid, and finally anti-Sm and anti-ribonucleoprotein (anti-RNP). These observations imply that immunoregulation of potentially pathogenic autoantibodies can occur for a sustained period, and that only in individuals whose regulation becomes “exhausted” does disease appear. Among autoantibodies, some are clearly pathogenic, such as certain subsets of anti-DNA that cause nephritis upon transfer to healthy animals. Antibodies to neurons (anti-N-methyl-aspartate receptor, a subset of anti-DNA) can cause neuronal death. Antibodies to platelets and erythrocytes can cause the cells to be phagocytized and destroyed. Antibodies to Ro/La (SSA/SSB) can cause fetal cardiac conduction defects. Human antibodies to phospholipids can cause fetal loss in mice and probably in humans. In addition, autoantibodies generate self-perpetuating cycles; the autoantibodies contain amino acid sequences that are T-cell determinants; these peptides activate T helper cells to further expand autoantibody production. Mechanisms of pathogenicity are discussed in detail in other chapters, and for many autoantibodies the mechanisms are not entirely known. Pathogenic ICs in patients with SLE are dominated by soluble complexes that avoid clearance by phagocytic mononuclear cells, and both size and charge of the complexes can cause them to be trapped in tissue, rather than continuing to circulate. In addition, complement products in ICs are bound by complement receptors; Ig in ICs is bound by FcR, and thus the ICs can fix to cells and tissues by those interactions. Defects in clearing the complexes characteristic of SLE are probably major causes of their persistence and enhance their quantities and potentially harmful properties.

Regulatory Mechanisms Fail to Control Autoimmune Responses

As shown in Box 3-1, several mechanisms that downregulate active immune responses are defective in SLE.

Abnormalities in T and B Lymphocytes in SLE

B- and T-cell interactions in SLE play a major role in production of IgG and complement-fixing autoreactive antibodies. It is likely that hyperactivation of T and/or B cells promotes SLE by making higher quantities of autoantibodies and proinflammatory cytokines, and that hypoactivation also promotes autoreactivity by allowing autoreactive B and T cells to escape apoptosis. Thus, tweaking of the T/B activation immunostat away from the “norm” promotes autoimmunity. B-cell surface antigen receptors (BCRs) are assembled from various combinations of Ig heavy and light chains in bone marrow; the vast majority of BCRs and their autoantibodies in people with SLE are assembled from a variety of Ig genes and combinations that do not differ from normal protective antibody assembly. The SLE autoantibody response has somewhat limited clonality (not different from antibody responses to external antigens), and somatic hypermutation, indicating that cells have been stimulated by antigens. A major difference between people with SLE and healthy individuals is abnormalities of B-cell tolerance. The end result is elevated quantities of activated B cells, of memory B cells, and of plasma cells in patients with active SLE.

There are several defects that permit survival of autoreactive B-cell subsets in SLE. The usual tolerance processes (apoptosis, anergy, ignorance, BCR editing) are blunted, allowing survival and maturation of dangerous autoreactive B cells. After normal B cells exit the bone marrow, they go through a series of checkpoints that normally remove autoreactive cells. There are defects in several of these checkpoints in SLE, including entry of early immature B to mature B and of transitional B to mature B, entry into germinal centers (GCs), and naïve B to activated B maturation. In addition, some patients have defective expression of FcγRIIB in memory B cells, a molecule that suppresses B-cell development. Thus, defects allow persistence of autoreactive cells that would be inactive or deleted in healthy individuals. Many patients with SLE have abnormally high levels of BLyS/BAFF cytokine, which promotes survival of B cells from the late transitional stage through mature activated and memory B cells. Genetic polymorphisms predisposing to SLE include several that affect signaling through the BCR, such as PTPN22 and BLK. Abnormally high quantities of Ca⁺⁺ are mobilized intracellularly after BCR activation in SLE. Overall, memory and activated B cells, as well as plasma cells, are increased in numbers in SLE, they require smaller-than-normal stimuli to be activated, and many pathways from BCR signaling to nuclear factor kappa B (NF-κB) activation may be altered.

Normally, in GCs, nonautoreactive B cells migrate into T zone areas, where they contact CD4⁺ T helper cells, which drive them into activated and memory subsets, with subsequent Ig class switching and plasma cell production. This process results in protective antibody responses. In SLE there is a blockade of whatever process prevents autoreactive B cells from travelling to T cell zones. Thus in GCs there is a tolerance defect that allows T-cell help for production of potentially harmful autoantibodies. Normal and SLE B cells can also produce autoantibodies with class switching and maturation independent of T-cell help, via activation of B-cell TLRs. In SLE this process may be enhanced, probably by autoantigens in ICs. This environmental exposure of B cells to autoantigens is probably influenced by the SLE genetic variants that promote activation of innate immunity and high IFN production by innate immune cells.

T cells in SLE are also abnormal. Like B cells, they respond to lesser stimuli than are required for healthy T cells. A major abnormality of SLE CD4⁺ T cells is assembly of an abnormal signaling apparatus after T-cell receptor (TCR) activation. Figure 3-3 shows some of these abnormalities. In health, TCR stimulation results in assembly of the CD3ζ chain into the surface activation cluster. In SLE, the FcRγ chain is substituted for CD3ζ, resulting in a different activation pathway. The end results are increased release of intracellular calcium, which promotes translocation of calcium/calmodulin-dependent protein kinase IV (CaMK4) to the nucleus, and upregulation of transcription repressor cyclic adenosine monophosphate (AMP) response–element modulator alpha (CREM-α), which on binding to promoter regions of DNA suppresses IL-2 production and enhances IL-17 production. Abnormally low secretion of IL-2 by T cells impairs production of regulatory T cells, whereas increased production of IL-17 promotes inflammation. Causes of the downregulation of CD3ζ include antibodies to T lymphocytes and mTOR activation in T cells resulting from increased levels of nitric oxide (NO) and elevated transmembrane potentials in the mitochondria of SLE T cells. SLE T-cell subsets have many other abnormalities: CD8⁺ cytotoxic T cells may be defective, adding to the persistence of autoreactive B cells. Regulatory T cells of CD4⁺ and CD8⁺ phenotypes also are abnormal in quantities and/or functions. Double-negative (DN) T cells (CD3⁺CD4⁻CD8⁻), which probably derive from CD8⁺ T cells, infiltrate tissue and secrete IL-17.