Pathogenesis of Systemic Sclerosis: Cause, Cascade, Effect

2.1 The Disease as a Story

Think of SSc pathogenesis as a three-act play performed on the stage of a genetically susceptible host:

1. Act I — The Insult: An environmental exposure or infection injures the vascular endothelium in a person carrying susceptibility genes.
2. Act II — The Cascade: Endothelial injury triggers immune activation, autoantibody production, and a self-sustaining cytokine loop.
3. Act III — The Scar: Chronic inflammation and tissue hypoxia drive fibroblasts to become myofibroblasts, laying down excess collagen that stiffens the skin and viscera, perpetuating a vicious cycle of ischemia and fibrosis.

This is formalized in Harrison’s as the pathogenic SSc triad: microangiopathy, immune dysregulation/autoimmunity, and fibrosis — three cardinal processes that may act synchronously but with variable dominance across individual patients and over the disease course. In general, autoimmunity and reversible vascular reactivity dominate early disease, whereas fibrosis and tissue atrophy dominate late disease. [Harrison’s, Ch. 360, Fig. 360-3]

2.2 Genetic Susceptibility

Twin studies show low disease concordance (4.7% monozygotic) but a markedly increased risk (13.6%) in monozygotic twins compared with the general population — genetics confer modest but real susceptibility, not deterministic risk. [Harrison’s, Ch. 360] A first-degree relative of an SSc patient has a 1.6% lifetime risk — markedly higher than the general population.

HLA associations

Genome-wide association studies (GWAS) implicate genes at the highly polymorphic HLA region and genes involved in innate/adaptive immune responses and interferon signaling, highlighting the importance of autoimmunity as the initial trigger. Common HLA class II haplotypes associated with SSc include HLA-DRB1*11:04, DQA1*05:01, and DQB1*03:01. [Harrison’s, Ch. 360]

Non-HLA candidate genes

Several gene variants also correlate with specific manifestations: CTGD, CD226 with ILD; TNIP1 with PAH; HLA-DPB1 with scleroderma renal crisis; the antigen-encoding HLA-DPB1*05:01 allele associates with anticentromere antibodies. [Harrison’s, Ch. 360] Many of these loci are shared with other autoimmune diseases (SLE, Sjögren’s, RA, MS, psoriasis), consistent with common pathogenic pathways across phenotypically distinct disorders.

2.3 Environmental & Occupational Triggers

Environmental factors, dietary/lifestyle factors, and drug exposures likely play a major etiologic role atop genetic predisposition:

• Viruses: parvovirus B19, Epstein–Barr virus (EBV), cytomegalovirus (CMV), and Rhodotorula glutinis have putative pathogenic roles.
• Occupational exposures: silica (quartz), polyvinyl chloride, epoxy resins, welding fumes, and organic solvents (trichloroethylene, toluene, xylene) — associated with cell type-specific, stable, and heritable epigenetic modifications (DNA methylation, histone changes) that drive pathogenic gene expression.
• Drug-associated SSc-like illness: bleomycin, pentazocine, cocaine, appetite suppressants (e.g., L-tryptophan — implicated in eosinophilia-myalgia syndrome), and immune checkpoint inhibitors (PD-1 blockers) used in cancer therapy.
• Historic epidemic exposures: toxic oil syndrome (Spain, 1980s, contaminated rapeseed oil) and eosinophilia-myalgia syndrome (USA, 1990, L-tryptophan supplements).
• Gadolinium-based contrast in patients with impaired renal function is associated with nephrogenic systemic fibrosis, an important scleroderma-mimic (see Module 8).

2.4 The Three Cardinal Pathomechanisms

2.4.1 Microangiopathy

Vascular injury is an early and possibly primary event. Endothelial cell injury, pericyte loss, and endothelial-to-mesenchymal transition (EndoMT) lead to enhanced vascular permeability, transendothelial leukocyte diapedesis, activation of coagulation cascades, elevated thrombin production, and impaired fibrinolysis. Platelets aggregate and release serotonin, platelet-derived growth factor (PDGF), and thromboxane (a potent vasoconstrictor). Smooth-muscle-like myointimal cells accumulate via EndoMT, thickening the basement membrane and causing perivascular adventitial fibrosis. Progressive luminal occlusion, sustained intimal/medial hypertrophy, and adventitial fibrosis create a vicious cycle underlying fibroproliferative vasculopathy and rarefaction (loss) of small blood vessels in late-stage disease. [Harrison’s, Ch. 360]

Paradoxically, revascularization that normally re-establishes blood flow to ischemic tissue is defective in SSc despite elevated levels of other angiogenic factors, and bone-marrow-derived circulating endothelial progenitor cells are reduced in number and function.

2.4.2 Immune Dysregulation & Autoimmunity

Circulating antinuclear antibodies (ANAs) are detected in virtually all SSc patients. Activated T cells (with oligoclonal antigen receptors), monocytes/macrophages, dendritic cells, mast cells, and eosinophils infiltrate skin and other organs. T cells show a T_H2-polarized profile driven by dendritic cells, producing IL-4, IL-13, IL-33, and TSLP that induce fibroblast activation and exhibit antifibrotic properties when T_H1/IFN-γ predominates instead. Regulatory T-cell (Treg) function is impaired despite increased Treg frequency in circulation and tissue.

B cells show elevated CD19 and co-stimulatory molecules CD80/CD86, indicating B-cell chronic activation; serum APRIL and BAFF (B-cell activating factor) are elevated and associate with extent of skin and lung involvement.

2.4.3 Fibrosis

Fibrosis is the process by which normal tissue architecture is replaced by rigid, avascular, relatively acellular connective tissue. Fibroblasts and mesenchymal cells respond to extracellular cues by proliferating, migrating, secreting collagens/matrix molecules, and transdifferentiating into contractile myofibroblasts. In SSc, this normally self-limited repair response becomes sustained and amplified, producing pathologic fibrosis. [Harrison’s, Ch. 360]

Key profibrotic cytokines: TGF-β (the master regulator), IL-6, IL-11, IL-13, IL-23, connective tissue growth factor (CTGF), PDGF, lysophosphatidic acid, endothelin-1, hypoxia, ROS, thrombin, and mechanical forces — signaling together to sustain fibroblast activation via maladaptive repair. Damage-associated endogenous ligands for TLR4 (fibronectin-EDA and tenascin-C) and TLR9 (mitochondrial DNA) further contribute to nonresolving fibrosis via aberrant TLR activation and innate signaling.

2.5 Integrated Progressive Model of Pathogenesis

Vascular injury (endothelial injury, pericyte loss, EndoMT, platelet activation, complement/coagulation cascade activation) triggers immune system responses (innate + adaptive T/B-cell activation, monocyte/macrophage and dendritic cell involvement, TLR signaling, mast cells, eosinophils) and generates SSc-specific autoantibodies (anticentromere, anti-topoisomerase I, anti-RNA polymerase III, anti-receptor antibodies). These converge on proinflammatory/profibrotic signaling (T_H2 cytokines, type I IFN, chemokines, TGF-β, IL-6/11/13, CTGF/CCN2, PDGF, Wnt), driving fibrotic cellular responses (myofibroblast differentiation from stromal progenitors, epigenetic reprogramming, accelerated cellular senescence, apoptosis evasion). The end result is tissue fibrosis — deposition/remodeling of ECM components (collagens, COMP, osteopontin, fibulin, tenascin, fibronectin) causing matrix rigidity and contraction — culminating in architectural disruption and organ failure (pulmonary fibrosis, renal failure, heart failure, GI dysmotility, tendon friction rubs, joint contractures). Tissue hypoxia feeds back to worsen vascular injury, closing the vicious cycle. [Harrison’s, Ch. 360, Fig. 360-4]