Ankylosing Spondylitis (AS) - Pathophysiology

1. Overview and Classification

2. Genetic Susceptibility

• ~90% of AS patients are HLA-B27 positive, making it the strongest genetic risk factor
• However, HLA-B27 is necessary but not sufficient - only ~5% of HLA-B27-positive individuals develop AS
• Over 30 HLA-B27 subtypes exist; B*2705 is the ancestral/primordial subtype
• Subtypes B*2706 and B*2709 do NOT confer disease susceptibility, suggesting subtype-specific peptide presentation drives pathogenesis
• HLA-B27 prevalence varies widely: absent in Australian Aboriginals, 1% in Japan, 7% in northern Europeans, up to 50% in some indigenous Canadian tribes
• This geographic variation parallels AS prevalence in those populations

• Polymorphisms in endoplasmic reticulum aminopeptidase-1 (ERAP1) on chromosome 5 are the second strongest genetic risk factor
• Crucially, the ERAP1 association is restricted to HLA-B27+ patients - a gene-gene interaction
• ERAP1 trims peptides in the ER before loading onto nascent MHC class I molecules; dysfunction alters the peptide repertoire presented to CD8+ T cells
• This firmly implicates MHC class I peptide presentation as central to AS pathogenesis

• IL-23R gene (chromosome 1) - associated with AS, IBD, and psoriasis (shared SpA pathway)
• Over 100 susceptibility genes identified, though each has a modest odds ratio except HLA-B27

3. The Three Mechanistic Hypotheses for HLA-B27's Role

4. Innate vs. Adaptive Immunity

• AS synovial histology: abundant neutrophils, macrophages, and hypervascularity
• RA synovial histology: lymphoid aggregates, dendritic cells, lining cell hyperplasia, citrullinated proteins
• This innate predominance explains why AS lacks autoantibodies (RF negative, ACPA negative) and why anti-TNF works effectively

5. Key Cytokine Pathways

• Central pro-inflammatory mediator; elevated in AS sacroiliac joint and spinal tissue
• Anti-TNF agents (adalimumab, etanercept, infliximab) dramatically reduce inflammation - validating its key role

• IL-23 (produced by macrophages and dendritic cells) drives differentiation of Th17 cells and innate lymphoid cells (ILCs) at the enthesis
• Th17 cells secrete IL-17A, which drives neutrophil recruitment, osteoclast activation, and local tissue inflammation
• Resident entheseal T cells (CD4+ and CD8+) that express the IL-23 receptor are activated at the enthesis
• Ustekinumab (anti-IL-12/23), secukinumab (anti-IL-17A), and ixekizumab all work via this axis

6. The Enthesis - The Primary Target

• Mechanical stress at entheses creates micro-trauma and a niche for immune cell residence
• CD4+ and CD8+ T cells expressing IL-23R reside at entheseal sites and respond to locally produced IL-23
• Enthesitis at the sacroiliac joint ligaments is the hallmark early lesion
• Inflammation at the annulus fibrosus-vertebral body junction (Sharpey fibers) triggers reactive bone formation - the syndesmophyte

7. The Bone Formation Paradox - Inflammation then Ankylosis

• Wnt signaling pathway: Normally inhibited by DKK-1 and sclerostin. In AS, after inflammatory destruction, residual mesenchymal cells receive aberrant Wnt signals that promote chondrocyte and osteoblast differentiation
• BMP (Bone Morphogenetic Protein) pathway: BMPs at entheseal sites promote periosteal osteoblastogenesis
• TNF paradox: TNF inhibits bone formation acutely, but when chronic TNF is blocked, bone formation may actually accelerate - explaining why anti-TNF drugs reduce inflammation but may not halt syndesmophyte progression
• The key sequence: Inflammation → Erosion → Repair/Ossification → Syndesmophyte → Ankylosis

8. The Gut-Joint Axis

• Up to 60% of AS patients have subclinical microscopic gut inflammation; 90% of those with enteric reactive arthritis do as well
• Gut inflammation fluctuates with arthritis activity
• AS develops in ~16% of IBD patients, and ~50% of those exhibit sacroiliac compromise
• Gut microbiome dysbiosis in AS: decreased Faecalibacterium prausnitzii and Desulfovibrio spp. (also seen in IBD), increased Proteobacteria with age in HLA-B27 transgenic rats
• The gut microbiome alters the Th1/Th17 cytokine environment (elevated IL-17A, IL-23, IFN-γ)
• Circulating anti-enteric microbial antibodies (e.g., anti-I2) found in both IBD and AS patients
• Sulfasalazine (poorly gut-absorbed) works in both peripheral SpA and IBD - further evidence of the gut-joint link

9. The Sacroiliac Joint and Spinal Progression

• The SIJ is a fibrocartilaginous joint subjected to significant mechanical loading
• Early: subchondral bone marrow edema (osteitis) visible on MRI - the earliest imaging finding
• Enthesitis of SIJ ligaments → erosions on the iliac side (lower synovial portion) → sclerosis → ankylosis

• Inflammation at the discovertebral junction (Romanus lesion) causes anterior corner erosions (shiny corners on X-ray)
• Reactive bone formation creates syndesmophytes (marginal, flowing, smooth in AS vs. coarse/nonmarginal in psoriatic/reactive arthritis)
• Progressive bridging syndesmophytes produce the bamboo spine
• Spinal fusion locks the spine into kyphotic posture

10. Extra-Articular Pathophysiology

11. Key Differences from Rheumatoid Arthritis (Pathophysiologically)

• Goldman-Cecil Medicine (International Ed.), Chapter 244 - Spondyloarthritis (Inman & Rahman), pp. 2799-2807
• Rheumatology, 2-Volume Set (Elsevier 2022) - Spondyloarthritis chapter, pp. 222+
• Firestein & Kelley's Textbook of Rheumatology (2-Volume Set)
• Emery's Elements of Medical Genetics and Genomics

What Does HLA-B27 Actually Do in AS?

First: What is HLA-B27 normally?

1. Pick up small peptide fragments (8-10 amino acids) from inside the cell - from proteins being made, degraded, or from invading viruses/bacteria
2. Present those peptides on the cell surface to CD8+ cytotoxic T lymphocytes
3. If the peptide is "foreign" (viral, bacterial, or tumour-derived), the CD8+ T cell kills the cell
4. If the peptide is "self," the T cell ignores it (tolerance)

The Key Abnormalities of HLA-B27

1. It Folds Very Slowly in the Endoplasmic Reticulum (ER)

• The heavy chain (HLA-B27 protein itself)
• β2-microglobulin (a support protein)
• A peptide fragment

2. This Triggers the Unfolded Protein Response (UPR) - Hypothesis 1

HLA-B27 misfolds in ER
        ↓
ER stress accumulates
        ↓
UPR activated (via XBP-1, ATF-6 pathways)
        ↓
NF-κB signalling turned on
        ↓
Macrophages release IL-23
        ↓
IL-23 drives Th17 cells
        ↓
Th17 cells release IL-17A
        ↓
Chronic inflammation at entheses + sacroiliac joints

3. After Leaving the ER, B27 Forms Dangerous Homodimers - Hypothesis 2

B27 at cell surface
        ↓
β2-microglobulin dissociates
        ↓
Free heavy chain
        ↓
Heavy chain homodimers form on surface
        ↓
Recognized by KIR3DL2 receptors on CD4+ T cells and NK cells
        ↓
IL-17 secretion → joint/entheseal inflammation

4. It Presents Arthritogenic Peptides - Hypothesis 3 (Currently Best Supported)

• HLA-B27 presents peptides to CD8+ T cells
• In susceptible individuals, B27 presents self-peptides (or microbial peptides that look like self) that trigger CD8+ T cell autoreactivity - called molecular mimicry
• These autoreactive T cells then attack joint/entheseal tissue

• Multiple studies have found expansion of CD8+ T cell clones with a characteristic CDR3 motif in AS patients
• This motif was found in 37 of 37 AS patients vs. only 4 of 19 HLA-B27-positive healthy controls in one landmark study
• It appears in both European and East Asian AS patients - suggesting a common antigenic driver
• The same motif appears in patients with bacterial-triggered reactive arthritis - linking gut microbes as the original trigger

Why Does HLA-B27 Matter vs. "Normal" HLA?

Why Does it NOT Cause Disease in Everyone?

1. ERAP1 gene variants (second biggest risk factor) - ERAP1 trims peptides before B27 loads them; certain variants change the peptide repertoire B27 presents, worsening autoreactivity. Importantly, ERAP1 variants only increase AS risk in HLA-B27+ people - a direct gene-gene interaction
2. IL-23R variants - alter the strength of the Th17 response downstream
3. Gut microbiome dysbiosis - germ-free B27 transgenic rats do NOT develop SpA; exposure to gut bacteria is required. HLA-B27 itself alters the gut microbiome, and dysbiosis precedes clinical disease
4. Mechanical stress - entheses under high mechanical strain (lower limbs, SI joints) are more susceptible. B27-positive individuals may have an exaggerated immune response to mechanical micro-damage
5. Barrier integrity - translocation of bacterial peptides across a leaky gut mucosa provides the initial antigenic trigger that B27 then "mishandles"

The Net Result: What HLA-B27 Does to the Joint

HLA-B27 (slow folding + abnormal peptide presentation)
                ↓
ER stress / UPR → IL-23 from macrophages
Arthritogenic peptide → CD8+ T cell activation
Homodimers → IL-17 from NK/CD4+ cells
                ↓
        IL-23 / IL-17 axis activated
                ↓
Innate immune inflammation at ENTHESIS
(not the synovium - that's RA's territory)
                ↓
Neutrophils + macrophages + hypervascularity
                ↓
Osteitis of SI joint → erosion → new bone (syndesmophyte)
                ↓
Progressive ankylosis (bamboo spine)

• Firestein & Kelley's Textbook of Rheumatology, HLA-B27 section, pp. 1602+
• Harrison's Principles of Internal Medicine 22E (2025), Chapter 374, pp. 2925+
• Roitt's Essential Immunology - Spondyloarthritis chapter
• Goldman-Cecil Medicine, Chapter 244 - Spondyloarthritis