Mechanisms of Regulation and Dysregulation of the Immune System¶
Chapter 361 | Part 11: Immune-Mediated, Inflammatory, and Rheumatologic Disorders
KEY CLINICAL POINTS¶
- Immune homeostasis is critical for preventing autoimmunity, immunodeficiency, and excessive inflammation.
- Checkpoint inhibitors like anti-CTLA-4 and anti-PD-1 antibodies have revolutionized cancer immunotherapy by reversing T-cell exhaustion.
- CAR T-cell therapy uses synthetic receptors to target malignant cells, with approvals for B-cell malignancies and solid tumors.
- Autoimmune diseases arise from immune dysregulation due to defects in central/peripheral tolerance mechanisms.
- Monoclonal antibodies (e.g., rituximab, adalimumab) are pivotal in treating autoimmune and inflammatory disorders.
1. DEFINITION & OVERVIEW¶
Immune homeostasis refers to the balanced state of immunity that protects against pathogens while preventing autoimmunity and immunodeficiency. Dysregulation of immune checkpoints (e.g., CTLA-4, PD-1) leads to autoimmune diseases, cancer, or chronic inflammation. Key concepts include T-cell exhaustion, regulatory T-cell (Treg) function, and cytokine signaling.
Table 361-1: Monogenetic Mutations Leading to Immune Dysregulation¶
| MUTATIONS AND FUNCTIONAL DEFICITS | DISEASES OR SYNDROME |
|---|---|
| RAG1, RAG2; lymphopenia with recombinase deficiency | Severe combined immune deficiency (Omenn’s syndrome) with autoreactive T cells |
| Fas, FasL, CASP10; apoptosis defects | Autoimmune lymphoproliferative disease |
| AIRE, deletion chromosome 22q11.2; decrease in central tolerance | DiGeorge’s syndrome with autoimmune T cells |
| FOXP3, CD25, CTLA-4, LRBA; decrease in peripheral immune tolerance | IPEX syndrome, enteropathy, dermatitis, autoimmunity |
| STAT-1, STAT-3; modulation of type 1 interferons | Hyper-IgE syndrome (Job’s syndrome), chronic mucocutaneous candidiasis |
Table 361-2: Approved Immune Checkpoint Blockade Therapies¶
| TUMOR TYPE | THERAPEUTIC AGENT | TARGET | FDA APPROVAL YEAR |
|---|---|---|---|
| Melanoma | Ipilimumab | CTLA-4 | 2011 |
| Melanoma | Nivolumab | PD-1 | 2014 |
| Non-small-cell lung cancer | Nivolumab | PD-1 | 2015 |
| Renal cell carcinoma | Nivolumab | PD-1 | 2015 |
| Hodgkin’s lymphoma | Nivolumab | PD-1 | 2016 |
| Urothelial carcinoma | Atezolizumab | PD-L1 | 2016 |
| Head and neck squamous cell carcinoma | Nivolumab | PD-1 | 2016 |
| MSI-high or MMR-deficient solid tumors | Pembrolizumab | PD-1 | 2017 |
| Pediatric melanoma | Ipilimumab | CTLA-4 | 2017 |
| Gastric and gastroesophageal carcinoma | Pembrolizumab | PD-1 | 2017 |
1.1 Immune Checkpoints¶
Immune checkpoints regulate T-cell activation to prevent excessive immune responses. Key molecules include CTLA-4, PD-1, and their ligands (B7-1/B7-2, PD-L1/PD-L2). Dysregulation of these pathways is central to autoimmune diseases and cancer progression.
1.2 T-Cell Exhaustion¶
Chronic antigen exposure leads to T-cell exhaustion, characterized by reduced effector function and increased expression of inhibitory receptors like PD-1 and TIM-3. This state limits antitumor immunity and is a target for checkpoint inhibitor therapy.
2. ETIOLOGY & PATHOPHYSIOLOGY¶
Immune dysregulation arises from defects in central (thymus) and peripheral (lymph nodes, tissues) tolerance mechanisms. Key pathways include T-cell receptor (TCR) editing, Treg dysfunction, and cytokine signaling (e.g., IL-1, IL-6, TNF- α ). Chronic viral infections and cancer can perturb immune checkpoints, leading to autoimmunity or immune evasion.
Table 361-3: Central vs. Peripheral Tolerance Mechanisms¶
| CENTRAL TOLERANCE | PERIPHERAL TOLERANCE |
|---|---|
| TCR editing by V(D)J recombination | B- and T-cell anergy, inhibitory signaling (CTLA-4, PD-1) |
| Thymic negative selection | T-cell competition for IL-2, IL-7, IL-15 |
| Treg differentiation | B-cell competition for BAFF |
| Elimination of antigen-bearing dendritic cells | Treg-mediated suppression via TGF-b, IL-10 |
2.1 T-Cell Regulation¶
T-cell activation requires two signals: TCR engagement with MHC/peptide and co-stimulation via CD28/B7-1/B7-2. Inhibitory signals (e.g., CTLA-4, PD-1) dampen T-cell responses to prevent autoimmunity.
2.2 Autoimmune Pathogenesis¶
Autoimmune diseases result from breakdowns in immune tolerance, including Treg dysfunction, B-cell autoreactivity, and cytokine dysregulation. Chronic antigen exposure (e.g., HIV, hepatitis C) drives T-cell exhaustion and autoantibody production.
3. CLINICAL FEATURES¶
Autoimmune diseases present with diverse clinical manifestations, including organ-specific damage (e.g., thyroiditis, myositis) and systemic inflammation (e.g., rheumatoid arthritis, lupus). Immune checkpoint inhibitor therapy can cause immune-related adverse events (irAEs) such as colitis, endocrinopathies, and pneumonitis.
Table 361-4: Autoantigens in Human Autoimmune Disorders¶
| AUTOANTIGEN | AUTOIMMUNE DISEASES |
|---|---|
| Acetylcholine receptor | Myasthenia gravis |
| Collagen (multiple types) | Rheumatoid arthritis, systemic lupus erythematosus |
| b2-Glycoprotein I (B2-GPI) | Primary antiphospholipid syndrome |
| Glutamate decarboxylase (GAD65) | Type 1 diabetes |
| Thyroid peroxidase | Autoimmune Hashimoto’s thyroiditis |
3.1 Autoimmune Disease Manifestations¶
Organ-specific autoimmunity (e.g., Hashimoto’s thyroiditis, celiac disease) and systemic autoimmunity (e.g., SLE, rheumatoid arthritis) are driven by autoantibodies and T-cell-mediated inflammation. Chronic viral infections (e.g., HIV) increase autoantibody production and immune dysregulation.
3.2 Immune-Related Adverse Events (irAEs)¶
Checkpoint inhibitors may induce irAEs, including colitis, hepatitis, pneumonitis, and endocrinopathies. These events are mediated by uncontrolled immune activation and are more common in patients with preexisting immune disorders.
4. MANAGEMENT & TREATMENT¶
Therapies target immune checkpoints (e.g., anti-CTLA-4, anti-PD-1), modulate cytokine signaling (e.g., TNF- α inhibitors), or use CAR T-cells for malignancies. Monoclonal antibodies (e.g., rituximab, adalimumab) are central to treating autoimmune diseases and inflammatory conditions.
Table 361-5: Monoclonal Antibodies in Autoimmune Diseases¶
| TARGET | FUNCTION | FDA-APPROVED mAbs | AUTOIMMUNE DISEASES |
|---|---|---|---|
| CD20 | B-cell depletion | Rituximab | Rheumatoid arthritis, lymphoma |
| TNF-a | Anti-inflammatory | Adalimumab | Crohn’s disease, rheumatoid arthritis |
| IL-6R | Anti-inflammatory | Tocilizumab | Systemic juvenile idiopathic arthritis |
| B7-1/B7-2 | Co-stimulation blockade | Belimumab | Systemic lupus erythematosus |
| TARGET | FUNCTION | FDA-APPROVED mAbs | AUTOIMMUNE DISEASES |
|---|---|---|---|
| FcRn | IgG clearance | Rozanolixizumab | Myasthenia gravis |
4.1 Checkpoint Inhibitor Therapy¶
Anti-CTLA-4 (ipilimumab) and anti-PD-1/PD-L1 (nivolumab, pembrolizumab) enhance T-cell anti-tumor activity by blocking inhibitory signals. These therapies are approved for melanoma, lung cancer, and other malignancies.
4.2 CAR T-Cell Therapy¶
Chimeric antigen receptor (CAR) T-cells redirect T-cells to target malignant cells via engineered receptors (e.g., anti-CD19 for B-cell malignancies). CAR T-cells are approved for relapsed/refractory lymphomas and leukemias.
5. SPECIAL CONSIDERATIONS¶
Immune dysregulation in pregnancy, pediatric populations, and the elderly requires tailored approaches. For example, CAR T-cell therapy carries risks of neurotoxicity (cytokine release syndrome) and infections in immunocompromised patients.
5.1 Pregnancy and Autoimmune Diseases¶
Autoimmune diseases (e.g., lupus, rheumatoid arthritis) may flare during pregnancy, requiring careful management of immunosuppressive therapies to balance maternal and fetal risks.
6. KEY POINTS & CLINICAL PEARLS¶
- Immune checkpoints (CTLA-4, PD-1) are critical for maintaining self-tolerance and antitumor immunity.
- Checkpoint inhibitors have transformed cancer therapy but carry risks of immune-related adverse events.
- CAR T-cell therapy is effective for B-cell malignancies but requires careful monitoring for cytokine release syndrome.
- Autoimmune diseases result from defects in central/peripheral tolerance mechanisms, often involving Treg dysfunction.
- Monoclonal antibodies targeting cytokines (e.g., TNF- α , IL-6) are cornerstone therapies for inflammatory and autoimmune disorders.