Exhaustive Study Notes on Tolerance, Autoimmunity, and Autoimmune Diseases

Introduction to Immunologic Tolerance and Autoimmunity

  • Definition of Immunologic Tolerance: This is the process of acquiring non-reactivity toward specific self-antigens.
  • Critical Nature of Self-Recognition: The ability to recognize "immunologic" self-antigens is vital for health. A failure in this process can lead to the development of autoimmune diseases.
  • Textbook References:
    • Chapter 2727 (6th6\text{th} edition).
    • Chapter 2222 (7th7\text{th} edition).

Layers of Immunologic Tolerance

  • Central Tolerance:
    • Thymus (Site of Action): The mechanism is deletion of autoreactive T-cells.
    • Bone Marrow (Site of Action): The mechanisms include deletion and receptor editing of B-cells.
  • Peripheral Tolerance:
    • Regulatory Cells: Occurs in secondary lymphoid tissue. The mechanism involves cellular inactivation by weak signaling in the absence of costimulus.
    • Clonal Deletion: Occurs in secondary lymphoid tissue and sites of inflammation. The mechanism is suppression via cytokines and intercellular signals.
    • Apopotic Pathways: Post-activation apoptosis occurs in secondary lymphoid tissue and sites of inflammation.
    • Cytokine Deviation: Occurs in secondary lymphoid tissue and sites of inflammation. The mechanism involves differentiation to TH2T_H2 cells, which limits the secretion of inflammatory cytokines.
    • Antigen Segregation: Occurs in peripheral organs such as the thyroid and pancreas. This acts as a physical barrier preventing self-antigen access to the lymphoid system.
    • Clearance of Apoptotic Bodies: Occurs in secondary lymphoid tissue and sites of inflammation. The mechanism involves the removal of self-antigen and the induction of negative signals.

Concepts in Autoimmunity

  • Autoreactive (Self-reactive): This refers to an immune system response directed against autoantigens (self-antigens).
  • Autoantibodies: These are specialized immunoglobulins produced specifically in response to self-antigens.
  • Autoimmune Disorder: A condition characterized by actual tissue injury resulting from an autoimmune response.

Maintenance of Self-Tolerance

  • Positive Selection: Selection process ensuring the survival of functional lymphocytes.
  • Negative Selection:
    • Occurs when lymphocytes lack affinity for self-peptides or self-MHC.
    • Autoreactive B cells are negatively selected based on their interaction with self-antigens.
  • Peripheral Tolerance: Managed within peripheral lymphoid tissues.
  • Independent Pathways: The pathways for B and T cells are affected independently and function differently to maintain tolerance.

T-Cell-Specific Tolerance

  • Thymic Deletion: The primary method of removing autoreactive T-cells.
  • Three Pathways of Tolerance:
    1. Clonal Abortion: Immature T-cell clones are aborted, a process similar to B-cell clonal abortion.
    2. Functional Deletion: Subsets of mature T-cells may be individually deleted. This results in the loss of only one specific function within a T-cell group rather than the whole group.
    3. T-Cell Suppression: Active suppression where T-cell suppressors inhibit the actions of other T-cell subsets or B-cells.

B-Cell-Specific Tolerance

  • Dependency Factors: B-cell tolerance depends on the maturity of the cell, the nature of the antigen, and the manner of antigen presentation.
  • Four Pathways of Tolerance:
    1. Clonal Abortion: Can be triggered by low concentrations of multivalent antigens, causing the immature clone to abort.
    2. Clonal Exhaustion: Occurs following repeated antigen challenge with a T-independent antigen, which can eventually remove all mature functional B-cell clones.
    3. Functional Deletion: Prevented by the combined absence of helper T-cell subsets and the presence of T-dependent antigens, or by an excess of T-independent antigens, which stops mature B-cells from functioning normally.
    4. Antibody-Forming Cell Blockade: Occurs when an excess of T-independent antigen interferes with the secretion of antibodies by the antibody-forming cells.

Factors Influencing the Development of Autoimmunity

  • Immunopathogenic Mechanisms:
    • Alterations in lymphocytes.
    • Abnormal selection processes for lymphocytes.
    • Cross-reactions with foreign antigens (molecular mimicry).
    • Increased production of cytokines.
    • Polyclonal stimulation of lymphocytes.
    • Local tissue alterations.
  • Disease Spectrum: Includes both organ-specific and non-specific types.
  • Genetic Factors: May reduce the rate of lymphocyte deletion or enhance the activation of autoreactivity.
  • Gender: Autoantibodies are notably more prevalent in females than in males.
  • Patient Age: Autoimmunity risk increases steadily with age, typically peaking between 60−7060-70 years of age.
  • Exogenous Factors: Exposure to radiation, drugs, viruses, and chronic infectious diseases.

GeneticSusceptibility and Disease Associations

  • Pathogenesis Flow:
    1. Genetic susceptibility combined with environmental stimuli leads to susceptibility genes.
    2. This results in a failure of self-tolerance.
    3. Self-reactive lymphocytes are activated, leading to self-reactive effector lymphocytes.
    4. Simultaneous activation of tissue Antigen Presenting Cells (APCs) contributes to tissue injury and inflammation.
    5. The final result is an autoimmune disease.
  • Table of Associated Abnormalities:
    • Addison’s Disease: Targeted autoantigen is P-450 enzymes.
    • Crohn’s Disease: Targeted autoantigens include p-ANCA and pancreatic acinar cells.
    • Ovarian Failure/Infertility: Targeted autoantigen is P-450 enzymes.
    • Pernicious Anemia: Targeted autoantigen is Parietal cells.
    • Ulcerative Colitis: Targeted autoantigen is p-ANCA.

Clinical Presentation and Laboratory Markers

  • Common Features:
    • Elevation of serum immunoglobulins.
    • Inflammatory lesions and tissue destruction.
    • Decreased levels of complement proteins.
    • Heavy predilection for the female sex.
    • Clinical improvement when treated with immunosuppressive therapy.
  • Major Autoantibodies and Targets:
    • Antinuclear Antibodies (ANA): DNA, histone, and non-histone proteins.
    • Thyroid: Anti-thyroglobulin and anti-thyroid microsome.
    • Gastric: Anti-intrinsic factor and anti-parietal cell.
    • Adrenocortical: Antiadrenal.
    • Striated Muscle: Anti-striational.
    • Others: Acetylcholine Receptor, Smooth Muscle, Salivary Gland, Mitochondrial, Reticulin, Myelin, Islet cell, and Skin.

Arms of Immunity in Autoimmune Response

  • Innate Immune System:
    • Non-specific and involves an excess inflammatory response.
    • Systemic Lupus Erythematosus (SLE): Often involves complement deficiency and monocytic activation where repair processes become damaging.
  • Adaptive Immune System:
    • Involves T and B cells responding to self-antigens.
    • B-cell Regulation: Faults in B-cell regulation account for the majority of autoimmune diseases.

Classification of Autoimmune Diseases

  • Organ-Specific: The immune response is directed strictly against antigens located within one particular organ.
  • Organ-Nonspecific: The immune response targets antigens found across many different tissues.
  • Mid-Spectrum Diseases:
    • Vasculitis: Involves circulating immune complexes.
    • Carditis: Involves both endogenous and exogenous cardiac antigens.
    • Collagen Vascular Diseases: Including Progressive Systemic Sclerosis (Scleroderma) and Eosinophilia-Myalgia Syndrome.
    • Endocrine Gland Diseases:
      • Hashimoto’s (Lymphoid) chronic thyroiditis.
      • Graves’ disease.

Progressive Systemic Sclerosis (PSS) / Scleroderma

  • Etiology: Often idiopathic, but can be triggered by drugs or occupational factors.
  • Epidemiology:
    • Affects all races.
    • 33 times more common in females than males.
  • Signs and Symptoms:
    • Fibrosis of the skin and various internal organs.
    • Raynaud’s Syndrome (arterial occlusions).
    • Hypo-motility within the gastrointestinal (GI) tract.
    • Disease progression can be slowly progressive or rapid and fatal.
  • Laboratory Findings:
    • Positive Antinuclear Antibodies (ANA) found in 40−90%40-90\% of cases.
    • Presence of Extractable Nuclear Antigens (ENAs) targeting the nucleolus, the centromere, and Scl-70.
  • Cellular Hyperactivity:
    • T-cells: Drive vascular changes and increased collagen production.
    • Cytokines: Facilitate inflammation and fibrosis.
    • Endothelial Cells: Involved in the vascular component of the disease.