2 Types of Autoimmune Diseases: Organ Specific and Systemic Autoimmune Diseases (2023)

The two broad types of autoimmune diseases are: 1. organ specific and 2. Systemic autoimmune diseases!

Autoimmune diseases are described under two broad categories: organ-specific autoimmune diseases and systemic autoimmune diseases (Tables 20.2 and 20.3). .

1. Organ-Specific Autoimmune Diseases:

In organ-specific autoimmune diseases, the autoimmune responses are directed against antigens present only in a particular organ (Table 20.2).

i. In some autoimmune diseases the autoantibodies bind to self-antigens in the organ cells and lead to destruction of cells.

ii. In some other autoimmune diseases, the autoanti­bodies bind to self-antigens on cells and lead to either overstimulation of the cell or suppression of the normal functioning of the cells.

Autoimmune Hemolytic Anemia:

In autoimmune hemolytic anemia, auto antibodies to self- RBCs are formed. The autoantibodies bind to antigens on RBCs and lead to the lysis of RBCs. Most drugs are not immunogenic by themselves, but they may act as haptens.

The drug forms a complex with protein antigens on RBCs. The drug-RBC antigen complex induces the immune system to produce antibodies. The antibodies formed against the drug-RBC antigen complex bind to the RBC and activate the complement cascade, resulting in RBC lysis.

Pernicious Anemia:

Pernicious anemia is a chronic disease resulting from the non-absorption of vitamin B₁₂, which is essential for the development of RBCs. Pernicious anemia, is most common in late adult life. The basic abnormality of the disease is severe atrophic gastritis, wherein there is extreme deficiency of all the gastric secretions, including intrinsic factor. The gastric lesion probably develops due to an autoimmune attack on gastric cells.

Several types of auto antibodies are found in pernicious anemia patients:

i. The parietal cell auto antibodies react with the beta unit of the gastric ATPase proton pump.

ii. About half the pernicious anemia patients have auto­antibodies to intrinsic factor in their serum. Auto antibodies to intrinsic factor are found in the gastric juice of 75 percent of patients.

Normally, the intrinsic factor in the gastric juice binds to vitamin B12 in food and forms a complex; and the intrinsic factor-vitamin B₁₂ complex is absorbed from the intestine and the absorbed vitamin B₁₂ is used for RBCs production.

The autoantibodies to intrinsic factor may interfere with the absorption of vitamin B₁₂ by the following mechanisms:

i. In pernicious anemia, the intrinsic factor autoanti­bodies in gastric juice may bind to intrinsic factor and block the binding of vitamin B₁₂ to intrinsic factor

ii. The autoantibody to intrinsic factor may bind to intrinsic factor-B₁₂ complex and interferes with the absorption of the complex. Consequently, the absorption of vitamin B₁₂ is interfered, resulting in decreased production of RBCs.

Pernicious anemia patients are treated by regular vitamin B₁₂ injections.

Idiopathic Autoimmune Thrombocytopenic Purpura:

Autoantibodies to platelets bind to many of the major platelet membrane glycoproteins. Platelet GpIIb / IlIa and GPIb/IX are the major antigens with which the platelet autoantibodies bind. The antibody-coated platelets are engulfed mainly by the splenic macrophages and destroyed .Consequently, the number of platelets decrease (known as thrombocytopenia) and the patient suffers from bleeding from skin, mucous membrane and other parts as well.

Goodpasture’s Syndrome:

Autoantibodies to certain antigens on the membrane of kidney glomeruli and lung alveoli are formed in a condition called Good pasture’s syndrome. Binding of autoantibodies to the membrane antigens in lung and kidney leads to complement activation, resulting in inflammatory reactions in lung and kidney. Consequently, the kidneys are damaged and the patient also suffers from pulmonary hemorrhage (i.e. bleeding from lungs).

Hashimoto’s Thyroiditis:

In Hashimoto’s thyroiditis autoantibodies against many thyroid proteins and T cells specific for thyroid antigens are formed. There is abundant infiltration of thyroid gland by lymphocytes, macrophages, and plasma cells. Thyroglobulin and thyroid peroxidase are necessary for the uptake of iodine by thyroid gland and subsequent production of thyroid hormones.

In Hashimoto’s thyroiditis, autoantibodies to thyroglobulin and thyroid peroxidase are formed. The binding of the auto antibodies to thyroglobulin and thyroid peroxidase interfere with the iodine uptake by thyroid gland. Consequently, production of thyroid hormones decreases and the patient develops hypothyroidism. (Hypo­thyroidism means decreased production of thyroid hormones.)

Autoantibodies may cause type II mediated damage to thyroid cells. The thyroid-bound thyroid auto­antibodies may bind to NK cells (through the Fc region) and lead to thyroid destruction by ADCC (antibody- dependent cellular cytotoxicity) mechanism.

Insulin-dependent Diabetes Mellitus:

Insulin-dependent diabetes mellitus (IDDM) is a T cell mediated autoimmune disease .IDDM is also known as juvenile diabetes because it usually appears in childhood or early adolescence. Insulin is produced by beta cells in the islets of Langerhans’ in pancreas.

The insulin secreted by the beta cells is essential for glucose metabolism. Decreased production or non-production of insulin results in diabetes mellitus. Insulin-dependent diabetes (IDDM) is a condition caused by autoimmune responses against the beta cells in pancreas resulting in destruction of beta cells. Consequently, insulin secretion is decreased and the patient develops diabetes.

Several factors may be involved in the destruction of beta cells by autoimmune responses:

i. Self-reactive cytotoxic T cells (CTLs) against the beta cells migrate to areas of beta cells in pancreas. The cytokines secreted by CTLs attract and activate macrophages. The cytokines of CTLs and the lytic enzymes released by macrophages may destroy the beta cells.

ii. Autoantibodies to beta cells may activate the complement cascade and destroy beta cells. Auto­antibodies may also mediate antibody-dependent cell-mediated cytotoxicity (ADCC), resulting in destruction of beta cells.

What is the involved antigen on beta cells and what predisposes the attack on the beta cells are not yet known.

IDDM occurs at a markedly higher frequency in individuals with certain MHC class II molecules. As with other autoimmune diseases the role of MHC molecules in the causation of disease is unknown. If one of the identical twins has IDDM, the chance that the other individual will develop IDDM is one in three. This suggests that apart from genetic factors (such as MHC molecules) some other environmental factors also are required for the development of IDDM.

iii. Non-obese diabetic (NOD) mouse is an excellent animal model for IDDM. Administration of immunosuppressive drugs to NOD mice or depletion of T cells from NOD mouse retards the progression of diabetes. Furthermore, transfer of T cells from NOD mouse to non-diabetic mouse causes transfer of diabetes. These observations suggest that T cell mediated immune mechanisms play important roles in the development of diabetes in NOD mice.

Graves Disease:

Graves’ disease is an example where binding of autoantibodies to cell surface receptors results in activation of the cell. In Graves’ disease, autoantibodies to thyroid stimulating hormone (TSH) receptors bind to TSH receptors on thyroid and lead to excessive production of thyroid hormones.

Myasthenia gravis:

Binding of autoantibodies to acetylcholine receptors on the muscle cells at the neuromuscular junction interferes with the action of acetylcholine on muscle cells, resulting in a condition called myasthenia gravis.

2. Systemic Autoimmune Diseases:

In systemic autoimmune diseases, the autoimmune responses are directed against self-antigens present in many organs and tissues of the body resulting in widespread tissue damage to the host (Table 20.3).

Systemic Lupus Erythematous:

Systemic lupus erythematous (SLE) is currently the most prevalent autoimmune disease in the developed countries. SLE patients develop autoantibodies to a number of self-antigens. The commonest autoantibody found in the serum of SLE patients is the autoantibody to double stranded DNA (dsDNA).

SLE is a chronic systemic inflammatory disease and it affects many organ systems (skin, joints, kidneys, lungs, heart, etc.). The cause of SLE is not known. SLE affects predominantly females.

SLE patients produce autoantibodies to a number of self-antigens, such as DNA, histones, RBCs, platelets, leukocytes, and clotting factors. The antibodies may belong to IgG or IgM class.

Autoantibodies in SLE:

a. Autoantibodies to nuclear components:

i. Anti-nuclear antibodies (against multiple nuclear antigens)

ii. Anti-histone antibodies

iii. Anti-ds (double stranded)-DNA antibodies

iv. Anti-ribo-nucleoprotein antibody

v. Anti-Sm (Smith) antibodies (Antibodies against ‘extractable nuclear proteins’ were originally designated by the initials of the patients in whom they were first found (e.g. Sm for Smith)

vi. Anti-ss (single stranded)-DNA antibodies.

b. Autoantibodies to non-nucleic acid antigens:

i. Anti-erythrocyte antibodies

ii. Anti-platelet antibodies

iii. Lymph cytotoxic antibodies (predominantly against T lymphocytes)

iv. Anti-neuronal antibodies

v. Anti-phospholipid antibodies.

c. Anti-cytoplasmic antibodies:

i. Anti-mitochondrial antibodies

ii. Anti-ribosomal antibodies

iii. Anti-lysosome antibodies.

Interaction of autoantibodies with specific self- antigens causes various clinical problems:

i. Autoantibodies to RBCs lead to lysis of RBCs and result in autoimmune hemolytic anemia

ii. Autoantibodies to platelets destroy the platelets and lead to thrombocytopenia and bleeding problems

iii. Deposition of circulating immune complexes in the blood vessels causes vacuities

iv. Deposition of circulating immune complexes in kidneys causes glomerulonephritis

v. Deposition of circulating immune complexes in synovial membranes of joints leads to arthritis.

In SLE, large amounts of circulating immune complexes are formed. The immune complexes in SLE are small and they get trapped mainly in the kidneys and synovial tissue of the joints. Therefore symptoms due to glomerulonephritis and arthritis are the most common presentations in SLE. Immune complex deposition and subsequent complement activation leads to the tissue destruction.

Since complement system is activated during immune complex mediated reactions, the serum levels of C3 and C4 are lower than normal in SLE. The serum levels of C3 and C4 are reported to be proportional to the severity of the disease in SLE.

Rheumatoid Arthritis:

Rheumatoid arthritis (RA) is a chronic systemic inflammatory disease primarily involving the joints. The cause of RA is unknown. 70 percent of patients with RA carry the HLA-DR4 haplotype. The serum and synovial fluids (the fluid in the joints) of the RA patients have rheumatoid factors. Rheumatoid factors of IgM, IgG and IgA are found in serum and synovial fluid of RA patients.

The rheumatoid factors in joints are thought to be synthesized locally within the lymphoid infiltrates of synovial membrane of the joint. Immune complexes consisting of the rheumatoid factor-immunoglobulin activate the complement cascade and lead to a number of inflammatory phenomenon’s affecting the joints. Initially swelling and pain occur in small joints and later it involves the larger joints.

In advanced cases permanent joint deformities occur and the patients are crippled.

i. Rheumatoid factors (RFs) are autoantibodies directed against antigenic determinants on the CH2 and CHS domains in the Fc region of IgG molecules. RFs are generally associated with rheumatoid arthritis. However RFs are present in normal individuals and RFs are elevated in a variety of other diseases.

Normal synovial membrane of a joint is relatively an acellular membrane. In RA, the synovial membrane is infiltrated with inflammatory cells and forms an invasive pannus consisting of macrophages, mast cells, and fibroblasts.

The junction between the invasive pannus and the joint cartilage is a focus for enzymatic degradation. Organized lymphoid tissues (CD4⁺ T cells, B cells and macrophages) are present around the synovial blood vessels. The synovial fluid of RA patients contains large numbers of neutrophils (upto 10⁵/ml). In RA, the antigen against which the autoimmune responses are directed is not known.

Multiple Sclerosis:

Multiple sclerosis is an autoimmune disease of central nervous system. Auto reactive T cells are implicated in the pathogenesis of the disease. The central nervous system (CNS) is relatively in an immunologically privileged site, because the CNS tissue antigens normally don’t enter the circulation. Therefore self-reactive T cells capable of reacting with self-CNS antigens are not deleted in the thymus; and consequently, such self-reactive T cells are present in the periphery of the host.

It is suggested that in multiple sclerosis (MS) patients, some unknown injurious agent of CNS (like virus) may injure the brain tissues and lead to the exposure of the brain tissues to self-reactive T cells; and consequently, the self-reactive T cells become activated and attack the brain tissues. The activated T cells infiltrate into the brain tissue and lead to destruction of myelin sheath that surrounds the nerve fiber. Destruction of myelin sheath leads to numerous neurological dysfunctions.

The MS patients develop multiple, hard (sclerotic) plaques throughout the white matter of CNS. The plaques show dissolution of myelin and presence of lymphocytes and macrophages. Activated T cells are present in the cerebrospinal fluid.

Ankylosing Spondylitis:

Ankylosing spondylitis is a chronic, progressive, inflammatory disease of unknown etiology. The disease primarily affects the sacroiliac joints, vertebral joints, and large peripheral joints. 90 percent of the affected individuals are males, whereas females are mostly affected in many other autoimmune diseases. A strong association between HLA-B27 and ankylosing spondylitis is seen. Individuals with HLA-B 27 have a 90 times greater likelihood of developing ankylosing spondylitis than individuals with a different HLA-B allele.