Cellular mechanisms of pancreatic damage
The development of type 1 diabetes mellitus (Diabetes mellitus typus 1) is based on the destruction of β-cells in the islets of Langerhans. The main executioners of this process are cellular elements of the immune system:
- Cytotoxic T-lymphocytes. They directly attack β-cells. Target cell killing occurs via two pathways: through the release of perforins (which form pores in the membrane) and via the induction of Fas-dependent apoptosis (programmed cell death).
- Macrophages (Th1-dependent pathway). T-cell aggression is not fully possible without the participation of macrophages. If macrophages are absent, the damaging effect of sensitized T-lymphocytes drops sharply. Macrophages actively release reactive oxygen species (ROS) and nitrogen species, and also secrete IL-1β. This interleukin is a potent cytotoxic factor for those β-cells that bear the corresponding receptors on their surface.
Regarding the humoral arm of the immune system, patients' blood frequently contains autoantibodies against various targets, including insulin itself. However, to date, their actual pathogenetic role in β-cell death has not been established.
Differences from type 2 diabetes mellitus
To understand type 1 immunology, it is important to contrast it with type 2 (insulin-independent) diabetes mellitus.
Unlike type 1, type 2 diabetes manifests primarily in older adults. Its pathogenesis is completely different: no destruction of pancreatic cells occurs. Instead, the body produces autoantibodies directed against insulin receptors.
As a result, insulin is produced in normal amounts, but target tissues lose the ability to perceive its signal. This is precisely why classical therapy with insulin preparations proves ineffective in this form of the disease.
Other examples of autoimmune endocrinopathies
In addition to the pancreas, the thyroid gland frequently becomes a target of autoimmune aggression. Organ-specific pathologies of this organ are usually accompanied by goiter formation (enlargement of the gland). Three main forms are distinguished:
- Basedow's disease (thyrotoxicosis, Graves' disease). Characterized by hyperthyroidism. Autoantibodies bind to thyrotropin (TSH) receptors on cell membranes. The uniqueness of the mechanism is that the antibodies do not block, but rather activate the cells, forcing them to synthesize hormones uncontrollably.
- Hashimoto's thyroiditis. Leads to hypothyroidism. The immune system attacks the intracellular protein thyroglobulin. Interestingly, predisposition to the disease is associated with single-nucleotide substitutions in the gene encoding this protein.
- Primary myxedema. Also presents with hypothyroidism. Autoantibodies are produced against the colloid antigen CA2 and other proteins.
A common feature of hypothyroid forms is the suppression of hormone secretion under the action of antibodies, while compensatory cellular hypertrophy leads to goiter growth.
Breakdown of self-tolerance as exemplified by multiple sclerosis
The fundamental mechanism of any autoimmune disease is the loss of self-tolerance. This process has been studied in detail using multiple sclerosis (a chronic demyelinating disease) as an example.
The key event in pathogenesis is the activation of autoreactive T-lymphocytes, which begin to recognize epitopes of native myelin. The triggering factor is believed to be molecular mimicry. Viral antigens can be structurally similar to myelin antigens. Consequently, virus-specific T-cells cross-react with the body's own tissues.
For the disease to transition into an active phase, an additional condition is necessary: a deficiency of regulatory T-cells and a failure of other immune control mechanisms. This same principle of loss of control is also relevant to the development of type 1 diabetes mellitus.
Briefly: answers to topical questions
Which specific β-cell antigens do autoantibodies form against in type 1 diabetes mellitus?
In type 1 diabetes mellitus, autoantibodies are detected against pancreatic islet cell antigens:
- GADA — against glutamate decarboxylase;
- IA-2 / IA-2A — against tyrosine phosphatase;
- ZnT8 — against zinc transporter 8;
- IAA — against insulin;
- ICA — against islet cell structures.
Which major histocompatibility complex (HLA) alleles are associated with a high risk of type 1 diabetes mellitus?
Type 1 diabetes mellitus is associated with diabetogenic HLA antigens:
- HLA-B8;
- HLA-B15;
- HLA-D;
- HLA-DR3;
- HLA-DR4.
It is also indicated that autoantigenic peptides from pancreatic proteins preferentially bind to HLA-B8 and HLA-DR3 molecules, which is associated with a high incidence of juvenile / type 1 diabetes mellitus.
Which viruses are considered the main triggers for the development of type 1 diabetes mellitus?
The main triggers for the development of type 1 diabetes mellitus are viral infections capable of causing the destruction of islets of Langerhans β-cells. These etiological factors include:
- Coxsackie virus;
- Mumps virus;
- Measles virus;
- Rubella virus;
- Smallpox virus;
- Cytomegalovirus;
- Adenovirus.
The impact of these infectious agents unfolds against a background of genetic predisposition and triggers the autoimmune component of pathogenesis.
What main pro-inflammatory cytokines, besides IL-1β, participate in the destruction of the islets of Langerhans?
In addition to IL-1β, the immunopathogenesis of β-cell damage in the islets of Langerhans involves:
- IFNγ — synthesized by Th1 cells and activates macrophages;
- IL-2 — primarily sourced from Th1 cells; under its influence, CD8+ T-lymphocytes proliferate and differentiate into cytotoxic T-lymphocytes that cause cell destruction.
Furthermore, tissue damage in delayed-type hypersensitivity is associated with products of activated macrophages, including pro-inflammatory cytokines.
Remember the role of cells in T1D: "T-lymphocytes are T-terminators (perforin, apoptosis), while Macrophages are M-throwers (throwing reactive oxygen species and IL-1β)."
Class and exam questions
What is the role of insulin antibodies in type 1 diabetes mellitus?
Autoantibodies to insulin and other β-cell antigens are frequently detected in the blood, but their destructive (pathogenetic) role has not been proven to date. T-lymphocytes and macrophages pose the main danger.
How does Graves' disease differ from Hashimoto's thyroiditis in the mechanism of antibody action?
In Basedow's disease, antibodies bind to TSH receptors and activate cells (hyperthyroidism). In Hashimoto's thyroiditis, antibodies attack thyroglobulin, leading to suppression of glandular function (hypothyroidism).
Why are insulin preparations ineffective in type 2 diabetes mellitus?
In type 2 diabetes, endogenous insulin production is unimpaired. The problem lies in the presence of autoantibodies to insulin receptors, causing tissues to simply fail to perceive the hormone's signal.
What else to study on this topic
- Mechanism of Fas-dependent apoptosis of β-cells
- Properties and functions of interleukin-1β (IL-1β)
- The phenomenon of autoantigen molecular mimicry
- The role of regulatory T-cells in maintaining self-tolerance
- Thyroglobulin genetic mutations in autoimmune thyroiditis
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Other topics in the section "Immunity in defense and damage of the organism. Pathology of immunity"
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