July 06, 2024
Type 1 diabetes mellitus is a disease that combines autoimmune (95%) and idiopathic (5%) variants of pancreatic β-cell death with absolute insulin deficiency, distinguishable by the detection or absence of autoantibodies. Depending on the degree and rate of β-cell destruction, three subtypes of diabetes can be distinguished.
In the fulminant form, autoantibodies are very rarely detected.
In acute and slowly progressive forms (latent adult-onset diabetes, LADA) the detection of autoantibodies helps in the differential diagnosis with type II diabetes.
A legitimate question, however, is whether antibody production is the cause of immune inflammation or merely a witness to the drama?
The short answer to this question is that they are considered witnesses (for now)
It is thought that the development of autoimmune type diabetes is based on a type IV hypersensitivity response (Delayed-Type Hypersensitivity), which is independent of antibodies.
The immune response is mediated by cytotoxic CD8 + T- lymphocytes and CD4 + T- lymphocyte cytotoxins,which together with macrophages support the inflammatory process. Read more about how cells recognize their own - strangers here.
Let us review the list of known autoantibodies.
- Insulin autoantibodies (IAA)
- antibodies to glutamic acid decarboxylase (GAD-65)
- antibodies to islet cell protein (islet cell antibodies, ICA)
- antibodies to tyrosine phosphatase (IA-2A)
- antibodies to Zn transporter 8 (ZnT8)
Let's try to remember them again by making logical connections. Let's pretend we are a a β-cell and imagine the tasks involved in producing insulin and some signaling molecules.
Briefly, the process can be described as follows: synthesize a large complex molecule, pack it compactly. And when a signal is given, the packaged molecule rolls into the external environment.
The first task is performed by the endoplasmic reticulum (EPR). It synthesizes pre-proinsulin, from which a signal peptide is further cut, converting pre-proinsulin into pro-insulin.
Then it is the turn of the EPR's indispensable assistant, the Golgi apparatus, where the pro-insulin molecule is completely and figuratively finished into insulin and C-peptide. The next step is to seal the vacuoles, with the peptides packed into them.
But we are a thrifty cell and we have no extra space, so we want to group and compactly organize the precious product.
For this purpose we need zinc ion Zn (β-cells contain unusually large zinc reserves compared to other cells of the organism).
On the surface of the membrane of the vacuole is a zinc transporter (ZnT8), which allows Zn molecules to enter the lumen of the vacuole and participate in the formation of crystalline hexameric aggregates of insulin. The result is a "mature" form of insulin that forms a dense granule core. The granules also contain C-peptide and amylin.
Now a signal 💥 is required to trigger the exocytosis mechanism, where insulin and C-peptide are secreted in small amounts into the extracellular space.
Triggers of insulin granule exocytosis
Glucose enters the β-cell through the GLUT2 channel and is recycled in the Krebs cycle, increasing the concentration of intracellular ATP.
ATP-sensitive potassium (K+) channels pick up this increase and close. A corresponding increase in intracellular K+ leads to membrane depolarization.
In response, potential-dependent Ca 2+ channels wake up and release calcium into the cell. This is considered to be the key player that initiates the process by which mature insulin granules move to and fuse with the surface membrane.
Thus, the granule filling (insulin, C-peptide and amylin) ends up outside the β-cell.
Other signals for exocytosis of insulin granules include some amino acids (leucine and arginine), and stimulation of the vagus nerve.
The β-cell as a versatile factory
In addition to insulin, the β cell can also synthesize a number of neurotransmitters and other signaling molecules packaged in synaptic-like microvesicles (SLMVs).
Why do islets need to make GABA?
Now it is time to mark antibodies on the scheme
It is striking that the majority of autoantibodies are directed against proteins of secretory granules, especially against antigens of granules containing mature insulin.
Antibodies to glutamic acid decarboxylase (the isoform GAD-65, анти-GAD-65)
The older the patient, the more likely it is that antibodies against GAD-65 will be detected. This type of antibody is particularly characteristic of LADA diabetes. In this case, it is not only predominant but also the longest-lived, being detected in patients even 10 years after the onset of the disease.
These antibodies are sometimes found in a fulminant course of diabetes mellitus.
In a person with a genetic predisposition to type 1 DM (or LADA) and no evidence of diabetes, the presence of antibodies to GAD-65 confers a 20-fold increased risk of developing diabetes within 10 years.
These antibodies are not specific to diabetes. They can occur in some rare neurological disorders. Their clinical picture (muscle cramps, seizure syndrome, "Restrained Man Syndrome") is based on the fact that in certain areas of the brain the balance of neurotransmitters is shifted toward the excitatory glutamate. In this case, levels of antibodies to GAD are 100 to 500 times higher than in diabetes.
In general, antibodies to GAD-65 are found in about 1% of healthy people in the population.
Antibodies to Zn transporter 8 (Zn transporter 8, ZnT8A)
Antibodies to the zinc transporter are formed relatively late from the onset of β-cell destruction and are not always detected. When they are detected, they are often detected together with other antibodies.
Their presence is more characteristic of LADA diabetes, sometimes they can be the only autoantibodies.
In general, the presence of these antibodies can predict the rapid manifestation and more aggressive course of diabetes.
Zinc transporter antibody levels often drop soon after the disease becomes manifest.
Testing for ZnT8A is used after pancreatic transplantation to assess the risk of rejection.
Insulin autoantibodies (IAA)
Autoantibodies to insulin are a specific marker of β-cell damage because these are the only cells in the body that produce insulin. These antibodies are more common in children at high risk for type 1 DM and are very rare in adults. IAA appear early and are present in high concentrations at the time of diagnosis. After the onset of diabetes, their levels decrease.
In patients diagnosed with type 1 DM who have started insulin therapy, antibodies to exogenously administered insulin may appear. Therefore, testing after insulin therapy has been started is of little use. The test is also not recommended for determination in the diagnosis of diabetes mellitus in adults.
Ig G antibodies to islet cell protein (ICA)
Determined by immunofluorescence and, unlike the tests for antibodies to GAD-65 and IA-2, requires an advanced laboratory level and sufficient experience to evaluate the result.
Autoantigens are found in the membrane of the secretory granule and in other β-cell compartments. They are probably involved in the maturation of insulin in the granule.
This is a frequent finding in the debut of type 1 DM in children. However, these antibodies can also be detected in other autoimmune diseases (Addison's disease, Hashimoto's thyroiditis, etc.) and are therefore not suitable as an initial test to assess disease risk in first-line relatives. In this case, it is recommended to test first for antibodies to GAD65 and IA-2, and in children also for antibodies to insulin, and only then for ICA.
After manifestation of the disease, the concentration of these antibodies decreases and after six months they may no longer be detected.
In the general population, these antibodies can be detected in 0.5% of cases and only a small percentage of these people will develop type 1 DM. .
Autoantibodies against tyrosine phosphatase (IA-2, tyrosine phosphatase‐related molecules‐2, protein tyrosine phosphatase-like protein (PTP))
IA-2 belongs to the tyrosine phosphatase family. It is a large transmembrane glycoprotein located in the membrane of the insulin vesicle.
This protein is thought to regulate the process of insulin granule exocytosis and β-cell growth.
Antibodies to this protein appear long before the disease, are detected more frequently in children and are a marker of a more aggressive immune process. At the time of clinical diagnosis of DM type 1, these autoantibodies are present in 60% of cases. Their presence in family members of the patient increases the risk of developing the disease.
Let's add a few general comments about autoantibodies
The HLA genotype determines the expression of autoantibodies and the type of autoantibodies produced in the patient.
Some antibodies are specific for pancreatic β-cell injury (IAA and ZnT8A) and others are not (anti-GAD-65)
Many autoantibodies can be detected in the body long before impaired glucose tolerance and/or clinical manifestations of the disease. Some occur very early, others at a later stage.
In children and adults, the palette of autoantibodies and the frequency with which they occur differ.
In children insulin antibodies (IAA) often appear first. By the time DM type 1 manifests clinically, the antibodies are often found in combination.
In adults with LADA anti-GAD-65 antibodies appear very early and detection approaches 100% at the time of diagnosis. About as often, at the onset of hyperglycemia, they have ICA antibodies. Less frequently IA-2, in this case almost always together with anti-GAD-65. Antibodies to zinc transporter 8 are detected much less frequently, but sometimes they are the only antibodies that allow the diagnosis of LADA diabetes. Unlike in children, antibodies against insulin, a specific marker of DM 1, are not particularly relevant in the case of LADA screening.
There are also differences in the dynamics of their concentration after diagnosis. Usually the antibody concentration drops to levels undetectable by laboratories. However, antibodies to GAD-65 are also detected after 10 years of illness.
It is difficult to say how this dynamic of AT formation reflects the stages of β-cell destruction. A more important factor is the presence of more than one antibody in plasma in a person without evidence of hyperglycemia. This usually indicates a significantly increased risk of developing diabetes. This is especially true for family members in the patient's 1st line. For example, the presence of two autoantibodies to GAD65 and IA2 in a sibling increases the risk of diabetes to 60-70% over the next five years, and the detection of three autoantibodies brings it close to 100%.
In general, assessing the risk of developing diabetes in children whose parents have diabetes is a separate issue. However, parental disease has been shown to increase the risk of diabetes in children slightly more (4-7%) than maternal diabetes (2-4%). The risk of developing type 1 DM in a patient's sibling is about 3-6%. In monozygotic twins from 35% to 50%.
This review lists only the antibodies most commonly used in the clinic. Other autoantibodies under investigation include Heat shock protein 60, antibodies to GLUT2, Tetraspanin-7 and others.
Concluding remarks
Type 1 diabetes mellitus is diagnosed based on blood glucose levels and the result of the glycosylated hemoglobin HbA1C test. The determination of antibodies plays a supporting role.
They are most likely not the cause of autoimmune β-cell damage, but a consequence of it.
Autoantibodies in type 1 DM and LADA diabetes help to confirm the diagnosis and distinguish it from other types and forms of diabetes. Antibody testing should be performed as early as possible after diagnosis because most antibody levels continue to decline.
Screening of first-line relatives makes no decisive contribution to the early detection of people at high risk of developing type 1 DM in the general population, as a significant proportion of patients have no family history of the disease.
The presence of more than one antibody in a person with normal or impaired glucose tolerance, but without symptoms of diabetes, indicates a rapidly approaching manifestation of the disease and a more aggressive course.
Since the clinical pictures of LADA and type 2 DM are often indistinguishable at the time of diagnosis, antibody detection can be crucial. Anti-GAD-65 is the most common autoantibody in LADA, but if it is not present, the second step is to determine other antibodies (ZnT8A and IA-2).
More specific tests may be available in the future to help identify a group of patients for taget immunotherapy.