Autoantibodies specific to type 1 diabetes

An autoantibody is an immunoglobulin-type protein, which your immune system (B lymphocytes) produces as a molecular weapon against aggressors. The essential difference compared to a regular antibody lies in its target. Normally, antibodies recognise foreign agents, such as bacteria or viruses, while an autoantibody attacks a structure of your own body, such as a protein, an enzyme, a receptor or a component of a cell. This confusion between what is self and what is foreign is the cause of autoimmune diseases [1].

In practice, autoantibodies are useful to you primarily as disease markers. If they appear in your blood, it means your immune system has started a reaction directed against one of your own tissues. In type 1 diabetes, autoantibodies are not responsible for the destruction of the beta cells in the pancreas, which is carried out by T lymphocytes and their friends. The presence of autoantibodies signals that the autoimmune process exists and allows the diagnosis of type 1 diabetes to be made even before your blood glucose rises above normal values [1].

Type 1 diabetes is an autoimmune disease, in which your immune system mistakenly identifies the beta cells in the pancreas (the ones that produce insulin) as a threat and starts a battle against them [1]. Autoantibodies appear because B lymphocytes (cells of the immune system) are activated and start producing antibodies against structures in the beta cells or even against insulin itself. The exact cause of this faulty reaction is not fully understood, but a combination is presumed to exist between a genetic predisposition (especially the HLA system) and certain environmental triggers, such as viral infections, alterations of the gut microbiota or other influences still insufficiently understood [2].

The appearance of autoantibodies is not a sudden event. They can appear several months or years before you show the clinical manifestations of type 1 diabetes, and the type and order of appearance vary greatly from one person to another [3]. This long interval between the onset of autoimmunity and the appearance of hyperglycaemia explains why type 1 diabetes has preclinical stages and why, if you are at high risk, autoantibody testing can identify the disease while the beta cells still function nearly normally.

The four autoantibodies internationally recognised as the standardised markers of type 1 diabetes are the anti-insulin antibodies (IAA), the anti-glutamic acid decarboxylase antibodies (GADA, directed especially against the GAD65 isoform), the anti-tyrosine phosphatase antibodies (IA-2A or anti-islet antigen 2) and the anti-zinc transporter 8 antibodies (ZnT8A) [4]. This is the antibody panel recommended both for presymptomatic screening in people at high risk and for classifying diabetes in adults, when there is suspicion of autoimmune diabetes [5] [6].

Each of the four autoantibodies recognises a different target in the pancreatic beta cells, which makes the panel complementary. If one antibody comes back negative, the positivity of the others can compensate in establishing the presence of autoimmunity. Of all of them, GADA is the most frequently encountered in adults, IAA appears earliest in young children, IA-2A signals an increased risk of rapid progression to hyperglycaemia, and ZnT8A is useful especially when the other three antibodies are absent [7]. When you are investigating the autoimmune nature of diabetes, evaluating all four autoantibodies together gives you the highest probability of confirming the diagnosis.

GADA antibodies are autoantibodies directed against the enzyme called glutamic acid decarboxylase, which exists in two main forms, GAD65 and GAD67. The clinical tests used in practice detect especially anti-GAD65 antibodies [4]. This enzyme is found in the beta cells of the pancreas, but also in the neurons that produce the neurotransmitter GABA, which is why the GADA level can also be elevated in a few rare neurological conditions, a fact that must be borne in mind when your result is interpreted.

In the context of type 1 diabetes, GADA is the most frequently identified autoantibody in adults and tends to persist longest in the blood after diagnosis [8]. GADA is the main autoantibody recommended for testing adults with suspected type 1 diabetes, being the first-line investigation. If you already have diabetes and discover only GADA positive, with no other autoantibodies present, progression toward insulin dependence is expected to be slow, and this profile is characteristic of the autoimmune form of adult-onset type 1 diabetes called LADA (latent autoimmune diabetes of the adult).

IA-2A antibodies are directed against an enzyme of the tyrosine phosphatase class (also called islet antigen 2 or ICA512), located on the membrane of the secretory granules in beta cells. The secretory granules are structures inside the beta cells that store insulin before it is released into the blood. The destruction of beta cells during the autoimmune process exposes this protein to the immune system, which reacts by producing IA-2A [4].

IA-2A has a particular clinical significance, indicating an independent and increased risk of progression to symptomatic type 1 diabetes (stage 3) [9]. For this reason, people with isolated IA-2A are monitored in the same way as those with several autoantibodies present. IA-2A is more frequent in children and adolescents compared to adults, and its combination with GADA, IAA or ZnT8A signals advanced autoimmunity and a high probability of rapid evolution toward clinically manifest hyperglycaemia [10].

IAA antibodies are autoantibodies directed against insulin, the hormone produced by the beta cells of the pancreas. Their particularity is that their target is also the final product of the attacked cells, which makes IAA a direct marker of the autoimmune recognition of insulin. In the young child with a genetic predisposition for type 1 diabetes, IAA is usually the first autoantibody to appear in the blood, sometimes many years before the clinical onset of the disease [3].

An important practical point for you is that IAA cannot be reliably measured after you start insulin treatment, because your body naturally produces antibodies against exogenous insulin, and these cannot be distinguished from the actual autoimmune ones. For this reason, IAA testing is useful only before starting insulin therapy, especially in children or in the context of presymptomatic screening for type 1 diabetes [4].

ZnT8A antibodies target a protein called zinc transporter 8, present in the membrane of the insulin granules in beta cells. This transporter has the role of bringing zinc ions into the interior of the granules, and zinc is essential for storing insulin locally in a stable crystallised form. Any defect of this structure affects the beta cell's ability to store and release insulin correctly [11].

ZnT8A is the most recently included of the four standardised autoantibodies, having been described in 2007, and is particularly useful clinically as a complementary test. If you are suspected of having a form of diabetes that is actually type 1, but GADA, IA-2A and IAA are negative, the presence of ZnT8A can confirm the autoimmune nature of your disease [7]. Including ZnT8A in the standard panel has increased the diagnostic sensitivity, reducing the number of cases mistakenly labelled as idiopathic type 1 diabetes or as atypical type 2 diabetes.

The autoantibodies specific to type 1 diabetes have high specificity, meaning that a confirmed positive result indicates an autoimmune process directed against the beta cells in the vast majority of cases [7]. The greater the number of positive autoantibodies, the higher the diagnostic value. If you have two or more autoantibodies persistently present, progression toward clinically manifest type 1 diabetes is considered almost certain in the long term, even if blood glucose is still normal at the time of testing [10].

On the other hand, the sensitivity of a single autoantibody is limited, which is why testing is always done for the whole standardised panel. A single positive result must be confirmed by a second test, ideally in a laboratory participating in the international standardisation programme (Islet Autoantibody Standardization Program), because a significant proportion of children with a single autoantibody can revert to negativity on subsequent testing [6]. This retesting prevents both underdiagnosis and the mistaken labelling as type 1 diabetes of a person with a false positive result.

Yes. The presence of a pancreatic autoantibody in your blood is not equivalent to a diagnosis of type 1 diabetes and does not guarantee that the disease will appear. A significant proportion of people who have a single positive autoantibody, especially in childhood, spontaneously revert to negativity on repeat testing, without diabetes ever appearing. Your risk of progression depends on several factors, such as the number of autoantibodies present, their titre (quantity), the age at which they appear, the type of autoantibody and the genetic predisposition [12].

If you persistently have two or more autoantibodies, your risk of progression toward symptomatic type 1 diabetes is very high in the long term, but the interval until the appearance of hyperglycaemia can be months, years or even decades [10]. This phase of autoimmunity without clinically manifest diabetes is called stage 1 (with normal blood glucose) or stage 2 (with dysglycaemia) of type 1 diabetes and allows close surveillance, in order to avoid the abrupt onset with diabetic ketoacidosis when hyperglycaemia eventually appears [13]. And let us not forget that autoantibodies do not destroy the pancreatic beta cells, they only draw our attention to the fact that someone else is destroying them.

Yes, it is possible. A small proportion of patients with type 1 diabetes have no detectable autoantibody at the moment of diagnosis [14]. If you are under 35 years of age and do not have clinical features of type 2 diabetes or of monogenic diabetes, a negative autoantibody test does not exclude a diagnosis of type 1 diabetes. In this case the clinical picture, the rapid progression toward insulin dependence and the low level of C-peptide point the diagnosis toward type 1 diabetes [6].

Some of these cases could be explained by the technical limits of the tests, such as autoantibodies present in concentrations that are too small or directed against structures (antigens) not yet included in the standard panel. This category corresponds to the form called idiopathic type 1 diabetes, in which there is permanent insulinopenia and a tendency to ketoacidosis, but without evidence of autoimmunity [15].

Yes, the level of autoantibodies can decrease progressively after you receive a diagnosis of type 1 diabetes stage 3, and some can become undetectable on subsequent testing. This phenomenon is explained by antigen depletion. As your beta cells are destroyed almost completely, the source that stimulates the immune system disappears, and the production of autoantibodies decreases [8]. The four standardised autoantibodies, however, behave differently. GADA persists longest, sometimes years or decades after diagnosis, while IA-2A and ZnT8A decrease more rapidly.

In the case of IAA antibodies, the situation is special. After you start insulin therapy, your body produces antibodies against exogenous insulin, which cannot be distinguished from the original autoimmune ones, which is why the test is no longer interpretable. This dynamic explains why, if you want to clarify the type of diabetes after several years of insulin treatment, the autoantibody panel should not include IAA. However, a negative result does not exclude the autoimmune origin of the disease.

Yes. Research in the field of autoimmunity in type 1 diabetes has identified several autoantibodies that are not yet included in the standardised routine panel but which are being actively studied. Among them are the anti-tetraspanin 7 antibodies (TSPAN7), a protein from the granules of the beta cells, related to the IA-2 antigen, the anti-chromogranin A antibodies, and also autoantibodies directed against new structures generated by changes occurring over time in some beta-cell proteins (like a premature ageing of theirs) [11].

The aim of adding these extra markers is, on the one hand, to identify cases in which type 1 diabetes is present but the four classic autoantibodies are negative (the apparently idiopathic form), and, on the other hand, to refine the estimation of the risk of progression in people with already confirmed autoimmunity [15]. For now, these tests remain at the research level. It is likely that in the coming years the list of autoantibodies in current use will broaden, which will be able to offer you a more secure diagnosis and a better assessment of risk.