📘 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 recognize 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 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 initiated a reaction directed against one of your own tissues. In the case of T1D, autoantibodies are not responsible for the destruction of beta cells in the pancreas, which is carried out by T lymphocytes and their friends. The presence of autoantibodies signals the existence of the autoimmune process and allows the diagnosis of T1D to be established even before your blood glucose rises above normal values [1].

T1D 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 elucidated, but a combination is presumed to exist between a genetic predisposition (especially the HLA system) and some triggering factors from the environment, such as viral infections, alterations of the intestinal microbiota or other influences still insufficiently understood [15].

The appearance of autoantibodies is not a sudden event. They can appear several months or years before you show the clinical manifestations of T1D, and the type and order of appearance varies greatly from one person to another [2]. This long interval between the onset of autoimmunity and the appearance of hyperglycemia explains why T1D has preclinical stages and why, if you are at high risk, autoantibody testing can identify the disease while beta cells still function nearly normally.

The four autoantibodies internationally recognized as standardized markers of T1D are anti-insulin antibodies (IAA), anti-glutamic acid decarboxylase antibodies (GADA, directed especially against the GAD65 isoform), anti-tyrosine phosphatase antibodies (IA-2A or anti-islet antigen 2) and anti-zinc transporter 8 antibodies (ZnT8A) [3]. This is the antibody panel recommended both for presymptomatic T1D screening in people at high risk and for diabetes classification in adults, when there is suspicion of autoimmune diabetes [8] [9].

Each of the four autoantibodies recognizes a different target in the pancreatic beta cells, which makes the panel complementary. If one antibody comes back negative, the positivity of 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 hyperglycemia, and ZnT8A is useful especially when the other three antibodies are absent [4]. When you are investigating the autoimmune nature of diabetes, the combined evaluation of all four autoantibodies offers 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 [3]. 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 known when your result is interpreted.

In the context of T1D, GADA is the most frequently identified autoantibody in adults and tends to persist the longest in the blood after diagnosis [10]. GADA represents the main autoantibody recommended for testing adults with suspected T1D, being the first-line investigation. If you already have diabetes and you only discover positive GADA, with no other autoantibodies present, the evolution 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 in the autoimmune process exposes this protein to the immune system, which reacts by producing IA-2A [3].

IA-2A has a particular clinical significance, indicating an independent and increased risk of progression to symptomatic T1D (stage 3) [7]. For this reason, the monitoring of people with isolated IA-2A is done in the same way as for 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 an advanced autoimmunity and a high probability of rapid evolution toward clinically manifest hyperglycemia [6].

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 genetic predisposition for T1D, IAA is usually the first autoantibody that appears in the blood, sometimes many years before the clinical onset of the disease [2].

A practical aspect, important 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 differentiated 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 T1D screening [3].

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 the local storage of insulin in stable crystallized form. Any defect of this structure affects the ability of the beta cell to correctly store and release insulin [11].

ZnT8A is the most recently included of the four standardized autoantibodies, having been described in 2007, and has a particular clinical utility as a complementary test. If you are suspected that the form of diabetes you have is actually type 1, but GADA, IA-2A and IAA are negative, the presence of ZnT8A can confirm the autoimmune nature of your disease [4]. The inclusion of ZnT8A in the standard panel has increased the diagnostic sensitivity, reducing the number of cases mistakenly labeled as idiopathic T1D or as atypical T2D.

Autoantibodies specific to T1D have a high specificity, meaning that a confirmed positive result indicates in the vast majority of cases an autoimmune process directed against the beta cells [4]. The greater the number of positive autoantibodies, the higher the diagnostic value. If you have two or more autoantibodies persistently present, the evolution toward clinically manifest T1D is considered almost certain in the long term, even if blood glucose is still normal at the time of testing [6].

On the other hand, the sensitivity of a single autoantibody is limited, which is why testing is always done for the entire standardized panel. A unique positive result must be confirmed by a second test, ideally in a laboratory participating in the international standardization program (Islet Autoantibody Standardization Program), because an important part of the children with a single autoantibody can revert to negativity at subsequent testing [9]. This retesting prevents both underdiagnosis and the mistaken labeling as T1D of a person with a false positive result.

Yes. The presence of a pancreatic autoantibody in your blood is not equivalent to the diagnosis of T1D and does not guarantee the appearance of the disease. An important part of the persons who have a single positive autoantibody, especially in childhood, spontaneously revert to negativity at repeated testing, without diabetes ever appearing. Your risk of progression depends on several factors, such as the number of autoantibodies present, their titer (quantity), the age at which they appear, the type of autoantibody and the genetic predisposition [5].

If you persistently have two or more autoantibodies, your risk of evolution toward symptomatic T1D is very high in the long term, but the interval until the appearance of hyperglycemia can be months, years or even decades [6]. This phase of autoimmunity without clinically manifest diabetes is called stage 1 (with normal blood glucose) or stage 2 (with dysglycemia) of T1D and allows you close surveillance, in order to avoid the abrupt onset with diabetic ketoacidosis when hyperglycemia eventually appears [12]. And let us not forget that autoantibodies do not destroy the pancreatic beta cells, they only draw our attention that someone else is destroying them.

Yes, it is possible. A small part of patients with T1D have no detectable autoantibody at the moment of diagnosis [13]. If you are under 35 years of age and do not present clinical features of T2D or of monogenic diabetes, a negative test for autoantibodies does not exclude your diagnosis of T1D. In this case the clinical picture, the rapid evolution toward insulin dependence and the low level of C-peptide orient the diagnosis toward T1D [9].

A part of these cases could be explained by the technical limits of the tests, such as autoantibodies present in concentrations too small or directed against structures (antigens) not yet included in the standard panel. This category corresponds to the form called idiopathic T1D, in which there is permanent insulinopenia and tendency to ketoacidosis, but without evidence of autoimmunity [14].

Yes, the level of autoantibodies can progressively decrease after you receive the diagnosis of T1D stage 3, and some can become undetectable at 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 [10]. The four standardized autoantibodies, however, have different behavior. GADA persists the 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 differentiated 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 assay must not include IAA. However, a negative result does not exclude the autoimmune origin of the disease.

Yes. Research in the field of autoimmunity in T1D has identified several autoantibodies that are not yet included in the standardized routine panel, but which are being actively studied. Among them are anti-tetraspanin 7 antibodies (TSPAN7), a protein from the granules of beta cells, related to the IA-2 antigen, anti-chromogranin A antibodies, but also autoantibodies directed against new structures generated by changes occurring over time in some proteins from beta cells (like a premature aging of theirs) [11].

The aim of integrating these additional markers is on the one hand the identification of cases in which T1D is present, but the four classic autoantibodies are negative (the apparently idiopathic form), and on the other hand the refinement of the estimation of the risk of progression in people with already confirmed autoimmunity [14]. Currently, these tests remain at the research level. It is likely that in the coming years the list of currently used autoantibodies will broaden, which will be able to offer you a more secure diagnosis and a better risk evaluation.