Disclaimer- The posts in this series are intended to improve awareness of this important and potentially deadly disease. I am not an Allergist or Immunologist and I am not offering any medical advice. Please consult with your own physician or health care provider for that. An educated patient and their family are the best advocates for their health care.

Alpha-gal syndrome (AGS) is an IgE-mediated allergic disease directed against the carbohydrate antigen galactose-α-1,3-galactose (alpha-gal), an oligosaccharide present on the surface of the cells of non-primate mammals. I have divided this post into three parts: the evolutionary background; the role of ticks in the sensitization and class switch of the antibody response from Ig G/M to IgE; and the mechanism of the delayed onset of symptoms.

Evolutionary Background

The α-gal epitope is a carbohydrate antigen with the structure Galα1-3Galβ1-4GlcNAc-R. It is abundantly expressed on the cell surface of non-primate mammals and New World monkeys. The synthesis of this epitope is catalyzed by the enzyme α1,3-galactosyltransferase encoded by the GGTA1 gene. The enzyme transfers a galactose molecule from UDP-galactose to a terminal N-acetyllactosamine acceptor on glycoproteins and glycolipids. Humans, apes (catarrhines), and Old World monkeys lack the α-gal epitope due to the inactivation of the gene. The inactivation is estimated to have occurred 20–30 million years ago. Because α-gal is absent from human tissues, the immune system recognizes it as foreign. As a result, all immunocompetent humans produce “natural” anti-α-gal antibodies (IgM, IgG, and IgA), constituting up to ~1% of total circulating IgG, driven by the continuous exposure to α-gal epitopes on commensal gut bacteria and dietary sources. Importantly, this baseline response does not include IgE. The structural similarity between α-gal and the blood group B antigen may explain why individuals with blood type B may have reduced anti-α-gal antibody levels due to partial immune tolerance, potentially lowering their risk of AGS. The evolutionary advantage of GGTA1 inactivation is thought to result from the prevention of sepsis from commensal intestinal bacterial that express α-gal on their surfaces.

The presence of these antibodies in humans, however, is a major immunological barrier to xenotransplantation, the transplantation of organs or tissues from other species. When a pig kidney, for example, is transplanted into a human, human anti-α-gal antibodies bind to α-gal epitopes on pig endothelial cells triggering hyperacute rejection and rapid graft failure.

Sensitization- Tick Bites and IgE Class Switching

The puzzling question is how does the “natural” immune response involving B cells that produce IgG, M and A antibodies to alpha-gal undergo a switch to IgE. Traditional responses to oligosaccharides were thought to be T-cell independent unlike IgE responses which are directed at protein epitopes and are T-cell dependent. The “natural” immune response is thought to be stimulated by gut bacteria. The tolerance to orally ingested alpha-gal in foods is felt to be due to a homeostasis between regulatory and effector T cells. This may be the result of the effect that bacterial unmethylated DNA and cell walls have in inhibiting IgE production. This oral tolerance to mammalian foods is broken with the injection of tick saliva into the skin. The AGS phenotype can be reproduced in knockout mice with the injection into the skin of Lone Star tick saliva or tick extracts. The IgE response required CD4+-T cells and was dependent on the MyD88 pathway, and the production of IL-4. Tick saliva also contains prostaglandins, sphingomyelinase, and cysteine protease inhibitors that create a milieu favorable to Th2 induction. Th2 cells (a subset of T-helper cells) mediate the activation and maintenance of the antibody-mediated immune response against extracellular parasites, bacteria, allergens, and toxins. They mediate these functions by producing a variety of cytokines such as IL-4, IL-5, IL-6, IL-9, IL-13, and IL-17E (IL-25) which activate eosinophils, and inhibit several macrophage functions, providing phagocyte-independent protective responses. Th2 cytokines have effects on many cell types in the body. They stimulate and recruit eosinophils and basophils, to the site of infection or in response to allergens or toxins leading to tissue eosinophilia and mast cell hyperplasia. They also induce mucus production, and airway hyper-responsiveness. Most importantly for AGS, Th2 cells also control the regulation of B cell class-switching from IgG/M to IgE. 

It is clear that the initiating event that converts a “normal” IgG and IgM antibody response into a pathological IgE response in AGS is tick bite exposure. In the United States, the tick almost always responsible is the Lone Star tick (Amblyomma americanum), different tick species have been implicated on other continents. Alpha-gal is synthesized by tick galactosyltransferases and is present in tick salivary glands, midgut, and hemolymph. Salivary gland α-gal is strongly upregulated during feeding.
Proteins in the tick saliva contain several bioactive molecules that create a local cutaneous environment that favors Th2 cell induction. Repeated tick bites amplify this Th2 signal, with production of IL-4 and IL-13, cytokines that drive IgE class switching. The Th2 cells in AGS patients respond to tick proteins (not directly to α-gal itself), functioning as carrier-specific helper T cells that provide help to pre-existing α-gal-reactive B cells. Blocking CD40L and IL-4 reduces B cell proliferation and CD23 expression, confirming the dependence on classical T-B cooperation. The hypothesis is that IgE-producing B cells, derived from pre-existing mature B cell clones already producing anti-α-gal IgM or IgG, undergo a class switch recombination to IgE in the Th2 milieu created by tick saliva. This explains why sensitization occurs through the skin. The first two steps are summarized in the figure below.

The Delayed Allergic Response

One of the hallmarks of AGS is the 2-6 hour delay between oral ingestion of mammalian meat or organs and symptom onset, which is very different from typical IgE-mediated food allergies. The “glycolipid hypothesis” has been advanced to explain this delay, shown in the figure.

Alpha-gal in mammalian meat is thought to be predominantly conjugated to glycolipids (not just glycoproteins). In the digestive process, glycolipids pass through the intestinal wall, and are packaged into chylomicrons in the lacteals, which enter the thoracic duct, and then the systemic circulation after about an hour. Chylomicrons which are large, about 300-1000 nm in size, are processed to VLDL and then LDL (12-25 nM) over the next several hours. LDL is small enough to pass between endothelial cells and reach tissue-resident mast cells. Alpha-gal epitopes on these LDL particles then cross-link IgE bound to the FcεRI (high affinity IgE receptor) on mast cells and basophils, triggering degranulation and release of histamine, tryptase, and other mediators. This lipid-associated processing hides the antigen during early digestion preventing it from interacting with free or cell-bound IgE accounting for the temporal delay. When α-gal is delivered intravenously, as with a cetuximab infusion, it bypasses the intestine and the reaction is immediate.

In summary, AGS results from a combination of events including: an evolutionary loss of self-tolerance to α-gal which provides a substrate of pre-existing B-cell clones that respond to the antigen with an IgG/M response; tick saliva that delivers both the antigen and a potent Th2 milieu through the skin which results in the class switching from IgG/M to IgE; and the lipid-associated digestion of meat glycolipids that results in the characteristic delayed effector response.

Next- The Alpha-gal syndrome and a Possible Association with Heart Disease