Alpha-Gal Syndrome: Environmental Sensitization and Delayed Meat Allergy

Alpha-gal syndrome did not emerge from a single experiment or a single clinic. It took shape from two initially separate observations on opposite sides of the world — one involving severe reactions to a cancer drug, the other involving tick bites and delayed allergy to red meat. Only later were they recognised as different faces of the same syndrome.

In the early 2000s, cetuximab (Erbitux), a chimeric mouse-human IgG1 monoclonal antibody against epidermal growth factor receptor, was adopted for colorectal cancer and squamous cell carcinoma of the head and neck. However clinicians in parts of the southeastern United States, particularly Tennessee, Arkansas, and North Carolina, reported seeing unusually high rates of immediate hypersensitivity reactions. The regional pattern was striking. Something had sensitised these patients before they ever received the drug. But what?

Chung CH, Platts-Mills TA et al. Cetuximab-induced anaphylaxis and IgE specific for galactose-alpha-1,3-galactose . N Engl J Med. 2008 Mar 13;358(11):1109-17.

Platts-Mills’ team at the University of Virginia showed that many patients who reacted to cetuximab already had pre-existing IgE antibodies before treatment. Among 76 cetuximab-treated patients, hypersensitivity reactions occurred in 25; IgE antibodies to cetuximab were detected in 17 of those 25, compared with only 1 of 51 non-reactors. The geographic signal was equally striking. IgE to cetuximab was found in 20.8% of control subjects in Tennessee, compared with 6.1% in northern California and just 0.6% in Boston. [ Chung et al, 2008 ]

The implication was clear. This was not a random drug reaction. Something in the environment, and something more common in the southeastern United States, had already primed susceptible patients.

The surprise was that the relevant IgE was not directed against a protein epitope, but against a carbohydrate… galactose-α-1,3-galactose, or alpha-gal. On cetuximab, this oligosaccharide was present on the Fab portion of the heavy chain as a result of production in a murine (mouse) cell line.

That finding mattered because humans, apes, and Old World monkeys lack the enzyme α-1,3-galactosyltransferase, and do not synthesise alpha-gal. Although naturally occurring IgM and IgG antibodies to alpha-gal were already recognised, the presence of specific IgE suggested a different kind of immune response. That implied prior sensitisation through some environmental exposure not yet understood.

Key point: In many patients with cetuximab hypersensitivity, alpha-gal–specific IgE was present before the first infusion.

Simultaneously, and independently of the cetuximab work in the United States, Professor Sheryl van Nunen and colleagues at Royal North Shore Hospital in Sydney noted an unusual pattern in their allergy clinic. Patients from Sydney’s northern beaches were presenting with allergic reactions, often severe, after eating red meat. Many also reported prior tick bites in an area endemic for Ixodes holocyclus , the Australian paralysis tick.

Van Nunen SA, Fernando SL, Clarke LR, Boyle RX. The association between Ixodes holocyclus tick bite reactions and red meat allergy [abstract]. Intern Med J. 2007;37(Suppl 5):A132.

This abstract is the first formal report linking tick bite reactions with mammalian meat allergy in humans. It described 25 adults from Sydney’s northern beaches, of whom 23 had experienced allergic reactions to red meat, 14 had suffered severe anaphylaxis, and 24 reported tick bites before the onset of their meat allergy.

The importance of the report was clinical rather than mechanistic. Van Nunen and colleagues had identified a reproducible syndrome: tick bites first, red meat reactions later.

Van Nunen SA, O’Connor KS, Clarke LR, Boyle RX, Fernando SL. An association between tick bite reactions and red meat allergy in humans . Med J Aust. 2009 May 4;190(9):510-1.

The subsequent Medical Journal of Australia paper expanded the case series. Of the 25 patients with red meat allergy, 24 reported significant local tick bite reactions, painful, pruritic lesions greater than 50 mm that persisted for at least a week, before the onset of their meat allergy. Seventeen had experienced severe reactions to tick bites themselves, including tongue swelling, throat tightness, or shortness of breath. All 25 patients lived in a region endemic for Ixodes holocyclus .

A retrospective control group of 29 patients from the same area with non-meat food allergy had also been exposed to ticks, but none had developed mammalian meat allergy. The comparison suggested that tick exposure alone was not sufficient, and that the nature of the tick bite response might matter.

At the time, the mechanism remained uncertain. The authors discussed possible cross-reactivity between tick-derived and meat-derived antigens, but the alpha-gal explanation had not yet been established. What the Australian work had provided was the first clear clinical description of the syndrome that the American studies would soon help explain.

Commins SP, Platts-Mills TA et al. Delayed anaphylaxis, angioedema, or urticaria after consumption of red meat in patients with IgE antibodies specific for galactose-alpha-1,3-galactose . J Allergy Clin Immunol. 2009 Feb;123(2):426-33.

The crucial synthesis came in 2009, when Scott Commins, then a doctoral researcher with Platts-Mills at the University of Virginia, showed that patients with delayed allergic reactions after eating red meat had IgE antibodies specific for alpha-gal.