Milk-Specific IgE Reactivity Without Symptoms in Albumin-Sensitized Cat Allergic Patients

To the Editor,

Serum albumin (Fel d 2) is a cross-reactive animal dander allergen, which can be found in animal-derived foods such as meat and milk.¹ In a population study conducted in Moscow, Russia, we noted that milk albumin, Bos d 6, was more frequently recognized than the major milk allergens caseins, lactalbumin and lactoglobulin.²,³,⁴

We hypothesized that immunoglobulin E (IgE) sensitization to Bos d 6 could be attributed to original IgE-sensitization to cat dander via the respiratory tract and may be caused by IgE cross-reactivity between Fel d 2 and Bos d 6. Therefore, we studied a cohort of 85 patients with clinically documented allergy to cat. Using a panel of purified cat allergen molecules (Supplementary Data S1), we identified 15 patients who showed specific IgE reactivity (0.45–100 kU_A/L) to cat albumin, Fel d 2 as tested by quantitative IgE ImmunoCAP (Supplementary Table S1, patients 1–15). These patients suffered from various symptoms of allergy upon contact with a cat; however, each of them regularly consumed cow's milk and albumin-containing food, such as pork and beef, without experiencing any allergic symptoms. Further testing of these 15 Fel d 2-positive patients' sera showed that 7 out of the 15 patients showed IgE reactivity > 0.1 kU_A/L to Bos d 6, 9 displayed IgE reactivity to cow's milk allergen extract and 5 were even positive to fx5, the mix of food allergen extracts including cow's milk in ImmunoCAP (Supplementary Table S1).

Interestingly, IgE levels were always higher to Fel d 2 than to Bos d 6, suggesting that respiratory sensitization to Fel d 2 may have induced IgE antibodies in these patients which cross-reacted with Bos d 6.

To investigate this, we performed a series of ELISA IgE inhibition experiments with sera from additional cat-allergic patients who were Fel d 2-sensitzed according to ELISA testing and from whom we had sufficient volumes of serum available (Supplementary Table S1, patients 16–19).⁵ Also, these patients regularly consumed cow's milk and meat without showing any allergic symptoms.

Pre-incubation of these sera with Fel d 2 inhibited IgE binding to BSA in the range of 44%–84% (Figure, left panels), whereas pre-incubation of sera with BSA had almost no effect on IgE binding to Fel d 2 with inhibitions rates of 3%–12% (Figure, right panels). BSA inhibited IgE binding to BSA (25%–75%) (Figure, left panels), while Fel d 2 inhibited IgE-binding to Fel d 2 almost completely (83%–93%) (Figure, right panels). Pre-incubation of sera with the unrelated control allergen, Alt a 1, had no effects on IgE binding to Fel d 2 and BSA giving similar OD values as buffer alone (Figure, Alt a 1 columns). As in the ImmunoCAP measurements, we noted that, in the ELISA experiments, the IgE levels specific for BSA (Figure, left panels, OD values Alt a 1) were much lower than those specific for Fel d 2 (right panels, OD values Alt a 1), corroborating that Fel d 2, but not Bos d 6, was the originally sensitizing allergen.

Figure. IgE cross-reactivity between Fel d 2 and Bos d 6 (BSA). IgE binding (y-axes: OD values corresponding to bound IgE) of cat allergic patients (#16, 17, 18, and 19) to BSA (left panels) or Fel d 2 (right panels) are shown after pre-incubation of sera with an unrelated control allergen (Alt a 1), Fel d 2 or BSA (x-axes). ODs are displayed as means ± standard deviation of duplicates. Percentages of the inhibition of IgE binding are indicated on the tops of the bars.

OD, optical density; IgE, immunoglobulin E.

A previous study by Vicente-Serrano et al.⁶ reported a group of patients who were sensitized first to milk and then showed cross-reactivity with dander-derived albumin without direct contact with animals. However, our findings are different from their results because we describe patients with first sensitization to cat albumin and subsequent cross-reactivity with milk, without milk-related clinical symptoms. In summary, cross-reactivity between Fel d 2 and Bos d 6 may give false positive IgE test results for albumin-sensitized cat allergic patients when cow's milk allergen extracts are used to determine allergen-specific IgE and insinuate IgE sensitization to cow's milk. Since a considerable proportion of the world population suffers from clinical symptoms caused by non-IgE-mediated cow's milk intolerance (10%–100%),⁷ our finding is clinically relevant and should be considered in the serological detection of IgE sensitization to cow's milk.⁸ According to birth cohort studies,⁹ almost 7% of the population in Sweden is sensitized to Fel d 2 and at the same time approximately 30% will suffer from adverse reactions to milk due to lactose intolerance⁷ making the differential diagnosis a relevant clinical problem.

SUPPLEMENTARY MATERIALS

Supplementary Data S1

Supplementary methods

Supplementary Table S1

Demographic, clinical and immunological characterization of Fel d 2-positive cat allergic patients