Immunological IgE Cross-Reactions of Bovine and Human α-Lactalbumins in Cow's Milk Allergic Patients

Abstract

Despite great homology with the equivalent human protein, bovine α-lactalbumin (B α-La), a major component of whey, has been identified as a major milk allergen. The aim of this study was to investigate the relationship between structure and IgE binding capacity in α-Las: (1) the importance of three-dimensional structure using native vs disulfide bridgereduced B α-La; and (2) the incidence of amino acid sequence divergence on specific IgE cross-reactivity to human vs bovine α-La. Purified native, reduced and S-carboxymethylated B α-La and human α-La (H alpha-LA) were prepared. Specific IgE of 20 sera from patients with clinically recognized cow's milk protein allergy and positive RAST tests to B α-La were measured in original direct and competitive ELISA inhibition tests. All sera containing specific anti-native B α-La IgE also reacted with denatured protein, but the IgE levels were generally lower, showing that three-dimensional structure is an important feature in B α-La allergenicity but that sequential epitopes are also exposed after protein denaturation. Despite lower IgE levels, all sera also gave significant IgE responses to H α-La. Competitive ELISA inhibition confirmed results obtained by direct ELISA. The demonstrated IgE cross-reactivity between B α-LA and H α-La could be related to the high degree of sequence homology between the two proteins but did not prove to have a clinical significance. However, it is of great interest for a study of the relationship between structure, IgE binding capacity and allergenicity in alpha-Las.