e204 nBos d 6, BSA Cow

Allergen

Cow's milk (CM) is popularly consumed throughout the world. Allergy to CM is prevalent among infants and young children during the first year of life, affecting 0.5-3% at the age of 1 year. Bos d 6 (bovine serum albumin) has been reported as a minor allergenic whey protein in CM. According to a number of studies, sensitization towards Bos d 6 ranges between 43-92.9% of patients with CM allergy (CMA). Bos d 6 is the most heat-labile protein among the other whey proteins and therefore is likely to be destroyed after boiling CM. Thus, specific immunoglobulin E to Bos d 6 might help evaluate tolerance to boiled milk. Clinical manifestations of Bos d 6-mediated allergy majorly include anaphylaxis. Moreover, sensitization to Bos d 6 is regarded as a predictive marker of CMA in children with beef allergies. Cross-reactivity has been observed between serum albumin of cow, deer, dog, sheep, pork, and cat

Epidemiology

Worldwide distribution

Cow’s milk (CM) is a popular food consumed worldwide (Muthukumar et al. 2020). However, CM allergy (CMA) is prevalent among infants and young children, with a high prevalence observed during the first year of life (Kaczmarski et al. 2013, Vandenplas 2017), affecting 0.5-3% at the age of 1 year (Flom et al. 2019, Skripak et al. 2007).

Bos d 6 is bovine serum albumin (BSA) and a minor allergenic whey protein in CM. According to a study, 43% of patients with CMA (n=92) are found to be sensitized to Bos d 6 (Hochwallner et al. 2014, Wal 2002). According to another clinical study, 61.3% of children allergic to CM (n=80) reported sensitization towards Bos d 6 (based on immunoblotting) (Restani et al. 1999).

In an Italian in-vitro study, 45% (9/20) of pediatric patients allergic to CM exhibited sensitization to Bos d 6 (Natale et al. 2004). As per another study conducted in Italy, sensitization to Bos d 6 was observed in 92.9% (26/28) of the children allergic to CM (Martelli et al. 2002).

Furthermore, specific immunoglobulin E (sIgE) against Bos d 6 was reported among 45.5% of patients with CMA (n=86) in an Iran-based study (Shoormasti et al. 2011). And in a study conducted in Denmark among 39 milk-allergic infants [21 infants with IgE-CMA (CMA) and 18 infants with non-IgE CMA (CMI)], the frequency of sIgEs to Bos d 6 was highest before (12/17 for CMA; 9/13 for CMI) and after (15/20 for CMA; 15/17 for CMI) the milk challenge at 12 months in comparison to other cow milk proteins (Host et al. 1992).

Moreover, a study conducted in Europe reported the presence of sIgE towards Bos d 6 in 3.8% of the patients (children and adults) with CMA (n=73) (Hochwallner et al. 2010). A study used crossed radioimmunoelectrophoresis technique to determine the serum IgE level to CM antigenic components in the sera of 21 CMA patients and detected IgE antibodies against bovine albumin in the sera of all patients (Gjesing et al. 1986).

Environmental Characteristics

Source and tissue

Cow milk comprises two main types of proteins, whey (constituting about 20% of the total milk protein) and casein (constituting about 80% of the total milk protein) (Flom et al. 2019, Villa et al. 2018).

Bos d 6 (type of whey protein) is present in minor quantities in CM (Villa et al. 2018). It (1.2% of whey content) penetrates the milk via secretory cells from the blood (Jaiswal et al. 2021) and helps in the transport, metabolism and distribution of substances (like fatty acids, drugs, and hormones). Furthermore, this protein also plays a role in regulating the colloidal osmotic pressure and protects from free radicals (Villa et al. 2018).

According to WHO/IUIS, Bos d 6 (serum albumin) is listed as a food allergen from CM (WHO/IUIS 2019).

Risk factors

Allery to Bos d 6 in patients with persistent CMA is considered a risk factor for developing asthma and rhinoconjunctivitis due to animal epithelia sensitization (Vicente-Serrano et al. 2007).

Clinical Relevance

Disease severity

CMA is considered as a major common pediatric food allergy (Agyemang et al. 2019). Milk allergy can be either IgE-mediated or non-IgE-mediated (Walsh et al. 2016). The clinical manifestations of CM protein allergy are reported to be of two types such as immediate (shortly after consumption) and delayed (after hours or days of consumption) responses (De Greef et al. 2012). The immediate reactions are generally IgE-mediated and can lead to angioedema, urticaria, aggravation of atopic dermatitis (AD), vomiting, in some cases, anaphylaxis, as well as respiratory symptoms (breathing, wheezing, coughing) (De Greef et al. 2012, Flom et al. 2019, Ito et al. 2012, Walsh et al. 2016). On the other hand, most delayed reactions are non-IgE mediated (Walsh et al. 2016) and can lead to gastrointestinal symptoms (like enteropathy or proctocolitis) or skin reactions (such as AD) (De Greef et al. 2012, Ito et al. 2012).

As per a study conducted in Iran, anaphylactic reactions were observed in 60% of the patients with CMA showing a positive test for sIgE towards Bos d 6 (Shokouhi Shoormasti et al. 2016).

Cross-reactive molecules

BSA is a minor allergen in milk (Bos d 6) but a major allergen in beef. Thus, patients with milk allergy sensitized to Bos d 6 may have a concomitant beef allergy, and BSA has also been found to cross-react with other BSA such as pork and sheep (Martelli et al. 2002, Vicente-Serrano et al. 2007, Werfel et al. 1997).

A prospective cohort study conducted on 38 children (age <14 years) with CMA-associated symptoms detected the rate of cross-sensitization (SPT-based) between goat milk and cow milk to be 63.2% (24/38) (Ehlayel et al. 2011).

According to a study, the cross-reactivity found between SA of cow, pork, deer, sheep, dog, and, cat (meat and/or epithelia) in the sera of all the patients with persistent milk allergy (n=8) was inhibited by pure Bos d 6 (Vicente-Serrano et al. 2007).

An Egypt-based study reported cross-reactivity between CM and beef in 30 patients with CMA (Refaat et al. 2011).

Molecular Aspects

Biochemistry

Bos d 6 (BSA) is a protein (molecular weight: 67 kDa) consisting of 583 amino acid residues and 35 Cys residues (Inge et al. 2022, WHO/IUIS 2019, Wynn et al. 2022). This protein is arranged into 3 homologous domains and comprises of 9 loops linked by 17 covalent disulfide bonds. Most of these disulfide bonds are protected inside the core of the protein, making them less accessible. Even under denaturing conditions, disulfide bonds help maintain the native antigenic determinants in BSA due to their stable tertiary structure (Farrell et al. 2004, Restani et al. 1999, Villa et al. 2018).

BSA is water-soluble and rich in amino acids recognized by gastrointestinal enzymes; however, it is comparatively resistant to digestion (Restani et al. 1999).

Isoforms, epitopes, antibodies

Bos d 6 has been reported to possess at least one sequential epitope, corresponding to amino acids 524-542 (crucial for antibody binding) (Chruszcz et al. 2013, Restani et al. 1999).

To date (25th August 2022), only one isoallergen of Bos d 6, i.e., Bos d 6.0101, has been identified and officially published by the WHO/IUIS (WHO/IUIS 2019).

Bos d 6 is the most heat-labile of all the milk proteins (El-Agamy 2007). In a study, 8 adults with a positive response to oral challenge with milk were subjected to boiled milk (10 minutes). Patients reactive to BSA exhibited no SPT response to boiled milk, thus showing reduced whey protein antigenicity (A Norgaard; H 1996, Restani et al. 2009).

Cross-reactivity due to structural similarity

Milk of different ruminant species (such as cow, goat, and sheep) is composed of homologous proteins with similar structures and functions, thereby leading to cross-reactivity between the milk proteins (Villa et al. 2018).

Bos d 6 from CM protein shows sequence homology with other mammalian milk, including ewe (92.4%), goat (71.2%), pig (79.9%), donkey (74.1%), horse (74.5%) and human (76.6%) (Tsabouri et al. 2014).

Diagnostic Relevance

Disease Severity

In an Italy-based study, 92.9% of children with beef allergy were also allergic to CM. Thus, sensitization to Bos d 6 was a predictive marker of CMA in children with beef allergy (Martelli et al. 2002).

Exposure

Cow milk allergy mainly occurs through ingestion (Kaczmarski et al. 2013).

Explained Results

Allergen information

Bos d 6 (bovine serum albumin) is a minor allergenic protein found in CM and is the most heat-labile protein of all the milk proteins (Villa et al. 2018).

Clinical relevance

Typical symptoms of CMA include skin reactions (angioedema, urticaria, and atopic dermatitis), GI symptoms, respiratory systems, and in some cases, anaphylaxis (De Greef et al. 2012, Flom et al. 2019, Ito et al. 2012, Shokouhi Shoormasti et al. 2016).

Cross-reactivity

Bos d 6 (BSA) from CM has been found to show cross-reactivity with SA of deer, pork, sheep, dog, and cat (meat and/or epithelia) (Vicente-Serrano et al. 2007). Moreover, cross-reactivity has also been reported between CM and beef (Refaat et al. 2011).

Compiled By

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Eva Södergren