e227 Equ c1, Horse dander

Summary

Equ c 1, a lipocalin, is a major horse allergen found in saliva, hair and to some extent, in the urine of horses (Equus caballus). It is a homodimer, glycosylated lipocalin protein (182 amino acids) weighing around 22 kDa with surfactant properties. It is found to be the most prevalent allergen (50% to 76%) among the horse-allergic patients. Sensitization to horse allergens has been reported to be between 3.6% and 16.5%, among the individuals associated with horse-related occupation, while sensitization of 5.38% has been reported among individuals with respiratory allergy. Additionally, Equ c 1 sensitization has been found to be associated with severe asthma in children as well as in adults and reportedly more prevalent in asthmatic individuals as compared to randomly-selected individuals. Further, it is also reported to be significantly associated with moderate-to-severe rhinitis among horse-sensitized patients. The main route of this horse allergen exposure is inhalation, even though it is not found to be dispersed by air to a great extent. Equ c 1 is species specific marker of sensitization. However, Equ c 1 has been found to cross-react with other lipocalins such as Can f 6 (dog), Fel d 4 (cat) and Mus m 4 (mouse) due to sequence homology. This cross-reactivity should be taken into consideration when interpreting sensitization to Equ c 1.

Epidemiology

Worldwide Distribution

Sensitization to horse allergens has been reported to be between 3.6% and 16.5%, among the individuals associated with horse-related occupation, while sensitization of 5.38% has been reported among individuals with respiratory allergy (1).

Equ c 1, a lipocalin, is a major horse allergen and considered as species specific marker of sensitization (2). A study conducted in Finland among 21 horse-allergic subjects reported sensitization to Equ c 1 in 76% of subjects (3). A study conducted in Sweden reported the rate of sensitization to Equ c 1 to be 50.6% among horse-allergic children (4), while the sensitization rate of Equ c 1 was found to be 60% among asthmatic children in another Swedish study (5).

The West Sweden Asthma Study was conducted among a random cohort of adults (964 asthmatics and 1042 non-asthmatics) for their sensitization to various aeroallergens. The sensitization of Equ c 1 was found to be 6.3% among males and 5.8% among females. Also, it was interesting to note that the sensitization rate significantly reduced with increasing age (p<0.01) with maximum sensitization in adults with age ≤30 years and minimum in 61 to 75 years elderly (6). Further, based on the same West Sweden Asthma Study among 1103 randomly selected adults, sensitization to Equ c 1 was found in 2.1% (6).

A retrospective study conducted among 1255 allergic individuals in Czech Republic found a sensitization of 6.2% to horse allergen Equ c 1 (7). Another study conducted in Spain among 159 animal-sensitized (dog/cat/horse) patients reported 70% to be sensitized to Equ c 1, while 60% were found to be monosensitized to Equ c 1 (8). Moreover, the pet sensitization profiles evaluated by component resolved diagnosis in two general pediatric populations found Equ c 1 sensitization to be 2% in a Japanese cohort (9, 10) as compared to 3% in a Swedish cohort (9, 11).

Environmental Characteristics

Source and tissue

Equ c 1 is a major allergen found in the saliva, hair and to some extent, in the urine of horses (Equus caballus) (12).

The natural Equ c 1 was isolated from horse hair by affinity chromatography and purified by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and silver staining (13). The recombinant protein was developed from cDNA clone obtained from sublingual horse tissue. The molecule was generated by recombinant systems in Pichia pastoris. Affinity chromatography and gel filtration were used for eluting the recombinant protein as a dimer and subjected to high-performance liquid chromatography-electrospray ionization mass spectrometry for further purification (3). 

Clinical Relevance

Disease severity

Equ c 1 is reported to be the major allergen of horse (14) and found to be associated with severe asthma in children (9) as well as in adults (6).

Sensitization to Equ c 1 was found to be significantly more prevalent (p<0.001) in asthmatic individuals (11.8%) as compared to randomly selected individuals (2%) as shown in the West Sweden Asthma Study (6). Also, sensitization to Equ c 1 was found to be significantly more frequent (p=0.03) in children with severe asthma as compared to children with controlled asthma (51% vs. 25%, respectively) (5).

Cross-reactive molecules

IgE-mediated cross-reactivity of Equ c 1 has been demonstrated with Fel d 4 (cat), Can f 6 (dog) and Mus m 4 (mouse) (14). A study using sera from 100 dog-allergic patients found that 73.8% of patients sensitized to Equ c 1 may be sensitized to all the three allergens i.e. Can f 6 from dog, Fel d 4 from cat and Equ c 1 from horse. They also reported that co-sensitization of Equ c 1 was co-related with Can f 6 (correlation coefficient, r = 0.41) and Fel d 4 (r=0.42). Moreover, potential cross-reactivity was predicted among the three allergens based on ELISA inhibition assays (15).

Interestingly, asymmetric type of cross-reactivity may be found between Equ c 1 and Mus m 1, and this may be attributed to certain unique epitopes in addition to common epitopes being observed among the pair. Equ c 1, being a dimeric protein may have some additional unique IgE epitopes as compared to a monomer protein, such as Mus m 1 (3).

Molecular Aspects

Biochemistry

Equ c 1 is part of lipocalin family of protein. These lipocalins are ubiquitous proteins having several bioactive functions. The tertiary structure consists of 8 anti-parallel β-strands forming a central β-barrel surrounded by α-helix. This forms a hydrophobic core for binding of molecules, such as pheromones, odorants or retinol (2, 14, 16). Lipocalins are small molecules consisting of 150 to 250 amino acids and are commonly found mammalian allergens in saliva, urine, sweat, sebum and dander. They can be transported to distant places by air as they attach to particles (9, 14).

Equ c 1 is a homodimer, glycosylated lipocalin protein weighing around 22 kDa. It consists of 187 amino acid long peptide, with the mature one being around 172 amino acids in length (14). The structure of Equ c 1 is similar to other lipocalins, however, additionally, a short β-strand and a helical turn at C-terminal end as well as a disulfide bond between the C-terminal and β-chains are present. Equ c 1 usually exist as a dimer, however the mode of dimerization is found to be different than other lipocalins reported like Mus m 1 (mouse) or Bos d 5 (cow) (16). Furthermore, Equ c 1 has been reported to possess surfactant properties (14).

Isoforms, epitopes, antibodies

Equ c 1.0101 is the only isoform of Equ c 1 (17).

The T-cell epitopes of Equ c 1 were found to be crowded in specific epitope regions in contrast to other allergen epitopes which were randomly distributed. A study has shown that Equ c 1 possesses a powerful, single immunodominant epitope region between 137 and 160 amino acids (Equ c 1137-160), that is bounded by most of the T-cell lines specific to Equ c 1 (18). Hence, this epitope may be developed for peptide-based immunotherapy as the T-cell response to this specific epitope was reported differently among allergic and non-allergic horse individuals (19).

Furthermore, studies have identified around 34 allergenic peptides from recombinant Equ c 1 (1).

Cross-reactivity due to structural similarity

The lipocalins usually share low sequence identity i.e. around 20-30%, however, recent data suggests that certain lipocalins may present with high sequence identity, leading to cross-reactivity between them. Equ c 1 has been found to show amino acid sequence homology with certain lipocalins, which include 34% for Equ c 2 (horse), 28%  for Can f 1 (dog), 26% for Can f 2 (dog), 29% for Can f 4 (dog), 57% for Can f 6 (dog), 68% for Fel d 4 (cat), 26% for Fel d 7 (cat), 46% for Mus m 1 (mouse), 47% for Rat n 1 (rat), 48% for Cav p 6 (guinea pig), 52% for Ory c 4 (rabbit), 28% for Bos d 23k (cow) and 33% for Bos d 2 (cow) (9, 14). Moreover, sequence identity between Equ c 1 and human tear lipocalin have been reported to be 22.7%, which may not presume any cross-reactivity (15).

Further, the cross-reactivity between Equ c 1 and Rat n 1 (rat) could also be attributed to the identical T-cell epitopes (20).

Diagnostic Relevance

Disease Severity

Equ c 1 was considered as a species-specific marker for sensitization to horse hair. However, due to recent knowledge of its cross-reactivity with cat and dog lipocalin (Fel d 4 and Can f 6 respectively), it is now considered for horse allergy relatively, along with sensitization to cat and dog allergens  (12, 21).

Exposure

The main route of horse allergen exposure is through inhalation (14), however, the allergen is not found to be dispersed by air to a great extent. The hidden exposure of Equ c 1 through secondary exposures, such as clothes, vehicles or dust may induce allergic reactions in susceptible individuals (1).

Further, it was considered that Curly horses or certain breeds of horses are hypoallergenic as they release less allergen levels in the air. However, a study conducted using 224 hair samples from 32 breeds of horses found no correlation between production of allergens or release of allergens in the air with the hypo-allergenicity of the breed of Curly horses. Also, the amount of allergens released in the air during grooming were not found to be significantly different for Curly horses as compared to other breeds which were not considered hypoallergic (13).

Compiled By

Author: Turacoz Healthcare Solutions

Reviewer: Dr Magnus Borres

Last reviewed: July  2021