Allergenic Molecules
The recognition of HDM-specific allergens helps in the diagnosis and treatment of allergic diseases, associated with HDM, especially identification and characterization of novel HDM allergens (36). The World Health Organization (WHO) and International Union of Immunological Societies’ Allergen Nomenclature Sub-Committee (IUIS) have characterized the following 14 allergens from B. tropicalis (as shown in the table below) (37).
Allergen |
Biochemical name |
Molecular Weight (kDa) |
Allergenicity |
---|
Blo t 1 |
Cysteine protease |
39 |
- Major allergen
- Based on immunoblot assay, Blo t 1 was identified in 62% (13 out of 21) of B. tropicalis skin test-positive patients (38).
- In a panel study with asthmatic patients, 92.5% (37 of 40) children and 60% (24 of 40) adults showed reactivity to proBlo t 1 (39).
|
Blo t 2 |
ML-domain protein |
14.5 |
- On ELISA, 6.1% (15 of 243) were found positive to r Blo t 2 (37).
|
Blo t 3 |
Trypsin |
23.8
|
- IgE reactivity to recombinant glutathione-S-transferase (GST)-Blo t 3 was reported in 51% of patients (23 of 45) positive to crude B. tropicalis extract (40).
|
Blo t 4 |
α-amylase |
56 |
- On SPT, 28% of allergic individuals (n=100) were found positive to Blo t 4 (37).
|
Blo t 5 |
Unknown |
14 |
- Major allergen
- Blo t 5 was prevalent in 91.8% of Taiwanese children (134 out of 146) and 73.5% of Malaysian children (36 out of 49), showing the IgE responses to Blo t 5 (41).
- According to a study conducted in 143 asthmatic Malaysian patients, 90% of children (n=49) and 37% of adults (n=94) were sensitive to Blo t 5 (28).
|
Blo t 6 |
Chymotrypsin |
25 |
- On SPT, 8% of allergic individuals (n=200) were found positive to Blo t 6 (37).
|
Blo t 7 |
Bacterial permeability-increasing like protein |
25 |
- 8 of 33 patients sensitized to B. tropicalis were found positive in IgE binding to r Blo t 7 (37).
|
Blo t 8 |
Glutathione-S- transferase |
27 |
- In a SPT with purified Blo t 8, 5 of 20 subjects (healthy and asthmatic individuals) reported positive reaction (37).
|
Blo t 10 |
Tropomyosin |
33 |
- In an analysis, 20% (7 out of 35) children with allergic sensitization to mite and 29% (27 out of 93) adults with AR, showed a positive skin prick reaction to Blo t 10 (42).
|
Blo t 11 |
Paramyosin |
110 |
- In a dot blot immunoassay, 52% (33 out of 63) patients with positive IgE to crude B. tropicalis mite extract, demonstrated IgE binding to Blo t 11 (43).
|
Blo t 12 |
Unknown |
14 |
- In asthmatic patients sensitized to B. tropicalis, 50% of patients (16 to 32) were reported to have IgE binding to rBlo t 12 (37).
|
Blo t 13 |
Fatty acid-binding protein |
14.8 |
- Of 45 patients sensitized to B. tropicalis, 11% were found to have IgE binding to rBlo t 13 (37).
|
Blo t 19 |
Anti-microbial peptide homologue |
7 |
- 10% of mite-sensitized patients, showed IgE binding to recombinant Blo t 19 (37).
|
Blo t 21 |
Unknown |
13 |
- Major allergen
- 93% (40 of 43) of adult patients with AR, demonstrated IgE binding to Blo t 21 (10).
|
Allergen |
Biochemical name |
Molecular Weight (kDa) |
Allergenicity |
---|
AR: Allergic rhinitis; B. tropicalis: Blomia tropicalis; ELISA: enzyme-linked immunosorbent assay; IgE: Immunoglobulin E; SPT: Skin prick test
Of interest, the major allergen, Blo t 5 is the best-characterized allergen, found in the tropical and sub-tropical areas (3, 41). Blo t 5 and its similar allergen, Blo t 21, are the most predominant allergens found in the B. tropicalis (44).
Cross-reactivity
The allergic sensitivity, following the ingestion of HDMs, show symptoms in two different forms - the ingestion of invertebrates demonstrating cross-reactivity with mite allergens, and the ingestion of foods that are contaminated with dust mites (1).
A high degree of cross-reactivity is noted between Dermatophagoides pteronyssinus and Dermatophagoides farinae extracts, however, the reactivity between Dermatophagoides and Blomia tropicalis is low (20).
In a study, co-sensitization and cross-reactivity had been reported between B. tropicalis and two Dermatophagoides species, i.e. Der p and Der f. In this study, 70.14% of allergenic patients (1050 out of 1497 patients) were found to be co‐sensitized to B. tropicalis, Der p, and Der f. However, the cross-reactivity between B. tropicalis and the other mites was limited (45).
A moderate degree of cross-reactivity has been shown between B. tropicalis and D. pteronyssinus, using inhibition assays. From sera of 12 allergic Venezuelan patients, 5 reacted to extracts of both mite species, while 4 reacted to only D. pteronyssinus and only 3 to B. tropicalis. In 2 of the 5 sera reactive to both mite species, it was seen that D. pteronyssinus could inhibit the binding of B. tropicalis, and vice-versa (12).
Amino acid sequence similarity of 30-50% has been reported between Blo t 1, Blo t 2 and Blo t 5, and Der p 1, Der p 2 and Der p 5 (D. pteronyssinus), hence suggesting low-to-moderate cross-reactivity between D. pteronyssinus and B. tropicalis (31).
Furthermore, between Blo t 3 and Der p 3, limited cross-reactivity has been reported. Blo t 3 showed a 54.8% amino acid sequence similarity with Der p 3 (trypsin-like serine protease) allergen in D. pteronyssinus (3). Blo t 11 has shown 90% similarity with Der f 11 (D. farinae) and 89% with Der p 11 (D. pteronyssinus) (3). Blo t 13 demonstrated moderate-to-high cross-reactivity with human fatty acid binding-proteins (FABPS) (46). Furthermore, Blo t 21 shares close to 40% of sequence homology with Blo t 5, however despite the similarities, Blo t 5 and Blo t 21 do not cross-react (3).
Marked cross-reactivity has been noted between Sui m 5 (from Suidasia medanensis) and Blo t 5 (2).
Tropomyosin from B. tropicalis has demonstrated 95% homology with allergens from Group 10 mites. The Blo t 10 showed high homology with tropomyosin from Lepidoglyphus destructor (Lep d 10), D. pteronyssinus (Der p 10) and D. farinae (Der f 10) (42).
Tropomyosin is a large family of heat-resistant, alpha-helical proteins. These proteins form a coiled-coil structure of two parallel helices, that includes two sets of seven alternating actin-binding sites. This feature plays a vital role in regulating the function of actin filaments (47).
One of the main important causes of cross-reactivity, among mites, shellfish, helminths, and cockroaches is the tropomyosin, although, glutathione transferase may also be included. In cases where genuine sensitization is unclear, specific allergen components can be useful to identify the primary allergy (20).
Tropomyosin allergens from HDMs are reported to show cross-reactivity with tropomyosin allergens of invertebrates, such as crustaceans (shrimp, lobster, crab, crayfish), mollusks (mussel, oyster, scallop, clams, abalone, snails, squid, octopus, cuttlefish) and insects (cockroaches) (1, 47).
Tyr p 5 (from Tyrophagus putrescentiae) showed 52.9% amino acid identity with Blo t 5. It also exhibited a 55.8% identity with Tyr p 21 and Blo t 21. Der f 21 was inhibited by both, group 5 and group 21 allergens (Tyr p 5, Tyr p 21, Blo t 5 and Blo t 21), reflecting cross-reactivity between the three (48).