Blo t 5
Summary
Blomia tropicalis 5 (Blo t 5), a major allergen found in the gut and fecal pellets of a common dust mite, B. tropicalis, causes allergic reactions in 70-92% of patients with mite allergy in tropical climates. The probable risk factors for Blo t 5 sensitizations include environmental and climatic factors, genetic predisposition, and exposure to indoor dust. The literature has demonstrated the association of Blo t 5 with several symptoms ranging from mild to severe reactions such as asthma, allergic rhinitis, conjunctivitis, atopic dermatitis, and even oral mite anaphylaxis. Studies have reported its correlation with disease severity in patients with asthma and atopic dermatitis. Blo t 5 is a 14 kDa monomeric allergen with a predominant α-helical structure Despite structural similarities with Blo t 21 and Der p 5, Blo t 5 shows limited to moderate cross-reactivity with Blo t 21 and Der p 5.
Epidemiology
Worldwide distribution
Blomia tropicalis 5 (Blo t 5) is one of the major allergens of Blomia tropicalis (B. tropicalis), a common dust mite, that mostly inhabits tropical, subtropical, and temperate regions characterized by elevated humidity and warm temperatures. In tropical climates, Blo t 5 has been found to induce allergic reactions in around 70-92% of individuals with mite allergies, establishing it as the primary group-5 allergen linked to reactions in this climate.
A prospective study from Singapore including 253 children (mean age: 7.3 years) with symptoms of allergic rhinitis (AR), asthma, and atopic dermatitis (AD) reported that 70% of these children were sensitized to B. tropicalis and among them, 45% showed positive IgE responses to Blo t 5.
Route Of Exposure
The primary pathway for exposure to Blo t 5 is through airway inhalation. However, Blo t 5 allergic manifestations can be triggered in numerous atopic individuals through the oral ingestion of contaminated food. A study carried out in Egypt confirmed Blo t 5 to be present in several types of raw and processed food products such as wheat, corn, rice, cakes, rusks, pasta, etc.
Clinical Relevance
Disease severity
Blo t 5 has been linked to sensitization and allergic symptoms in patients with AR and asthma. Cutaneous manifestations such as AD are also found to be associated with Blo t 5.
A sub-analysis of a nested case-control study was conducted in Columbia in 570 individuals (272 cases with current or past symptoms of asthma and 298 control) to report the prevalence of sensitization of different allergens from B. tropicalis. The findings suggested that sensitization to Blo t 5 was significantly greater in the asthmatic patients than the control group, thus confirming a strong association of Blo t 5 with asthma.
Evidence suggest a correlation between Blo t 5 level and disease severity in asthma patients with greater IgE reactions to Blo t 5 observed in severe asthma as compared to milder disease.
B. tropicalis allergens are also often associated with AR and conjunctivitis.
B. tropicalis allergens are also commonly linked to AD. A study from Brazil enrolled 59 patients with AD (median age: 8.4 years; mild AD, n=29; severe AD, n=30) to determine the patterns of sensitization in mild and severe forms of AD. The positivity rate for recombinant Blo t 5 (rBlo t 5) in immunoassay was significantly higher in patients with severe AD (83.3%) as compared to mild AD (41.4%). Furthermore, rBlo t 5 showed maximum correlation with the disease severity.
The influence of B. tropicalis allergens is also found to be particularly prominent in children. Another rare but severe symptom related to Blo t 5 allergen is oral mite anaphylaxis (OMA), also known as pancake anaphylaxis. OMA is comparatively a newer syndrome that was found to occur immediately following the consumption of pancake made of flour contaminated with mites and is more common in tropical and subtropical areas where warm and humid climate is conducive for mites.
Cross-reactive molecules
A limited IgE cross-reactivity has been observed between Blo t 5 and Der p 5 from Dermatophagoides pteronyssinus.
A study examined degree of cross-reactivity among recombinant group 5 and group 21 allergens of B. tropicalis (rBlo t 5 and rBlo t 21), Dermatophagoides farina (rDer f 5 and rDer f 21), and Tyrophagus putrescentiae (rTyr p 5 and rTyr p 21) and IgE inhibition to each allergen using a competitive ELISA technique. The IgE reactivity to rBlo t 5 was suppressed by approximately 86.99% due to rDer f 21, around 77.35% with rTyr p 5, and by 73.44% with rTyr p 21. The study results suggested cross-reaction is attributed to most of the IgE reactivity to group-5 and group-21 allergens of mite species.
Diagnostics
Disease severity
An African study with 59 children and young adults (age 2-18) with confirmed asthma, found a high IgE-binding frequency for rBlo t 5 (37%). The study also confirmed that the majority of the patients with IgE reactivity to rBlo t 5 contained high levels of Blo t 5-specific IgE.
A study from Brazil has reported that Blo t 5 may be associated with severe AD manifestation.
Blo t 5 had higher rate of sensitization as compared to Der p 5 from D. pteronyssinus in a population from South China. Blo t 5 has been reported as a potential marker for accurate discrimination of IgE sensitization to Blomia and Dermatophagoides species.
Cross-reactivity
Analysis of cross-reactivities of rBlo t 5 and rBlo t 21 with the B. tropicalis extracts suggests that these allergens can be used as valuable markers for diagnosing B. tropicalis-sensitized patients.
Cross-Reactivity
Blo t 5 shares 39% identity with Blo t 21 with similarities in their primary and secondary structure with typical features of α-helical proteins. An average cross-reactivity between Blo t 5 and Blo t 21 lies between low to moderate; however, in some cases, it may be higher. Both allergens harbor significant conserved IgE epitopes identified by IgE antibodies in B. tropicalis-sensitized patients.
The co-sensitization rate is reported to be high between Blo t 5 and Blo t 21. A Colombian study confirmed 80.3% Blo t 21 co-sensitization in Blo t 5-sensitized patients.
Blo t 5 exhibits 43% amino acid sequence identity with Der p 5 but shows low-to-moderate cross-reactivity.
A study by Kim and colleagues showed that Tyr p 5 and Blo t 5 displayed 52.9% sequence identity. The IgE reactivity of Blo t 5 was inhibited by up to 77.35% by rTyr p 5, suggesting high levels of cross-reactivity among the allergens.
Explained Results
Allergen Information
Blo t 5, a 14 kDa allergen, is found in the gut and fecal pellets of B. tropicalis and presents as one of the significant allergens from the species. It is responsible for allergic manifestations in patients with mite allergy in tropical, subtropical, and temperate regions with warm and humid climates.
Clinical relevance
Patients with sensitization to Blo t 5 may experience problems including asthma, AR, and AD.
Cross-reactivity
Cross-reactivity of IgE is seen among group-5 and group-21 allergens of different mite species. However, Blo t 5 exhibits low-to-moderate cross-reactivity with Blo t 21 and Der p 5, despite sharing 39% and 41% sequence similarity with the allergens, respectively.
Author: Turacoz
Reviewed: Dr. Ulrica Olsson
References
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