Representatives of the family Chironomidae (non-biting midges; order Diptera) are found worldwide. Freeze-dried chironomid larvae, predominantly of the species Chironomus thummi thummi are frequently used as fish food and are an allergen source for fish keepers and persons employed in the manufacture of fish food. At present, 9 allergens of C. thummi thummi have been assigned an official designation by the WHO/IUIS allergen nomenclature sub-committee: Chi t 1 to 9. All of them are hemoglobins with molecular weights of 16 kDa. IgE binding and cross-reactivity was clearly demonstrated for all these proteins. However, the assignment of 9 distinct allergen numbers to members of the same protein family is quite unusual.
Currently, the IUIS allergen database contains 12 allergen and isoallergen sequences from C. thummi thummi. The Uniprot database has demerged entry P02225, listed in the database for Chi t 7, into 7 entries, 5 from C. thummi thummi and 2 from C. thummi piger that are identical to 2 of the sequences from C. thummi thummi. Consequently, the 16 unique amino acid sequences of the mature C. thummi allergens were aligned using ClustalX2, a neighbor-joining tree was generated from the alignment and a percent sequence identity matrix was built to evaluate appropriate nomenclature.
Pairwise sequence alignments showed that sequences belonging to allergens Chi t 5, 6, 7 and 8 possess sequence identities to Chi t 3 of between 51 and 63%. Chi t 1, 2, 4, and 9 diverge to a greater extent from Chi t 3 (<50% identical) and from each other. Phylogenetic tree analysis suggests the clustering of Chi t 3, 6, 7, and 8, while Chi t 1, 2, 4, 5, and 9 form separate clades.
Based on these analyses, the IUIS Allergen Nomenclature Sub-Committee renames Chi t 5, 6, 7 and 8 isoallergens of Chi t 3, even though their sequence identities to Chi t 3 are below the 67% threshold previously defined for isoallergens. The remaining hemoglobins, previously designated Chi t 1, 2, 4 and 9 will retain their previous names.