House Dust Mites (HDM) are arachnids that infest bedding, carpet, upholstered furniture and fabric. Like many other allergens, exposure to HDM allergens in sensitised patients is associated with poorer lung function, greater medication requirements and more asthma symptoms as well as chronic rhinosinusitis symptoms. In contrast to other allergens, there is evidence that HDMA leads to the development of asthma, in addition to exacerbating pre-existing asthma in HDM-sensitised patients.

ToleroMune HDM is currently being developed for the treatment of HDM allergy.

The Purpose of this optional observational follow-on study is to evaluate the continued efficacy of TM-HDM in HDM allergic subjects based on change in TRSS from TH002 baseline approximately two years after the completion of the baseline EEC visit in TH002 following challenge with HDM allergen in an EEC.

Subjects who completed all dosing visits and the post treatment challenge (PTC) in study TH002 will be invited to attend the Screening visit for TH002a. Subjects will attend for 3 visits to the EEU on successive days. Following the last EEC visit a follow-up visit will be performed 3-10 days later.

Inclusion Criteria:

• previously randomised in study TH002, completed all dosing visits and the PTC

Exclusion Criteria:

• "Partly controlled" and "uncontrolled" asthma
• History of anaphylaxis to House Dust Mite allergen
• FEV1 <80% of predicted.
• Symptoms of a clinically relevant illness
• Subjects who cannot tolerate allergen challenge in the EEC

Subjects previously randomised in study TH002 and completed all dosing visits and the PTC