[1] Kings Coll London, Div Asthma Allergy & Lung Biol, London SE1 9RT, England
[2] Univ London Birkbeck Coll, Dept Biol Sci, Inst Struct & Mol Biol Crystallog, London WC1E 7HX, England
[3] Univ Cambridge, Cambridge Inst Med Res, Dept Med, Cambridge CB2 0XY, England
[4] UCL, Div Med, London W1T 7NF, England