1. Department of Medicine, Imperial College, London W2 1PG, UK
2. Clinical Infectious Diseases Research Initiative, Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, Observatory 7925, South Africa
3. Tuberculosis Research Section, NIAID, NIH, Bethesda, MD 20892, USA
4. MRC National Institute for Medical Research, London NW7 1AA, UK