Use of a whole blood assay to evaluate in vitro T cell responses to new leprosy skin test antigens in leprosy patients and healthy subjects

Development of an immunological tool to detect infection with Mycobacterium leprae would greatly benefit leprosy control programmes, as demonstrated b y the contribution of the tuberculin test to tuberculosis control. In a new approach to develop a 'tuberculin-like' reagent for use in leprosy, two ne w fractions of M. leprae depleted of cross-reactive and immunomodulatory li pids-MLSA-LAM (cytosol-derived) and MLCwA (cell wall-derived)-have been pro duced in a form suitable for use as skin test reagents. T cell responses (i nterferon-gamma (IFN-gamma) and lymphoproliferation) to these two new fract ions were evaluated in a leprosy-endemic area of Nepal using a simple in vi tro whole blood test. The two fractions were shown to be highly potent T ce ll antigens in subjects exposed to M. leprae-paucibacillary leprosy patient s and household contacts. Responses to the fractions decreased towards the lepromatous pole of leprosy. Endemic control subjects also showed high resp onses to the fractions, indicating high exposure to M. leprae, or cross-rea ctive mycobacterial antigens, in this Nepali population. The new fractions, depleted of lipids and lipoarabinomannan (LAM) gave enhanced responses com pared with a standard M. leprae sonicate. The cell wall fraction appeared a more potent antigen than the cytosol fraction, which may be due to the pre dominance of the 65-kD GroEL antigen in the cell wall. The whole blood assa y proved a robust field tool and a useful way of evaluating such reagents p rior to clinical trials.