Biomarkers for leprosy: would you prefer T (cells)?

The need for rapid diagnostic tests that can be applied in non-expert settings may now be greater than ever before, due to changes in leprosy control programmes and the decrease in special expertise required for (early) diagnosis of leprosy. However, there is no test available that can detect asymptomatic Mycobacterium leprae infection or predict progression of infection to clinical disease. The characteristics of the leprosy disease spectrum, in which both cellular and humoral immunity against M. leprae determine the outcome of infection, are pre-eminently suitable for development of tests that simultaneously detect biomarkers specific for both types of immune responses, thereby covering the complete immunological leprosy spectrum. Since publication of the M. leprae genome, many research groups have investigated the potential of M. leprae-unique antigens in serologic as well as T cell assays. Despite the early promise of the genome towards identifying antigens that induce M. leprae-specific T cells, it took more than a decade before promising candidates were identified. This review describes recent studies on M. leprae-specific T cell responses and discusses the application of measuring specific cellular mediated immune responses possibly combined with serologic responses in field-friendly tests for early diagnosis of M. leprae exposure and infection.