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Leprosy is a chronic disease caused by M. leprae infection that can cause severe neurological complications and physical disabilities. A leprosy-specific vaccine would be an important component within control programs but is still lacking. Given that multifunctional CD4 T cells [i.e., those capable of simultaneously secreting combinations of interferon (IFN)-γ, interleukin (IL)-2, and tumor necrosis factor (TNF)] have now been implicated in the protective response to several infections, we tested the hypothesis if a recombinant M. leprae antigen-specific multifunctional T cells differed between leprosy patients and their healthy contacts. We used whole blood assays and peripheral blood mononuclear cells to characterize the antigen-specific T cell responses of 39 paucibacillary (PB) and 17 multibacillary (MB) leprosy patients and 31 healthy household contacts (HHC). Cells were incubated with either crude mycobacterial extracts (M. leprae cell sonicate–MLCS) and purified protein derivative (PPD) or recombinant ML2028 protein, the homolog of M. tuberculosis Ag85B. Multiplex assay revealed antigen-specific production of IFN-γ and IL-2 from cells of HHC and PB, confirming a Th1 bias within these individuals. Multiparameter flow cytometry then revealed that the population of multifunctional ML2028-specific T cells observed in HHC was larger than that observed in PB patients. Taken together, our data suggest that these multifunctional antigen-specific T cells provide a more effective response against M. leprae infection that prevents the development of leprosy. These data further our understanding of M. leprae infection/leprosy and are instructive for vaccine development.

Leprosy is a chronic infectious disease caused by Mycobacterium leprae. According to official reports from 121 countries across five WHO regions, there were 213 899 newly diagnosed cases in 2014. Although leprosy affects the skin and peripheral nerves, it can present across a spectrum of clinical and histopathological forms that are strongly influenced by the immune response of the infected individuals. These forms comprise the extremes of tuberculoid leprosy (TT), with a M. leprae-specific Th1, but also a Th17, response that limits M. leprae multiplication, through to lepromatous leprosy (LL), with M. leprae-specific Th2 and T regulatory responses that do not control M. leprae replication but rather allow bacterial dissemination. The interpolar borderline clinical forms present with similar, but less extreme, immune biases. Acute inflammatory episodes, known as leprosy reactions, are complications that may occur before, during or after treatment, and cause further neurological damages that can cause irreversible chronic disabilities. This review discusses the innate and adaptive immune responses, and their interactions, that are known to affect pathogenesis and influence the clinical outcome of leprosy. Electronic supplementary material The online version of this article (doi:10.1186/s40249-016-0229-3) contains supplementary material, which is available to authorized users.

Leprosy is a chronic infection caused by Mycobacterium leprae. It affects the skin and peripheral nerves and can cause irreversible chronic disabilities. The worldwide registered number of cases in 2009 was 213,036. This review discusses clinical aspects of the disease, including leprosy reactions and neuronal damage, as well as immunological and immunogenetic aspects influencing disease susceptibility and outcome. The cardinal signs of leprosy are skin lesions with altered sensation, thickened peripheral nerves, and presence of alcohol acid-resistant bacilli in skin biopsy or lymph. Confirmatory examinations include (1) bacteriological examination, which allows patients' classification into two operational groups, multibacillary (MB) and paucibacillary (PB); and (2) histopathological examination, which permits stratification in different clinical forms. These clinical forms differ not only by histopathology but also according to the host's immune response to M. leprae. These forms comprise the extremes of (1) tuberculoid leprosy (TT), with a specific Th1 response and control of M. leprae multiplication; (2) lepromatous leprosy (LL) without Th1 response and preserved Th2 response; and (3) the interpolar clinical forms, borderline tuberculoid (BT), borderline borderline (BB), borderline lepromatous (BL), and indeterminate form (IL). Appropriate treatment is based on smear examination or the number of lesions at diagnosis. In the evolution of leprosy, acute inflammation, known as reactions, may occur during or after treatment. These reactions are classified into two main types: the type I reaction or reversal reaction (RR), and the type II reaction or erythema nodosum leprosum (ENL). The role of innate immune response to control the infection is supported by immunological and genetic studies. Drug Dev Res 72:509–527, 2011. © 2011 Wiley-Liss, Inc.