Abstract
BackgroundLeprosy is a contagious and chronic systemic granulomatous disease caused by Mycobacterium leprae. In the pathogenesis of leprosy, granulomas play a key role, however, the mechanisms of the formation and maintenance of M. leprae granulomas are still not clearly understood.MethodsTo better understand the molecular physiology of M. leprae granulomas and the interaction between the bacilli and human host cells, we developed an in vitro model of human granulomas, which mimicked the in vivo granulomas of leprosy. Macrophages were differentiated from human monocytes, and infected with M. leprae, and then cultured with autologous human peripheral blood mononuclear cells (PBMCs).ResultsRobust granuloma-like aggregates were obtained only when the M. leprae infected macrophages were co-cultured with PBMCs. Histological examination showed M. leprae within the cytoplasmic center of the multinucleated giant cells, and these bacilli were metabolically active. Macrophages of both M1 and M2 types co-existed in the granuloma like aggregates. There was a strong relationship between the formation of granulomas and changes in the expression levels of cell surface antigens on macrophages, cytokine production and the macrophage polarization. The viability of M. leprae isolated from granulomas indicated that the formation of host cell aggregates benefited the host, but the bacilli also remained metabolically active.ConclusionsA simple in vitro model of human M. leprae granulomas was established using human monocyte-derived macrophages and PBMCs. This system may be useful to unravel the mechanisms of disease progression, and subsequently develop methods to control leprosy.
Highlights
Leprosy is a contagious and chronic systemic granulomatous disease caused by Mycobacterium leprae
We developed an in vitro model of M. leprae granulomas, which mimicked the human granulomatous skin lesion with progressive recruitment of monocytes around macrophages infected by M. leprae, and their differentiation into epithelioid cells (ECs) and multinucleated giant cells (MGCs) as well as recruitment of activated lymphocytes
Granuloma-like aggregates formed by co-culture of M. leprae infected macrophages and autologous peripheral blood mononuclear cells (PBMCs) When PBMCs were incubated with M. leprae infected macrophages in a 24-well tissue culture plate, the cells aggregated to form a multilayered granuloma-like aggregates by day 9 as shown in Figure 1A, whereas control groups did not recruit any cells at this stage (Figure 1B, C)
Summary
Leprosy is a contagious and chronic systemic granulomatous disease caused by Mycobacterium leprae. Leprosy is a chronic mycobacterial infection that presents an extraordinary range of cellular immune responses in humans. Regulation of cell-mediated immunity against Mycobacterium leprae through the fine-tuning between cells, cytokines and chemokines continues to be unraveled. Similar to other mycobacterial infections, granulomatous inflammation in the skin lesion defines certain forms of leprosy [1,2]. The bacilli enter and replicate within macrophages, resulting in the production of cytokines and chemokines, which in turn triggers an inflammatory response leading to the recruitment of macrophages and lymphocytes at the infectious site. Granulomas have long been believed to benefit the host by containing and restricting the growth of mycobacteria in a localized area, to prevent the spread of the disease to other parts of the tissue or organs [5]. Some studies in zebra fish infected with M. marinum and M. tuberculosis suggested that the granulomas contribute to early bacterial growth and expanding infection [6,7,8,9,10]
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