Interaction of Mycobacterium leprae with the HaCaT human keratinocyte cell line: new frontiers in the cellular immunology of leprosy

Abstract

Leprosy is a chronic granulomatous disease caused by Mycobacterium leprae affecting the skin and peripheral nerves. Despite M. leprae invasion of the skin and keratinocytes importance in innate immunity, the interaction of these cells in vitro during M. leprae infection is poorly understood. Conventional and fluorescence optical microscopy, transmission electronic microscopy, flow cytometry and ELISA were used to study the in vitro interaction of M. leprae with the HaCaT human keratinocyte cell line. Keratinocytes uptake of M. leprae is described, and modulation of the surface expression of CD80 and CD209, cathelicidin expression and TNF-α and IL-1β production of human keratinocytes are compared with dendritic cells and macrophages during M. leprae interaction. This study demonstrated that M. leprae interaction with human keratinocytes enhanced expression of cathelicidin and greatly increased TNF-α production. The highest spontaneous expression of cathelicidin was by dendritic cells which are less susceptible to M. leprae infection. In contrast, keratinocytes displayed low spontaneous cathelicidin expression and were more susceptible to M. leprae infection than dendritic cells. The results show, for the first time, an active role for keratinocytes during infection by irradiated whole cells of M. leprae and the effect of vitamin D on this process. They also suggest that therapies which target cathelicidin modulation may provide novel approaches for treatment of leprosy.

Importance of the immune response to Mycobacterium leprae in the skin

“…Because keratinocytes express mannose-binding receptors (KCMR), TLRs and Class II MHC antigens as well as produce cytokines, chemokines and antimicrobial peptides, they may play an important role and participate in the epidermal immune response to M. leprae (Mutis et al 1993). It has been demonstrated in vitro that human keratinocytes can phagocytose M. leprae and subsequently exhibit the expression of the surface molecules CD80, CD209 and cathelicidin as well as secrete TNF-α and IL-1β (Lyrio et al 2015). M. leprae induces the production of cytokines and chemokines in keratinocytes, which mediates mycobacterial killing and results in a more efficient immune response that is important for the recruitment of LCs, DCs, T cells and neutrophils.…”

“…In addition, keratinocytes may play an important role in leprosy by participating in the epidermal immune response to M. leprae (Mutis et al 1993). Thus, this cell type possesses a highly sophisticated innate pattern recognition system in which the simultaneous recognition of a pathogen by different classes of pattern recognition receptors can provide a specific immune response or, in the case of commensals, a lack of a response to microorganisms (Lyrio et al 2015). Furthermore, keratinocytes can distinguish between pathogenic and commensal microorganisms (Pivarcsi et al 2005).…”

“…Seo et al reported that the bacilli are located within typical epidermal cells, which exhibit tonofilaments and melanosomes in their cytoplasm as well as desmosomes at the junction of each of the cells (Seo et al 1995). Human keratinocytes have been shown to phagocytose M. leprae in vitro, which induces changes in the expression of various surface molecules and cathelicidin as well as the secretion of TNF-α and IL-1β by these cells (Lyrio et al 2015). Cytokines and chemokines derived from keratinocytes are important for the mobilisation of LCs, DCs, T cells and neutrophils to the site of infection, where they can mediate microbial killing and initiate a more efficient immune response against M. leprae.…”

“…Leprosy is a disease that manifests as skin lesions, and keratinocytes and the epidermis play an important role in the innate immune response to M. leprae (Lyrio et al 2015). Moreover, while some evidence indicates a role for dendritic cells (DCs) in the immune response to M. leprae (Santos et al 2001), another cell type important for epidermal defence, the keratinocyte, is also a source of cytokines and chemokines, which are critical for recruiting DCs, T cells and neutrophils to the site of infection (Lyrio et al 2015).…”

“…Moreover, while some evidence indicates a role for dendritic cells (DCs) in the immune response to M. leprae (Santos et al 2001), another cell type important for epidermal defence, the keratinocyte, is also a source of cytokines and chemokines, which are critical for recruiting DCs, T cells and neutrophils to the site of infection (Lyrio et al 2015). In addition, human keratinocytes have been shown to phagocytose M. leprae in vitro and subsequently exhibit changes in the expression of surface molecules and cathelicidin as well as secrete tumour necrosis factor (TNF)-α and IL-1β (Lyrio et al 2015). In addition, the invasion of keratinocytes and the secretion of cytokines and chemokines by immune cells were reported.…”