Animal model of leprous neuritis using transfer of sensitized lymphocytes into M. leprae-inoculated nude mice

BackgroundLeprosy is curable with multidrug therapy, but preventing peripheral neuropathy remains challenging. In leprosy, reversal reactions (Type 1 reactions) are characterized by sudden enhanced cell-mediated immunity against Mycobacterium leprae (M. leprae), leading to acute neuritis that may cause irreversible nerve damage, paralysis, sensory impairment, and muscle atrophy. Armadillo and nude mouse models are used to study M. leprae infection, but detailed neuropathological models for immune-mediated reactions in leprosy are lacking. This study aimed to establish a reproducible animal model to analyze leprous neuritis pathogenesis. Methodology/Principal FindingsBALB/c nude mice were inoculated with M. leprae and six months later received transfers of naive CD4+ cells, sensitized CD4+ cells, or sensitized whole spleen cells from BALB/c mice. Histological and quantitative analyses were performed four weeks post-transfer. Mice that received transfer of sensitized CD4+ cells or sensitized spleen cells exhibited footpad swelling up to 88% greater than in animals with no cell transfer. Mice that received the sensitized cells also had massive inflammatory cell infiltration into nerve fascicles. Quantitative imaging confirmed a significant reduction in the percentage of myelinated area in mice that received transfer of sensitized CD4+ (p < 0.05) or whole spleen cells (p < 0.01) compared to control mice with no transfer. Furthermore, bacterial fragmentation within the endoneurium coincided with myelinated axon destruction, indicating that the immune response targeted bacilli but simultaneously damaged nerve structures. Conclusions/SignificanceThis study successfully reproduced leprous peripheral neuritis that resembles the reversal reaction in humans. The findings indicate that CD4+ cells are primary drivers of inflammation, whereas interactions between multiple immune cell populations in whole spleen cell transfers further exacerbate disease pathology. This model confirms that host immune responses are crucial for nerve injury progression. This model provides a valuable tool for investigating neuroprotective therapies to prevent permanent disability in patients with leprosy. Author SummaryAlthough leprosy can be treated as an infectious disease, many patients still suffer from permanent peripheral nerve damage, known as leprous neuritis. A particularly dangerous condition called the "reversal reaction" occurs when the immune system of an infected individual suddenly overreacts, causing severe acute inflammation in the nerves. Without prompt treatment, this inflammatory response can result in irreversible nerve damage, paralysis, and muscle atrophy. In this study, we developed a new animal model using "nude mice", which lack their own T-cells, to understand how this damage happens. We infected these mice with leprosy bacteria and then transferred specific immune cells, specifically sensitized CD4+ T cells, isolated from M. leprae-immunized mice. The results showed that this transfer triggered significant footpad swelling and caused inflammatory cells to invade the nerve fascicles. Most importantly, we observed a significant reduction in myelin, the protective coating that promotes nerve signaling. These findings confirm that nerve damage is largely driven by the hosts own immune response attempting to clear the bacteria. By successfully recreating this reaction, our research provides a vital tool for developing new therapies aimed at preventing permanent disability and protecting the physical functions of patients.