Palsy of the rear limbs in Mycobacterium lepraemurium-infected mice results from bone damage and not from nerve involvement

A small but relatively constant proportion (3-5%) of mice chronically infected with Mycobacterium lepraemurium (MLM) develops bilateral paralysis of the rear limbs. The aim of the study was to investigate whether or not the bilateral leg palsy results from nerve involvement. Direct bacterial nerve infection or acute/delayed inflammation might possibly affect the nerves. Therefore, palsied animals were investigated for the presence of: (a) histopathological changes in the leg tissues including nerves, bones and annexes, and (b) serum antibodies to M. lepraemurium and M. leprae lipids, including phenolic glycolipid I from M. leprae. Histopathological study of the palsied legs revealed that the paralysis was not the result of direct involvement of the limb nerves, as neither bacilli nor inflammatory cells were observed in the nerve branches studied. Antibodies to brain lipids and cardiolipin were not detected in the serum of the palsied animals, thus ruling out an immune response to self-lipids as the basis for the paralysis. Although high levels of antibodies to MLM lipids were detected in the serum of palsied animals they were not related to limb paralysis, as the nerves of the palsied legs showed no evidence of inflammatory damage. In fact, nerves showed no evidence of damage. Paralysis resulted from severe damage of the leg bones. Within the bones the bone marrow became replaced by extended bacilli-laden granulomas that frequently eroded the bone wall, altering the normal architecture of the bone and its annexes, namely muscle, tendons and connective tissue. Although this study rules out definitively the infectious or inflammatory damage of nerves in murine leprosy, it opens a new avenue of research into the factors that participate in the involvement or the sparing of nerves in human and murine leprosy, respectively.