Immune responses in mice with murine leprosy.

Mice infected with Mycobacterium lepraemurium showed marked histological changes in the thymus and lymph nodes. In the thymus there was a progressive depletion of lymphoid cells and replacement by macrophages, many of which contained bacilli in the advanced stages of the disease. In lymph nodes there was depletion of paracortical immunoblasts and accumulation of macrophages. Infected mice showed impaired cell-mediated immune responses, including delayed rejection of skin grafts and depression or absence of contact sensitivity. Humoral immune responses to bovine serum albumin and sheep erythrocytes in infected mice were normal. The defect in cell-mediated immune responses is thought to be secondary to the massive infection, and is discussed in relation to impaired immune responses in human lepromatous leprosy.