Lymphocyte programmed cell death is mediated via HLA class II DR.

HLA class II molecules are constitutively expressed on human B lymphocytes and are induced on human T lymphocytes after activation, through which signal transduction via these molecules has been extensively described. We have observed cell death of as many as 60% after stimulation of lymphocytes via HLA class II DR molecules, but not via DP or DQ. Propidium iodide fluorescence, DNA fragmentation and morphology of the dead cells were examined. The reported cell death was very rapid, and independent of Fc receptors and of complement. A morphologically distinct sub-population of activated B cells was sensitive to HLA-DR mediated death, while resting B lymphocytes from the same donor did not die as a result of HLA class II mediated stimulation. Death via HLA-DR was distinguishable from necrosis. Cytoskeletal integrity, serine/threonine phosphatase activity and endonucleases were required for the pathway leading to HLA-DR mediated death. The 'ladder' pattern of DNA fragmentation which typically characterizes apoptosis was not observed, despite the observation of cell and nuclear shrinkage normally associated with apoptosis. These data suggest that HLA class II mediated death is a means of rapidly removing either T or B lymphocytes which have already served their role in the immune response, thereby avoiding the inflammatory responses associated with necrosis and concentrating the ligands for new TCR and/or CD4 interactions.

Immune cell defects affect bone remodelling in osteopetrotic op/op mice.

The aim of the present work was to further characterize immunological defects in osteopetrosis. The op/op mutant mouse is of particular interest since a marrow cavity develops spontaneously in older animals. The interleukin production (IL-1, IL-2, IL-3, IL-4, IL-6), the presence of macrophage differentiation antigens and the evolution of the bone lesions were studied in osteopetrotic and normal mice. Low levels of IL-1, IL-3 and IL-4 were observed at the age of 6 weeks in the op/op mice. However, at 22 weeks of age, the level of IL-1 and IL-4 returned to normal value in these op/op mice whereas the level of IL-3 remained partially decreased at the same age. Furthermore, macrophage expression of MAC-2 antigen, reduced at 12 weeks of age was found to be normal 10 weeks later. These immunological defects and their recovery seems to be concomitant with the healing of the bone lesions.

Monocytic origin of fibroblasts: spontaneous transformation of blood monocytes into neo-fibroblastic structures in osteomyelosclerosis and Engelmann's disease.

We describe here two pathological situations, osteomyelosclerosis and Engelmann's disease, in which HLA-DR blood monocytes modulate to the fibroblastic class, in long-term culture. Monocytes/macrophages were identified by immunofluorescence, using monoclonal antibodies against surface markers (Leu M3, CD 68, and HLA-DR) and the neo-fibroblasts by electron microscopy and immunofluorescence using monoclonal antibodies against a cytoplasmic enzyme specifically involved in the synthesis of collagen (5B5). Macrophages makers were found on the neo-fibroblasts, whereas HLA-DR macrophages expressed the cytoplasmic marker 5B5. Since osteoblasts are classically derived from fibroblasts, the significance of the in vitro differentiation of monocytes/macrophages into fibroblasts to the in vivo mechanism leading to excessive osteoblastic proliferation in both osteomyelosclerosis and Engelmann's disease, is discussed. The possible involvement of this pathway leading from monocytes to fibroblasts and osteoblasts in the normal process of bone modeling and remodeling in questioned.