Inappropriate Expression of CSF-1 in CSF-1R-Expressing Cells in Mice Leads to Osteoporosis, Macrophage Activation and Early Death.

The CSF-1R is selectively expressed on mononuclear phagocytes, osteoclasts and cells of the female reproductive tract. CSF-1 is normally expressed in a wide variety of cell types, excluding those expressing the CSF-1 receptor (CSF-1R). Autocrine/paracrine regulation by CSF-1 plays an important role in the development of disease in mouse models of acute myeloid leukemia and mammary tumor progression to metastasis. To determine the consequences of autocrine regulation by CSF-1 in the absence of other interventions, we generated transgenic mice that expressed CSF-1 in CSF-1R bearing cells. In contrast to mice that expressed the CSF-1R in CSF-1-expressing cells, which had no gross phenotype at 1 year of age, mice expressing CSF-1 under the control of mouse CSF-1R promoter and second intron (TgRC mice), had a short lifetime, starting to lose weight and mobility after 3 weeks of age. CSF-1 mRNA was expressed in peritoneal macrophages (PM) and sequence analysis of the RT-PCR products verified the expression of a chimeric mRNA of the CSF-1R exon 2, 3 and CSF-1 mRNAs under control of the CSF-1R promoter. Quantitative real-time RT-PCR analysis revealed that CSF-1 mRNA expression was increased by 19-fold in PM and 980-fold in bone marrow derived macrophages (BMM) from a TgRC founder mouse, compared to these populations from non-transgenic mice. Consistent with autocrine regulation by CSF-1, in contrast to BMM from non-transgenic mice, TgRC BMM proliferated in the absence of CSF-1. However, there was no elevation of the circulating CSF-1 in TgRC mice, indicating that transgene-regulated expression of CSF-1 only affected local CSF-1 concentrations. X-ray and histological analysis demonstrated that TgRC mice developed osteoporosis with increased numbers of femoral TRAP-positive osteoclasts. Macrophage densities were significantly increased in bone marrow, liver, spleen and brain. In several tissues, there were local regions of increased macrophage density, consistent with effects of locally produced CSF-1 on macrophage survival and/or proliferation. Also, macrophages in several tissues had a more dendritic appearance than in non-transgenic mice, suggesting that they may be activated. TgRC PM, cultured in the presence of CSF-1 exhibited a highly dendritic morphology with multiple protruding pseudopods also suggesting an activated state. Studies of the macrophage cytokine expression profile by the real-time quantitative RT-PCR revealed elevated levels of mRNA for the proinflammatory cytokines, IL-1β (33-fold) (in BMM) and IL-10 (10-fold) and IL-6 (2.6-fold) (in PM), consistent with the priming effect of CSF-1 on proinflammatory cytokine release by macrophages. These studies indicate that inappropriate local expression of CSF-1 by or near CSF-1R-expressing cells can result in activation of macrophages and osteoclasts and severe disease. Supported by NIH grant CA32551 (ERS) and an ASH Fellow Scholar Award and Leukemia and Lymphoma Society Special Fellow Award (X-MD).