We focused on the crucial step in the pathogenesis of osteolysis – the macrophage phagocytosis of particulate wear debris – as an in vitro model of this disease. Using nucleotide microarrays and a variety of analysis techniques including Self-Organizing Maps, we simultaneously probed the gene expression time-courses of thousands of genes following macrophage culture with polyethylene (UHMWPE) particles, titanium alloy (TiAlV) particles, LPS as a positive control, and no stimulatory additives as a negative control.

Inflammation-related genes displayed the most dramatic clustering with the Self-Organizing Map. They were almost always over-represented in groups with significant up-regulation at 30 min. and 4 h. and unchanged or slightly down-regulated at 8 and 24 h., a time-course consistent with previous RT-PCR studies.

In order to isolate the most important genes involved in the macrophage response, we identified the top 0.5% and top 2% up- and down-regulated genes at each time-point. While the genes known to be important in this process, such as TNFalpha, IL-1beta, and MIP-1alpha were expressed where and when expected from previous ELISA and RT-PCR studies, a variety of genes with putative, but previously under-appreciated roles, were also identified.

These top genes identified processes that validate and logically extend the current model of the pathogenesis of osteolysis. The osteolytic lesions are made up of an area of bone resorption filled with inflammatory tissue histologically marked by granulomas, granulation tissue, and fibrosis. Cytokines and other genes involved in macrophage and lymphocyte activation may explain the granuloma formation, while the prominence of angiogenic factors and a gene expression bias toward the TGFbeta pathway may explain the granulation tissue and fibrosis respectively. Finally, the sine qua non of osteolysis, the lytic bone lesions, were indicated by the prominence of genes involved in osteoclast formation, activation, and proliferation.

Thus, the macrophage gene expression in this in vitro model strongly indicated processes that make up the histology and pathology of osteolysis.