Abstract
Abstract Roughly 400,000 people have bone metastases in the U.S. with the vast majority of these occurring in the spine. The etiology of bone metastasis still remains to be fully elucidated. This study explored the differences in immune landscape between long bone and spine that may contribute to higher rates of bone metastasis to the spine. Spines and femurs from male C57BL6/J mice (N=10) were processed for flow cytometry and immunophenotyping using Mass Cytometry by Time-Of-Flight (CyTOF). The cells were analyzed with CyTOF using a 33-surface protein marker mouse antibody panel. Spines (N=3) and femurs (N=2) from patients were analyzed with CyTOF using the Maxpar Complete Human T cell Immuno-Oncology Panel Set. There are global differences in the immune cell composition between the long bone and spine microenvironment. Flow cytometry revealed slight increases in the CD45+ and Cd11b+ cell populations in the bone marrow of murine spines compared to murine long bone, which are markers for myeloid-derived suppressor cells (MDSCs). Using CyTOF, significant differences in the immune cell landscape between long bone and spine were observed. In the murine long bone, an increase in monocytes/macrophages, myeloid progenitors, granulocytic MDSCs, granulocytes, and mast cells was observed compared to the spine. In the murine spine an elevation of CD8a+ DC cells, classical monocytes, MDSCs, pDCs, memory T helper cells, and NK T cells was seen. Evaluation of human long bone and spine revealed similar trends with a predominance of myeloid progenitor cells and monocytes in the human vertebra compared to the human long bone marrow. Significant differences in the immune microenvironment exist between the spine and long bone marrow in both murine and human samples. This is the first report of significant differences in immune cell populations between different skeletal locations. However, the functional significance of these differences has yet to be determined.
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