Introduction of Mature Mast Cells into Bone Marrow Alters Bone Metabolism in Growing Mice

There is evidence that mast cells contribute to skeletal response to injury, but it is less clear whether these immune cells directly influence normal bone growth and turnover. Mature mast cells are common in the bone marrow of humans and rats, but have not been convincingly demonstrated to be present in the bone marrow of healthy mice, potentially limiting the mouse as a model for characterizing the full range of mast cell/bone cell interactions. An initial goal of this investigation was to comprehensively screen seven strains of mice for mature mast cells in bone marrow. Finding none, we then investigated three approaches to home these cells to the marrow of mice unable to generate mast cells: (1) administration of soluble kit ligand to membrane kit ligand-deficient <i>Kit^Sl/Sld</i> mice, (2) adoptive transfer of wild-type hematopoietic stem cells to kit receptor-deficient <i>Kit^W/W</i>^-<i>v</i> mice, and (3) adoptive transfer of wild-type mouse bone marrow-derived mast cells generated in vitro and delivered intravenously to <i>Kit^W/W-v</i> mice. Only the third approach was successful. Using this method, we then evaluated the impact of bone marrow-derived mast cells on bone mass, architecture, turnover, and gene expression. The adoptive transfer of mast cells resulted in alterations in cancellous bone microarchitecture and cell populations in the vertebra, and in differential expression of genes associated with bone metabolism in the tibia. Taken together, our results support the concept that bone marrow mast cells influence bone metabolism and suggest that homing mast cells to the bone marrow of mice is a useful model to understand the role of these cells in skeletal health and disease.