Effects of Pasteurella multocida toxin on porcine bone marrow cell differentiation into osteoclasts and osteoblasts

The effect of Pasteurella multocida toxin (PMT) on porcine osteoclast and osteoblast differentiation was studied using in vitro cell culture systems. When grown in the presence of Vitamin D3, isolated porcine bone marrow cells formed multinucleated cells with features characteristic of osteoclasts. Exposure of bone marrow cells to Vitamin D3 and PMT during growth resulted in formation of increased numbers and earlier appearance of osteoclasts compared to controls. Ultrafiltered medium form PMT-treated cells likewise increased osteoclast numbers, suggesting that a soluble mediator may be involved in the action of PMT. When cell cultures were treated with fluorescein-labeled PMT, fluorescence was found within the cytoplasm of small, round cells that did not resemble either osteoclasts or osteoclastic precursor cells. Cultures of porcine bone marrow cells exposed to dexamethasone, ascorbic acid, and beta-glycerophosphate developed into osteoblastic cells that formed multilayered, mineralized nodules. Exposure of osteoblastic cultures to low concentration of PMT resulted in retarded cell growth, formation of decreased numbers of nodules and minimal to no mineralization in the nodules; higher concentration of PMT resulted in increased cellular debris and poor growth of cells, with no nodule formation. These findings suggest that PMT may induce turbinate atrophy in pigs by increasing osteoclast numbers and inhibiting osteoblastic bone formation. The effect of PMT on osteoclastic differentiation and growth may not be due to a direct effect on preosteoclastic cells, but rather due to alterations in the soluble mediator secretion by marrow stromal cells.