Osteopontin and the immune system:another brick in the wall

The innate and adaptive arms of the immune system are finely tuned to efficiently react to either endogenous or exogenous noxiae that perturb the function of our tissues and organs,thus rapidly returning to homeostasis with minimal collateral damage.The molecular mechanisms by which this occurs have been only partially elucidated.The work by Kanayama et al.1 contributes to the building of this wall of knowledge by adding an important brick.They show that osteopontin(OPN),a phosphorylated protein of the extracellular matrix(ECM),has a dominant role in balancing lymphoid and myeloid cells during emergency granulopoiesis(that is,an increased cellular output from the bone marrow to meet the acute demand for the respective blood cell type).2 OPN that is produced and released by bone marrow(BM)stromal cells negatively impacts the pool size of hematopoietic stem cells(that is,Lin−Sca-1+c-kit+).