Assessing The Role of Spleen in Development of Myeloid Bias Amongst Haematopoietic Stem Cells

Myeloproliferative disorders are common in aged individuals, and ageing is associated with a skewing of haematopoiesis towards myeloid lineage. The spleen houses a distinct resident population of HSC primed for myelopoiesis which retain this myeloid bias even after migration to bone marrow niches. The importance of spleen in the phenotype of myeloid bias with ageing has therefore been investigated here. Two animal models were studied using CD150hi expression as an indicator of long term (LT)-HSC with a myeloid-biased phenotype. A steady-state model investigated a role for spleen in the age-related increase in myeloid-biased HSC by comparing splenectomised aged female mice with age-matched normal animals. This study showed no significant reduction in the proportion of CD150hi myeloid-biased LT-HSC in splenectomised aged mice compared with age-matched controls, discounting a role for spleen. The second model tested whether extramedullary haematopoiesis in spleen during pregnancy accelerates the redistribution of HSC and increases the myeloid-bias amongst bone marrow LT-HSC. The model utilised exbreeder mice were tested and shown to contain higher numbers of LT-HSC in bone marrow than did age-matched controls, indicating an important role for pregnancy in myeloid bias. The importance of spleen on myeloid bias detected in exbreeder mice was also tested. Young female mice were splenectomised and then exposed to multiple rounds of pregnancy. However, no significant role for spleen was indicated in myeloid bias. A further study compared the number of migratory CD150hi myeloid biased LT-HSC in bone marrow in young and old mice, and in old mice which had undergone splenectomy. The results showed that old mice had no more CCR2+ LT-HSC than young mice, and no significant difference was detected between old and splenectomised old mice, indicating that spleen was not important in expanding the number of migratory CCR2+ LT-HSC. This study identifies a subset of quiescent, bone marrow resident primitive LT-HSC which acquire myeloid bias with ageing. These are present in higher number in old mice, and in exbreeder mice which have undergone multiple rounds of pregnancy leading to extramedullary hematopoiesis. Data shown here also indicate that spleen plays no significant role in the maintenance of this subset in bone marrow or in the peripheral migration of these cells.