Haematopoiesis During Native Conditions and Immune Thrombocytopenia Progression

The maintenance of haematopoietic stem cell (HSC) self-renewal and differentiation throughout life is essential for ongoing haematopoiesis and is highly dependent upon cytokine-cytokine receptor interactions and direct cell-cell contact between the HSC and components of the perivascular bone marrow (BM) microenvironment. Thrombopoietin (TPO) is one of two such cytokines essential for HSC self-renewal. Although the majority of TPO is produced distally by the liver, lower amounts of TPO are thought to be produced locally in the BM, directly at the site of utilisation. However, the exact cellular sources of BM derived TPO are unclear and remains an active area of research. Contrary to previous studies, the results in this thesis indicate that megakaryocytes do not express Thpo, and instead LepR+/Cxcl12-DsRedhigh BM stromal cells (BMSCs) are major sources of Thpo in mice.

Immune thrombocytopenia (ITP) is an acquired autoimmune condition characterised by reduced platelet production and increased platelet destruction by sustained immune attack. In this thesis, a novel mouse model of sustained ITP was generated and the effect on the immune and haematopoietic system was assessed. Platelet destruction was antibody dependent and appeared to be primarily driven by splenic macrophages. Additionally, ITP progression was associated with considerable progenitor expansion and BM remodelling. Single cell assays using Lin-Sca1+c-Kit+CD48-CD150+ long-term HSCs (LT-HSCs) revealed elevated LT-HSC activation and proliferation in vitro. However, LT-HSC functionality was maintained as measured by in vivo serial transplantations. ITP progression was associated with considerable BM vasodilation and angiogenesis, as well as a 2-fold increase in local production of CXCL12; a cytokine essential for LT-HSC function and BM homing expressed at high levels by LepR+ BMSCs. This was associated with a 1.5-fold increase in LepR+ BMSCs and a 5.5-fold improvement in progenitor homing to the BM. Whereas the increase in BMSCs was transient and reverted back to baseline after platelet count returned to normal, vasculature changes in the BM persisted. Together, these studies demonstrate that LT-HSCs expand in response to ITP, and that LT-HSC functionality during sustained haematopoietic stress is maintained through an adapting BM microenvironment.