Susan K. Nilsson1,2


1Biomedical Manufacturing, Commonwealth Scientific and Industrial Research Organization, Melbourne, Victoria 3169, Australia
Australian Regenerative Medicine Institute, Monash University, Clayton, Victoria 3800, Australia


Haematopoietic stem cells (HSC) are rare, multipotent adult stem cells that are responsible for producing all circulating blood cells throughout life and are the key cellular component of a stem cell transplant, which are used in the treatment of blood cancers such as leukaemia. These stem cells reside in a specialised microenvironment in the bone marrow (BM) termed the “stem cell niche”. Over the last 20 years, our laboratory have identified several niche interactions which we have shown to be important in regulating HSC function and maintenance with the aim of exploiting these findings to improve clinical stem cell transplants.

Mobilised peripheral blood (PB) HSC have largely replaced bone marrow as the primary source of HSC for transplant and is clinically achieved using a specific growth factor (G-CSF) with and without small molecules such as the CXCR4 antagonist AMD3100. However, AMD3100 is not sufficiently effective when used alone and G-CSF is associated with several side effects (e.g. splenomegaly and bone pain) and also fails to mobilise clinically usable numbers of HSC in a significant number of patients. These shortcomings can greatly compromise patient outcomes and result in large costs associated with re-mobilisation. To address these issues, significant effort has focused on developing small molecules as rapid, safe and effective mobilisation agents. Herein, we demonstrate that the novel small molecule antagonist “BOP”, which potently targets both α9β1 and α4β1 integrins expressed by BM HSC, rapidly mobilises long-term multi-lineage reconstituting HSC, either alone or synergistically when used in combination with AMD3100 or G-CSF. Notably, the significantly greater augmentation of HSC in PB using BOP+AMD3100 was also recapitulated in a humanised NSG xenograft model, demonstrating its applicability in the clinical context. To acquire greater understanding of integrin-mediated HSC mobilisation, we synthesised a water soluble, fluorescent integrin antagonist, r-BOP (R-BC154), which we show has high affinities to both α4β1 and α9β1. Using this fluorescent probe, we demonstrate that BOP preferentially binds HSC and progenitors through endogenously primed/activated integrins within the endosteal niche. Differential targeting of endosteal HSC/progenitors was attributed to greater concentrations of integrin-activating metal ions (calcium, magnesium and manganese), with a metal ion concentration gradient emanating from the endosteal BM demonstrated. Together, we show that integrin α9β1 is a novel target for stem cell mobilisation and highlights the combination of the small molecule integrin antagonist BOP with AMD3100 as an effective and rapid mobiliser of long-term reconstituting HSC with promising clinical applications.