Objective

Haematopoiesis is the complex developmental process maintaining the turnover of all blood cell lineages, i.e. erythrocytes, immune (white) cells and platelets. It takes place primarily in the bone marrow, where it critically depends on the correct functioning of rare, quiescent haematopoietic stem cells (HSCs) and more numerous HSC-derived, highly proliferative and differentiating haematopoietic progenitor cells (HPCs). This is regulated by complex interactions between stem cells and the bone marrow microenvironment, and it was suggested to change in settings involving stress. Indeed, using intravital microscopy, we showed that HSCs injected in irradiated recipients steadily engage with endosteal niches. Moreover, if phenotypically identical HSCs are harvested from mice harbouring a natural infection, such as that caused by T.spiralis, they show a unique migratory behaviour and engage larger niches, yet remain in the proximity of osteoblastic cells. Based on these and other studies, it has been hypothesised that also malignant haematopoietic cells rely on interactions with specific bone marrow microenvironments to grow, and especially to develop chemoresistance. To investigate this further, we performed longitudinal imaging of mice injected with murine and human T acute lymphoblastic leukaemia (T-ALL) cells over the course of hours or days, including during response to multiple chemotherapy treatments. Strikingly, we observed a very different behaviour from that of HSCs, with T-ALL cells showing a highly migratory behaviour throughout all stages of disease, from initial bone marrow infiltration to response to chemotherapy. Moreover, at advanced stages of disease the bone marrow microenvironment is heavily remodelled, especially in ednosteal areas. This raises the question whether leukaemia-induced bone marrow remodelling may be the cause of the collapse of healthy haematopoiesis observed in leukaemia patients. To address this, we have used a combination of intravital microscopy, quantitative flow cytometry and mathematical modelling.