The biological and clinical behaviors of hematological malignancies can be influenced by the active crosstalk with an altered bone marrow (BM) microenvironment. In the present study, we provide a detailed picture of the BM vasculature in acute myeloid leukemia using intravital two-photon microscopy. We found several abnormalities in the vascular architecture and function in patient-derived xenografts (PDX), such as vascular leakiness and increased hypoxia. Transcriptomic analysis in endothelial cells identified nitric oxide (NO) as major mediator of this phenotype in PDX and in patient-derived biopsies. Moreover, induction chemotherapy failing to restore normal vasculature was associated with a poor prognosis. Inhibition of NO production reduced vascular permeability, preserved normal hematopoietic stem cell function, and improved treatment response in PDX.

Here, leukaemia cells are followed by intravital microscopy as they infiltrate mouse bone marrow and respond to chemotherapy, revealing that at all stages analysed they are highly motile and do not display any associations with particular bone marrow sub-compartments. Cancer cells are thought to rely on tight interactions with their micro-environment for survival. Using intravital microscopy, Cristina Lo Celso and colleagues have followed leukaemia cells isolated from a human T-cell acute lymphoblastic leukaemia (T-ALL) in a mouse model. The cells were implanted in mouse bone marrow. Surprisingly, they observe that leukaemia cells are highly motile and do not display any association with a particular sub-compartment of the bone marrow microenvironment. The leukaemia cells caused a disruption of the bone marrow itself, however, resulting in a loss of osteoblasts. It is widely accepted that complex interactions between cancer cells and their surrounding microenvironment contribute to disease development, chemo-resistance and disease relapse. In light of this observed interdependency, novel therapeutic interventions that target specific cancer stroma cell lineages and their interactions are being sought. Here we studied a mouse model of human T-cell acute lymphoblastic leukaemia (T-ALL) and used intravital microscopy to monitor the progression of disease within the bone marrow at both the tissue-wide and single-cell level over time, from bone marrow seeding to development/selection of chemo-resistance. We observed highly dynamic cellular interactions and promiscuous distribution of leukaemia cells that migrated across the bone marrow, without showing any preferential association with bone marrow sub-compartments. Unexpectedly, this behaviour was maintained throughout disease development, from the earliest bone marrow seeding to response and resistance to chemotherapy. Our results reveal that T-ALL cells do not depend on specific bone marrow microenvironments for propagation of disease, nor for the selection of chemo-resistant clones, suggesting that a stochastic mechanism underlies these processes. Yet, although T-ALL infiltration and progression are independent of the stroma, accumulated disease burden leads to rapid, selective remodelling of the endosteal space, resulting in a complete loss of mature osteoblastic cells while perivascular cells are maintained. This outcome leads to a shift in the balance of endogenous bone marrow stroma, towards a composition associated with less efficient haematopoietic stem cell function1. This novel, dynamic analysis of T-ALL interactions with the bone marrow microenvironment in vivo, supported by evidence from human T-ALL samples, highlights that future therapeutic interventions should target the migration and promiscuous interactions of cancer cells with the surrounding microenvironment, rather than specific bone marrow stroma, to combat the invasion by and survival of chemo-resistant T-ALL cells.

Clonal hematopoiesis of indeterminate potential (CHIP) is an age-related condition associated with increased mortality among patients with cancer. CHIP mutations with high variant-allele frequencies can be detected in tumors, a phenomenon we term tumor-infiltrating clonal hematopoiesis (TI-CH). The frequency of TI-CH and its effect on tumor evolution are unclear. We characterized CHIP and TI-CH in 421 patients with early-stage non-small-cell lung cancer (NSCLC) from the TRACERx study and in 49,351 patients from the MSK-IMPACT pan-cancer cohort. We studied the association of TI-CH with survival and disease recurrence and evaluated the functional effect of TET2-mutant CHIP on the biologic features of lung tumors. Among patients with NSCLC, 42% of those with CHIP had TI-CH. TI-CH independently predicted an increased risk of death or recurrence, with an adjusted hazard ratio of 1.80 (95% confidence interval [CI], 1.23 to 2.63) as compared with the absence of CHIP and an adjusted hazard ratio of 1.62 (95% CI, 1.02 to 2.56) as compared with CHIP in the absence of TI-CH. Among patients with solid tumors, 26% of those with CHIP had TI-CH. TI-CH conferred a risk of death from any cause that was 1.17 times (95% CI, 1.06 to 1.29) as high as the risk with CHIP in the absence of TI-CH. TET2 mutations were the strongest genetic predictor of TI-CH; such mutations enhanced monocyte migration to lung tumor cells, fueled a myeloid-rich tumor microenvironment in mice, and resulted in the promotion of tumor organoid growth. TI-CH increased the risk of disease recurrence or death among patients with NSCLC and the risk of death from any cause among patients with solid tumors. TI-CH remodeled the tumor immune microenvironment and accelerated tumor organoid growth, findings that support a role for an aging-related hematologic clonal proliferation in cancer evolution. (Funded by the Royal Society and others.).

Xenotransplantation of patient-derived samples in mouse models has been instrumental in depicting the role of hematopoietic stem and progenitor cells in the establishment as well as progression of hematological malignancies. The foundations for this field of research have been based on the development of immunodeficient mouse models, which provide normal and malignant human hematopoietic cells with a supportive microenvironment. Immunosuppressed and genetically modified mice expressing human growth factors were key milestones in patient-derived xenograft (PDX) models, highlighting the importance of developing humanized microenvironments. The latest major improvement has been the use of human bone marrow (BM) niche–forming cells to generate human–mouse chimeric BM tissues in PDXs, which can shed light on the interactions between human stroma and hematopoietic cells. Here, we summarize the methods used for human hematopoietic cell xenotransplantation and their milestones and review the latest approaches in generating humanized BM tissues in mice to study human normal and malignant hematopoiesis.