We have utilized molecular and imaging techniques to gain new insights into the behavior of hematopoietic stem and progenitor cells (HSPCs) in vivo. Utilizing lentiviral vectors carrying genes for 5 distinct fluorescent proteins (FPs) termed LEGO vectors, we have utilized a combinatorial color approach to be able to uniquely mark and then track output from individual HSPCs in time and space in vivo. We have generated significant interest in this approach and were invited to publish a video journal article demonstrating the approach technically. We have continued active development and utilization of lentiviral barcoding with high-diversity 31-35bp genetic barcodes to study hematopoiesis in the non-human primate model. Our collaborator Rong Lu first devised this very powerful approach and applied it to study murine hematopoiesis. We have now transplanted 12 macaques with barcoded autologous CD34+ cells, and have been able to track hematopoietic output from thousands of individual HSPCs over time (up to 3.5 years) and in multiple lineages in a quantitative and highly reproducible manner, for the first time. We have already made a number of important and novel discoveries, including the lack of evidence for a common lymphoid progenitor producing T and B cells in primates, with no shared clonal derivation of B and T cells until late after transplant, and much earlier shared clonal derivation of myeloid and B cells. We have also for the first time discovered the unique lineage derivation of the major fraction of natural killer (NK) cells. CD16+CD56- cytotoxic NK cells did not share barcodes with B, T or myeloid cells even 24 months post-transplant. In vitro and murine models have not previously been able to shed light on NK cell lineage relationships. We have continued to use barcoded cells from these macaques to further dissect in vivo NK cell ontogeny, and the process of ex vivo expansion of NK cells, highly relevant for adoptive cell therapy development, in collaboration with Dr. Rick Childs' group. We have discussed a unique self-renewing population able to regenerate CD56+ NK cells that is present in a double negative population of peripheral blood cells. We continue to search for the precursor to the ontologically-unique CD16+ mature NK subpopulation, tracking dominant clones in phenotypically purified samples from blood, bone marrow, lymph nodes, and in the future liver, vaginal and intestinal lymphoid aggregates. We have evidence that the clonally-distinct NK cells have an adaptive phenotype, and may have characteristics of memory NK cells that are of great current interest in the NK field, however their clonal characteristics and derivation have not previously been able to be investigated in humans, given the lack of clonal markers. We have extended our analysis of the geographic segregation of individual HSPCs long term in specific marrow sites, confirming the findings described above using LEGO imaging techniques in the macaque model utilizing barcoding. We can directly demonstrate in situ production of B cells, CD56+ NK cells and myeloid cells in localized marrow niches, and surprisingly, we now have strong evidence for in situ marrow production of T cells. We have recently extended the barcoding model in a number of new directions, including: 1) Comparison of the clonal behavior of young versus aged HSPC, with preliminary data from transplants of two aged macaques with barcoded cells demonstrating a very different kinetic and clonal pattern compared to young animals. 2) Analysis of novel methodologies for stem cell expansion, with quantitative and lineage analytics performed on expanded versus non-expanded cells in vivo. 3) Investigation of the clonal ontogeny of erythroid and platelet lineages. 4) Collaboration with Rahul Sajita at NYU to apply single cell RNAseq to barcoded populations in order to further define ontogeny as well as identify rare precursor cell populations. We are completing an investigation of the relationship between normal HSPCs and leukemia engrafting cells using competitive repopulation in the murine model, asking whether co-infusion of increasing doses of HPSCs can compete directly with leukemic cells for marrow niches, thus slowing leukemic progression. We have data indicating competition for the same niches, with confocal imaging results also backing up these functional findings.
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