In Vivo Lentiviral Marking Demonstrates Long-Term Myeloid and Lymphoid Lineage Contribution of Individual Hematopoietic Stem Cell Clones to Murine Steady-State Hematopoiesis.

Abstract 813

Most of the knowledge to date on the in vivo blood forming activity of individual hematopoietic stem and progenitor cells was gained in transplantation experiments of defined cell populations into syngeneic or xenogeneic murine hosts. Consequently, stem and progenitor cells are solely defined by their role in post-transplant reconstitution and very little is known on their clonal activity in steady-state hematopoiesis. To gain new insights into the clonal activity of stem and progenitor cells under steady-state conditions we used a genetic in vivo lentiviral marking strategy and subsequently monitored the clonal activity of marked hematopoietic cells for up to one year by highly sensitive integration site amplification using LAM-PCR. Highly concentrated GFP-expressing lentiviral vectors (LV) were injected intravenously (IV, n=10) or intrafemorally (IF, n=15) into GFP-tolerant B6.Cg-Tg (Krt1-15-EGFP) 2Cot/J (Krt15) mice to directly mark hematopoietic stem and progenitor cells. 5 mice from each of the two cohorts were treated with 5-Fluorouracil (5-FU, 150 mg/kg) to mobilize hematopoietic stem cells prior to LV-marking. The clonality of the transduced myelopoiesis and lymphopoiesis was analyzed by LAM-PCR. A small proportion of all peripheral blood cells in LV-injected mice consistently expressed GFP for up to one year (5-100 GFP⁺ cells per 20000 PB cells analyzed). Pre-treatment with 5-FU did not affect the percentage or lineage distribution of marked blood cells even when the vector was injected intravenously. Even though the initial percentage of marked cells was similar after IV and IF vector injection (p>0.05) the marking kinetics were different. Whereas the percentage of GFP expressing cells in PB of IF-marked mice remained stable over the whole observation period for up to 1 year, a 2-fold decline of the percentage of marked cells was detected two weeks after IV-marking indicating that predominantly short-lived more mature cells were transduced after IV vector injection. LAM-PCR analyses of sorted cell lineages showed that multiple clones contributed to the marked myeloid and lymphoid long-term hematopoiesis after IF-injection. In summary, our data demonstrate stable marking of steady-state hematopoiesis for up to one year. Our results demonstrate that remarkably stable stem cell clones with myeloid and lymphoid differentiation potential contribute to murine steady-state long-term hematopoiesis. In vivo marking will further allow to directly address the response of individual stem cell clones to hematopoietic stress including chemotherapy.