Abstract We report that hematopoietic progenitor cells expressing the CD34 antigen (CD34+ cells) transiently circulate in the peripheral blood (PB) of cancer patients treated with 7 g/m2 cyclophosphamide (HD-CTX) with or without recombinant human granulocyte macrophage-colony stimulating factor (rHuGM-CSF). In adult humans, CD34+ cells represent a minor fraction (1% to 4%) of bone marrow (BM) cells, comprising virtually all hematopoietic colony-forming progenitors in vitro and probably also stem cells capable of restoring hematopoiesis of lethally irradiated hosts. We show that CD34+ cell circulation is fivefold enhanced by rHuGM-CSF 5.5 protein micrograms/kg/day by continuous intravenous infusion for 14 days after HD-CTX. During the third week after HD-CTX (ie, when CD34+ cells peak in the circulation), large- scale collection of PB leukocytes by three to four continuous-flow leukaphereses allows the yield of 2.19 to 2.73 x 10(9) or 0.45 to 0.56 x 10(9) CD34+ cells depending on whether or not patients receive rHuGM- CSF. The number of CD34+ cells retrieved from the circulation by leukaphereses exceeds the number that can be harvested by multiple BM aspirations under general anesthesia. Thus, after therapy with HD-CTX and rHuGM-CSF, PB represents a rich source of hematopoietic progenitors possibly usable for restoring hematopoiesis after myeloablative chemoradiotherapy. To determine whether CD34+ cells found in the PB are equivalent to their marrow counterpart, we evaluated their in vitro growth characteristics and immunological phenotype by colony assays and dual-color immunofluorescence, respectively. We show that PB CD34+ cells possess qualitatively normal hematopoietic colony growth and high cloning efficiency comparable to that observed with BM CD34+ cells. In addition, PB CD34+ cells display heterogeneous surface membrane differentiation antigens analogous to BM CD34+ cells. The availability of large quantities of CD34+ cells by leukapheresis is relevant to the field of stem cell transplantation and possibly to genetic manipulations of the hematopoietic system in humans.

Siena, S., Bregni, M., Brando, B., Ravagnani, F., Bonadonna, G., & Gianni, A. (1989). Circulation of CD34+ hematopoietic stem cells in the peripheral blood of high-dose cyclophosphamide-treated patients: enhancement by intravenous recombinant human granulocyte-macrophage colony-stimulating factor. Blood, 74(6), 1905–1914.