Mobilized peripheral blood CD34+ cells express more amphotropic retrovirus receptor than bone marrow CD34+ cells

Background and Objective. The increased susceptibility to gene transfer by amphotropic retroviral vectors of mobilized peripheral blood (PB) CD34⁺ cells compared to their bone marrow (BM) counterparts may depend, among other factors, on the level of expression of the amphotropic receptor on the progenitor cell. Using a previously described flow cytometry strategy, we have studied retrovirus binding to mobilized CD34⁺ cells, derived from cancer patients treated with high-dose chemotherapy and growth factor(s), that are efficiently transduced by N2 retro-virus vector. Design and Methods. We measured the binding of the retrovirus to the cells using a rat monoclonal antibody reactive with the gp70 envelope glycoprotein, common to all replication-defective amphotropic retroviruses. Antibody-virus-cell complexes were indirectly labeled and analyzed by flow cytometry. We compared the binding of PA317-N2 vector to CD34⁺ cells derived from steady-state BM, steady-state PB and mobilized PB from cancer patients treated with high-dose chemotherapy and cytokine. Results. The fluorescence intensity of mobilized CD34⁺ cells was approximately one log higher than that of steady-state BM or PB CD34⁺ cells, indicating that the expression of the amphotropic receptor was increased. Moreover, the virus binding was proportional to the gene transfer rate, as assessed by G418 resistance into mobilized PB-derived CFU- GM. The increase in fluorescence intensity appeared to be restricted to CD34⁺ cell subset, neither CD2⁺ nor CD14⁺ cells bound the virus in an appreciable amount. Interpretation and Conclusions. Virus binding, as assessed by indirect immunofluorescence assay, is increased in mobilized CD34⁺ cells. The increased binding may contribute to their high susceptibility to retrovirus vector infection.