Manipulation of haemopoietic stem cells for clinical use

Abstract

Normal haemopoiesis derives from a bone marrow pool of pluripotent stem cells capable of self renewal and multilineage differentiation. Further developments in autografting and gene therapy will rely on a clearer definition and understanding of these primitive cells and an ability to manipulate them effectively. Phenotypic and functional differences between bone marrow and peripheral blood stem cells have been identified and exploited. Techniques for the mobilisation and harvesting of stem cells are established in clinical practice but consensus is lacking on the optimum approach for different diseases. In malignancy, there is great interest in removing contaminating tumour cells from harvests in the hope of reducing relapse rates. Stem cell selection using the CD34 antigen has been commonly used for such purging strategies. The concentration of stem cells produced by CD34 selection provides an ideal product for further manipulation by cell expansion and /or genetic modification. These techniques should facilitate the future engineering of more specific therapeutic cell products.

I have examined three specific aspects of the manipulation of haemopoietic stem cells with the aim of generating clinically useful therapeutic products. These were: a disease -specific mobilisation regimen in CML, clinical scale CD34 selection and transplantation in myeloma and genetic modification of stem cells to alter their function using PNH as a model disease.

A disease -specific mobilisation procedure was studied in patients with CML in an attempt to collect mainly normal progenitors for subsequent transplantation. This in vivo purging strategy relies on the differential mobilisation of Ph +ve/Ph -ve stem cells following myelosuppression. The Hydroxyurea /G-CSF regimen used was markedly less toxic than existing methods and the results compared favourably, with 28% of the harvests entirely Ph -ve and 56% showing a major response. Cytogenetic responses have been demonstrated post transplant in some patients.

Clinical scale ex vivo tumour purging was performed in myeloma patients by CD34 selection using an immunoaffinity column (CeprateTM). This procedure can effect a 3 -4 log tumour reduction and also yields cells suitable for further laboratory manipulation. I wished to confirm that the method was clinically acceptable. All but one patient achieved an adequate CD34 +ve cell dose for transplant post selection and engraftment was normal, confirming the safety and efficacy of the approach. Genetic modification of peripheral blood stem cells was achieved using a retroviral construct (CD59 -TM) capable of ameliorating the complement sensitivity in FNn. This was used successfully to transduce CFU -GM and BFU-E in peripheral blood and stem cell harvests from patients with haematological malignancy. Transduction efficiency was maximised by multiple cycles of infection in the presence of 113, 116 and SCF. A technique was developed for the simultaneous assessment of cell complement lysis and surface phenotype by flow cytometry to detect the presence and function of the novel construct. In FMI, the most primitive peripheral blood stem cells are of normal phenotype which has prompted suggestions from some authors that they might be collected and used for autologous transplantation. I have shown however, that G-CSF mobilises mainly cells of PM phenotype in these patients. This, combined with the observation that the overall progenitor numbers in these hypoplastic patients are low suggests that the prospects for autografting or gene therapy are poor in this disease.