As a cure for the life-threatening hemoglobinopathy of sickle cell disease (SCD), gene therapy has immense
potential, avoiding the burdens with allogeneic transplant of finding an appropriate donor and the risk of graft
versus host disease. Collecting an adequate number of hematopoietic stem cells (HSCs) for gene modification,
however, often requires at least 2 days and sometimes multiple cycles of leukapheresis, imposing a cost and
logistical burden. Chemotherapy conditioning prior to infusion of the modified HSCs is myeloablative with weeks
until neutrophil, lymphoid, and platelet recovery, imposing infectious and transfusion-related risks. These issues
are significant quality of life concerns in the United States, where about 100,000 people live with SCD, and are
likely to limit access to SCD gene therapy to highly experienced transplant centers. In lower and middle income
countries such as in sub-Saharan Africa, where about 236,000 babies are born each year with SCD, however,
these issues may be insurmountable barriers to SCD gene therapy implementation. This proposal aims to
address these impediments to equity of access to SCD gene therapy by evaluating a likely-to-be cost-effective
and easy method to increase HSC homing and engraftment. In the non-SCD autologous transplant setting, the
flow-through fraction after CD34+ HSC immunomagnetic selection is known to contain accessory/immune cells
associated with improved HSC homing and engraftment. We hypothesize that, in the SCD autologous transplant
setting, co-infusing with the CD34+ cells part of the CD34-negative fraction, which is normally discarded after
immunomagnetic CD34+ cell selection, will assist with HSC homing and engraftment. Using the immune and
accessory cells left over from CD34+ cell selection of SCD cord blood units and SCD plerixafor-mobilized
apheresis products, we will immunophenotype the accessory fraction and determine in vitro homing efficiency of
a product’s CD34+ cells incubated alone (control arm) and with (experimental arm) its flow through cells (Aim
1). We will also perform transplant studies in immunodeficient mice with CD34+ cells alone (control arm) and
with (experimental arm) its flow through cells to look for improvement in human cell engraftment (Aim 2).
Altogether, we believe that evaluating the effectiveness of adding back the CD34- flow through after CD34+ gene
modification may offer a cost-effective method for increasing equity of access to SCD gene therapy.
