Improving the outcome of cord blood transplantation: use of mobilized HSC and other cells from third party donors

We developed the strategy of umbilical cord blood transplants (UCBT) with co-infusion of a limited number of highly purified mobilized haematopoietic stem cells (MHSC) from a human leucocyte antigen (HLA) unrestricted third party donor (TPD). Short post-transplant periods of neutropenia were usually observed in adults with haematological neoplasms receiving UCBT with a relatively low cell content and 0–3 HLA mismatches after myeloablative conditioning. This resulted from an early and initially predominant engraftment of the TPD–MHSC. After a variable period of double complete TPD + UCB chimerism, final full UCB chimerism was achieved (cumulative incidence >90%) within 100 d. Early recovery of the circulating neutrophils resulting from the 'bridge transplant' of the TPD–MHSC reduced the incidence of serious neutropenia-related infections, also facilitating the use of drugs with myelosuppressive side effects to combat other infections. The observed incidence of graft- versus -host disease and relapses was low, with overall and disease-free survival curves comparable to those of HLA identical sibling transplants. Post-transplant recovery of natural killer cells occurred soon after the transplant and B cells recovered around 6 months, but T-cell recovery took more than 1 year. Available data show that T cell recovery derives from UCB–HSC through thymic differentiation and that cytomegalovirus (CMV)-specific lymphocytes develop following CMV reactivations.