Early Multilineage Chimerism Predicts The ‘Winning’ Unit In Double Cord Blood Transplantation

Umbilical cord blood (UCB) is an established alternative haemopoietic cell source for use in clinical transplantation for life-threatening malignant and non-malignant disorders. One challenge in using UCB in adolescents and adults has been delayed engraftment due to the finite and limited cell dose of a single unit, which has been shown to be a key determinant of engraftment, transplant related mortality and survival. The use of double unit transplantation, pioneered at the University of Minnesota, has become one of the most successful approaches to overcome this limitation to date. This group have also developed a reduced intensity conditioning (RIC) regimen, successfully broadening access to UCB transplant to older patients and those with co-morbidities. The kinetics of granulocyte, T cell and B cell chimerism in this setting require detailed study.

Since 2009, the British Society of Blood and Marrow Transplantation have conducted a prospective, phase II study of UCB transplantation using the Minnesota RIC conditioning regimen (Fludarabine 200mg/m2, Cyclophosphamide 50mg/kg and TBI 2Gy), with Ciclosporin and Mycophenolate Mofetil graft versus host disease prophylaxis. Lineage specific chimerism was performed at days 7, 14, 21, 28, 35, 60, 100, 180, 360 and 720 post transplant and analysed at laboratories associated with participating centres.

28 consecutive adult trial patients who have received a double unit transplant, have engrafted and have chimerism data up to at least day 35 are included in this analysis. The ‘winning' unit had a median unit:recipient match of 4/6 (range 4-6/6), with a median pre freeze total nucleated cell (TNC) count of 189x10⁷ (range 83-250) and CD34 of 84x10⁵ (range 23-169). The ‘losing' unit had a median unit:recipient match of 5/6 (range 4-6/6), with a median pre freeze total nucleated cell (TNC) count of 183x10⁷ (range 127-303) and CD34 of 55x10⁵ (range 42-95). Despite the low white count early post transplant, peripheral blood (PB) lineage specific chimerism for mononuclear cells (PBMC), T cells and granulocytes was feasible in nearly all patients. B cell chimerism was unsuccessful or not available in 55% of time points. The pattern of early T cell and granulocyte chimerism is summarised in the table.

From day 60 onwards, the median granulocyte and T cell chimerism remained 100% winning unit. However, T cell chimerism at day 14 identified the winning unit in all patients with a result at this time point (n=25). Contribution to the B cell compartment was 100% winning unit by day 35, with 83% recipient at day 7 and 24% at day 14 and 4% losing unit at day 7 and 14.

Lineage specific chimerism is technically feasible in the immediate post transplant period and gives important insights into the kinetics of double cord blood unit engraftment. Although the ‘losing' unit may contribute to the B and T cell compartments in the first 2 weeks, it contributes little after day 21. The winning unit is clearly identifiable by day 14 in all lineages. Early granulocyte recovery (driven by G-CSF) in the RIC setting is seen to be primarily due to autologous recovery until around day 21 after which the winning unit predominates. These data provide an insight into the biology of engraftment that may also inform additional novel strategies such as ex vivo CD34 expansion and adding in haploidentical stem cells.

No relevant conflicts of interest to declare.