In the end, there can be only one … cord blood unit

In this issue of Blood, Lamers et al¹  used elegant methods to dissect how CD4⁺ T cells may contribute to the determination of the long-term predominant unit and the graft-versus-leukemia (GVL) effect after double umbilical cord blood transplantation (dUCBT).

The dUCBT platform was developed as a tool to test graft manipulations (eg, ex vivo expansion) and to allow larger patients with no available suitable single unit to proceed to a potentially curative allograft with this donor type. Some skeptically suggested that such a strategy would result in cross-rejection between the 2 donors and higher risk of graft failure. However, early studies demonstrated the safety and efficacy of dUCBT as measured by improved time to neutrophil recovery and lower risk of graft failure as compared with low-cell-dose single umbilical cord blood (UCB) grafts.²  These studies also showed that, despite an early period of dual donor chimerism (presence of both donor units), most patients had hematopoiesis derived from only a single UCB unit by day 100 posttransplantation as determined by chimerism assays.³  However, the question of how the 2 UCB units interacted and contributed to outcomes remained open. More recently, it has been shown that a UCB unit with a higher CD3⁺ cell content⁴  or UCB units containing interferon-γ–secreting CD8⁺ T cells in response to stimulation from the nonengrafting (NE) unit were more likely to become the long-term predominant unit.

Lamers et al pursued a different path of investigation, building on in their observation that there was an increase in circulating CD4⁺ T cells early after dUCBT and that CD4⁺ chimerism was associated with a higher likelihood of identifying the long-term predominant unit.⁵  In their current research, they enhanced the analysis of the small number of circulating CD4⁺ T cells early after dUCBT by propagating these cells in vitro. Briefly, peripheral blood mononuclear cells from patients with single-unit chimerism were cocultured with HeLa cells transduced to express HLA class II molecules similar to those expressed by the NE unit. The cellular products resulting from the propagation and enriched for CD4⁺ T cells were tested for alloreactivity against HLA class II antigens by coculture with HeLa cells expressing class II of the NE unit and measurement of activation and effector markers by flow cytometry. While most samples reacted to HLA class II of the NE unit, greater alloreactivity was observed in response to the NE unit HLA-DR and HLA-DQ molecules than HLA-DP molecules.

One of the many controversies surrounding dUCBT is whether it provides a more potent GVL effect.⁶  While recent reports suggest that the low relapse rate observed after dUCBT results from greater HLA mismatch than is typical in adults receiving this graft type,⁷  early speculations suggested that alloreactivity between the 2 units could also enhance GVL potency. The study by Lamers et al provides evidence that, at least in some cases, alloreactivity between the two units enhances GVL. In a subset where the patient and the NE unit shared HLA class II alleles but were mismatched to the predominant unit, they then cocultured the product from 5 T-cell cultures that were alloreactive to HLA-DR and HLA-DQ with 6 samples of primary leukemia cells from these patients. All but one of the primary leukemia cells were recognized by the propagated UCB donor CD4⁺ T cells.

In summary, the report by Lamers et al provides another building block for understanding the biology of graft-versus-graft interactions and further supports the importance of T cells in establishing graft predominance. This report also provides new insights on a potential mechanism of enhanced GVL after dUCBT.