We prospectively studied the chimaerism status in the bone marrow (BM) and peripheral blood (PB) of 23 patients receiving umbilical cord (UCB, 14 cases) or BM (nine cases) transplants from unrelated donors by PCR amplification of four individual-specific VNTR genetic loci. Haematological engraftment, with persistent full donor pattern, was observed in 10/14 (72%) patients receiving UCB and in 9/9 (100%) patients transplanted with marrow from an unrelated donor (MUD). In contrast, the remaining four patients converted to an autologous pattern. Three out of these four patients had an early autologous haematological reconstitution reaching a neutrophil level >0.5 x 109/l at days 27, 33 and 37 after transplant, respectively. In all three of these patients, chimaerism analysis demonstrated an early appearance of donor cells (i.e. within 35 d after UCB transplant) showing a transient full donor (one case) or mixed chimaerism condition (two cases). Despite the early autologous haemopoietic reconstitution, one of the three patients died of GVHD at day 60, which was explained by the demonstration of low levels of donor lymphoid cells. In the MUD group all nine patients converted to a persistent full donor pattern with haematological reconstitution, accompanied in two of them by transient mixed chimaerism lasting to days 60 and 270 after transplant. Our data show that monitoring of chimaerism may predict graft failure with or without early autologous haemopoietic reconstitution in patients receiving unrelated UCB transplants. Furthermore, chimaerism analysis may identify, in patients with autologous reconstitution, those at risk of severe GVHD in whom immunosuppressive therapy should not be discontinued.
Umbilical cord blood (UCB) has been proved a potential source of haemopoietic stem cells (HSC) for transplantation following a successful graft in a patient with Fanconi's anaemia (Broxmeyer et al, 1989; Gluckman et al, 1989) [2,9].Following these first reports the use of UCB rapidly moved from related to an unrelated setting, and to transplant patients with malignant and non-malignant haematological disease (Wagner et al, 1995, 1996; Kurtzberg et al, 1996; Gluckman et al, 1997) [28,29,12,10]. Due to the relative immaturity of UCB lymphocytes, the incidence and severity of GVHD is less when compared to bone marrow transplants, allowing a greater HLA disparity between donor and recipient (Cairo & Wagner, 1997) . Moreover, compared to adult HSC, those of UCB present distinctive proliferative advantages including higher number of colonies in culture, increased cell cycle rate, autocrine production of growth factor and longer telomeres (Mayani & Lansdorp, 1994; Vaziri et al, 1994) [15,27]. However, the limited number of nucleated cells collected in a UCB unit raises concern about rate and time to engraftment and restricts this therapeutic option mostly to paediatric patients. The assessment of chimaerism status of allografted patients provides a useful tool to investigate important clinical events such as engraftment, graft rejection and leukaemia relapse. To this purpose, several methods with variable sensitivity have been used to evaluate chimaerism. In particular, polymerase chain reaction (PCR) amplification of individual specific genetic loci, such as the variable number of tandem repeats (VNTR) and short tandem repeats (STR), is a sensitive technique identifying up to 1-0.1% and 0.1-0.01% of individual specific DNA, respectively (Uguzzoli et al, 1991; Lawler et al, 1991; Mackinnon et al, 1992; Rapanotti et al, 1997) [25,13,14,19].
We have prospectively monitored engraftment in 23 patients receiving UCB or BM from an unrelated donor (MUD) and found that in UCB transplant, graft failure with or without early autologous haemopoietic reconstitution, is predicted by early detection of a mixed chimaeric condition.