Definition of a critical T cell threshold for prevention of GVHD after HLAnon-identical PBPC transplantation in children

The aim of this study was to reduce the rate of graft failure after HLA non -identical stem cell transplantation by using G-CSF mobilized CD34(+) perip heral blood progenitor cells (PBPC), either in combination with bone marrow or as single grafts, To prevent GVHD, PBPC were highly purified, resulting in a 5 to 6 log T cell depletion. In additon to T cell depletion no furthe r GVHD prophylaxis was used, We transplanted 23 pediatric patients with lif e-threatening malignant or non-malignant hematological disorders, who had n o available matched donor. Engraftment was obtained in 18 of 21 evaluable p atients, Five patients developed acute GVHD of grade II and III, which beca me chronic in four cases and was fatal in four. The use of highly purified PBPC allowed the exact quantification of residual T cells in the grafts and a strict correlation between the residual T cell load and the development of GVHD was observed: patients with GVHD had received numbers of T cells be tween 8 and 20 x 10(4)/kg, whereas patients without GVHD were grafted with T cell numbers ranging from 0.7 to 6.0 x 10(4)/kg. We therefore clearly dem onstrate that a residual T cell content of <5 x 10(4)/kg is safe for preven tion of GVHD after HLA non-identical PBPC transplantation in children.