REGENERATION OF TdT+, PRE-B, AND B CELLS IN BONE MARROW AFTER ALLOGENEIC BONE MARROW TRANSPLANTATION1

In 15 children transplanted with allogeneic bone marrow for acute leukemia and in complete remission, regeneration of the early stages of the B cell system was studied. Bone marrow aspirates taken before and longitudinally after BMT were investigated for pre-B and B cells by immunofluorescence techniques; in some cases, TdT+ cells were also determined. Normal values were derived from bone marrow samples taken from 23 healthy individuals who served as bone marrow donors. In normal bone marrow, B cells outnumber pre-B cells and the latter are more numerous than TdT+ cells. Before BMT, the numbers of BM pre-B were outside the normal range in all cases; B cell numbers were abnormal in most of the 11 patients studied, probably due to the antileukemic remission induction/consolidation therapy. After BMT, two distinct patterns of regeneration of the B cell system were observed. In 9 patients, TdT+ cells were considerably increased early after BMT. This was followed by a rise in pre-B cells, with values well above the normal range, and resulting in ratios of TdT+:pre-B cells and of pre-B cells:B cells that were transiently greater than 1. In the other 6 patients, the regeneration of TdT+ cells varied and the reconstitution of the pre-B cells was more gradual than in the first group, with pre-B-to-B cell ratios less than 1 during the whole observation period. The only consistent difference between the patients of the two groups, possibly relevant to the regeneration of the B cell lineage, was the duration of corticosteroid therapy, which was much longer in the 6 patients with slow-pace reconstitution. The pace of regeneration of the B cell system in the bone marrow was correlated with the recovery of the humoral immunity, as indicated by a significant increase in specific antibody titers after the second vaccination with diphtheria-tetanus-poliomyelitis vaccine in 7 of 9 patients in the rapid-pace group, versus 2 of 6 patients in the slow-pace group.