Reconstitution of lymphocyte subpopulations after paediatric bone marrow transplantation

We prospectively studied the reconstitution of lymphocyte subpopulations in a group of 22 children, who survived disease-free at least 6 months after allogeneic BMT for a haematological malignancy. Absolute counts of total lymphocytes, B lymphocytes, T lymphocytes, and CD4(+) helper T lymphocytes reached the 5th percentile (p(5)) Of age-matched reference values within 6 months after BMT in 15, 17, 7 and 2 patients, respectively. In particular, CD4(+) helper T lymphocyte reconstitution was very slow. Unexpectedly, CMV reactivation had a profound positive influence upon the number of CD4(+) helper T lymphocytes in the children. In five patients, absolute B lymphocyte counts above the 95th percentile were reached from 6 months after BMT onwards, mimicking normal ontogeny, Unlike normal ontogeny, the percentages of helper T lymphocytes expressing the 'naive' CD45RA isoform were low and those expressing the 'memory' CD45RO isoform were high in the first 3 months after BMT, as described before. Thereafter, the CD45RA:CD45RO ratio slowly normalised, Also, CD7 expression was absent on up to 90% of T lymphocytes in the first months after BMT, and on a steadily decreasing percentage thereafter, as recently described in adults. However, the absolute counts of CD45RO(+)/CD4(+) and CD7(-)/CD4(+) helper T lymphocytes did not change significantly. So, we found no evidence of peripheral expansion of previously primed donor-derived 'memory' T lymphocytes during the follow-up period which spanned 1-18 months after BMT, The absolute counts of 'naive' CD45RA(+) helper T lymphocytes did not show a faster increase after BMT than in adults, despite the presumed presence of a non-involuted thymus in children.