The identity and lineage potential of the cells that initiate thymopoiesis remain controversial. The goal of these studies was to determine, at a clonal level, the immunophenotype and differentiation pathways of the earliest progenitors in human thymus. Although the majority of human CD34 ؉ lin ؊ thymocytes express high levels of CD7, closer analysis reveals that a continuum of CD7 expression exists, and 1% to 2% of progenitors are CD7 ؊ . CD34 ؉ lin ؊ thymocytes were fractionated by CD7 expression and tested for lineage potential in B-lymphoid, T-

Successful autologous hematopoietic stem cell (HSC) transplantation in childhood acute lymphoblastic leukemia (ALL) requires the ability to either selectively kill the leukemia cells or separate normal from leukemic HSC. Based on previous studies showing that more than 95% of childhood B-lineage ALL express CD38, this study evaluated whether normal CD34 ؉ CD38 ؊ progenitors from children with B-lineage ALL could be isolated by flow cytometry. CD34 ؉ cells from bone marrow samples from 10 children with B-lineage ALL were isolated at day 28 of treatment, when clinical remission had been attained. The CD34 ؉ progenitor cells were flow cytometrically sorted into CD34 ؉ CD38 ؉ and CD34 ؉ CD38 ؊ populations. The absolute numbers of CD34 ؉ CD38 ؊ cells that could be isolated ranged from 401 to 6245. The cells were then analyzed for the presence of clonotypic rearrangements of the T-cell receptor (TCR) V␦2-D␦3 locus. Only patients whose diagnostic marrow had an informative TCR V␦2-D␦3 rearrangement were included in this study. Detection thresholds were typically 10 ؊4 to 10 ؊5 leukemic cells in normal marrow. In 6 of 10 samples analyzed, the sorted CD34 ؉ CD38 ؊ cells had no detectable V␦2-D␦3 rearrangements. In 4 cases, the clonotypic leukemic V␦2-D␦3 rearrangement was detected in the CD34 ؉ CD38 ؊ population, indicating that the putative normal HSC population also contained leukemic cells. The data indicate that although most childhood ALL cells express CD34 and CD38, leukemic cells are also frequently present in the CD34 ؉ CD38 ؊ population. Therefore, strategies to isolate and transplant normal HSC from children with ALL will require a more stringent definition of the normal HSC than the CD34 ؉ CD38 ؊ phenotype. (Blood. 2001;97:3925-3930)

Mycobacterium marinum is a photochromogenic mycobacterium that is ubiquitous in the aquatic environment. In the general population, exposure to aquaria is the most common cause of M. marinum infection. Known as "swimmer's granuloma" or "fish tank granuloma," M. marinum is an occupational hazard for aquarium cleaners and fishermen. There are several reports in the literature of M. marinum infection in immunocompromised hosts, including those with solid organ transplants, but none in patients who have received stem cell transplants (SCTs). To our knowledge, this is a first report of disseminated M. marinum infection in an SCT recipient who continued to develop new skin lesions even after months of targeted therapy. The implications are that elderly patients who receive T-cell-depleted SCTs may be at prolonged risk for pathogens dependent on cellular immunity, and the presentation of illness with such pathogens may be more severe and widely disseminated than might otherwise be expected.

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