We describe here a technique to study the clonal contribution of primitive stem cells that account for long-term hematopoiesis in the same mouse over a 14-month period. Specifically, irradiated recipient female mice were transplanted with retrovirally marked male hematopoietic progenitors. Bone marrow was then collected repeatedly from local sites from the same mice throughout a 14-month period and injected into secondary irradiated recipients for analysis of donor retrovirally marked day-11 colony-forming unit-spleen (CFU-S-11). We have tracked the temporal in vivo fate of 194 individual CFU-S-derived cell clones in 38 mice reconstituted with such retrovirally marked bone marrow cells. Our data show that long-term hematopoiesis is maintained by a large number of simultaneously functioning small, shortlived (1 to 3 months) clones that usually grow locally with little or no dispersion between different regions of the hematopoietic system. Furthermore, the clones that disappeared were never detected again. The data suggest that normal hematopoiesis is supported by the sequential recruitment of marrow repopulating cells into a differentiation mode.

A portion of the human cellular homolog of v-rel, the transforming gene of the leukemogenic retrovirus reticuloendotheliosis virus, strain T, was used to survey RNAs from several mouse tissues, selected lymphocyte populations, and hematopoietic cell lines for c-rel expression. Relatively high levels of a high-molecular-weight transcript were observed in peripheral B and T cells, whereas lower levels were detectable in functionally immature thymocytes. These results suggested that, unlike c-myb and c-ets, the c-rel proto-oncogene plays a role in later stages of lymphocyte differentiation.REV-T, a highly leukemogenic virus of galliform birds, is believed to have arisen by recombination between a replication-competent type C retrovirus and a portion of a turkey cellular gene designated c-rel (30, 31). Evolutionary comparisons between cloned segments of the turkey and chicken c-rel homologs indicate that the rel locus resembles many other cellular proto-oncogenes in being both large and complex (6, 30). The nine exons from which v-rel was derived, for example, are dispersed over more than 20 kilobase (kb) pairs of DNA and range in size from -60 to over 500 base pairs (30). Very little is known about the normal expression of this cellular gene except that elevated levels of rel-related transcripts can be detected in avian hematopoietic tissues, suggesting a potential role for c-rel in the differentiation of lymphoid and myeloid cell lineages (6,14).Recently, we presented evidence that unique c-rel loci could also be identified in humans, mice, and cats, all of which provide potentially useful genetic model systems for studying the function of this proto-oncogene (3, 4). To date, however, no clear picture has emerged regarding c-rel transcription in any mammalian group, a problem based partly on the fact that the rel-specific probes used in previous studies were derived from the distantly related avian viral rel oncogene (19,23,25,28). In this study, we used a recently described cloned DNA fragment containing two exons of the human c-rel proto-oncogene (4) to screen RNAs from various postnatal mouse tissues, fractionated lymphocytes, and cell lines. We detected large (-7.5 kb) rel-homologous transcripts in thymic and splenic RNAs from 9-, 18-, and 28-day-old mice, whereas RNAs from kidney and liver tissues from the same mice were essentially negative in comparison. Analyses of RNA samples from fractionated lymphocytes showed high levels of rel transcripts in surface immunoglobulin-positive splenic B cells, followed closely by Lyt2-L3T4+ and Lyt2+ L3T4-splenic T lymphocytes. RNA samples from thymocyte populations obtained by differential agglutination with peanut lectin (PNA) were weakly positive, although the more mature, medullary-type PNA-fraction showed a slightly higher level of expression. We corroborated this observation by analyzing RNAs from thymocyte subsets obtained by positive selection and from * Corresponding author.various established murine lymphocyte cell lines. Judging from the elevated levels found i...

A portion of the human cellular homolog of v-rel, the transforming gene of the leukemogenic retrovirus reticuloendotheliosis virus, strain T, was used to survey RNAs from several mouse tissues, selected lymphocyte populations, and hematopoietic cell lines for c-rel expression. Relatively high levels of a high-molecular-weight transcript were observed in peripheral B and T cells, whereas lower levels were detectable in functionally immature thymocytes. These results suggested that, unlike c-myb and c-ets, the c-rel proto-oncogene plays a role in later stages of lymphocyte differentiation.