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Cell lines and clones were established from PB76‐positive mouse fetal liver at day 13 and 14 of gestation, which proliferated with division times of a day in serum‐substituted cultures under the stimulatory influence of adherent stromal cells and the cytokine IL‐7 for periods longer than half a year. These lines expressed varying levels of the B lymphocyte lineage related markers PB76, B220, BP‐1, VpreB and lambda 5, but no surface Ig or MHC class II molecules. All clones expressed PB76, VpreB and lambda 5 in a high percentage of cells, while B220 and/or BP‐1 expression was low or undetectable in some. A cell line, and several clones established from it, all had kappa and lambda light chain genes in germ‐line configuration. Either one or both of their H‐chain‐gene containing chromosomes carried a DH to JH. These pre B cell lines and clones could be induced to VH to DH and VL to JL rearrangements. This resulted in the development of varying percentages of sIg‐positive surface, MHC class II negative, LPS‐reactive B cells within 2–3 days, in the absence of contacts with stromal cells and/or IL‐7. When injected into SCID mice, the cultured pre B cells populated the spleen of these mice to 5% with surface Ig‐, MHC class II‐positive LPS‐reactive cells for greater than 25 weeks. The long‐term in vitro proliferative capacity of these DH‐JH rearranged pre B cell clones makes them major candidates for committed stem cells of the B lineage.

The RAD52 epistasis group is required for recombinational repair of double-strand breaks (DSBs) and shows strong evolutionary conservation. In Saccharomyces cerevisiae, RAD52 is one of the key members in this pathway. Strains with mutations in this gene show strong hypersensitivity to DNA-damaging agents and defects in recombination. Inactivation of the mouse homologue of RAD52 in embryonic stem (ES) cells resulted in a reduced frequency of homologous recombination. Unlike the yeast Scrad52 mutant, MmRAD52 ؊/؊ ES cells were not hypersensitive to agents that induce DSBs. MmRAD52 null mutant mice showed no abnormalities in viability, fertility, and the immune system. These results show that, as in S. cerevisiae, MmRAD52 is involved in recombination, although the repair of DNA damage is not affected upon inactivation, indicating that MmRAD52 may be involved in certain types of DSB repair processes and not in others. The effect of inactivating MmRAD52 suggests the presence of genes functionally related to MmRAD52, which can partly compensate for the absence of MmRad52 protein.Double-strand breaks (DSBs) in the DNA of living organisms occur during several physiological processes including meiotic recombination, mating-type switching in yeast, and V(D)J rearrangement in developing B and T lymphocytes. Agents such as ionizing radiation and certain chemicals also lead to the induction of DSBs in the genome. If left unrepaired, DSBs result in broken chromosomes and cell death, as has been shown convincingly in yeast (5). Alternatively, incorrect repair of DSBs may generate deletions, chromosome rearrangements, and cell transformation and eventually lead to the formation of tumors.Two main pathways are known to be involved in the repair of DSBs in eukaryotes: end-to-end rejoining, a homology-independent but error-prone process, and error-free repair via (homologous) recombination. Repair of DSBs in the yeast Saccharomyces cerevisiae occurs predominantly via recombination, whereas a contribution of end-to-end rejoining can be observed only in a recombination-deficient background (9, 27, 47). Recombinational repair in S. cerevisiae involves the genes of the RAD52 epistasis group, of which nine members have been identified thus far (ScRAD50, ScRAD51, ScRAD52, ScRAD54, ScRAD55, ScRAD57, ScRAD59, ScMRE11, and ScXRS2) (2,11,15,16,44). Interestingly, it has been shown that ScRAD50, ScMRE11, and ScXRS2 are also involved in end-to-end rejoining (10,28,55). Mutations in genes of the RAD52 group result in an increased sensitivity to ionizing radiation and defects in one or more types of recombination. Among these mutants, the Scrad51, Scrad52, and Scrad54 mutants display the most severe radiation sensitivity and defects in recombination.Biochemical experiments with S. cerevisiae have shown that the ScRad51 protein forms nucleoprotein filaments with single-stranded DNA and promotes pairing and limited strand exchange (51). The ScRad52 protein alone or a heterodimer of ScRad55 and ScRad57 functions as a cofactor in this reaction, ...

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SummaryBone marrow of both normal and rearrangement-deficient mice contains a small population of B220(CD45R) + cells, which do not express the B lineage marker CD19. Instead, part of this population coexpresses the surface marker CD43 and lacks or expresses very low levels of heat stable antigen (HSA) and BP-1, thus representing a part of Hardy's fraction A (B220 +-CD43+HSA -, BP-1-) of B lineage development. However, some 20-40% of these B220 +-CD19-cells also coexpress the NKI.1 surface molecule and do not express genes like Vp~B or B29 restricted to the B cell lineage. These cells respond to recombinant interleukin 2 in vitro, and develop into killer cells that can lyse the prototypic NK target tumor cell, YAC-1, as well as syngeneic nomaal lipopolysaccharide or concanavalin A blasts, providing they lack the surface expression of major histocompatibility complex class I molecules. The implications of these findings for studies on B lymphopoiesis are discussed. It is suggested that the CD19-specific monoclonal antibody is more reliable, as in humans, than B220(CD45R) to detect B lineage cells in mice.

To study the influence of immunoglobulin heavy-chain (HC) and light-chain (LC) expression in promoting

A systemic graft-vs.-host reaction (GVHR) 1 induced in nonirradiated recipients can give rise to a variety of pathological symptoms. One of the possible outcomes is acute GVH disease (GVHD). After an initial brief phase of lymphoid stimulation (1-3), acute GVHD rapidly produces suppressive pathological symptoms, such as pancytopenia accompanied by aplastic anemia and hypogammaglobulinemia (1-5). A different possible consequence of the GVHR is stimulatory GVHD, also referred to as chronic GVHD. The symptoms of chronic GVHD include a persistent lymphoid hyperplasia (1-4, 6), hypergammaglobulinemia (2-4, 7, 8), and the formation of autoantibodies and pathological lesions reminiscent of systemic lupus erythematosus (SLE) and other types of vascular collagen disease (2, 3, 6, 8, 9). 2' 3 Both types of GVHD can be experimentally induced in nonirradiated F1 hybrid recipients by the injection of lymphocytes from one of the parental strains. A common requirement for the induction of both acute and chronic GVHD is the presence of T lymphocytes in the donor-cell inoculum (3-5, 8, 9). 2, 3 Which type of GVHD will develop after the injection of parental T cells appears to depend on the functional subset of donor T cells activated in the F1 host. Work from this laboratory has shown that the stimulatory type of GVHD can be induced by alloreactive donor T helper (TH) cells (2-5, 8, 9) carrying the Lyt-1 +2-phenotype, 3 whereas the induction of acute GVHD requires both alloreactive TH and T suppressor (Ts) cells (2-5).

Previous work from this laboratory has led to the hypothesis that the stimulatory pathological symptoms of chronic graft-vs.-host disease (GVHD) are caused by alloreactive donor T helper (TH) cells, whereas the suppressive pathological symptoms of acute GVHD are caused by alloreactive T suppressor (TS) cells of the donor. In the present paper we analyzed the Lyt phenotypes of B10 donor T cells required for the induction of either acute or chronic GVHD in H-2-different (B10 X DBA/2)F1 recipients. First, nonirradiated F1 mice were used as the recipients. We found that unseparated B10 T cells induced only a moderate formation of systemic lupus erythematosus (SLE)-like autoantibodies, but a high percentage of lethal GVHD (LGVHD). In contrast, Lyt-1+2- donor T cells were unable to induce LGVHD in these recipients; these cells were capable, however, of inducing a vigorous formation of SLE-like autoantibodies and the formation of severe immune-complex glomerulonephritis. Lyt-1-2+ T cells were incapable of inducing either acute or chronic GVHD. In another experiment, the sensitivity and accuracy of the GVH system were increased by using irradiated F1 mice as recipients and by comparing donor-cell inocula that contained similar numbers of T lymphocytes. In addition, donor-cell inocula were used that had been tested for their allohelper and allosuppressor effects on F1 B cells in vitro. In the irradiated F1 recipients, too, unseparated donor T cells were superior to T cell subsets in inducing LGVHD; Lyt-1-2+ donor cells were completely and Lyt-1+2- donor cells were almost incapable of doing so. In contrast, Lyt-1+2- T cells, but neither unseparated T cells nor Lyt-1-2+ T cells, were capable of inducing a vigorous formation of SLE-like auto-antibodies. We conclude that the stimulatory pathological symptoms of chronic GVHD are caused by Lyt-1+2- allohelper T cells. In contrast, the development of the suppressive pathological symptoms of acute GVHD appears to involve alloreactive Lyt-1+2+ T suppressor cells.