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Mice homozygous for the scid mutation on chromosome 16 have a severe combined immune deficiency as a result of their inability to correctly rearrange their immunoglobulin and T-cell receptor genes. In scid mice, when precursors for B and T lymphocytes reach the stage of development requiring expression of these surface receptors, a defective recombinase system aberrantly cuts and rejoins the receptor gene segments greatly reducing the efficiency of producing functional receptors. As a result, most scid mice have no detectable B or T lymphocytes. We have demonstrated that the scid defect is not specific to lymphocyte development. Myeloid cells and fibroblasts from scid mice show a marked increase in sensitivity to ionizing radiation, indicating that the scid mutation leads to an inability to repair DNA damage induced by ionizing radiation as well as interfering with rearrangement of the immunoglobulin and T-cell receptor genes.

The murine intestinal epithelium contains a heterogeneous population of intraepithelial leukocytes (IEL) most of which are granulated, Thy-1-CD5-CD8+. In order to assess the lineage relationship of this subgroup of IEL to peripheral T cells, we examined IEL in mice with the severe combined immunodeficiency (scid/scid) mutation, which lack T and B cells in peripheral lymphoid tissues. Electron and light microscopy showed that the intestine from scid/scid mice had granulated IEL similar to IEL in normal C.B-17 mice. Flow cytometry of isolated IEL stained with monoclonal antibodies against Thy-1, CD3, CD4, CD5 and CD8 showed that scid/scid mice IEL contained cells with the Thy-1-CD4-CD5-CD8+ phenotype. Immunohistochemical staining of IEL in tissue sections with antibodies to Thy-1 and CD8 confirmed that the Thy-1-CD8+ cells were in the intestinal epithelium. These scid/scid IEL also lacked CD3 expression and mRNA for the V gamma 7 V region gene of the gamma T cell receptor. We conclude that scid/scid mice contain precursors for IEL that can differentiate into a granulated Thy-1-CD5-CD8+ IEL in the intestine. The absence of CD8+ peripheral T cells in these mice suggests that these IEL differ from classical T cells in their ability to differentiate and express CD8 and do not require T cell receptor expression for their localization to the intestine.

Consistent with an ordered immunoglobulin (Ig) gene assembly process during precursor (pre-) B cell differentiation, we find that most Abelson murine leukemia virus (A-MuLV)-transformed pre-B cells derived from scid (severe combined immune deficient) mice actively form aberrant rearrangements of their Ig heavy chain locus but do not rearrange endogenous kappa light chain variable region gene segments. However, we have identified several scid A-MuLV transformants that transcribe the germline Ig kappa light chain constant region and actively rearrange the kappa variable region gene locus. In one case progression to the stage of kappa light chain gene rearrangement did not require expression of Ig mu heavy chains; furthermore, this progression could not be efficiently induced following expression of mu heavy chains from an introduced vector. As observed in pre-B cell lines from normal mice, attempted V kappa-to-J kappa rearrangements in scid transformants occur by inversion at least as frequently as by deletion. The inverted rearrangements result in retention of both products of the recombination event in the chromosome, thus allowing their examination. scid kappa coding sequence joins are aberrant and analogous in structure to previously described scid heavy chain coding joins. In contrast, the recognition signals that flank involved coding segments frequently are joined precisely back-to-back in normal fashion. The scid VDJ recombinase defect therefore does not significantly impair recognition of, site-specific cutting at, or juxtaposition and appropriate ligation of signal sequences. Our finding that the scid defect prevents formation of correct coding but not signal joins distinguishes these events mechanistically.

Most Abelson murine leukemia virus (A-MuLV)-transformed cell lines derived from scid (severe combined immune deficient) mice actively rearrange their endogenous immunoglobulin (Ig) heavy (H), but not light (L) chain variable region genes. Such cell lines express germline VH segments and other RNA transcripts that are characteristically produced by early precursor (pre)-B lymphocytes, but do not express high levels of transcripts from the germline kappa (k) constant region (C kappa) locus. However, we have derived scid A-MuLV transformants that express germline C kappa transcripts and attempt kappa gene assembly. In one case kappa gene expression and rearrangement occurred in the absence of mu H chain expression, and in another was not induced efficiently by introduction of a mu-expression vector. Although the vast majority of scid H and L chain coding sequence joins are grossly aberrant, scid A-MuLV transformants can form normal coding joints at a very low frequency. In contrast, these cells form generally normal signal sequence joins at an approximately normal efficiency. Thus, these findings mechanistically distinguish coding and signal join formation. Subcloning analyses suggest that scid A-MuLV transformants that do not attempt chromosomal coding sequence joining may have a relative survival advantage, and therefore that these events may often result in unrepaired chromosomal breakage and cell death.

Mice homozygous for an autosomal recessive scid (severe combined immune deficiency) mutation on chromosome 16 exhibit a defect that specifically impairs lymphoid differentiation but not myelopoiesis. Consequently such mice are deficient in both humoral and cell-mediated immune functions. Despite their defect, scid mice survive under pathogen-free conditions and are fertile. The mutation does not impair the hematopoietic microenvironment necessary for lymphoid differentiation, since these mice can be cured with grafts of normal bone marrow (BM) or cells from long-term BM cultures (LTBMC); however, reconstitution requires sublethal (400 cGy) irradiation of recipients. Engraftment with cells from LTBMC gave near-normal levels of colony-forming B cells (CFU-B) in spleen and BM of the recipients by 6 weeks postgrafting. Since LTBMC are devoid of all mature B and pre-B cells but contain stem cells that restore lymphoid function in scid mice, we used a limiting-dilution assay to characterize and enumerate the number of stem cells in LTBMC capable of restoring lymphoid function. Curing was determined by the CFU-B-cell assay, since CFU-B are not detectable in normal scid mice. The results indicate that fewer cells from LTBMC than from fresh BM are required to obtain lymphoid reconstitution. As few as 10(3) LTBMC cells can repopulate significant B- and T-cell function in scid recipients. From these results we conclude that scid mice can be used as recipients to quantify lymphoid-restricted stem cells and that there is a functional separation of lymphoid- and myeloid-restricted stem cells in LTBMC with an enrichment for lymphoid-restricted stem cells in these cultures.

A human acute lymphoblastic leukemia (ALL) cell line that was transplanted into immune-deficient SCID mice proliferated in the hematopoietic tissues, invaded various organs, and led to the death of the mice. The distribution of leukemic cells in SCID mice was similar to the course of the disease in children. A-1 cells marked with a retrovirus vector showed clonal evolution after the transplant. SCID mice that were injected with bone marrow from three patients with non-T ALL had leukemic cells in their bone marrow and spleen. This in vivo model of human leukemia is an approach to understanding leukemic growth and progression and is a novel system for testing new treatment strategies.

Mice homozygous for an autosomal recessive mutation for the scid gene exhibit a defect that specifically impairs lymphoid differentiation but not myelopoiesis. Such mice can be cured of their lymphoid deficiency by grafts with normal bone marrow, although full reconstitution of lymphoid function is seldom obtained. Long-term bone marrow cultures (LTBMC) are devoid of all mature B and pre-B cells but contain lymphoid stem cells. We therefore reconstituted scid mice with LTBMC cells to study the kinetics of B lymphocyte reconstitution in normal and irradiated (4 Gy) scid recipients and in irradiated (9.5 Gy) co-isogenic C.B-17 mice. Detectable colony-forming B cells rapidly increased in the spleen and bone marrow of irradiated C.B-17 and irradiated scid recipients, reaching normal levels between 4 and 6 wk post-grafting. Unirradiated scid recipients showed limited reconstitution in spleen and very poor reconstitution in bone marrow. Unirradiated scid recipients also had relatively few surface Ig+ cells in spleen or bone marrow, whereas both groups of irradiated recipients had normal numbers between 4 and 6 wk post-reconstitution. Normal levels of cytotoxic T cell activity by 8 wk after reconstitution were observed only in the irradiated C.B-17 and irradiated scid recipients. Analysis of mice reconstituted with cells from LTBMC indicates that these cultures contain lymphoid stem cells with significant proliferative and self-renewal potential, and that full reconstitution of lymphoid function requires prior irradiation of the scid recipient.

scid mice lack detectable B and T lymphocytes; there are no typical pre-B cells as defined by c mu and surface markers in their bone marrow and their thymus contains only 1% of the normal number of cells. In these characters scid mice seem to lack lymphoid stem cells. However, some mice have detectable serum immunoglobulin and others develop thymomas; both observations indicate that the block in lymphoid development is not absolute. To determine whether scid mice have any B-cell precursors, we looked for pre-B cells by their ability to be transformed by Abelson murine leukemia virus (A-MuLV). Surprisingly, scid mice contain as many B-cell precursors transformable with A-MuLV as normal control mice. Cell-surface markers specific for pre-B and B cells were detected on the A-MuLV-transformed bone marrow cells of both scid and normal mice, indicating that the A-MuLV-transformed cells belong to the B lineage. Interestingly, the same surface markers were undetectable on nontransformed scid bone marrow cells. We conclude from these results that scid mice have normal numbers of early B-cell precursors but that their differentiation into functional B cells is severely impaired.

Mice with severe combined immunodeficiency syndrome (SCID) exhibit an impairment in both T and B cell maturation, whereas myelopoiesis remains unaffected. We report here that spleens from SCID mice have undergone phenotypic expansion of cells bearing the NK-2 and asialo GM1 markers (70-80%) characteristic of NK cells and this expansion is accompanied by a 3-4-fold enrichment in NK cytolytic activity over their normal C.B-17 littermates. Furthermore, the NK cells from SCID mice do not rearrange or express T cell receptor alpha or beta genes, or a third T cell rearranging gene, gamma. These findings suggest that (a) T cell receptors are not necessary for NK-mediated cytolysis, and (b) either NK cells constitute an entirely distinct lineage or NK cell function is acquired in pre-T cells prior to the expression of T cell receptor genes.

Scid mice are excellent recipients for studying the characteristics of stem cells. Sublethal irradiation not only enhances engraftment of stem cells but enables one to graft limiting numbers of cells without compromising the survival of the recipient. This enables one to estimate the frequency of stem cells by limiting dilution analysis. Compared to fresh bone marrow, LTBMC are slightly enriched for stem cells capable of reconstituting lymphoid function in scid recipients. The stem cells have self-renewal ability since bone marrow from cured primary scid recipients can cure secondary recipients. Our results indicate that lymphoid reconstitution following engraftment with LTBMC occurs from a lymphoid-restricted stem cell; similar restricted stem cells also exist in normal bone marrow.