Pauciclonal B cell involvement in production of immunoglobulin in scid Ig+ mice

V(D)J recombination in peritoneal B cells of leaky scid mice

Developing lymphocytes in immune-deficient severe combined immunodeficient (scid) mice express a defective recombinase activity and rarely succeed in making an antigen receptor; those cells that do succeed account for the known B and T cell leakiness in this mutant mouse strain. To gain more insight into the nature of the scid defect, we assessed the status of heavy (H) and light (L)k, chain genes in immunoglobulin (Ig)Mk-secreting B cells from the peritoneal cavity of old leaky scid mice, the only lymphoid site where scid B cells have been routinely detected. We found these cells to be unusual in that their nonexpressed H chain alleles were either abnormally rearranged or in germline configuration (wild-type B cells generally show normal rearrangements at both H chain alleles). The VDJH junctions of the expressed alleles showed little or no nontemplated (N) addition, similar to neonatal B cells from wild-type mice. About half of the V(D)J junctions lacking N additions contained nucleotides that could have been encoded by either of the participating coding elements (VDH, DJH, or VJk), indicating that the recombination occurred between short stretches of homology. Unusually long templated (P) additions were seen in both VDJH and VJK junctions, and many recombinations appeared to involve P-based homologies. These findings suggest that: (a) B cell leakiness results from a low frequency of coding joint formation in cells expressing the defective scid recombinase activity; (b) joining of scid coding ends is facilitated when the ends contain short stretches of sequence homology, where in many cases, one of the homologous sequences results from a P addition; and (c) scid peritoneal B cells may arise early in ontogeny.

High frequency of normal DJH joints in B cell progenitors in severe combined immunodeficiency mice

The severe combined immunodeficiency (scid) mouse has a defective V(D)J recombinase activity that results in arrested lymphoid development at the pro-B cell stage in the B lineage. The defect is not absolute and scid mice do attempt gene rearrangement. Indeed, approximately 15% of all scid mice develop detectable levels of oligoclonal serum immunoglobulin and T cell activity. To gain more insight into the scid defect and its effect on V(D)J rearrangement, we analyzed DJH recombination in scid bone marrow. We determined that DJH structures are present in scid bone marrow and occur at a frequency only 10-100 times less than C.B-17+/+. The scid DJH repertoire is limited and resembles fetal liver DJH junctions, with few N insertions and predominant usage of reading frame 1. Moreover, 70% of the DJH structures were potentially productive, indicating that normal V(D)J recombinants should be arising continually.

CD3+ T cells in severe combined immune deficiency (scid) mice. I. Transferred purified CD4+ T cells, but not CD8+ T cells are engrafted in the spleen of congenic scid mice

Young (less than 3 months of age) and old (greater than 1 year of age) C.B-17 scid/scid mice were tested for the presence of immunoglobulin in serum and CD3+ T cells in spleen and peritoneal cavity. In all old severe combined immune deficiency (scid) mice tested we found detectable, but very variable levels of serum immunoglobulin as well as splenic and peritoneal CD3+ T cells comprising 3% to 10% of the nonfractionated cell populations of these organs (n = 10). In contrast, none of the analyzed young scid mice showed any evidence of peripheral lymphocytes. Low numbers (2 x 10(5) to 5 x 10(5) cells/mouse) of highly purified CD4+ cells from congenic C.B-17 or BALB/c donor mice were injected intravenously into young scid recipient mice. A CD4+ T cell population was clearly engrafted when transplanted scid mice were analyzed 8 to 13 weeks after T cell transfer: (a) a CD3+CD4+CD8- T cell population was detectable in the spleens of all recipient scid mice by flow microfluorometry analyses; (b) CD3+CD4+CD8 T cell lines could be grown out of these spleens in vitro; (c) the histological examination revealed evidence of lymphoid cell repopulation in the spleens of all transplanted scid mice and (d) transplanted CD4+ T cell populations could be serially transferred into secondary and tertiary recipient scid mice. These data indicate that scid mice can be constructed in which only the CD4+ T cell compartment is selectively reconstituted. In contrast to the successful engraftment of CD4+ T cell, highly purified congenic CD8+ T cells could not be engrafted into the spleen of scid mice.