B lymphocyte-restricted expression of prion protein does not enable prion replication in prion protein knockout mice.

Prion replication in spleen and neuroinvasion after i.p. inoculation of mice is impaired in forms of immunodeficiency where mature B lymphocytes are lacking. In spleens of wild-type mice, infectivity is associated with B and T lymphocytes and stroma but not with circulating lymphocytes. We generated transgenic prion protein knockout mice overexpressing prion protein in B lymphocytes and found that they failed to accumulate prions in spleen after i.p. inoculation. We conclude that splenic B lymphocytes are not prion-replication competent and that they acquire prions from other cells, most likely follicular dendritic cells with which they closely associate and whose maturation depends on them.

Cytokines: IL-21 joins the gamma(c)-dependent network?

The discovery of the cytokine IL-21 adds another member to the ever-growing list of small secreted molecules that have potent effects on lymphoid cells. Initial characterization of the IL-21 receptor complex suggests that IL-21 may belong to the cytokine family whose receptors share the common gamma chain, gamma(c).

Common cytokine receptor gamma chain (gammac)-deficient B cells persist in T cell-deficient gammac-mice and respond to a T-independent antigen.

Defects in the common cytokine receptor gamma chain (gammac) in man result in X-linked severe combined immunodeficiency disease (SCIDX1) characterized by an absence of alphabeta T cells, gammadelta T cells and NK cells, with the presence of circulating B cells. Mice made deficient for gammac lack gammadelta T cells and NK cells, but in contrast to SCIDX1 patients have appreciable numbers of alphabeta T cells, while B cells are reduced about tenfold in numbers and disappear with age. Here we show that when gammac- mice are rendered T cell deficient, B cell numbers are still reduced but the age-dependent loss of B cells does not occur. The peripheral B cells which persisted in gammac-/ nude and gammac-/TCRbeta-/- mice were able to respond to mitogen stimulation in vitro and to mount antigen-specific T-independent Ig responses in vivo. These results demonstrate that gammac- B cells are functionally competent and suggest that residual alphabeta T cells are implicated in the B cell loss in gammac mice. The gammac-/nude and gammac-/TCRbeta-/- mice provide new models to dissect the role of gammac-dependent receptors during murine B cell differentiation.

Bypass of lethality with mosaic mice generated by Cre-loxP-mediated recombination.

BACKGROUND: The analysis of gene function based on the generation of mutant mice by homologous recombination in embryonic stem cells is limited if gene disruption results in embryonic lethality. Mosaic mice, which contain a certain proportion of mutant cells in all organs, allow lethality to be circumvented and the potential of mutant cells to contribute to different cell lineages to be analyzed. To generate mosaic animals, we used the bacteriophage P1-derived Cre-loxP recombination system, which allows gene alteration by Cre-mediated deletion of loxP-flanked gene segments. RESULTS: We generated nestin-cre transgenic mouse lines, which expressed the Cre recombinase under the control of the rat nestin promoter and its second intron enhancer. In crosses to animals carrying a loxP-flanked target gene, partial deletion of the loxP-flanked allele occurred before day 10.5 post coitum and was detectable in all adult organs examined, including germ-line cells. Using this approach, we generated mosaic mice containing cells deficient in the gamma-chain of the interleukin-2 receptor (IL-2R gamma); in these animals, the IL-2R gamma-deficient cells were underrepresented in the thymus and spleen. Because mice deficient in DNA polymerase beta die perinatally, we studied the effects of DNA polymerase beta deficiency in mosaic animals. We found that some of the mosaic polymerase beta-deficient animals were viable, but were often reduced in size and weight. The fraction of DNA polymerase beta-deficient cells in mosaic embryos decreased during embryonic development, presumably because wild-type cells had a competitive advantage. CONCLUSIONS: The nestin-cre transgenic mice can be used to generate mosaic animals in which target genes are mutated by Cre-mediated recombination of loxP-flanked target genes. By using mosaic animals, embryonic lethality can be bypassed and cell lineages for whose development a given target gene is critical can be identified. In the case of DNA polymerase beta, deficient cells are already selected against during embryonic development, demonstrating the general importance of this protein in multiple cell types.