Shaping of the autoreactive regulatory T cell repertoire by thymic cortical positive selection.

The main function of regulatory T lymphocytes is to keep autoimmune responses at bay. Accordingly, it has been firmly established that the repertoire of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) is enriched in autospecific cells. Differences in thymic-positive and/or -negative selection may account for selection of the qualitatively distinct regulatory and conventional T cell (Tconv) repertoires. It has previously been shown that precursors for Tregs are less sensitive to negative selection than Tconv precursors. Studies with TCR/ligand doubly transgenic mice suggested that an agonist ligand might induce positive selection of Treg (but not Tconv) cells. However, massive deletion of Tconv (but not Treg) cell precursors observed in these mice renders interpretation of such data problematic and a potential role for positive selection in generation of the autospecific Treg repertoire has remained therefore incompletely understood. To study this important unresolved issue and circumvent use of TCR/ligand-transgenic mice, we have developed transgenic mice expressing a single MHC class II/peptide ligand on positively selecting thymic cortical epithelial cells. We found that functional Treg (but not Tconv) cells specific for the single ligand were preferentially selected from the naturally diverse repertoire of immature precursors. Our data therefore demonstrate that thymic cortical positive selection of regulatory and Tconv precursors is governed by distinct rules and that it plays an important role in shaping the autoreactive Treg repertoire.

The rat Toxo1 locus directs toxoplasmosis outcome and controls parasite proliferation and spreading by macrophage-dependent mechanisms.

Toxoplasmosis is a healthcare problem in pregnant women and immunocompromised patients. Like humans, rats usually develop a subclinical chronic infection. LEW rats exhibit total resistance to Toxoplasma gondii infection, which is expressed in a dominant mode. A genome-wide search carried out in a cohort of F(2) progeny of susceptible BN and resistant LEW rats led to identify on chromosome 10 a major locus of control, which we called Toxo1. Using reciprocal BN and LEW lines congenic for chromosome 10 genomic regions from the other strain, Toxo1 was found to govern the issue of T. gondii infection whatever the remaining genome. Analyzes of rats characterized by genomic recombination within Toxo1, reduced the interval down to a 1.7-cM region syntenic to human 17p13. In vitro studies showed that the Toxo1-mediated refractoriness to T. gondii infection is associated with the ability of the macrophage to impede the proliferation of the parasite within the parasitophorous vacuole. In contrast, proliferation was observed in fibroblasts whatever the genomic origin of Toxo1. Furthermore, ex vivo studies indicate that macrophage controls parasitic infection spreading by a Toxo1-mediated mechanism. This forward genetics approach should ultimately unravel a major pathway of innate resistance to toxoplasmosis and possibly to other apicomplexan parasitic diseases.

Studies of congenic lines in the Brown Norway rat model of Th2-mediated immunopathological disorders show that the aurothiopropanol sulfonate-induced immunological disorder (Aiid3) locus on chromosome 9 plays a major role compared to Aiid2 on chromosome 10.

Brown Norway (BN) rats treated with aurothiopropanol-sulfonate (Atps) constitute a model of Th2-mediated immunological disorders associated with elevated IgE responses and renal IgG deposits. Using F(2) offspring between Atps-susceptible BN and Atps-resistant Lewis rats, we had previously mapped three quantitative trait loci on chromosomes 9, 10, and 20 for which BN alleles increased susceptibility to Atps-induced immunological disorders (Aiid). In this study we have used congenic lines for the latter two quantitative trait loci, formerly called Atps2 and Atps3 and now named Aiid2 (chromosome 10) and Aiid3 (chromosome 9), for fine mapping and characterization of their impact on Atps-triggered reactions. In Aiid2 congenic lines, the gene(s) controlling part of the IgE response to Atps was mapped to an approximately 7-cM region, which includes the IL-4 cytokine gene cluster. Two congenic lines in which the introgressed segments shared only a portion of this 7-cM region, showed an intermediate IgE response, indicating the involvement of several genes within this region. Results from BN rats congenic for the Lewis Aiid3 locus, which we mapped to a 1.2-cM interval, showed a stronger effect of this region. In this congenic line, the Atps-triggered IgE response was 10-fold lower than in the BN parental strain, and glomerular IgG deposits were either absent or dramatically reduced. Further genetic and functional dissections of these loci should provide insights into pathways that lead to Th2-adverse reactions.