Susceptible MHC alleles, not background genes, select an autoimmune T cell reactivity.

To detect and characterize autoreactive T cells in diabetes-prone NOD mice, we have developed a multimeric MHC reagent with high affinity for the BDC-2.5 T cell receptor, which is reactive against a pancreatic autoantigen. A distinct population of T cells is detected in NOD mice that recognizes the same MHC/peptide target. These T cells are positively selected in the thymus at a surprisingly high frequency and exported to the periphery. They are activated specifically in the pancreatic LNs, demonstrating an autoimmune specificity that recapitulates that of the BDC-2.5 cell. These phenomena are also observed in mouse lines that share with NOD the H-2g7 MHC haplotype but carry diabetes-resistance background genes. Thus, a susceptible haplotype at the MHC seems to be the only element required for the selection and emergence of autoreactive T cells, without requiring other diabetogenic loci from the NOD genome.

Adaptation of TCR repertoires to self-peptides in regulatory and nonregulatory CD4+ T cells.

Currently, it is not understood how the specificity of the TCR guides CD4(+) T cells into the conventional lineage (Tconv) vs directing them to become regulatory (Treg) cells defined by the Foxp3 transcription factor. To address this question, we made use of the "Limited" (LTD) mouse, which has a restricted TCR repertoire with a fixed TCRbeta chain and a TCRalpha chain minilocus. The TCR repertoires of Tconv and Treg cells were equally broad, were distinct, yet overlapped significantly, representing a less strict partition than previously seen between CD4 and CD8 T cells. As a group, the CDR3alpha motifs showed a significant trend to higher positive charge in Treg than in Tconv cells. The Tconv and Treg repertoires were both reshaped between thymus and periphery. Reducing the array of peptides presented by MHC class II molecules by introducing the H2-DM(o/o) mutation into the LTD mouse led to parallel shifts in the repertoires of Tconv and Treg cells. In both cases, the CDR3alpha elements were entirely different and strikingly shortened, relative to normal LTD mice. These peculiar sequences conferred reactivity to wild-type MHC class II complexes and were excluded from the normal repertoire, even among Treg cells, indicating that some forms of self-reactivity are incompatible with selection into the Treg lineage. In conclusion, the Treg repertoire is broad, with distinct composition and characteristics, yet significantly overlapping and sharing structural constraints with the repertoire of conventional CD4(+) T cells.

Hepcidin antimicrobial peptide transgenic mice exhibit features of the anemia of inflammation.

The anemia of inflammation is an acquired disorder affecting patients with a variety of medical conditions, and it is characterized by changes in iron homeostasis and erythropoiesis. Mounting evidence suggests that hepcidin antimicrobial peptide plays a primary role in the pathogenesis of the anemia of inflammation. To evaluate which features of this anemia can be attributed to hepcidin, we have generated mice carrying a tetracycline-regulated hepcidin transgene. Expression of the hepcidin transgene resulted in down-regulation of endogenous hepcidin mRNA. The transgenic mice developed a mild-to-moderate anemia associated with iron deficiency and iron-restricted erythropoiesis. Similar to the anemia of inflammation, iron accumulated in tissue macrophages, whereas a relative paucity of iron was found in the liver. Circulating erythrocytes in transgenic animals had normal survival rates, but transgenic animals had an impaired response to erythropoietin. Thus, hepcidin transgenic mice recapitulate each of the key features of anemia of inflammation in human patients and serve as a useful model of this prevalent disorder.