Loss of RT6 message and most circulating T cells after thymectomy of diabetes prone BB rats

The GIMAP Family Proteins: An Incomplete Puzzle

Overview: Long-term survival of T lymphocytes in quiescent state is essential to maintain their cell numbers in secondary lymphoid organs and in peripheral circulation. In the BioBreeding diabetes-prone strain of rats (BB-DP), loss of functional GIMAP5 (GTPase of the immune associated nucleotide binding protein 5) results in profound peripheral T lymphopenia. This discovery heralded the identification of a new family of proteins initially called Immune-associated nucleotide binding protein (IAN) family. In this review we will use ‘GIMAP’ to refer to this family of proteins. Recent studies suggest that GIMAP proteins may interact with each other and also be involved in the movement of the cellular cargo along the cytoskeletal network. Here we will summarize the current knowledge on the characteristics and functions of GIMAP family of proteins.

Keratinocyte growth factor induces expansion of murine peripheral CD4+Foxp3+ regulatory T cells and increases their thymic output

Keratinocyte growth factor (KGF) has been shown to reduce the incidence and severity of graft-versus-host disease by prevention of epithelial damage and by modulating alloreactivity. Since regulatory T cells (Treg) play a crucial role in immune modulation, we evaluated the effects of exogenous KGF on peripheral CD4(+)Foxp3(+) Treg and the generation of Treg in the thymus of normal mice. A 3-day course of KGF induced a rapid selective increase in the number of highly suppressive CD4(+)Foxp3(+) Treg. Blood Treg numbers remained elevated for >2 mo, but the frequency normalized after 2 wk due to a concomitant increase in CD4(+)Foxp3(-) T cells. Analysis of single joint TCR excision circles frequency and Ki-67 expression in peripheral blood Treg showed that the early selective increase of Treg was predominantly accounted for by peripheral expansion. Thymectomy before KGF administration did not affect the early selective increase of Treg but abrogated the late increase in CD4(+) T cell numbers, thereby showing its dependence on thymic output. Collectively, these results show that KGF induces an increase in blood CD4(+)Foxp3(+) Treg numbers via two independent mechanisms. First by selective peripheral expansion of Treg and thereafter by enhanced thymic output of newly developed Treg.

The diabetes-prone BB rat carries a frameshift mutation in Ian4, a positional candidate of Iddm1

Diabetes-prone (DP) BB rats spontaneously develop insulin-dependent diabetes resembling human type 1 diabetes. They also exhibit lifelong T-cell lymphopenia. Functional and genetic data support the hypothesis that the gene responsible for the lymphopenia, Lyp, is also a diabetes susceptibility gene, named Iddm1. We constructed a 550-kb P1-derived artificial chromosome contig of the region. Here, we present a corrected genetic map reducing the genetic interval to 0.2 cM and the physical interval to 150-290 kb. A total of 13 genes and six GenomeScan models are assigned to the homologous human DNA segment on HSA7q36.1, 8 of which belong to the family of immune-associated nucleotides (Ian genes). Two of these are orthologous to mouse Ian1 and -4, both excellent candidates for Iddm1. In normal rats, they are expressed in the thymus and T-cell regions of the spleen. In the thymus of lymphopenic rats, Ian1 exhibits wild-type expression patterns, whereas Ian4 expression is reduced. Mutational screening of their coding sequences revealed a frameshift mutation in Ian4 among lymphopenic rats. The mutation results in a truncated protein in which the COOH-terminal 215 amino acids-including the anchor localizing the protein to the outer mitochondrial membrane-are replaced by 19 other amino acids. We propose that Ian4 is identical to Iddm1.

A sequence-ready PAC contig of a 550-kb region on rat chromosome 4 including the diabetes susceptibility gene Lyp

The Lyp locus controls diabetes development in rats. The diabetogenic allele in diabetes-prone BB rats is responsible for T cell lymphopenia characterized by the absence of regulatory T cells. We present refined genetic and radiation hybrid maps of the Lyp region on rat chromosome 4, a single 800-kb rat yeast artificial chromosome and a rat P1-derived artificial chromosome (PAC) contig corresponding to approximately 550 kb, both encompassing the entire candidate region. The contig, consisting of 48 PACs, gives 3- to 12-fold coverage. Genetic, radiation hybrid, and physical data were all in agreement and supported the same marker order. Nine genes and ESTs were identified in the contig in addition to a rat EST from the University of Iowa rat EST database-all possible candidate genes for Lyp. Alignment of our rat PAC contig with sequenced human PAC/BAC contigs confirms the position within the region of 3 of the 10 candidates and identifies an additional 8 genes/ESTs as candidates. These data will facilitate identification of Lyp.

Selective expression of RT6 superfamily in human bronchial epithelial cells

RT6 proteins are glycosylphosphatidylinositol (GPI)-linked alloantigens that are localized to cytotoxic T lymphocytes and that have nicotinamide adenine dinucleotide glycohydrolase and adenosine diphosphate (ADP)-ribosyltransferase activities. In view of the importance of GPI-linked surface proteins in mediating interactions of cells with their milieu, and the varied functions of airway cells in inflammation, we undertook the present study to determine whether human homologues of the RT6 superfamily of ADP-ribosyltransferases (ART) are expressed in pulmonary epithelial cells. We hypothesized that these surface proteins or related family members may be present in cells that interact with inflammatory cells, and that they may thereby be involved in intercellular signaling. Using in situ analysis and Northern blot analysis, we identified ART1 messenger RNA (mRNA) in airway epithelial cells. As expected for GPI-anchored proteins, the localization of ART1 at the apical surface of ciliated epithelial cells was demonstrated by staining with polyclonal anti-ART1 antibody, and was confirmed by loss of this immunoreactivity after treatment with phosphatidylinositol-specific phospholipase C (PI-PLC), which selectively cleaves GPI anchors and releases proteins from the plasma membrane. Using in situ hybridization with specific ART3 and ART4 oligonucleotides, we also identified two additional members of the RT6 superfamily in epithelial cells. In accord with these findings, we identified ART3 and ART4 mRNAs through reverse transcription- polymerase chain reaction of polyadenine-positive RNA from human trachea. Interestingly, these proteins appeared to be preferentially localized to the airway epithelium. The localized expression of these members of the RT6 superfamily in human pulmonary epithelial cells may reflect a role for them in cell-cell signaling during immune responses within the airway.

A diabetogenic gene prevents T cells from receiving costimulatory signals

T cell fate following antigen encounter is determined by several intracellular signals generated by the interaction of the T cell with an antigen-presenting cell. In the periphery activation requires T cell receptor signaling (signal one) in combination with costimulatory signals (signal two), usually provided through the cognate interaction of CD28 and B7 molecules. Provision of signal one alone to purified murine peripheral T cells in vitro induces apoptosis or anergy rather than promoting activation. These T cells can be rescued from apoptosis if they are provided with costimulation supplied, for example, by engaging the CD28 co-receptor with an anti-CD28 monoclonal antibody or by adding an exogenous source of interleukin-2. However, a majority of peripheral T cells from autoimmune, diabetes-prone Biobreeding (BB) rats exhibited different responses to these stimuli. T cells from these rats could not be rescued from apoptosis by costimulation. This was not due to the inability of BB-DP T cells to upregulate CD28 and the IL-2 receptor in response to TCR crosslinking. The failure of these costimulatory interactions to rescue BB-DP T cells segregated with the diabetes-susceptibility gene iddm1. Iddm1 in the rat causes peripheral T cell lymphopenia, which is associated with a dramatically shortened peripheral T cell life span. Our results indicate that a diabetogenic gene may contribute to autoimmunity by negating costimulatory signals important for the survival of long-lived peripheral T cells.

T Cell Reconstitution of BB/W Rats After the Initiation of Insulitis Precipitates the Onset of Diabetes

One of the diabetes susceptibility genes of the BB/W (Biobreeding/Worcester) rat maps to the lyp locus on chromosome 4. The BB/W lyp allele is responsible for a severe peripheral T lymphopenia. Correction of this lymphopenia by transfer of normal, histocompatible T cells prevents diabetes, providing T cell reconstitution is initiated before insulitis. We have analyzed this time-dependent regulation of the diabetogenic process by normal T cells. We demonstrate that T cell reconstitution after the initiation of insulitis precipitates the onset of diabetes through the recruitment of donor T cells to the autoimmune process. This inability of normal T cells to regulate primed diabetogenic BB/W T cells and their own autoreactive potential were observed when normal T cells outnumbered pathogenic T cells by approximately 1000-fold. Analysis of donor-derived T cells recovered from BB/W rats that were reconstituted before insulitis, and hence protected from diabetes, demonstrates that early T cell reconstitution of BB/W rats does not result in a long term physical or functional depletion of islet cell-specific T cell precursors among donor cells or in the expansion of T cells that can regulate the activation and expansion of diabetogenic T cells.

Antigen Activation Rescues Recent Thymic Emigrants from Programmed Cell Death in the BB Rat

One of the diabetes susceptibility genes of the BB rat is a mutation at the lyp locus that decreases the thymic output of T cells and the life span of most recent thymic emigrants (RTE). Consequently, there is a 10-fold reduction in the number of CD4+ and CD8+ T cells in secondary lymphoid organs. Results presented in this work demonstrate that the BB rat lyp mutation is associated with an accelerated apoptotic death in vitro of mature CD4+8− and CD4−8+ thymocytes and peripheral T cells. The stability of the pool of recirculating T cells (PRL) of BB rats over time results from a >10-fold increase in the mitotic activity of T cells as assessed in vivo by bromodeoxyuridine incorporation. This increased mitotic activity is not observed when BB T cells develop in the context of a normal sized PRL. MHC haploidentical WF and BB rats differ at minor histocompatibility loci. Intravenous injection of (WF × BB)F1 T cells into euthymic BB rats led to the rejection of donor T cells within 3 wk by unprimed recipients and within 1 wk by primed recipients. This secondary immune response was unaffected by postpriming thymectomy. F1 T cells were not rejected, but rather expanded after their injection into thymectomized BB rats that had been primed as early as 48 h after thymectomy. These results strongly suggest that the BB rat PRL is devoid of long-lived naive T cells and that rescue of recent thymic emigrants from programmed cell death is initiated by Ags, exclusively.

Abnormalities in the export and fate of recent thymic emigrants in diabetes-prone BB/W rats

Abnormalities in postthymic T cell development in the BB/W rat model of autoimmune insulin-dependent diabetes mellitus (IDDM) result in part from a lymphopenia (lyp) gene defect. To better characterize these abnormalities, the phenotypes of T cells from diabetes-prone (DP) and diabetes-resistant (DR) coisogenic rats were analyzed by multiparameter flow immunocytometry (FCM). Marked decreases in the numbers of Thy1- RT6+ T cells, most of which are CD8+, were documented in DP rats by live-gating. Conversely, an approximately 3-fold increase was observed in the percentage of Thy1+ RT6- T cells, which normally serve as the precursors of both Thy1- RT6+ and Thy1- RT6- T cell subsets in rats. These results suggested that, at a minimum, an arrest in maturation of the Thy1+ precursors of RT6+ T cells occurs postthymically in DP rats. To determine more precisely the stage(s) in T cell development at which lymphopenia occurs, the export and fate of recent thymic emigrants (RTE's) and their immediate descendants in DP rats was traced after intrathymic (i.t.) labelling with fluorescein isothiocyanate (FITC). The results showed that in DP, as compared with DR, rats: 1) 5-fold fewer RTE's are exported from the thymus per 24 hr; 2) more than 80% of the RTE's are CD4+; 3) most of the immediate descendants of RTE's disappear from the peripheral lymphoid tissues within one week after export from the thymus; and 4) few of the descendants of the RTE's that do survive differentiate into RT6+ T cells. Staining with propidium iodide revealed that a significantly higher proportion of Thy1+ T cells in DP than in DR rats are in cycle (S/G2/M), thereby accounting for their disproportionately high numbers relative to RTE's. These results indicate that, in addition to defective thymic export, most of the immediate descendants of RTE's in DP rats undergo non-productive proliferation and death at the time (3-7 days postthymic) at which their counterparts in DR rats differentiate into Thy1- RT6+ T cells. The resulting deficiency of immunoregulatory T cells, acting in concert with defective intrathymic selection of effector T cell precursors, appears to conspire to markedly enhance the predisposition of DP rats to autoimmunity.