Induction of Immunological Tolerance to Islet Allografts

T-cell dependent activation of resting B cells involves the interaction of gp39 on T cells with its receptor, CD40, on B cells. We administered either a combination of T-cell-depleted splenic lymphocytes and anti-gp39 monoclonal antibody or antibody alone to establish islet allografts in mice without continuous immunosuppression. Fully allogeneic H-2q FVB islets were permanently accepted by chemically diabetic H-2b C57BL/6 mice provided that the recipients were pretreated with both T-cell-depleted donor spleen cells and anti-gp39 antibody. Antibody alone was less effective in prolonging allograft survival, but we did observe that anti-gp39 mAb alone can exert an independent, primary effect on islet allograft survival that was dose dependent. Targeting gp39, in combination with lymphocyte transfusion, might prove suitable for tolerance induction and allotransplantation without immunosuppression.

Blockade of Cd40-Cd154 Interferes with Human T cell Engraftment in Scid Mice

Antibodies to the ligand for CD40 (CD154) have been shown to exert profound effects on the development of cell-mediated immune responses in mice. The present study shows that an antibody to human CD154 (hCD40L) inhibits in vivo Tetanus toxoid (TT) specific secondary antibody responses in hu-PBL-scid mice, as well as the expansion of xenoreactive human T cells in the scid mice. A possible cause for the reduced expansion of xenoreactive, human T cells, was the decreased expression of murine B7.1 and B7.2 caused by the administration of anti-hCD40L. Therefore, it may be that defective maturation of murine antigen-presenting cells impeded the priming and expansion of human xenoreactive T cells.

Human peripheral blood leucocyte non-obese diabetic-severe combined immunodeficiency interleukin-2 receptor gamma chain gene mouse model of xenogeneic graft-versus-host-like disease and the role of host major histocompatibility complex

Immunodeficient non-obese diabetic (NOD)-severe combined immune-deficient (scid) mice bearing a targeted mutation in the gene encoding the interleukin (IL)-2 receptor gamma chain gene (IL2rgamma(null)) engraft readily with human peripheral blood mononuclear cells (PBMC). Here, we report a robust model of xenogeneic graft-versus-host-like disease (GVHD) based on intravenous injection of human PBMC into 2 Gy conditioned NOD-scid IL2rgamma(null) mice. These mice develop xenogeneic GVHD consistently (100%) following injection of as few as 5 x 10(6) PBMC, regardless of the PBMC donor used. As in human disease, the development of xenogeneic GVHD is highly dependent on expression of host major histocompatibility complex class I and class II molecules and is associated with severely depressed haematopoiesis. Interrupting the tumour necrosis factor-alpha signalling cascade with etanercept, a therapeutic drug in clinical trials for the treatment of human GVHD, delays the onset and progression of disease. This model now provides the opportunity to investigate in vivo mechanisms of xenogeneic GVHD as well as to assess the efficacy of therapeutic agents rapidly.

Non-obese diabetic-recombination activating gene-1 (NOD-Rag1 null) interleukin (IL)-2 receptor common gamma chain (IL2r gamma null) null mice: a radioresistant model for human lymphohaematopoietic engraftment

Immunodeficient hosts engrafted with human lymphohaematopoietic cells hold great promise as a preclinical bridge for understanding human haematopoiesis and immunity. We now describe a new immunodeficient radioresistant non-obese diabetic mice (NOD) stock based on targeted mutations in the recombination activating gene-1 (Rag1(null)) and interleukin (IL)-2 receptor common gamma chain (IL2rgamma(null)), and compare its ability to support lymphohaematopoietic cell engraftment with that achieved in radiosensitive NOD.CB17-Prkdc(scid) (NOD-Prkdc(scid)) IL2rgamma(null) mice. We observed that immunodeficient NOD-Rag1(null) IL2rgamma(null) mice tolerated much higher levels of irradiation conditioning than did NOD-Prkdc(scid) IL2rgamma(null) mice. High levels of human cord blood stem cell engraftment were observed in both stocks of irradiation-conditioned adult mice, leading to multi-lineage haematopoietic cell populations and a complete repertoire of human immune cells, including human T cells. Human peripheral blood mononuclear cells also engrafted at high levels in unconditioned adult mice of each stock. These data document that Rag1(null) and scid stocks of immunodeficient NOD mice harbouring the IL2rgamma(null) mutation support similar levels of human lymphohaematopoietic cell engraftment. NOD-Rag1(null) IL2rgamma(null) mice will be an important new model for human lymphohaematopoietic cell engraftment studies that require radioresistant hosts.

A new immunodeficient hyperglycaemic mouse model based on the Ins2Akita mutation for analyses of human islet and beta stem and progenitor cell function

AIMS/HYPOTHESIS

To develop and validate a new immunodeficient mouse strain that spontaneously develops a non-autoimmune hyperglycaemia to serve as a diabetic host for human islets and human beta stem and progenitor cells without the need for induction of hyperglycaemia by toxic chemicals with their associated side effects.

METHODS

We generated and characterised a new strain of immunodeficient spontaneously hyperglycaemic mice, the NOD-Rag1null Prf1null Ins2Akita strain and compared this strain with the NOD-scid Il2rgammanull (also known as Il2rg) immunodeficient strain rendered hyperglycaemic by administration of a single dose of streptozotocin. Hyperglycaemic mice were transplanted with human islets ranging from 1,000 to 4,000 islet equivalents (IEQ) and were monitored for normalisation of blood glucose levels.

RESULTS

NOD-Rag1null Prf1null Ins2Akita mice developed spontaneous hyperglycaemia, similar to Ins2Akita-harbouring strains of immunocompetent mice. Histological examination of islets in the host pancreas validated the spontaneous loss of beta cell mass in the absence of mononuclear cell infiltration. Human islets transplanted into spontaneously diabetic NOD-Rag1null Prf1null Ins2Akita and chemically diabetic NOD-scid Il2rgammanull mice resulted in a return to euglycaemia that occurred with transplantation of similar beta cell masses.

CONCLUSIONS/INTERPRETATION

The NOD-Rag1null Prf1null Ins2Akita mouse is the first immunodeficient, spontaneously hyperglycaemic mouse strain described that is based on the Ins2Akita mutation. This strain is suitable as hosts for human islet and human beta stem and progenitor cell transplantation in the absence of the need for pharmacological induction of diabetes. This strain of mice also has low levels of innate immunity and can be engrafted with a human immune system for the study of human islet allograft rejection.

Different mechanisms control peripheral and central tolerance in hematopoietic chimeric mice

Regulatory T cells (Treg) are important in peripheral tolerance, but their role in establishing and maintaining hematopoietic mixed chimerism and generating central tolerance is unclear. We now show that costimulation blockade using a donor-specific transfusion and anti-CD154 antibody applied to mice given bone marrow and simultaneously transplanted with skin allografts leads to hematopoietic chimerism and permanent skin allograft survival. Chimeric mice bearing intact skin allografts fail to generate effector/memory T cells against allogeneic targets as shown by the absence of IFNgamma-producing CD44(high)CD8+ T cells and in vivo cytotoxicity. Depletion of Tregs by injection of anti-CD4 or anti-CD25 antibody prior to costimulation blockade prevents chimerism, shortens skin allograft survival and leads to generation of effector/memory cytotoxic T cells. Depletion of Tregs by injection of anti-CD4 or anti-CD25 antibody two months after transplantation leads to loss of skin allografts even though mice remain chimeric and exhibit little in vivo cytotoxicity. In contrast, chimerism is lost, but skin allografts survive following naïve T-cell injection. We conclude that hematopoietic chimerism and peripheral tolerance may be maintained by different mechanisms in mixed hematopoietic chimeras.

Costimulation and autoimmune diabetes in BB rats

Costimulatory signals regulate T-cell activation. To investigate the role of costimulation in autoimmunity and transplantation, we studied the BB rat model of type 1 diabetes. Diabetes-prone BB (BBDP) rats spontaneously develop disease when 55-120 days of age. We observed that two anti-CD28 monoclonal antibodies (mAb) with different functional activities completely prevented diabetes in BBDP rats. Anti-CD154 mAb delayed diabetes, whereas treatment with CTLA4-Ig or anti-CD80 mAb accelerated disease. Anti-CD86 or anti-CD134L mAbs had no effect. Diabetes resistant BB (BBDR) rats are disease-free, but >95% of them develop diabetes after treatment with polyinosinic-polycytidylic acid and an mAb that depletes Treg cells. In the induced BBDR model, anti-CD154 mAb delayed onset of diabetes, whereas CTLA4-Ig, anti-CD134L or either of the anti-CD28 mAbs had little or no effect. In contrast, blockade of the CD134-CD134L pathway was highly effective for preventing autoimmune recurrence against syngeneic islet grafts in diabetic BBDR hosts. Blockade of the CD40-CD154 pathway was also effective, but less so. These data suggest that the effectiveness of costimulation blockade in the treatment of type 1 diabetes is dependent on both the costimulatory pathway targeted and the mechanism of induction, stage, intensity and duration of the pathogenic process.

Islet cell transplantation tolerance

Curative islet transplantation for type 1 diabetes currently requires lifelong systemic immunosuppression. Induction of islet transplantation tolerance would be far preferable. We have previously demonstrated that blockade of costimulation by the administration of a donor-specific transfusion in combination with anti-CD154 monoclonal antibody leads to permanent islet and prolonged skin allograft survival in mice. The protocol requires the presence of CD4+ T cells, interferon-gamma, and CTLA4, and involves the deletion of CD8+ alloreactive T cells. Translation of this strategy into clinical practice will, however, require attention to at least two issues. First, we have observed that the presence of viral infection during tolerance interferes with tolerance induction. Second, we have observed that our tolerance induction protocol is ineffective in autoimmune nonobese diabetic mice. We hypothesize that resistance to tolerance induction in nonobese diabetic mice is due to the presence of memory autoreactive cells. To overcome the deleterious effects of viral infection and of primed memory responses, it may be necessary to modify current tolerance induction strategies based on costimulatory blockade. These modifications may require patient isolation, the generation of hematopoietic chimerism, or treatments that target the specific T-cell populations, cytokines, and/or costimulatory factors responsible for resistance. Such modifications may make it possible to extend tolerance induction to the "real world" situation of individuals with type 1 diabetes who are likely to harbor both memory allo-and autoreactive immune cells.

An atypical population of NK cells that spontaneously secrete IFN-gamma and IL-4 is present in the intraepithelial lymphoid compartment of the rat

The intestinal lymphoid compartment of the rat is large and diverse, but the phenotype and functions of its constituent cell populations are not fully characterized. Using new methodology for the isolation and purification of rat intestinal intraepithelial lymphocytes (IELs), we previously identified a population of alphabeta- and gammadelta-TCR- NKR-P1A+ NK cells. These cells were almost completely restricted to the CD4-CD8- IEL population, and unlike peripheral NK cells in the rat, they were CD2-. We now report that rat intraepithelial NK (IENK) and peripheral NK cells are similar in morphology, in their ability to lyse NK-sensitive targets, and in their ability to suppress a one-way mixed lymphocyte culture. In contrast, however, intraepithelial and splenic NK cells differ markedly in two respects. First, IENK cells express high levels of ADP-ribosyltransferase 2 (a marker of regulatory T cells in the rat) and CD25, whereas peripheral NK cells do not. Second, unlike splenic NK cells, a substantial fraction of IENK cells appear to spontaneously secrete IL-4 and/or IFN-gamma. We conclude that the rat IEL compartment harbors a large population of NKR-P1A+CD3- cells that function as NK cells but display an activated phenotype and unusual cytokine profile that clearly distinguish them from splenic NK cells. Their phenotypic and functional characteristics suggest that these distinctive IENK cells may participate in the regulation of mucosal immunity.

Requirement of the JIP1 scaffold protein for stress-induced JNK activation

The c-Jun N-terminal kinase (JNK) signal transduction pathway is activated in response to the exposure of cells to environmental stress. Components of the JNK signaling pathway interact with the JIP1 scaffold protein. JIP1 is located in the neurites of primary hippocampal neurons. However, in response to stress, JIP1 accumulates in the soma together with activated JNK and phosphorylated c-Jun. Disruption of the Jip1 gene in mice by homologous recombination prevented JNK activation caused by exposure to excitotoxic stress and anoxic stress in vivo and in vitro. These data show that the JIP1 scaffold protein is a critical component of a MAP-kinase signal transduction pathway.

Nicotinamide adenine dinucleotide (NAD) and its metabolites inhibit T lymphocyte proliferation: role of cell surface NAD glycohydrolase and pyrophosphatase activities

The presence of NAD-metabolizing enzymes (e.g., ADP-ribosyltransferase (ART)2) on the surface of immune cells suggests a potential immunomodulatory activity for ecto-NAD or its metabolites at sites of inflammation and cell lysis where extracellular levels of NAD may be high. In vitro, NAD inhibits mitogen-stimulated rat T cell proliferation. To investigate the mechanism of inhibition, the effects of NAD and its metabolites on T cell proliferation were studied using ART2a+ and ART2b+ rat T cells. NAD and ADP-ribose, but not nicotinamide, inhibited proliferation of mitogen-activated T cells independent of ART2 allele-specific expression. Inhibition by P2 purinergic receptor agonists was comparable to that induced by NAD and ADP-ribose; these compounds were more potent than P1 agonists. Analysis of the NAD-metabolizing activity of intact rat T cells demonstrated that ADP-ribose was the predominant metabolite, consistent with the presence of cell surface NAD glycohydrolase (NADase) activities. Treatment of T cells with phosphatidylinositol-specific phospholipase C removed much of the NADase activity, consistent with at least one NADase having a GPI anchor; ART2- T cell subsets contained NADase activity that was not releasable by phosphatidylinositol-specific phospholipase C treatment. Formation of AMP from NAD and ADP-ribose also occurred, a result of cell surface pyrophosphatase activity. Because AMP and its metabolite, adenosine, were less inhibitory to rat T cell proliferation than was NAD or ADP-ribose, pyrophosphatases may serve a regulatory role in modifying the inhibitory effect of ecto-NAD on T cell activation. These data suggest that T cells express multiple NAD and adenine nucleotide-metabolizing activities that together modulate immune function.

Translating data from animal models into methods for preventing human autoimmune diabetes mellitus: caveat emptor and primum non nocere

Type 1 diabetes in humans is a serious autoimmune disorder of children that is still poorly understood, unpreventable, and irreversible. Study of its animal models, notably the NOD mouse and BB rat, has generated a wealth of information concerning genetics and immunopathogenesis, but that information has still not altered the way in which we treat children with diabetes. In this review we attempt to identify the most promising avenues of continuing research in these models and the most important issues that must be faced by the designers of human therapies based on the animal dataset.

BB rat thymocytes cultured in the presence of islets lose their ability to transfer autoimmune diabetes

Thymocytes from adult BB rats can adoptively transfer autoimmune diabetes to athymic recipients. It is also known that the development of BB rat T-cells is recapitulated in adult thymus organ cultures (ATOCs). Based on these observations, we tested the hypothesis that cells capable of the adoptive transfer of diabetes would be present in long-term ATOCs but could be rendered nondiabetogenic by co-culture with appropriate antigens. We observed that cells recovered from adult diabetes-resistant BB (BBDR) rat thymi cultured for up to 14 days can adoptively transfer disease to athymic WAG-rnu/rnu rats treated with polyinosinic: polycytidylic acid and a monoclonal antibody to preclude development of ART2a+ regulatory T-cells. Co-culture of adult BBDR thymi in the presence of BBDR thyrocytes had no effect on the ability of recovered cells to induce diabetes in 70-80% of adoptive recipients. In contrast, co-culture in the presence of islets prevented transfer of diabetes, on average, in >90% of recipients. Fresh islets, frozen islets, and islets pretreated with streptozotocin to deplete insulin were equally effective in preventing diabetes, but none prevented insulitis in nondiabetic recipients. Co-culture in the presence of islets was not associated with detectable alterations in phenotype or in the secretion of gamma-interferon or interleukin-4, either in cultures or in cells recovered from adoptive recipients. We conclude that islet antigens involved in the initiation of autoimmune diabetes in BB rats may be absent or deficient in BB rat thymi. Exposure of ATOCs to exogenous islets may lead to deletion or anergy of diabetogenic T-cells or to the positive selection of regulatory T-cells.

Levels of Art2+ cells but not soluble Art2 protein correlate with expression of autoimmune diabetes in the BB rat

ART2a and ART2b are isoenzymes expressed on the surface of mature T cells and intraepithelial lymphocytes (IELs) in the rat. They exhibit both adenosine diphosphoribosyltransferase and nicotine adenine dinucleotide (NAD) glycohydrolase activities, and both can generate a transmembrane signal that modulates T cell activation. The presence or absence of ART2+ T cells modulates the expression of autoimmune diabetes in the BB rat. ART2 also circulates in a soluble form whose function is unknown. We tested the hypothesis that circulating ART2 protein regulates the expression of autoimmunity. We compared the kinetics, regulation, and source of soluble ART2 in normal rats and in rats with autoimmune diabetes. Basal levels of soluble ART2 varied greatly among strains of rats and were lowest in the diabetes-prone BB (BBDP/Wor) rat. In diabetes-resistant BB (BBDR/Wor) rats, administration of anti-ART2a antibody, which is known to induce diabetes, resulted in transient clearing of soluble ART2a that was followed rapidly by a rebound increase. Repeated treatment of BBDR/Wor rats with anti-ART2a antibody resulted in sustained supraphysiologic levels of soluble ART2a. Although the number of peripheral ART2a+ T cells is known to correlate with the expression of diabetes in BBDR/Wor rats, the level of soluble ART2a protein did not. The source of the soluble ART2 protein in the rat appeared to be the gut. The results suggest that ART2+ T cells and soluble ART2 protein may subserve different immunomodulatory functions.

Fetal thymi from diabetes-prone but not diabetes-resistant BB/Wor rats fail to generate mature ART2+ T-cells in organ culture

Diabetes-prone (BBDP) BB rats develop spontaneous autoimmune diabetes mellitus. They are lymphopenic and severely deficient in ART2+ T-cells. Diabetes-resistant BB (BBDR) rats do not develop spontaneous diabetes and have normal numbers of ART2+ T-cells. T-cell lymphopenia in BBDP rats results from hematopoietic stem cell defects leading to abnormal intrathymic T-cell maturation. To study this process, we established rat fetal thymic organ cultures (FTOC). Like mouse FTOC, cultures of BBDR rat thymi yielded approximately 10(5) cells per lobe. The majority of cells were CD8+ART2+ T-cells. In contrast, BBDP rat FTOC yielded 60% fewer cells (approximately 0.3 x 10(5)/lobe), a smaller percentage of CD8+ and TcRalphabeta+ T-cells, and almost no detectable ART2+ T-cells. ART2 mRNA was detectable in BBDR but not BBDP FTOC. In contrast, expression of mRNAs encoding bcl-2 and a panel of cytokines was comparable in BBDP and BBDR FTOC. Addition of anti-ICAM-1 (CD54) antibody reduced T-cell number in BBDR rat FTOC by approximately 70%, but addition of IL-7 or IL-1beta had no effect. The data demonstrate that BBDP thymocytes fail to generate mature ART2+ T-cells in rat FTOC, a system that can now be used to study the mechanism of this process.

Rat xenograft survival in mice treated with donor-specific transfusion and anti-CD154 antibody is enhanced by elimination of host CD4+ cells

BACKGROUND

Treatment with a donor-specific transfusion (DST) and a brief course of anti-mouse CD154 (anti-CD40-ligand) monoclonal antibody (mAb) prolongs the survival of both allografts and rat xenografts in mice. The mechanism by which allograft survival is prolonged is incompletely understood, but depends in part on the presence of CD4+ cells and the deletion of alloreactive CD8+ T cells. Less is known about the mechanism by which this protocol prolongs xenograft survival.

METHODS

We measured rat islet and skin xenograft survival in euthymic and thymectomized mice treated with combinations of DST, anti-CD154 mAb, anti-CD4 mAb, and anti-CD8 mAb. Recipients included C57BL/6, C57BL/6-scid, C57BL/6-CD4null, and C57BL/6-CD8null mice.

RESULTS

Pretreatment with a depleting anti-CD4 mAb markedly prolonged the survival of both skin and islet xenografts in mice given DST plus anti-CD154 mAb. Comparable prolongation of xenograft survival was obtained in C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb. In contrast, anti-CD8 mAb did not prolong the survival of either islet or skin xenografts in mice treated with DST and anti-CD154 mAb. Thymectomy did not influence xenograft survival in any treatment group. Adoptive transfer of splenocytes from C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb and bearing long-term skin xenografts revealed the presence of residual xenoreactive cells.

CONCLUSIONS

These data suggest that treatment with DST and anti-CD154 mAb induces a state of "functional" transplantation tolerance. They also support the hypothesis that both the induction and maintenance of graft survival based on this protocol depend on different cellular mechanisms in allogeneic and xenogeneic model systems.

Viral infection abrogates CD8(+) T-cell deletion induced by costimulation blockade

BACKGROUND

Treatment with a single donor-specific transfusion (DST) plus a brief course of anti-CD154 monoclonal antibody (mAb) prolongs skin allograft survival in mice. It is known that prolongation of allograft survival by this method depends in part on deletion of alloreactive CD8(+) T cells at the time of tolerance induction. Recent data suggest that infection with lymphocytic choriomeningitis virus (LCMV) abrogates the ability of this protocol to prolong graft survival.

METHODS

To study the mechanism by which viral infection abrogates allograft survival, we determined (1) the fate of tracer populations of alloreactive transgenic CD8(+) T cells and (2) the duration of skin allograft survival following treatment with DST and anti-CD154 mAb in the presence or absence of LCMV infection.

RESULTS

We confirmed that treatment of uninfected mice with DST and anti-CD154 mAb leads to the deletion of alloreactive CD8(+) T cells and is associated with prolongation of skin allograft survival. In contrast, treatment with DST and anti-CD154 mAb in the presence of intercurrent LCMV infection was associated with the failure to delete alloreactive CD8(+) T cells and with the rapid rejection of skin allografts. The number of alloreactive CD8(+) cells actually increased significantly, and the cells acquired an activated phenotype.

CONCLUSIONS

Interference with the deletion of alloreactive CD8(+) T cells mediated by DST and anti-CD154 mAb may in part be the mechanism by which viral infection abrogates transplantation tolerance induction.

Allogeneic hematopoietic chimerism in mice treated with sublethal myeloablation and anti-CD154 antibody: absence of graft-versus-host disease, induction of skin allograft tolerance, and prevention of recurrent autoimmunity in islet-allografted NOD/Lt mice

We describe a tolerance-based stem cell transplantation protocol that combines sublethal radiation with transient blockade of the CD40-CD154 costimulatory pathway using an anti-CD154 antibody. With this protocol, we established hematopoietic chimerism in BALB/c mice transplanted with fully allogeneic C57BL/6 bone marrow. The percentage of donor-origin mononuclear cells in recipients was more than 99%. In addition, all chimeric mice treated with anti-CD154 antibody remained free of graft-versus-host disease (GVHD) and accepted donor-origin but not third-party skin allografts. It was similarly possible to create allogeneic hematopoietic chimerism in NOD/Lt mice with spontaneous autoimmune diabetes. Pancreatic islet allografts transplanted into chimeric NOD/Lt mice were resistant not only to allorejection but also to recurrence of autoimmunity. We conclude that it is possible to establish robust allogeneic hematopoietic chimerism in sublethally irradiated mice without subsequent GVHD by blocking the CD40-CD154 costimulatory pathway using as few as 2 injections of anti-CD154 antibody. We also conclude that chimerism created in this way generates donor-specific allograft tolerance and reverses the predisposition to recurrent autoimmune diabetes in NOD/Lt mice, enabling them to accept curative islet allografts. (Blood. 2000;95:2175-2182)

Virus-induced abrogation of transplantation tolerance induced by donor-specific transfusion and anti-CD154 antibody

Treatment with a 2-week course of anti-CD154 antibody and a single transfusion of donor leukocytes (a donor-specific transfusion or DST) permits skin allografts to survive for >100 days in thymectomized mice. As clinical trials of this methodology in humans are contemplated, concern has been expressed that viral infection of graft recipients may disrupt tolerance to the allograft. We report that acute infection with lymphocytic choriomeningitis virus (LCMV) induced allograft rejection in mice treated with DST and anti-CD154 antibody if inoculated shortly after transplantation. Isografts resisted LCMV-induced rejection, and the interferon-inducing agent polyinosinic:polycytidylic acid did not induce allograft rejection, suggesting that the effect of LCMV is not simply a consequence of nonspecific inflammation. Administration of anti-CD8 antibody to engrafted mice delayed LCMV-induced allograft rejection. Pichinde virus also induced acute allograft rejection, but murine cytomegalovirus and vaccinia virus (VV) did not. Injection of LCMV approximately 50 days after tolerance induction and transplantation had minimal effect on subsequent allograft survival. Treatment with DST and anti-CD154 antibody did not interfere with clearance of LCMV, but a normally nonlethal high dose of VV during tolerance induction and transplantation killed graft recipients. We conclude that DST and anti-CD154 antibody induce a tolerant state that can be broken shortly after transplantation by certain viral infections. Clinical application of transplantation tolerance protocols may require patient isolation to facilitate the procedure and to protect recipients.

Treatment of allograft recipients with donor-specific transfusion and anti-CD154 antibody leads to deletion of alloreactive CD8+ T cells and prolonged graft survival in a CTLA4-dependent manner

A two-element protocol consisting of one donor-specific transfusion (DST) plus a brief course of anti-CD154 mAb greatly prolongs the survival of murine islet, skin, and cardiac allografts. To study the mechanism of allograft survival, we determined the fate of tracer populations of alloreactive transgenic CD8+ T cells in a normal microenvironment. We observed that DST plus anti-CD154 mAb prolonged allograft survival and deleted alloreactive transgenic CD8+ T cells. Neither component alone did so. Skin allograft survival was also prolonged in normal recipients treated with anti-CD154 mAb plus a depleting anti-CD8 mAb and in C57BL/6-CD8 knockout mice treated with anti-CD154 mAb monotherapy. We conclude that, in the presence of anti-CD154 mAb, DST leads to an allotolerant state, in part by deleting alloreactive CD8+ T cells. Consistent with this conclusion, blockade of CTLA4, which is known to abrogate the effects of DST and anti-CD154 mAb, prevented the deletion of alloreactive transgenic CD8+ T cells. These results document for the first time that peripheral deletion of alloantigen-specific CD8+ T cells is an important mechanism through which allograft survival can be prolonged by costimulatory blockade. We propose a unifying mechanism to explain allograft prolongation by DST and blockade of costimulation.

Diabetes prone BB rats are severely deficient in natural killer T cells

Diabetes prone (DP) BB rats develop spontaneous autoimmune hyperglycemia. Coisogenic diabetes resistant (DR) BB rats develop diabetes in response to immunological and environmental perturbants, but not spontaneously. Both are used to model human insulin-dependent diabetes mellitus (IDDM). Deficiencies in natural killer (NK) T cells have been implicated in the expression of human IDDM, but little is known of their phenotype or function in the rat. We now report that the phenotype of NK T cells in the rat is alphabetaTcR+ CD8+ CD4-, comparable to the NK T cell phenotype reported for humans, which is alphabetaTcR+ CD4- Valpha24-JalphaQ, and either CD8- or CD8alphaalpha+. We also report that DP- but not DR-BB rats are severely deficient in splenic and intrahepatic NKR-P1+ alphabetaTcR+ (NK T) cells. Because RT6+ T cells are deficient in DP-BB rats, and because depletion of cells expressing RT6 induces IDDM in DR-BB rats, we studied NK T cells for expression of this antigen. We observed that the majority of rat NK T cells express RT6+. In addition, injection of cytotoxic anti-RT6.1 monoclonal antibody depleted splenic and intrahepatic RT6+ NK T cells, T cells, and NK cells, but left intact the RT6- subset of each population. These results suggest that deficiencies in NK T cells may play a role in the susceptibility of DP- and DR-BB rats, respectively, to spontaneous and induced autoimmune IDDM.

NOD mice have a generalized defect in their response to transplantation tolerance induction

A protocol consisting of a single donor-specific transfusion (DST) plus a brief course of anti-CD154 monoclonal antibody (anti-CD40 ligand mAb) induces permanent islet allograft survival in chemically diabetic mice, but its efficacy in mice with autoimmune diabetes is unknown. Confirming a previous report, we first observed that treatment of young female NOD mice with anti-CD154 mAb reduced the frequency of diabetes through 1 year of age to 43%, compared with 73% in untreated controls. We also confirmed that spontaneously diabetic NOD mice transplanted with syngeneic (NOD-Prkdc(scid)/Prkdc(scid)) or allogeneic (BALB/c) islets rapidly reject their grafts. Graft survival was not prolonged, however, by pretreatment with either anti-CD154 mAb alone or anti-CD154 mAb plus DST. In addition, allograft rejection in NOD mice was not restricted to islet grafts. Anti-CD154 mAb plus DST treatment failed to prolong skin allograft survival in nondiabetic male NOD mice. The inability to induce transplantation tolerance in NOD (H2g7) mice was associated with non-major histocompatibility complex (MHC) genes. Treatment with DST and anti-CD154 mAb prolonged skin allograft survival in both C57BL/6 (H2b) and C57BL/6.NOD-H2g7 mice, but it was ineffective in NOD, NOD.SWR-H2q, and NOR (H2g7) mice. Mitogen-stimulated interleukin-1beta production by antigen-presenting cells was greater in strains susceptible to tolerance induction than in the strains resistant to tolerance induction. The results suggest the existence of a general defect in tolerance mechanisms in NOD mice. This genetic defect involves defective antigen-presenting cell maturation, leads to spontaneous autoimmune diabetes in the presence of the H2g7 MHC, and precludes the induction of transplantation tolerance irrespective of MHC haplotype. Promising islet transplantation methods based on overcoming the alloimmune response by interference with costimulation may require modification or amplification for use in the setting of autoimmune diabetes.

CTLA4 signals are required to optimally induce allograft tolerance with combined donor-specific transfusion and anti-CD154 monoclonal antibody treatment

Sensitization to donor Ags is an enormous problem in clinical transplantation. In an islet allograft model, presensitization of recipients through donor-specific transfusion (DST) 4 wk before transplantation results in accelerated rejection. We demonstrate that combined DST with anti-CD154 (CD40L) therapy not only prevents the deleterious presensitization produced by pretransplant DST in the islet allograft model, it also induces broad alloantigen-specific tolerance and permits subsequent engraftment of donor islet or cardiac grafts without further treatment. In addition, our data strongly indicate that CTLA4-negative T cell signals are required to achieve prolonged engraftment of skin allograft or tolerance to islet allograft in recipients treated with a combination of pretransplant DST and anti-CD154 mAb. We provide direct evidence that a CD28-independent CTLA4 signal delivers a strong negative signal to CD4+ T cells that can block alloimmune MLR responses. In this study immune deviation into a Th2 (IL-4) response was associated with, but did not insure, graft tolerance, as the inopportune timing of B7 blockade with CTLA4/Ig therapy prevented uniform tolerance but did not prevent Th2-type immune deviation. While CTLA4-negative signals are necessary for tolerance induction, Th1 to Th2 immune deviation cannot be sufficient for tolerance induction. Combined pretransplant DST with anti-CD154 mAb treatment may be attractive for clinical deployment, and strategies aimed to selectively block CD28 without interrupting CTLA4/B7 interaction might prove highly effective in the induction of tolerance.

Cutting edge: sustained expansion of CD8+ T cells requires CD154 expression by Th cells in acute graft versus host disease

Brief treatment with alphaCD154 Ab has been shown to prevent acute graft versus host disease (aGvHD). We extend these data to show that in the absence of CD154 function, donor T cells are unable to expand or generate high level anti-host CTL activity. Using transgenic (Tg) alloreactive CD8+ T cells adoptively transferred into allogeneic recipients, we show that short-term expansion of the CD8+ Tg T cells occurred in the absence of Th cells, and this short-term expansion could be facilitated with an agonistic alphaCD40. While CD40 agonism could enhance short-term expansion, sustained expansion of CD8+ Tg T cells required bona fide CD154-expressing CD4+ alloreactive Th cells. While CD154 was necessary for CD8+ Tg T cell sustained expansion, IL-2 was also implicated as essential. These observations suggest alphaCD154 therapy in GvHD is effective because the treatment causes an abortive CD8 alloresponse leading to the exhaustion or deletion of alloreactive CD8+ clones preventing the development of disease.

Recapitulation of normal and abnormal BioBreeding rat T cell development in adult thymus organ culture

Congenitally lymphopenic diabetes-prone (DP) BioBreeding (BB) rats develop spontaneous T cell-dependent autoimmunity. Coisogenic diabetes-resistant (DR) BB rats are not lymphopenic and are free of spontaneous autoimmune disease, but become diabetic in response to depletion of RT6+ T cells. The basis for the predisposition to autoimmunity in BB rats is unknown. Abnormal T cell development in DP-BB rats can be detected intrathymically, and thymocytes from DR-BB rats adoptively transfer diabetes. The mechanisms underlying these T cell developmental abnormalities are not known. To study these processes, we established adult thymus organ cultures (ATOC). We report that cultured DR- and DP-BB rat thymi generate mature CD4 and CD8 single-positive cells with up-regulated TCRs. DR-BB rat cultures also generate T cells that express RT6. In contrast, DP-BB rat cultures generate fewer CD4+, CD8+, and RT6+ T cells. Analysis of the cells obtained from ATOC suggested that the failure of cultured DP-BB rat thymi to generate T cells with a mature phenotype is due in part to an increased rate of apoptosis. Consistent with this inference, we observed that addition of the general caspase inhibitor Z-VAD-FMK substantially increases the number of both mature and immature T cells produced by DP-BB rat ATOC. We conclude that cultured DR-BB and DP-BB rat thymi, respectively, recapitulate the normal and abnormal T cell developmental kinetics and phenotypes observed in these animals in vivo. Such cultures should facilitate identification of the underlying pathological processes that lead to immune dysfunction and autoimmunity in BB rats.

The RT6 (Art2) family of ADP-ribosyltransferases in rat and mouse

Recent evidence suggests that a new member of the mono-ADP-ribosyltransferase/NAD glycohydrolase family, RT6, may be important in immune regulation. RT6 is expressed in two allelic forms and is present on post-thymic T cells in the rat. RT6-expressing T cells in the rat may have a regulatory role, a conclusion based on their ability to prevent autoimmune diabetes in the BB rat model of insulin-dependent diabetes mellitus. This observation led to investigation of RT6 at a molecular and biochemical level resulting in the determination that RT6 protein exists as both glycosylated and non-glycosylated glycosylphosphatidylinositol (GPI)-linked cell surface molecules. RT6, like many GPI-linked proteins, can mediate cell signal transduction events associated with T cell activation, and is also present in a soluble form in the circulation. The discovery that RT6 is an NAD glycohydrolase and auto-ADP-ribosyltransferase led to the ongoing investigations into the role that enzymatic activity may have in the immunoregulatory function of rat RT6+ T cells. A homologue of rat RT6, termed Rt6, has been identified in the mouse. Rt6 is predominately an ADP-ribosyltransferase enzyme as determined using simple guanidino compounds (e.g. arginine) as ribose acceptors. Abnormalities in mouse Rt6 mRNA are associated with the expression of autoimmunity. In the present manuscript, we review recent data on RT6/Rt6, and discuss the potential mechanisms by which RT6-expressing cells, and perhaps RT6 protein itself, may mediate immune regulation.

Induction of immunologic tolerance for transplantation

In the second half of the 20th century, the transplantation of replacement organs and tissues to cure disease has become a clinical reality. Success has been achieved as a direct result of progress in understanding the cellular and molecular biology of the immune system. This understanding has led to the development of immunosuppressive pharmaceuticals that are part of nearly every transplantation procedure. All such drugs are toxic to some degree, however, and their chronic use, mandatory in transplantation, predisposes the patient to the development of infection and cancer. In addition, many of them may have deleterious long-term effects on the function of grafts. New immunosuppressive agents are constantly under development, but organ transplantation remains a therapy that requires patients to choose between the risks of their primary illness and its treatment on the one hand, and the risks of life-long systemic immunosuppression on the other. Alternatives to immunosuppression include modulation of donor grafts to reduce immunogenicity, removal of passenger leukocytes, transplantation into immunologically privileged sites like the testis or thymus, encapsulation of tissue, and the induction of a state of immunologic tolerance. It is the last of these alternatives that has, perhaps, the most promise and most generic applicability as a future therapy. Recent reports documenting long-term graft survival in the absence of immunosuppression suggest that tolerance-based therapies may soon become a clinical reality. Of particular interest to our laboratory are transplantation strategies that focus on the induction of donor-specific T-cell unresponsiveness. The basic biology, protocols, experimental outcomes, and clinical implications of tolerance-based transplantation are the focus of this review.

Prolonged survival of rat islet and skin xenografts in mice treated with donor splenocytes and anti-CD154 monoclonal antibody

Treatment of C57BL/6 mice with one transfusion of BALB/c spleen cells and a brief course of anti-CD154 (anti-CD40 ligand) antibody permits BALB/c islet grafts to survive indefinitely and BALB/c skin grafts to survive for approximately 50 days without further intervention. We now report adaptation of this protocol to the transplantation of islet and skin xenografts. We observed prolonged survival of rat islet xenografts in mice treated with donor-specific spleen cell transfusion and anti-CD154 monoclonal antibody (mAb). Challenge islet xenografts placed on these animals indicated that graft acceptance was species-specific but not strain specific. Spleen cells from recipients bearing intact grafts led to rejection of rat islet xenografts in scid mice, suggesting that graft acceptance was not due to complete clonal deletion of xenoreactive cells. We also observed prolonged survival of rat skin xenografts in mice treated with donor-specific transfusion and anti-CD154 mAb. Prolonged survival of skin xenografts was also species specific. We conclude that treatment with appropriately timed donor-specific transfusion and anti-CD154 mAb induces durable survival of both islet and skin xenografts in mice. Because this procedure is targeted directly at CD154, a co-activation molecule expressed predominantly by activated CD4+ T-cells, the results suggest that CD4+ cells have a major role in the cellular immune response to xenografts.

Characterization of high density lipoprotein-bound and soluble RT6 released following administration of anti-RT6.1 monoclonal antibody

RT6 is a rat lymphocyte glycosylphosphatidylinositol (GPI)-anchored alloantigen with nicotinamide adenine dinucleotide (NAD) glycohydrolase (NADase) and auto-ADP-ribosyltransferase activities. RT6 may have immunoregulatory properties based in part on the observation that injection of diabetes-resistant (DR)-BB rats with depleting doses of anti-RT6.1 mAb induced autoimmune diabetes and thyroiditis. We now report that injection of DR-BB rats with anti-RT6.1 mAb increased plasma NADase activity, which localized, by fluid phase liquid chromatography fractionation, to the high density lipoprotein (HDL) fraction. Following ultracentrifugation in high salt, however, RT6 was found in the nonlipoprotein fraction, where it existed, under nondenaturing conditions, as a 200-kDa complex and, by SDS-PAGE, as a 30- to 36-kDa species. Thy-1, another GPI-linked protein, and proteins that reacted with anti-GPI-oligosaccharide Abs also translocated from HDL to the nonlipoprotein fraction under similar conditions. Injection of anti-RT6.1 mAb into thymectomized DR and diabetes-prone-BB rats increased soluble RT6 to levels comparable to those observed in euthymic DR-BB rats, suggesting that HDL-bound RT6 is not derived from peripheral lymphocytes. In agreement, NADase activity in the plasma of eviscerated DR-BB rats did not increase following injection of anti-RT6 mAb. These data suggest that HDL is a carrier of plasma RT6 and other GPI-linked proteins, with equilibrium between the lipoprotein and nonlipoprotein fractions being salt dependent. Since GPI-linked proteins in HDL can transfer to cells in a functionally active form, the presence of RT6 in HDL is consistent with it having a role in signaling in nonlymphoid cells.

High frequency apoptosis of recent thymic emigrants in the liver of lymphopenic diabetes-prone BioBreeding rats

Diabetes-prone (DP) BioBreeding (BB) rats develop spontaneous autoimmune diabetes. DP-BB thymocyte export is reduced, and most thymic emigrants disappear rapidly from peripheral lymphoid tissues. DP-BB rats are consequently lymphopenic and circulate severely reduced numbers of T cells. Peripheral T cells present are phenotypically immature (Thy1+) and appear activated. We hypothesized that DP-BB recent thymic emigrants have a shortened life span and disappear by apoptosis. The percentage of T cells with an alphabetaTCR(low) B220+ CD4- CD8- phenotype was increased in DP peripheral lymphoid tissues when compared with normal, nonlymphopenic diabetes-resistant (DR) BB rat tissues. There was no evidence of DNA fragmentation in freshly isolated DP- or DR-BB rat cells, but, after 24 h of culture, a higher proportion of DP- than DR-BB splenic T cells underwent apoptosis. We then tested the hypothesis that BB rat T cells with the alphabetaTCR(low) B220+ CD4- CD8- phenotype accumulate and undergo apoptosis in the liver. Such cells were observed undergoing apoptosis in both DP- and DR-BB rats, but comprised approximately 80% of intrahepatic T cells in DP vs approximately 20% in DR-BB rats. Most alphabetaTCR(low) B220+ CD4- CD8- cells in the liver were also Thy1+. The data suggest that T cell apoptosis in the DP-BB rat is underway in peripheral lymphoid tissues and is completed in the liver. Increased intrahepatic apoptosis of recent thymic emigrants appears in part responsible for lymphopenia in DP-BB rats and the concomitant predisposition of these animals to autoimmunity.

Long-term survival of skin allografts induced by donor splenocytes and anti-CD154 antibody in thymectomized mice requires CD4(+) T cells, interferon-gamma, and CTLA4

Treatment of C57BL/6 mice with one transfusion of BALB/c spleen cells and anti-CD154 (anti-CD40-ligand) antibody permits BALB/c islet grafts to survive indefinitely and BALB/c skin grafts to survive for approximately 50 d without further intervention. The protocol induces long-term allograft survival, but the mechanism is unknown. We now report: (a) addition of thymectomy to the protocol permitted skin allografts to survive for > 100 d, suggesting that graft rejection in euthymic mice results from thymic export of alloreactive T cells. (b) Clonal deletion is not the mechanism of underlying long-term graft survival, as recipient thymectomized mice were immunocompetent and harbor alloreactive T cells. (c) Induction of skin allograft acceptance initially depended on the presence of IFN-gamma, CTLA4, and CD4(+) T cells. Addition of anti-CTLA4 or anti-IFN-gamma mAb to the protocol was associated with prompt graft rejection, whereas anti-IL-4 mAb had no effect. The role of IFN-gamma was confirmed using knockout mice. (d) Graft survival was associated with the absence of IFN-gamma in the graft. (e) Long-term graft maintenance required the continued presence of CD4(+) T cells. The results suggest that, with modification, our short-term protocol may yield a procedure for the induction of long-term graft survival without prolonged immunosuppression.

Blockade of CD40-CD154 interferes with human T cell engraftment in scid mice

Antibodies to the ligand for CD40 (CD154) have been shown to exert profound effects on the development of cell-mediated immune responses in mice. The present study shows that an antibody to human CD154 (hCD40L) inhibits in vivo Tetanus toxoid (TT) specific secondary antibody responses in hu-PBL-scid mice, as well as the expansion of xenoreactive human T cells in the scid mice. A possible cause for the reduced expansion of xenoreactive, human T cells, was the decreased expression of murine B7.1 and B7.2 caused by the administration of anti-hCD40L. Therefore, it may be that defective maturation of murine antigen-presenting cells impeded the priming and expansion of human xenoreactive T cells.

Expression in BALB/c and C57BL/6 mice of Rt6-1 and Rt6-2 ADP-ribosyltransferases that differ in enzymatic activity: C57BL/6 Rt6-1 is a natural transferase knockout

Several proteins with NAD+:arginine ADP-ribosyltransferase (ART) activity are expressed in T cells and affect their function. Rat T cells that express the ART designated RT6 are determinants of the expression of autoimmune diabetes. In the mouse, a 35-kDa ecto-ART modulates the proliferation and functional activity of CTL. Here we report on mouse ARTs designated Rt6-1 and Rt6-2 in BALB/c and C57BL/6 mice. mRNAs for Rt6-1 and Rt6-2 were found in spleen, thymus, and intestinal tissue of both strains, but Rt6-1 mRNA in C57BL/6 mice was detected only at low levels. Rt6-1 and Rt6-2 cDNAs from both strains were cloned and sequenced. Predicted amino acid sequences of Rt6-2 were identical in both strains, but there was an in-frame stop codon in the sequence of Rt6-1 in C57BL/6 mice not present in BALB/c mice. Recombinant C57BL/6 Rt6-2 and BALB/c Rt6-1 proteins expressed in COS1 cells exhibited ART activity and were documented to be glycosylphosphatidylinositol-linked membrane proteins. COS-1 cells transfected with a C57BL/6 Rt6-1 cDNA construct expressed a truncated protein consistent in size with that predicted by the presence of the stop codon. This approximately 21-kDa protein appeared not to be glycosylphosphatidylinositol linked to the cell surface and lacked ART activity. C57BL/6 Rt6-1 therefore appears to be a naturally occurring ART knockout. The expression of Rt6-1 and Rt6-2 mRNAs in lymphoid tissues suggests that these ARTs may regulate immune system functions. Expression of Rt6-2 or another redundant ART may compensate for the lack of enzymatically active Rt6-1 in C57BL/6 mice.

Expression in BALB/c and C57BL/6 mice of Rt6-1 and Rt6-2 ADP-ribosyltransferases that differ in enzymatic activity: C57BL/6 Rt6-1 is a natural transferase knockout

Several proteins with NAD+:arginine ADP-ribosyltransferase (ART) activity are expressed in T cells and affect their function. Rat T cells that express the ART designated RT6 are determinants of the expression of autoimmune diabetes. In the mouse, a 35-kDa ecto-ART modulates the proliferation and functional activity of CTL. Here we report on mouse ARTs designated Rt6-1 and Rt6-2 in BALB/c and C57BL/6 mice. mRNAs for Rt6-1 and Rt6-2 were found in spleen, thymus, and intestinal tissue of both strains, but Rt6-1 mRNA in C57BL/6 mice was detected only at low levels. Rt6-1 and Rt6-2 cDNAs from both strains were cloned and sequenced. Predicted amino acid sequences of Rt6-2 were identical in both strains, but there was an in-frame stop codon in the sequence of Rt6-1 in C57BL/6 mice not present in BALB/c mice. Recombinant C57BL/6 Rt6-2 and BALB/c Rt6-1 proteins expressed in COS1 cells exhibited ART activity and were documented to be glycosylphosphatidylinositol-linked membrane proteins. COS-1 cells transfected with a C57BL/6 Rt6-1 cDNA construct expressed a truncated protein consistent in size with that predicted by the presence of the stop codon. This approximately 21-kDa protein appeared not to be glycosylphosphatidylinositol linked to the cell surface and lacked ART activity. C57BL/6 Rt6-1 therefore appears to be a naturally occurring ART knockout. The expression of Rt6-1 and Rt6-2 mRNAs in lymphoid tissues suggests that these ARTs may regulate immune system functions. Expression of Rt6-2 or another redundant ART may compensate for the lack of enzymatically active Rt6-1 in C57BL/6 mice.

Prolonged survival of mouse skin allografts in recipients treated with donor splenocytes and antibody to CD40 ligand

Combined treatment with antibody against CD40 ligand and one transfusion of donor splenocytes prolonged survival of fully mismatched BALB/c skin allografts on C57BL/6 recipients, with approximately 20% of grafts surviving > 100 days. In vitro alloresponsiveness in treated animals was reduced in the immediate post-transplantation period, but by day 100 was increased despite the presence of a successful allograft. The presence of alloreactivity on day 100 was confirmed in vivo by adoptive transfer, which suggests that our protocol had induced either a state of "split tolerance" or "graft accommodation." Mice with skin grafts that had survived for > or = 100 days revealed no evidence of lymphoid chimerism. Treatment with donor splenocytes and antibody against CD40 ligand permits long-term survival of highly antigenic donor skin allografts despite the presence of functionally intact alloreactive lymphocytes.

Dietary cow's milk protein does not alter the frequency of diabetes in the BB rat

One theory of the pathogenesis of IDDM proposes that exposure to cow's milk proteins triggers the disease in genetically susceptible individuals. We tested this hypothesis in the BB/Wor rat model of human IDDM. Diabetes-prone (DP) BB/Wor rats spontaneously develop IDDM. Coisogenic diabetes-resistant (DR) BB/Wor rats do not develop diabetes spontaneously, but IDDM can readily be induced by treatment with polyinosinic:polycytidylic acid and depletion of RT6+ T-cells. Pregnant BB/Wor rats were fed one of four experimental diets or a standard Purina commercial rat chow (5010) that was certified to be free of cow's milk protein. Offspring were maintained on the maternal diet after weaning. DP-BB/Wor rats, fed either of two experimental diets based on hydrolyzed casein and free of intact milk protein (Nutramigen or D11236), developed diabetes at only half the rate of animals fed Purina 5010 chow. Neither the addition of bovine serum albumin (BSA) to Nutramigen nor the substitution of total milk protein for the hydrolyzed casein in the D11236 diet increased the frequency of spontaneous diabetes. In contrast, there was no relationship between diet and susceptibility of DR-BB/Wor rats to IDDM induction. However, the methods used to induce IDDM in DR-BB/Wor animals were found to induce antibodies against BSA. We conclude the following: 1) Dietary modification can reduce spontaneous IDDM expression in DP-BB/Wor rats, but the agent of protection is not elimination of cow's milk protein. 2) The addition of BSA or intact milk protein does not abrogate the effectiveness of a protective diet. 3) The genetic susceptibility of the DR-BB/Wor rat to autoimmune diabetes is unaffected by any of the tested diets, but a role of anti-BSA-like autoreactivity in IDDM expression cannot be excluded.

Characterization of mouse Rt6.1 NAD:arginine ADP-ribosyltransferase

Rat RT6 proteins, and perhaps mouse Rt6, identify a set of immunoregulatory T lymphocytes. Rat RT6.1 (RT6.1) and rat RT6.2 (RT6. 2) are NAD glycohydrolases, which catalyze auto-ADP-ribosylation, but not ADP-ribosylation of exogenous proteins. Mouse Rt6.1 (mRt6.1) also catalyzes auto-ADP-ribosylation. The activity of mouse cytotoxic T lymphocytes is reportedly inhibited by ADP-ribosylation of surface proteins, raising the possibility that mRt6 may participate in this process. The reactions catalyzed by mRt6, would, however, need to be more diverse than those of the rat homologues and include the ADP-ribosylation of acceptors other than itself. To test this hypothesis, mRt6.1 and rat RT6.2 were synthesized in Sf9 insect cells and rat mammary adenocarcinoma (NMU) cells. mRt6.1, but not rat RT6.2, catalyzed the ADP-ribosylation of guanidino-containing compounds (e.g. agmatine). Unlike RT6.2, mRt6.1 was a weak NAD glycohydrolase. In the presence of agmatine, however, the ratio of [adenine-14C]ADP-ribosylagmatine formation from [adenine-14C]NAD to [carbonyl-14C]nicotinamide formation from [carbonyl-14C]NAD was approximately 1.0, demonstrating that mRt6.1 is primarily a transferase. ADP-ribosylarginine hydrolase, which preferentially hydrolyzes the alpha-anomer of ADP-ribosylarginine, released [U-14C]arginine from ADP-ribosyl[U-14C]arginine synthesized by mRT6.1, consistent with the conclusion that mRt6.1 catalyzes a stereospecific Sn2-like reaction. Thus, mRt6.1 is an NAD:arginine ADP-ribosyltransferase capable of catalyzing a multiple turnover, stereospecific Sn2-like reaction.

The rat T-cell surface protein RT6 is associated with src family tyrosine kinases and generates an activation signal

RT6 is a glycosyl-phosphatidylinositol-linked surface molecule present on most mature rat T-cells. RT6+ T-cells can prevent the expression of autoimmune diabetes in the BB rat, but the mechanism is unknown. Because cross-linking of other glycosyl-phosphatidylinositol-linked T-cell proteins is known to activate T-cells, we investigated the signaling properties of RT6. Antibody cross-linking of RT6 enhanced expression of the alpha subunit of the interleukin-2 (IL-2) receptor and potentiated the proliferation of rat T-cells cultured in the presence of phorbol ester plus recombinant IL-2 (rIL-2) and/or rIL-4. RT6 was found to coimmunoprecipitate with five tyrosine phosphorylated proteins including p60fyn and p56lck, members of the src tyrosine kinase family. Pretreatment of T-cells with phorbol ester increased the phosphorylation of proteins that coimmunoprecipitated with RT6, altered the electrophoretic mobility of several of these coimmunoprecipitated phosphoproteins, and increased the amount of p60fyn and p56lck coimmunoprecipitated with RT6. These data indicate that RT6-mediated signaling events may prime T-cells to respond to exogenous cytokines, suggesting a possible mechanism by which surface RT6 may influence T-cell function.

Rat RT6.2 and mouse Rt6 locus 1 are NAD+: arginine ADP ribosyltransferases with auto-ADP ribosylation activity

RT6 is a glycosylphosphatidylinositol-linked protein found on the surface of mature rat T lymphocytes. Cells that express RT6 have an immunoregulatory function and modulate the expression of autoimmune diabetes mellitus in the BioBreeding rat. A homologue of the rat RT6 gene, designated Rt6, has been identified in the mouse, but expression of mouse Rt6 protein has not been documented. Rat RT6 is known to be a nicotinamide adenine dinucleotide (NAD+) glycohydrolase. We now report that rat RT6.2 and recombinant mouse Rt6 locus 1 proteins possess auto-ADP ribosylation activity. In addition, mouse Rt6 but not rat RT6, catalyzes the ADP ribosylation of exogenous acceptors such as histones. The ADP-ribosyl-protein bonds in auto-ADP-ribosylated rat RT6.2, auto-ADP-ribosylated mouse Rt6, and ADP-ribosylhistone synthesized by Rt6 were stable to HgCl2 and HCl, but labile to NH2OH, consistent with ADP ribosylarginine linkages. To determine if these enzymatic activities could affect the function of rat T cells, the effect of substrate availability on lymphocyte proliferation was examined. An inverse correlation was observed between NAD+ concentration in the medium and the ability of rat T cells to respond to anti-CD3, ConA, and PMA plus ionomycin. The data suggest that lymphocyte surface ADP ribosyltransferases could be involved in signaling and immunoregulatory processes.

Rat RT6.2 and mouse Rt6 locus 1 are NAD+: arginine ADP ribosyltransferases with auto-ADP ribosylation activity

RT6 is a glycosylphosphatidylinositol-linked protein found on the surface of mature rat T lymphocytes. Cells that express RT6 have an immunoregulatory function and modulate the expression of autoimmune diabetes mellitus in the BioBreeding rat. A homologue of the rat RT6 gene, designated Rt6, has been identified in the mouse, but expression of mouse Rt6 protein has not been documented. Rat RT6 is known to be a nicotinamide adenine dinucleotide (NAD+) glycohydrolase. We now report that rat RT6.2 and recombinant mouse Rt6 locus 1 proteins possess auto-ADP ribosylation activity. In addition, mouse Rt6 but not rat RT6, catalyzes the ADP ribosylation of exogenous acceptors such as histones. The ADP-ribosyl-protein bonds in auto-ADP-ribosylated rat RT6.2, auto-ADP-ribosylated mouse Rt6, and ADP-ribosylhistone synthesized by Rt6 were stable to HgCl2 and HCl, but labile to NH2OH, consistent with ADP ribosylarginine linkages. To determine if these enzymatic activities could affect the function of rat T cells, the effect of substrate availability on lymphocyte proliferation was examined. An inverse correlation was observed between NAD+ concentration in the medium and the ability of rat T cells to respond to anti-CD3, ConA, and PMA plus ionomycin. The data suggest that lymphocyte surface ADP ribosyltransferases could be involved in signaling and immunoregulatory processes.

Ontogeny and immunohistochemical localization of thymus-dependent and thymus-independent RT6+ cells in the rat

RT6 is a cell surface alloantigen that identifies a regulatory subset of peripheral T cells in the rat. Diabetes-prone BB rats are deficient in peripheral RT6+ T cells and develop spontaneous autoimmune insulin-dependent diabetes mellitus. Diabetes-resistant BB rats have normal numbers of RT6+ T cells, and insulin-dependent diabetes mellitus can be induced in these animals by in vivo depletion of peripheral RT6+ cells. Athymic rats are also severely deficient in peripheral RT6+ T cells. Although very different with respect to the peripheral RT6+ cell compartment, normal, athymic, and diabetes-prone BB rats all generate RT6+ intestinal epithelial lymphocytes (IELs). The goal of these studies was to analyze the ontogeny of RT6+ IELs and peripheral lymphoid cells by in situ immunohistochemistry. We observed the following. 1) RT6+ IELs appear before alpha(beta) T-cell-receptor- expressing IELs in diabetes-prone BB, diabetes-resistant BB, and athymic WAG rats. 2) In vivo depletion of peripheral RT6+ T cells in diabetes-resistant BB rats using a cytotoxic monoclonal antibody is not accompanied by depletion of RT6+ IELs. 3) A population of RT6+ T-cell-receptor-negative IELs is present in normal, euthymic diabetes-resistant BB rats, constitutes a larger percentage of the euthymic but lymphopenic diabetes-prone BB rat IEL population, and is the predominant IEL phenotype in athymic WAG rats. These results suggest that RT6+ cells are composed of both thymus-dependent and thymus-independent cell subsets that have different developmental characteristics and may differ in function.