Prevention of diabetes in the BB rat by essential fatty acid deficiency. Relationship between physiological and biochemical changes.

Essential fatty acid (EFA) deficiency exerts a striking protective effect in several animal models of autoimmune disease. We now report that EFA deprivation prevents diabetes in the BB rat, an animal model of human insulin-dependent diabetes mellitus. In diabetes-prone (DP)-BB rats, the incidences of spontaneous diabetes and insulitis (the pathological substrate of autoimmune diabetes) were greatly reduced by EFA deficiency. This beneficial effect of the deficiency state was also seen in diabetes-resistant (DR)-BB rats that, after treatment with antibody to eliminate RT6+ T cells, would otherwise have become diabetic. The susceptibility of EFA-deprived DP-BB rats to spontaneous diabetes was restored when they were given dietary supplements of linoleate at 70 d of age (during the usual period of susceptibility), but not when they were repleted beginning at 120 d (after the peak incidence of diabetes). EFA deficiency did lead to growth retardation, but calorically restricted control rats demonstrated that the protective effect of the deficiency state was not a function of decreased weight. To examine the relationship between the biochemical changes of EFA deficiency and its physiological effects in this system, we compared the fatty acid changes that occurred in EFA-deficient animals that did and did not develop diabetes. Nondiabetic animals had significantly lower levels of (n-6) fatty acids (i.e., linoleate and arachidonate) and higher levels of oleate, an (n-9) fatty acid, than did diabetic animals. Levels of 20:3(n-9), the fatty acid that uniquely characterizes EFA deficiency, were similar in both groups, however. Among diabetic EFA-deficient rats, the age at onset of diabetes was found to correlate inversely with the level of (n-6) fatty acids, the least depleted animals becoming diabetic earliest, whereas there was no correlation with levels of 20:3(n-9). Among animals repleted with linoleate beginning at 70 d, restoration of susceptibility to diabetes correlated with normalization of the level of arachidonate. In summary, EFA deprivation reduced the frequency of diabetes in both DP and RT6-depleted DR-BB rats. This protective effect was strongly associated with depletion of (n-6) fatty acids, particularly arachidonate, but not with accumulation of the abnormal 20:3(n-9). Conjecturally, arachidonate and/or a metabolite may play a key role in mediating inflammatory injury in this animal model of autoimmune diabetes.

Depletion of RT6.1+ T lymphocytes induces diabetes in resistant biobreeding/Worcester (BB/W) rats.

To investigate the role of RT6+ T cells in the pathogenesis of diabetes in BB/W rats, we treated animals from the diabetes-resistant (DR) subline with anti-RT6.1 lymphocytotoxic mAb. This depleted greater than 95% of peripheral RT6+ T cells but did not substantially reduce levels of circulating T cells or the in vitro response of spleen cells to mitogen. Treatment of 30-d-old DR BB/W rats in this way: induced insulitis and diabetes, rendered nondiabetic RT6-depleted DR rats susceptible to the adoptive transfer of diabetes by spleen cells from acutely diabetic BB/W rats, and yielded DR spleen cell populations capable of the adoptive transfer of diabetes to diabetes-prone (DP) or DR recipients. Treatment of DR rats beginning at 60 d of age failed to produce these effects. These results suggest that both susceptibility and resistance to diabetes in the BB/W rat are in part regulated by the RT6+ T cell subset and provide evidence for the importance of regulatory T lymphocytes in the pathogenesis of autoimmunity and diabetes in BB/W rats.

Induction of type I diabetes by Kilham's rat virus in diabetes-resistant BB/Wor rats.

Type I diabetes mellitus is an autoimmune disease resulting from the interaction of genetic and environmental factors. A virus that was identified serologically as Kilham's rat virus (KRV) was isolated from a spontaneously diabetic rat and reproducibly induced diabetes in naive diabetes-resistant (DR) BB/Wor rats. Viral antigen was not identified in pancreatic islet cells, and beta cell cytolysis was not observed until after the appearance of lymphocytic insulitis. KRV did not induce diabetes in major histocompatibility complex-concordant and discordant non-BB rats and did not accelerate diabetes in diabetes-prone BB/Wor rats unless the rats had been reconstituted with DR spleen cells. This model of diabetes may provide insight regarding the interaction of viruses and autoimmune disease [corrected]

Diabetes in the Bio-Breeding/Worcester rat. Induction and acceleration by spleen cell-conditioned media.

Injections of media conditioned by concanavalin A-activated spleen cells from acutely diabetic rats accelerated the appearance of diabetes in young Bio-Breeding/Worcester (BB/W) rats. Activity was also found in media conditioned by spleen cells from nondiabetic, W-line Wistar Furth and Buffalo rats. Unconditioned media containing mitogen had no activity. Conditioned media also induced diabetes in resistant W-line BB/W rats but not in Wistar Furth rats. A soluble factor may activate a BB lymphocyte population that promotes diabetes.

Spleen cell transfusion in the Bio-Breeding/Worcester rat. Prevention of diabetes, major histocompatibility complex restriction, and long-term persistence of transfused cells.

We report that transfusions of RT1u Wistar-Furth (WF) spleen cells prevented spontaneous diabetes in the RT1u BB/W rat while RT1b Buffalo rat spleen cells did not. In addition, donor origin WF T lymphocytes were detected in nondiabetic-susceptible BB/W recipients 5 mo after transfusion. Survival of donor-origin lymphocytes may provide the cellular mechanism by which major histocompatibility complex-compatible WF spleen cell transfusions prevent BB rat diabetes.

Recapitulation of normal and abnormal BB rat immune system development in scid mouse/rat lymphohemopoietic chimeras.

Mice homozygous for the mutation "severe combined immune deficiency" (C.B17-scid/scid) lack functional T and B lymphocytes and readily accept tumor xenografts. Partial lymphohemopoietic scid/human and mouse/rat chimeras have been described, but complete chimerization with thymic engraftment and generation of donor-origin thymocytes has not been achieved. We now report that low-dose irradiation permits the engraftment of BB rat fetal liver stem cells in scid recipients. We observed that BB rat fetal liver cells injected into irradiated scid mice establish a rat hemopoietic system in the scid mouse bone marrow and populate the scid mouse thymus. These stem cells generated rat-origin thymocytes that migrated to the scid mouse spleen, a peripheral lymphoid organ. Finally, we found that xenogeneic chimeras created using fetal liver cells from the abnormal (lymphopenic, diabetes prone) subline of BB rats recapitulated both the quantitative and phenotypic abnormalities of the donor rat. Xenogeneic lymphohemopoietic chimeras established in scid mice may provide a powerful new tool in the study of immune system development and autoimmunity.

High stimulatory activity of dendritic cells from diabetes-prone BioBreeding/Worcester rats exposed to macrophage-derived factors.

Dendritic cells (DC) present antigen and initiate T cell-mediated immune responses. To investigate the possible association of autoimmunity with DC function, we compared the accessory activity of splenic DC from Wistar/Furth (WF) and diabetes-prone (DP) BioBreeding (BB) rats. The latter develop autoimmune diabetes and thyroiditis. DC function was quantified in vitro by measuring T cell proliferation in mitogen-stimulated and mixed lymphocyte reactions. When purified without macrophage coculture, WF and DP DC displayed similar levels of accessory activity. In contrast, when purified by a method involving coculture with macrophages, DC from DP rats consistently displayed greater accessory activity. This finding could not be explained by morphological or phenotypic differences between DP and WF DC. In accessory activity assays performed after reciprocal DC cocultures with DP and WF macrophages, DP DC exhibited higher accessory activity irrespective of macrophage donor strain. We also compared the accessory activity of WF and DP DC cultured in the presence of conditioned medium and a mixture of IL-1 and GM-CSF. In all assays, DP DC exhibited higher accessory activity. In studies of (WF x DP) F1 hybrids, the high accessory activity of DP DC was observed to be heritable, and studies of WF and DP radiation chimeras indicated that the effect was an intrinsic property of the DP hematopoietic system. We conclude: (a) splenic DC from DP and WF rats possess similar basal levels of accessory potency; (b) after interaction with macrophages, DC of DP origin are capable of greater stimulatory activity than are WF DC; and (c) the mechanism responsible for this phenomenon involves differential responsiveness of DP and WF DC to macrophage-derived factors such as IL-1 and GM-CSF.

In vitro erythroid colony formation in acute myelogenous leukemia in the rat.

Erythroid colonies were grown in vitro in plasma clot cultures. Normal adult rat bone marrow responded to exogenous erythropoietin with the formation of an average of 2 colonies/10(3) cells plated. No erythroid colonies were observed in cultured normal spleen preparations. Shay chloro-leukemia cells administered iv induced an acute myelogenous leukemia. During the progressive stages of the disease, the numbers of erythrocyte colony forming units (CFU-E) in the marrow decreased; concomitantly, these progenitors appeared in leukemic spleen cultures. Paralleling changes in CFU-E, the numbers of nucleated red blood cells in the marrow declined but increased in the leukemic spleen. However, compensatory spleen erythropoiesis was transient, due to continued leukemia cell colonization. The loss of erythroid progenitor cells from the bone marrow played a significant role in the anemia associated with this leukemia.

Leukemic host influence on normal erythrocytic and granulocytic colony formation in in vivo plasma clot diffusion chamber cultures.

The effect of a leukemic environment on normal erythroid and granulocytic colony formation was examined in in vivo plasma clot diffusion chamber cultures implanted into Shay chloroleukemic rat hosts at varying stages of the disease. Normal bone marrow cells isolated in plasma clot diffusion chamber cultures in leukemic hosts displayed significant differences in the pattern of normal bone marrow colony growth. Granulocyte colony-forming units were significantly inhibited by leukemic hosts throughout the course of the disease. The size of developing colonies was reduced to under 100 cells; however, maturation within these clusters appeared unaffected. Erythroid colonies showed a slight inhibition during the early stages of the leukemia, a significant stimulation of 100 to 350% in the midleukemic period, and a significant inhibition of 50 to 65% during the terminal stages of the disease. Burst formation was also inhibited in the late leukemic stages. The transient increase in erythroid colony-forming units on Days 7 and 8 of the leukemia was concomitant with the onset of the anemia associated with the disease. Since the normal bone marrow cells were compartmentalized within the plasma clot diffusion chamber cultures, the suppression of erythroid and granulocytic colony development appears to be directly due to the release of diffusible inhibitory substances from the leukemic animal.

Speculations on etiology of diabetes mellitus. Tumbler hypothesis.

Although clinically useful, the conventional partition of diabetes mellitus into two major classes, insulin dependent and non-insulin dependent, has become obsolete from an etiological standpoint. Contemporary research, particularly that with advanced cellular and molecular methodologies, suggests that the expression of diabetes depends on a wide range of factors. We suggest that the etiology of diabetes has become analogous to the cylinder of a lock containing many tumblers. Each tumbler, e.g., environment, genetics, or cellular interactions, must be aligned before the key can be turned and an understanding of the etiologic process claimed.

T-lymphocyte requirement for diabetes in RT6-depleted diabetes-resistant BB rats.

Diabetes-prone (DP) BB rats develop spontaneous autoimmune insulin-dependent diabetes mellitus (IDDM). The cell populations involved in the expression of diabetes are not precisely known but probably include natural killer (NK) cells, macrophages, and T lymphocytes. Because the DP rat has few lymphocytes of the CD5+/CD+ phenotype, cytotoxic T lymphocytes (Tc) are not believed to be important in the process. Diabetes-resistant (DR) BB rats that are depleted of RT6+ T lymphocytes also become diabetic and provide an additional model of IDDM. We report that diabetes in DR rats depleted of RT6+ T lymphocytes is prevented by the concomitant depletion of either the CD5+ or the CD8+ population. In contrast, coadministration of anti-asialogangliosideM1 (alpha-ASGM1), an antiserum that principally recognizes NK cells, failed to prevent hyperglycemia in RT6-depleted rats. We propose that the initiation of diabetes in both DP and RT6-depleted DR rats is T-lymphocyte dependent. However, the final common pathway leading to autoimmune beta-cell destruction in IDDM may be different in these models. The RT6-depleted DR rat requires a cell that is sensitive to anti-CD8 (possibly a Tc), whereas the DP rat requires an anti-ASGM1-sensitive cell.

Role of host immune system in BB/Wor rat. Predisposition to diabetes resides in bone marrow.

The role of the immune system in the development of autoimmune diabetes mellitus in the BB/Wor rat was studied with bone marrow transplantation methodology. In the first experiment, diabetes-prone (DP) and diabetes-resistant (DR) BB/Wor rats were irradiated and reconstituted with bone marrow to create both reciprocal (DP donor----DR host; DR donor----DP host) and syngeneic (DR----DR; DP----DP) histocompatible chimeras. Both susceptibility and resistance to subsequent spontaneous diabetes in these chimeras were found to be a function of the type of donor bone marrow transplanted and not the genetic background of the host. In a second experiment, rats from three strains that share the RT1u major histocompatibility complex haplotype of the BB/Wor and rats from three non-RT1u strains were lethally irradiated and reconstituted with DP BB/Wor bone marrow. To rapidly induce diabetes and/or insulitis, they were then injected with mitogen-activated spleen cells from acutely diabetic DP BB/Wor donors, with standard passive-transfer methods. Diabetes and pancreatic insulitis were observed in RT1u recipients, whereas non-RT1u rats developed insulitis but not diabetes. The data suggest that predisposition to spontaneous diabetes in BB rats resides in bone marrow cells.

Altered expression of diabetes in BB/Wor rats by exposure to viral pathogens.

Autoimmune diabetes mellitus affects greater than 50% of diabetes-prone BB (DP BB) rats but less than 1% of diabetes-resistant BB (DR BB) rats. We report an outbreak of spontaneous diabetes among DR BB rats that coincided with serologic evidence of the onset of viral infection. This apparent link between a change in the environment and the expression of diabetes then led us to study the interaction of environmental exposure to viral pathogens in this disorder with virally seropositive and seronegative populations of BB rats and polyinosinic-polycytidylic acid (poly I:C), an interferon inducer known to accelerate diabetes onset in DP rats. We administered a cytotoxic anti-RT6 monoclonal antibody, poly I:C, or both to DR rats. Depletion of the RT6.1+T-lymphocyte population has previously been shown to induce diabetes and thyroiditis in DR rats. RT6 alone did not induce diabetes in seronegative DR rats, and poly I:C was only weakly effective, but nearly all animals given both reagents became diabetic. When given to seropositive DR rats, either reagent alone induced diabetes; when given to non-BB rats, neither agent was effective. Poly I:C also accelerated the onset of DP diabetes to a greater extent in seropositive than in seronegative rats. We conclude that expression of the genetic predisposition to diabetes present in all BB rats depends on cellular factors that include the presence or absence of regulatory (RT6+) T lymphocytes and modulatory environmental factors including exposure to viral pathogens.

Biochemical studies of RT6 alloantigens in BB/Wor and normal rats. Evidence for intact unexpressed RT6a structural gene in diabetes-prone BB rats.

Lymphocytes bearing the T-lymphocyte differentiation antigen RT6 play an important immunoregulatory role in the development of autoimmune diabetes in BB rats. Immunofluorescence studies suggest that diabetes-prone (DP)- but not diabetes-resistant (DR)-BB rat lymphocytes fail to express RT6 antigen during ontogeny. Two alloantigenic forms of the molecule exist, i.e., RT6.1 and RT6.2; both are linked to cell membranes by a phosphatidylinositol (PI) linkage. In these studies, PI-phospholipase C (PLC) treatment of lymphocytes from BB and normal rats followed by immunoabsorption and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of released proteins with anti-RT6 allotype-specific monoclonal antibodies was performed. RT6.1 in several nondiabetic rat strains was found to consist of a family of nonglycosylated and variably glycosylated molecules: an N-Glycanase-resistant 24,000- to 26,000-Mr peptide and four N-Glycanase-sensitive peptides of 29,000, 31,000, 33,000, and 34,000 Mr. In contrast, RT6.2 was found to be a 24,000- to 26,000-Mr nonglycosylated polypeptide. The electrophoretic pattern of RT6.1 was observed to be the same when the antigen was extracted from W3/25+ (CD4+) versus W3/25- T lymphocytes or from resting versus mitogen-activated cells. A pattern of bands characteristic of the RT6.1 antigen found in normal rat strains was detected after PLC treatment or detergent solubilization of lymphocytes obtained from DR rats. In contrast, no evidence of either RT6 species was found after PLC or detergent treatment of comparable numbers of T lymphocytes from DP-BB rats. Interestingly, T lymphocytes from Wistar-Furth (RT6.2+) x DP (RT6-) F1 crosses were observed to coexpress both RT6.2 and RT6.1 molecules, with the electrophoretic pattern of RT6.1 being similar to that obtained in DR and other rat strains. This study provides biochemical evidence that DP rats may have an intact RT6a structural gene.

Intrinsic cytotoxicity of natural killer cells to pancreatic islets in vitro.

BB rats develop spontaneous autoimmune insulin-dependent diabetes mellitus that is similar to human insulin-dependent diabetes. In this study, we used an in vitro islet cell cytotoxicity assay to study the possible role of natural killer (NK) cells and their soluble effector molecules in this disorder. First, the results demonstrated that in vivo treatment of acutely diabetic BB rats with anti-asialogangliosideM1 (an NK cell antiserum) but not with anti-T-lymphocyte antibodies reduces spleen cell cytotoxic activity to islets in vitro. Flow microfluorometry (FMF)-sorting experiments were then used to confirm that the splenic cytotoxic effector cell in acutely diabetic BB rats is a CD8+/CD5- NK cell. Further analysis demonstrated that both FMF-sorted NK cell populations from Wistar-Furth rats and unfractionated spleen cells from athymic nu/nu rats with high intrinsic NK cell activity also exhibit high islet cell cytotoxic activity in vitro. Finally, we found that the kinetics and differential cytotoxic activity of NK cells toward islets in vitro could be mimicked by NK cell culture supernatants containing high levels of NK cytotoxic factor (NKCF). The islet cytotoxic activity of these culture supernatants was specifically inhibited by the addition of anti-NKCF monoclonal antibody. These results demonstrate that NK cells from diabetic and nondiabetic rats are cytotoxic to islet cells in vitro. They further suggest that this cytotoxic effect may be mediated in part through the production and release of soluble factors such as NKCF.

An RT6a gene is transcribed and translated in lymphopenic diabetes-prone BB rats.

T-cells expressing the RT6 surface alloantigen appear to perform important immunoregulatory functions in the rat. Diabetes-prone BB rats lack circulating RT6+ T-cells and spontaneously develop autoimmune diabetes mellitus and thyroiditis. The coisogenic diabetes-resistant BB rat does circulate RT6+ T-cells and is free of disease. Transfusions leading to engraftment of RT6+ T-cells prevent both diabetes and thyroiditis in the diabetes-prone rat. To investigate the absence of this subset in the lymphopenic BB rat, we used both molecular and biochemical procedures and made the following observations: 1) an mRNA encoding RT6 protein is present in diabetes-prone spleen cells; 2) nucleotide sequencing of this transcript reveals an intact coding sequence for the RT6.1 alloantigen; 3) sensitive chemiluminescent assay of diabetes-prone lymph node cell detergent extracts shows that diabetes-prone RT6 mRNA is translated in vivo; 4) quantitatively, diabetes-prone lymph node cells express < or = 10% of the RT6.1 protein found on similar numbers of diabetes-resistant BB cells; and 5) finally, we obtained evidence of an intact phosphatidylinositol linkage of the molecule to the cell surface and successfully immunoprecipitated the phosphatidylinositol-linked protein with DS4.23 monoclonal antibody, indicating that the RT6.1 antigen is correctly processed and folded in diabetes-prone lymph node cells. We conclude that the near total absence of RT6+ T-cells in the diabetes-prone BB rat is unlikely to be because of a defect in RT6 gene expression per se. Defects in RT6 gene regulation or other cellular defects leading to premature cell death in the T-cell lineage, alone or in combination, may instead be responsible.

Prevention of diabetes in BB/Wor rat by single transfusion of spleen cells. Parameters that affect degree of protection.

Previous studies have shown that multiple transfusions of spleen cells from histocompatible nondiabetic donors prevent autoimmune diabetes mellitus in diabetes-prone (DP) BioBreeding/Worcester (BB/Wor) rats. In this study, a single transfusion of greater than or equal to 50 x 10(6) cells from either diabetes-resistant (DR) BB/Wor or Wistar-Furth (WF) rats substantially reduced the incidence of diabetes when given to DP rats 27 or 46 days old but not 61 days old. Transfusion and protection were associated with the appearance of RT6+ donor lymph node cells in recipient rats. In vivo depletion of RT6+ T-lymphocytes in 150-day-old protected animals did not produce diabetes. DR BB/Wor and WF spleen cells were equally efficacious when given either intraperitoneally or intravenously. Mitogen-activated spleen cells were relatively less effective than untreated cells. We conclude that BB rat diabetes can be prevented by one transfusion of spleen cells from histocompatible DR and WF donors, and that the protective effect is dependent on recipient age and cell dose. The effect may be mediated by a population of RT6+ T-lymphocytes that, during a critical developmental period, regulate the expression of autoimmunity in these animals.

Autoimmune destruction of islets transplanted into RT6-depleted diabetes-resistant BB/Wor rats.

We report a novel animal model of islet transplantation that distinguishes recurrence of autoimmunity from allograft rejection. In this study, diabetes-resistant (DR) BB rats, less than 1% of which develop spontaneous diabetes, were made hyperglycemic by either a single injection of streptozocin (STZ) or in vivo immune elimination of a regulatory T-lymphocyte subset that expresses the RT6 alloantigen. DR islet grafts were then transplanted into both groups. DR transplants into STZ-induced diabetic DR rats produced long-term normoglycemia. In contrast, DR transplants into DR rats that had been treated with anti-RT6 monoclonal antibody were all destroyed within an average of 4 days. Allogeneic islets transplanted into both STZ-induced and RT6-depleted diabetic DR rats were rejected within a mean of 3 days. We conclude that failure of DR islet grafts in RT6-depleted diabetic DR BB rats represents recurrent autoimmunity.

Evidence that intrathymic islet transplantation does not prevent diabetes or subsequent islet graft destruction in RT6-depleted, diabetes-resistant BioBreeding/Worcester rats.

Pancreatic islet allografts transplanted intrathymically are accepted and restore normoglycemia in streptozotocin-diabetic rats given one injection of antilymphocyte serum. Intrathymic allografts similarly restore normoglycemia in diabetes-prone (DP) Bio-Breeding (BB) rats that have developed spontaneous autoimmune diabetes. Intrathymic islets also reduce the frequency of subsequent diabetes when transplanted prophylactically into young DP rats. These findings suggest that intrathymic transplantation can prevent not only allograft rejection, but also the appearance and recurrence of autoimmune tissue destruction. To explore these hypotheses further, we attempted both to confirm previous studies and to extend them to another model of autoimmune diabetes, the RT6-depleted diabetes-resistant (DR) BB rat. Fewer than 1% of DR-BB rats develop spontaneous diabetes, but most become hyperglycemic after in vivo immune elimination of RT6+ T cells. Using the protocols described in the literature, we observed the following: (1) Consistent with previous reports, intrathymic islet allografts survived indefinitely in streptozotocin-diabetic, antilymphocyte serum-treated, non-BB recipient rats. (2) Consistent with previous reports, intrathymic islet grafts produced long-term normoglycemia in diabetic DP-BB rats and also reduced the frequency of spontaneous diabetes in young animals transplanted prophylactically. (3) In contrast, intrathymic islets (iso- and allografts) neither prevented nor reversed diabetes in RT6-depleted DR rats. We hypothesize that intrathymic islet grafts survive in DP-BB rats because they are lymphopenic and immunocompromised, whereas immunocompetent diabetic DR rats successfully recapitulate the autoimmune disease process. Although intrathymic allograft transplantation is postulated to induce a state of tissue-specific tolerance, our results indicate that this tolerant state may not extend to autoimmune destruction of either isografts or allografts.

Effects of irradiation on diabetes in the BB/Wor rat.

Lymphoid irradiation is known to prevent spontaneous autoimmune diabetes in susceptible BB rats. The present studies investigated further the effects of radiation in diabetes prone (DP) and resistant (DR) BB/Wor rats, and histocompatible Yoshida (YOS) rats. Single doses of total body gamma irradiation (125-600 rads) induced diabetes within 22-44 days in 20 of 102 (20%) 30 day old DR rats, less than 1% of which develop the disease. Radiation was also associated with (1) a reduction in the ratio of W3/25+ to OX8+ peripheral blood lymphocytes within 2 weeks, and (2) a decreased percentage of lymph node cells expressing the RT6.1 surface alloantigen 3-4 weeks after treatment. Similar doses of irradiation did not alter the frequency or age at onset of diabetes in DP rats, and did not induce diabetes in YOS rats. When a single dose of 250 or 500 rads of gamma irradiation was followed by injection of mitogen activated spleen cells from acutely diabetic rats to adoptively transfer diabetes, 16 of 19 (84%) DR and 8 of 14 (57%) YOS rats became diabetic. Long term exposure to ultraviolet irradiation (UVB) did not alter the frequency or age at onset of diabetes in either DP or DR rats. We conclude that there may exist a population of regulatory cells relatively sensitive to gamma irradiation that play a role in determining the susceptibility of rats to autoimmune diabetes mellitus.