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MHC Class II Molecules Play a Role in the Selection of Autoreactive Class I-Restricted CD8 T Cells That Are Essential Contributors to Type 1 Diabetes Development in Nonobese Diabetic Mice¹

David V. Serreze^2,*, T. Matthew Holl^*, Michele P. Marron^*, Robert T. Graser^*, Ellis A. Johnson^*, Caroline Choisy-Rossi^*, Robyn M. Slattery, Scott M. Lieberman and Teresa P. DiLorenzo^,

^* The Jackson Laboratory, Bar Harbor, ME 04609; John Curtin School of Medical Research, Canberra, Australia; and Departments of Microbiology and Immunology and Medicine, Division of Endocrinology, Albert Einstein College of Medicine, Bronx, NY 10461

	Abstract

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Development of autoreactive CD4 T cells contributing to type 1 diabetes (T1D) in both humans and nonobese diabetic (NOD) mice is either promoted or dominantly inhibited by particular MHC class II variants. In addition, it is now clear that when co-expressed with other susceptibility genes, some common MHC class I variants aberrantly mediate autoreactive CD8 T cell responses also essential to T1D development. However, it was unknown whether the development of diabetogenic CD8 T cells could also be dominantly inhibited by particular MHC variants. We addressed this issue by crossing NOD mice transgenically expressing the TCR from the diabetogenic CD8 T cell clone AI4 with NOD stocks congenic for MHC haplotypes that dominantly inhibit T1D. High numbers of functional AI4 T cells only developed in controls homozygously expressing NOD-derived H2^g7 molecules. In contrast, heterozygous expression of some MHC haplotypes conferring T1D resistance anergized AI4 T cells through decreased TCR (H2^b) or CD8 expression (H2^q). Most interestingly, while AI4 T cells exert a class I-restricted effector function, H2^nb1 MHC class II molecules can contribute to their negative selection. These findings provide insights to how particular MHC class I and class II variants interactively regulate the development of diabetogenic T cells and the TCR promiscuity of such autoreactive effectors.

	Introduction

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Type 1 diabetes (T1D)³ in both humans and nonobese diabetic (NOD) mice results from T cell-mediated autoimmune destruction of insulin-producing pancreatic cells and is under complex polygenic control (reviewed in Refs. 1 and 2). However, particular MHC haplotypes provide the primary genetic component for susceptibility or resistance to T1D. Within the human MHC, particular combinations of HLA-DQ and DR class II alleles provide a large component of T1D susceptibility by mediating the selection and functional activation of cell-autoreactive CD4 T cells (reviewed in Ref. 3). In contrast, other class II variants, in particular DQ6, provide dominant T1D resistance (reviewed in Ref. 3). Similarly, T1D development in NOD mice is also dependent upon the class II variants characterizing their H2^g7 MHC haplotype (reviewed in Refs. 1 and 2). This includes both the expression of H2-A^g7 and an absence of H2-E class II molecules on NOD APC. Such a class II expression pattern contributes to T1D in a recessive manner because disease is inhibited in NOD mice carrying transgenes encoding H2-A variants derived from other MHC haplotypes or that restore H2-E expression (4, 5, 6, 7, 8, 9). Reported mechanisms of T1D resistance mediated by dominantly protective MHC class II alleles transgenically or congenically expressed in NOD mice include both the intrathymic deletion of cell-autoreactive CD4 T cells (10, 11, 12) and the selection of regulatory T cells (7).

It is becoming increasingly clear that certain MHC class I alleles can also contribute to susceptibility or resistance to T1D. Indeed, while they represent common alleles found in many strains lacking autoimmune proclivity, when the K^d and D^b class I variants characterizing the H2^g7 haplotype are expressed in NOD mice, they aberrantly mediate CD8 T cell responses against pancreatic cells that are absolutely essential to T1D development (13, 14, 15, 16). Similarly, there is increasing evidence from extended HLA haplotype analyses that the risk of T1D development in humans is further increased when certain common MHC class I variants are expressed in conjunction with particular class II alleles (17, 18, 19, 20, 21, 22, 23). It was subsequently shown that when transgenically expressed in NOD mice, the common human HLA-A2.1 class I variant mediates cell-autoreactive T cell responses that accelerate the onset of T1D (24). This provided the first functional evidence that, in the proper genetic context, some common human MHC class I variants can aberrantly exert diabetogenic activities. However, T1D was suppressed in NOD mice transgenically expressing the human HLA-B27 class I variant (24). Hence, there is clearly a need to more completely understand how the development and function of MHC class I-restricted diabetogenic CD8 T cells is regulated.

In the present study, a previously developed genetic resource was employed to gain an increased understanding about the MHC regulation of diabetogenic CD8 T cell development and function. This resource is a stock of NOD mice transgenically expressing the TCR from the CD8 T cell clone, AI4, originally identified as a K^d MHC class I-restricted effector contributing to the earliest phases of autoimmune cell destruction leading to TID (designated NOD.AI4 transgenic (Tg) mice) (16, 25). Due to allelic exclusion, these NOD.AI4 Tg mice mainly produce AI4 clonotypic CD8 T cells that are capable of independently and rapidly mediating T1D development (25). T cells expressing the AI4 TCR can be easily followed by flow cytometry. This allowed us to cross NOD.AI4 Tg mice with a series of other NOD stocks congenic or transgenic for non-H2^g7 MHC alleles to determine whether they could influence the development or function of a diabetogenic CD8 T cell clonotype. It was found that when forced to mature in the presence of APC expressing various collections of MHC genes conferring resistance to T1D, cell-autoreactive AI4 CD8 T cells either underwent intrathymic deletion or were functionally inactivated. Of greatest interest was the finding that while the cell-autoreactive effector function of AI4 T cells is MHC class I dependent, this clonotype can be deleted through interactions with certain class II variants.

	Materials and Methods

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Mice

NOD/LtDvs mice (H2^g7 = K^d, A^g7, E^null, D^b) are maintained at The Jackson Laboratory by brother-sister mating. Currently, T1D develops in 90% of female and 63% of male NOD mice before a year of age. T and B lymphocyte-deficient NOD-scid mice (26) are maintained at the N11 backcross generation. Previously described stocks (27, 28) of T1D-resistant NOD mice congenic for either the H2^nb1 (K^b, A^nb1, E^k, D^b) or H2^q (K^q, A^q, E^null, D^q) haplotypes are both maintained at the N21 backcross generation. A previously described stock (29) of T1D-resistant NOD mice congenic for the H2^b (K^b, A^b, E^null, D^b) haplotype is maintained at the N15 backcross generation. T1D-resistant NOD mice congenic for a previously described (30) class II-deficient H2^b haplotype (K^b, A^null, E^null, D^b) are maintained at the N11 backcross generation (designated NOD.H2^b-Ab⁰ mice). NOD mice transgenically expressing the AI4 TCR (V8/V2), and a substock congenic for a functionally inactivated Rag1 gene (designated NOD.AI4 Tg or NOD.Rag1^null.AI4 Tg mice) have also both been previously described (25, 31). A NOD stock transgenically expressing the murine H2-K^b class I variant was generated for use in some experiments (designated NOD-K^b Tg). The H2-K^b transgene was derived from a 7.5-kb genomic clone (32). Dormant 150-bp loxP sites were inserted into the HincII site 5' of exon 1 and the BamHI site in intron 3. At the 3' end of the construct there is a 74-bp tail containing the 5' end of the HSV-1 thymidine kinase gene. The K^b transgene construct was microinjected directly into NOD zygotes as previously described (33). Expression of the K^b transgene does not alter the incidence of T1D development in NOD mice.

Assessment of T1D development

T1D development in the indicated mice was defined by glycosuric values of 3 as assessed with Ames Diastix (kindly supplied by Miles Diagnostics, Elkhart, IN).

T cell subset enumerations

The proportion of PBL or splenocytes from the indicated mice that expressed the transgenic AI4 TCR was assessed by multicolor flow cytometric techniques (FACScan; Becton Dickinson, San Jose, CA) using the CellQuest 3.0 data reduction system. Expression of the transgenic AI4 TCR - and -chain was detected by respective use of red fluorescent PE- or green fluorescent FITC-conjugated Abs specific for TCR V8 (B21.14) or V2 (B20.6) elements. Other studies enumerated various T cell populations within spleens and thymii of the indicated bone marrow chimeras. Three-color flow cytometric analysis of thymocytes was performed using Abs directed against CD4, CD8, and TCR to compare T cell differentiation in each chimeric class. CD8 expression was detected with the mAb 53-6.72 conjugated to a green fluorescent FITC tag. CD4 expression was detected with biotinylated mAb GK1.5 that was developed subsequently with a red fluorescent streptavidin-Red-670 tag (Life Technologies, Rockville, MD). Presence of a rearranged TCR complex on CD4/CD8 double-positive thymocytes was detected with the mAb H57-597 conjugated to a red fluorescent PE tag whose fluorescence intensity can be readily distinguished from that of Red-670. For detection of total peripheral T cells, splenocytes were stained with the FITC-conjugated TCR-specific mAb. Total T cells were further characterized for CD4 expression using the mAb GK1.5 conjugated to the red fluorescent tag Cy3.18-OSu (Cy3; Biological Detection Systems, Pittsburgh, PA) or for CD8 expression with the mAb 53-6.72 conjugated to PE whose red fluorescence intensity can easily be distinguished from that of Cy3. Splenic or peripheral blood CD8 T cells expressing the transgenic AI4 TCR were detected by costaining with the FITC-labeled CD8-specific Ab and the PE-conjugated TCR V8-specific Ab. Less than 1% of CD8 T cells in standard NOD mice express the TCR V8 element.

Bone marrow chimeras and secondary adoptive transfer of CD8 T cells

The indicated mice were lethally irradiated (1200 rad from a ¹³⁷Cs source) at 4–6 wk of age and reconstituted as previously described (34) with single or 1:1 mixtures of various populations of bone marrow cells (5 x 10⁶ cells of each type). Chimeras were assessed for diabetes development through 6 wk postreconstitution. Upon diabetes development or at 6 wk reconstitution, the chimeras were assessed for proportions of various splenic T cell populations as described above. In some experiments, CD8 T cells were purified from spleens of the indicated chimeras and assessed for ability to adoptively transfer T1D to NOD-scid recipients (10⁶ CD8 T cells/recipient). CD8 T cells were purified by the previously described magnetic bead-based negative selection approach (35). The only change was that the negative selection mixture included a biotinylated Ab specific for CD4 (GK1.5) rather than CD8. Flow cytometric analyses indicated CD8 T cell purity was >95%.

AI4 T cell proliferation assay

CD8 T cells (>90% expressing the AI4 TCR) were purified from spleens of the indicated mice by the magnetic bead system described above. Triplicate aliquots of 5 x 10⁴ CD8 T cells were then seeded into flat-bottom 96-well plates in 200 µl of the previously described medium (16) also containing 2 x 10⁵ irradiated (2000 rad) NOD splenic leukocytes and an AI4 mimotope peptide (YFIENYLEL; S.M.L. and T.P.D., manuscript in preparation) at concentrations ranging from 0 to 100 nM. Following a 48-h incubation at 37°C in a 95% air/5% CO₂ humidified atmosphere, the cultures were pulsed with 1 µCi/well [³H]thymidine for an additional 24 h. The cultures were then harvested and [³H]thymidine incorporation determined using a LKB Betaplate 1205 system (LKB Instruments, Gaithersburg, MD). Data are presented as mean cpm ± SEM of the triplicate cultures.

MHC interaction assay

The ability of various MHC gene products associated with TID resistance to interact with the AI4 TCR was assessed through an in vitro-mixed leukocyte assay system. Triplicate aliquots of 5 x 10⁵ splenic leukocytes from NOD.Rag1^null.AI4 Tg mice were seeded in flat-bottom 96-well plates either in 200 µl of medium alone or with an equal number of irradiated (2000 rad) splenic leukocytes from the indicated NOD MHC congenic stocks and then incubated for 5 days at 37°C. In some experiments, the stimulatory splenocytes were pretreated with mAbs directed against the indicated MHC variants. The extent of activation through the AI4 TCR was then assessed by measuring IFN- levels in pooled supernatants from each triplicate culture with a commercially available ELISA kit (PharMingen, San Diego, CA).

	Results

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Heterozygous expression of the diabetes protective H2^nb1 MHC haplotype blocks the development of AI4 T cells

One mechanism by which expression of a non-H2^g7 MHC haplotype dominantly inhibits T1D development in NOD mice is by inducing intrathymic negative selection of class II-restricted cell-autoreactive CD4 T cells (10, 11). However, it was unknown if T1D protection elicited by expression of non-H2^g7 MHC gene products also entails the negative selection of class I-restricted cell-autoreactive CD8 T cells. To initially test this possibility, we determined if heterozygous expression of the H2^nb1 haplotype led to decreased development of AI4 T cells. As expected based on previously published results (25), 90% (9/10) of standard NOD.AI4 Tg females developed T1D by 10 wk of age (Table I). In contrast, 0% (0/5) of (NOD.H2^nb1 x NOD.AI4 Tg)F₁ females developed T1D through 30 wk of age. T1D resistance in (NOD.H2^nb1 x NOD.AI4 Tg)F₁ mice was associated with significantly lower levels of AI4 CD8 T cells among PBL (0.93 ± 0.38%) than in the standard NOD.AI4 Tg controls (20.9 ± 1.0%). However, PCR analyses confirmed these F₁ hybrids carried both the AI4 TCR - and -chain transgenes. Two experiments indicated the ability of the H2^nb1 haplotype to block the development of AI4 T cells is unlikely to be a superantigen-mediated event. Both were based on previous reports that any given endogenous superantigen preferentially engages and induces the deletion of all T cells expressing particular TCR V elements (reviewed in Ref. 36). However, heterozygous H2^nb1 expression did not induce a reduction in T cells only expressing the transgenic V2 chain of the AI4 TCR (data not shown). Furthermore, the total proportion of T cells expressing the TCR V2 element in nontransgenic (NOD.H2^nb1 x NOD)F₁ hybrids (5.3 ± 0.5%, n = 4) was actually slightly higher than in NOD parental controls (4.5 ± 0.2%, n = 4). These collective findings indicated one mechanism by which MHC haplotypes other than H2^g7 confer dominant protection against T1D in NOD mice is by limiting the production of pancreatic cell-autoreactive CD8 T cells such as the AI4 clonotype.

View this table: [in this window] [in a new window]   Table I. Heterozygous H2nb1 expression blocks the development of T1D and AI4 clonotypic T cells in NOD.AI4 Tg mice

A bone marrow chimera system was employed to determine whether heterozygous H2^nb1 expression limited the development of AI4 T cells through impaired positive selection or enhanced negative selection. The approach taken was to determine whether AI4 T cell development was inhibited when H2^nb1 molecules were heterozygously expressed on thymic epithelium or bone marrow-derived APC that respectively are the most efficient mediators of positive and negative selection (37). Positive controls consisted of NOD mice reconstituted with NOD.AI4 Tg marrow. As expected, at 6 wk postreconstitution this positive control group was characterized by high numbers of AI4 clonotypic CD8 T cells in the spleen (Fig. 1A). Negative controls consisted of (NOD.H2^nb1 x NOD)F₁ mice reconstituted with (NOD.H2^nb1 x NOD.AI4 Tg)F₁ marrow. These negative controls heterozygously expressed H2^nb1 on both thymic epithelium and APC and were characterized by significantly fewer splenic AI4 CD8 T cells than in the positive controls. One test group consisted of (NOD.H2^nb1 x NOD)F₁ mice reconstituted with NOD.AI4 Tg marrow. In this group, developing AI4 T cells only encountered H2^nb1 molecules on nonhemopoietically derived cells including thymic epithelium. The number of splenic AI4 CD8 T cells detected in this group did not statistically differ from the positive controls. Hence, heterozygous expression of H2^nb1 molecules on thymic epithelium does not limit the positive selection of AI4 CD8 T cells. The second experimental group consisted of NOD mice reconstituted with (NOD.H2^nb1 x NOD.AI4 Tg)F₁ marrow. In this case, developing AI4 T cells only encountered H2^nb1 molecules on hemopoietically derived cells including thymic APC. The number of splenic AI4 CD8 T cells in this latter group was significantly less than in the positive controls. Thus, heterozygous expression of H2^nb1 on hemopoietically derived cells appeared to induce the negative selection of AI4 CD8 T cells.

View larger version (20K): [in this window] [in a new window]   FIGURE 1. Heterozygous H2nb1 expression on hemopoietically derived APC results in the negative selection of AI4 T cells. The ability of AI4 T cell development to be blocked when H2nb1 MHC molecules were separately or jointly expressed on thymic epithelial and hemopoietically derived cells was assessed through a bone marrow chimera approach. Positive control chimeras lacking H2nb1 expression consisted of NOD mice reconstituted with NOD.AI4 Tg marrow (n = 5). Negative controls expressing H2nb1 molecules on both thymic epithelial and hemopoietically derived cells consisted of (NOD.H2nb1 x NOD)F1 mice reconstituted with (NOD.H2nb1 x NOD.AI4 Tg)F1 marrow (n = 6). Chimeras in which thymic expression of H2nb1 molecules was restricted to epithelial cells consisted of (NOD.H2nb1 x NOD)F1 mice reconstituted with NOD.AI4 Tg marrow (n = 3). Chimeras in which H2nb1 molecules were only expressed on hemopoietically derived cells consisted of NOD mice reconstituted with (NOD.H2nb1 x NOD.AI4 Tg)F1 marrow (n = 6). At 6 wk postreconstitution, the number of splenic CD8 (A) and thymic CD4/CD8 double-positive cells (B) expressing the AI4 TCR was determined by flow cytometry.

The H2^nb1 class I-matched H2^b and H2^b-Ab⁰ haplotypes do not mediate the negative selection of AI4 T cells

Given the fact they are a CD8 clonotype, we originally hypothesized it was a class I variant encoded within the H2^nb1, but not the H2^g7 haplotype, that provided a negatively selecting ligand for AI4 T cells. K^b represents the only known H2^nb1-encoded class I variant not shared with the NOD H2^g7 haplotype (both encode a D^b class I gene product) (38). Thus, we tested if heterozygous expression of another K^b-encoding MHC haplotype (H2^b) could also mediate the negative selection of AI4. Surprisingly, AI4 T cell numbers were not reduced in (NOD.H2^b x NOD.AI4 Tg)F₁ mice (Table II). This unexpected outcome led us to hypothesize that while the peripheral effector function of AI4 T cells is MHC class I restricted, during thymic maturation the AI4 TCR can engage class II variants encoded by the H2^nb1 but not the H2^b haplotype in a negatively selecting fashion.

Expression of a second TCR does not account for the negative selection of class I-restricted AI4 diabetogenic T cells by an H2^nb1 class II variant

Because the process of allelic exclusion is incomplete, T cells from TCR Tg mice, including the NOD.AI4 Tg stock, can express an additional endogenously derived TCR (25, 39, 40, 41). Hence, it was possible that engagement of such a second TCR allowed AI4 T cells to be negatively selected by an H2^nb1 class II variant. To address this issue, we generated a NOD.AI4 Tg stock that was also homozygous for a functionally inactivated Rag1 allele that blocks all endogenous TCR gene rearrangements. The diabetes frequency by 6 wk of age in NOD.Rag1^null.AI4 Tg female mice was found to be 88.2% (30/34). A mixed bone marrow chimera system was then employed to determine whether T cells only expressing the AI4 TCR were negatively selected by hemopoietically derived APC-expressing H2^nb1 class II or other MHC variants.

Experimental chimeras consisted of female F₁ hybrids between NOD mice and NOD stocks congenic for the indicated MHC haplotypes that were reconstituted with a 1:1 mixture of syngeneic F₁ and NOD.Rag1^null.AI4 Tg bone marrow cells, both obtained from female donors. This was done to eliminate any possibility of residual host vs graft responses that might limit the engraftment of APC heterozygously expressing non-H2^g7 MHC gene products with a potential to modulate AI4 T cell development. Positive controls consisted of NOD mice reconstituted with a 1:1 mixture of NOD and NOD.Rag1^null.AI4 Tg bone marrow. It had been our experience in similar studies that maximal, stable levels of chimerization are achieved by 6 wk postreconstitution (42). However, eight of 12 mice in this positive control group developed T1D before the 6-wk postreconstitution time point (Fig. 2A). At the time of disease onset, the proportion of CD8 T cells that expressed the AI4 TCR in these eight control mice ranged from 28 to 52%. As expected, in the four control chimeras that did not develop T1D by 6 wk postreconstitution, approximately half (43.5 ± 7.6%) of the CD8 T cells expressed the AI4 TCR.

View larger version (12K): [in this window] [in a new window]   FIGURE 2. Expression of a second TCR does not account for the negative selection of class I-restricted AI4 diabetogenic T cells by an H2nb1 class II variant. A, Controls consisting of standard NOD females reconstituted with NOD.Rag1null.AI4 Tg bone marrow cells. B, Female (NOD x NOD.H2nb1)F1 mice reconstituted with a 1:1 mixture of syngeneic F1 and NOD.Rag1null.AI4 Tg bone marrow cells. C, Female (NOD x NOD.H2b-Ab0)F1 mice reconstituted with a 1:1 mixture of syngeneic F1 and NOD.Rag1null.AI4 Tg bone marrow cells. D, Female (NOD x NOD.H2b)F1 mice reconstituted with a 1:1 mixture of syngeneic F1 and NOD.Rag1null.AI4 Tg bone marrow cells. Chimeras were assessed for proportions of splenic CD8 T cells expressing the AI4 TCR at the time of T1D onset (filled symbols) or at 6 wk postreconstitution (open symbols).

We also analyzed (NOD x NOD.H2^b)F₁ hybrids reconstituted with a 1:1 mixture of syngeneic F₁ and NOD.Rag1^null.AI4 Tg bone marrow. All four of these chimeras remained diabetes free through 6 wk postreconstitution (Fig. 2D). The proportion of CD8 T cells expressing the AI4 TCR in these chimeras (57.3 ± 6.9%) was also not different from that of the positive controls. Theseresults indicated the AI4 TCR can engage class II variants encoded by the H2^nb1 but not the H2^b haplotype in a negatively selecting fashion.

AI4 T cells are anergized when maturing in the presence of H2^b-Ab⁰- and H2^b-expressing APC

While present at equivalently proportional levels, it was unclear why the AI4 T cells that developed in H2^b-Ab⁰- or H2^b-expressing chimeras elicited a lower frequency of TID (43 and 0%) than those in the positive controls (67%). Furthermore, while AI4 T cells were not deleted in the (NOD.H2^b x NOD.AI4 Tg)F₁ or (NOD.H2^b-Ab⁰ x NOD.AI4 Tg)F₁ mice depicted in Table II, none of them developed T1D. One possibility was that in both the chimeras and F₁ hybrids, H2^b-Ab⁰- or H2^b-expressing APC did not block the development of AI4 T cells, but such effectors failed to become efficiently activated because in each case the cell targets, and perhaps other tissues such as pancreatic APC, expressed H2^g7 MHC class I molecules in a heterozygous rather than homozygous fashion. To test this possibility, we assessed the ability of 10⁶ CD8 T cells purified from the spleens of chimeras with H2^b-Ab⁰- or H2^b-expressing APC to adoptively transfer T1D to female NOD-scid recipients, which unlike the donors homozygously expressed H2^g7 MHC molecules on pancreatic cells and other tissues such as APC. As shown in Fig. 2, about half of the adoptively transferred CD8 T cells from these chimeras were of NOD.Rag1^null.AI4 Tg origin. Hence, positive controls consisted of NOD-scid recipients of 0.5 x 10⁶ CD8 T cells directly transferred from NOD.Rag1^null.AI4 Tg female donors. These positive control AI4 T cells transferred T1D to four of five NOD-scid recipients (Table III). In contrast, T1D failed to develop in any NOD-scid recipients infused with Rag1^null AI4 T cells that had developed in H2^b-Ab⁰ (0/3)- or H2^b (0/3)-expressing chimeras. This ruled out our original hypothesis that it was heterozygous rather than homozygous expression of H2^g7 on pancreatic cells and perhaps other sites that prevented AI4 T cells from efficiently activating and inducing T1D in these donor chimeras.

View larger version (18K): [in this window] [in a new window]   FIGURE 3. AI4 T cells are anergized when maturing in the presence of H2b-Ab0- and H2b-expressing APC. Triplicate aliquots of 5 x 104 CD8 T cells from NOD.AI4 Tg (•), (NOD.H2b x NOD.AI4 Tg)F1 (), or (NOD.H2b-Ab0 x NOD.AI4 Tg)F1 () mice were cocultured with 2 x 105 irradiated (2000 rad) NOD splenic leukocytes and the indicated concentration of the AI4 mimotope peptide (YFIENYLEL). After 48 h of incubation, the cultures were pulsed with [3H]thymidine for an additional 24 h. Proliferative responses are presented as mean cpm ± SEM. Asterisks designate a significantly higher (p < 0.05, Student’s t test) proliferative response by CD8 T cells from NOD.AI4 Tg than (NOD.H2b x NOD.AI4 Tg)F1 or (NOD.H2b-Ab0 x NOD.AI4 Tg)F1 mice. Similar results were obtained in a second experiment.

Similar to the case with the H2^nb1 and H2^b haplotypes, NOD mice heterozygously expressing the H2^q haplotype are dominantly protected from T1D (34). However the H2^q haplotype shares no class I or class II alleles with the H2^nb1 and H2^b haplotypes. Thus, we were curious if H2^q expression inhibited the development of functional AI4 T cells in ways similar or dissimilar to the negative selection and anergy induction processes respectively mediated by the H2^nb1 and H2^b haplotypes. The proportion of AI4 TCR-expressing CD8 T cells in PBL was actually greater in (NOD.H2^q x NOD.AI4 Tg)F₁ hybrids (41.8 ± 4.0%) than in standard NOD.AI4 Tg mice (21.3 ± 1.5%). However, as shown in Fig. 4A, the levels of CD8 coreceptor expression was significantly less on AI4 TCR-positive cells from the (NOD.H2^q x NOD.AI4 Tg)F₁ hybrids (MFI of Ab staining = 46.5 ± 0.6) than from standard NOD.AI4 Tg mice (MFI of Ab staining = 128.3 ± 3.2). This decreased level of CD8 expression rendered AI4 T cells from (NOD.H2^q x NOD.AI4 Tg)F₁ mice functionally anergic as evidenced by their suppressed ability to proliferate in response to the mimotope peptide (Fig. 4B). Hence, another mechanism by which some MHC molecules dominantly inhibit T1D development is to only allow the development of class I-restricted cell-autoreactive T cells that express low levels of the CD8 coreceptor that in turn causes them to be functionally impaired.

View larger version (23K): [in this window] [in a new window]   FIGURE 4. The T1D-protective H2q haplotype inactivates AI4 T cells through the down-regulation of CD8 expression. A, CD8 expression on AI4 TCR-positive cells from (NOD.H2q x NOD.AI4 Tg)F1 hybrids (open histogram) and standard NOD.AI4 Tg mice (shaded histogram). CD8 expression levels do not differ in standard NOD mice and (NOD x NOD.H2q)F1 hybrids. B, AI4 mimotope stimulated proliferative responses of CD8 T cells from NOD.AI4 Tg () and (NOD.H2q x NOD.AI4 Tg)F1 mice () were assessed as described for Fig. 3. Asterisks designate a significantly higher (p < 0.05, Student’s t test) proliferative response by CD8 T cells from NOD.AI4 Tg than (NOD x NOD.H2q)F1 mice. Similar results were obtained in a second experiment.

T cell-negative selection requires that a high threshold level of TCR signaling be achieved upon engagement of a MHC/peptide structure on hemopoietically derived APC (reviewed in Ref. 37). Hence, to further test whether AI4 T cell development is limited through the induction of negative selection upon engagement of a H2^nb1 class II variant, we assessed if such an interaction induced a strong TCR signaling response. This was done by determining the extent of IFN- release from NOD.Rag1^null.AI4 Tg T cells following stimulation with irradiated splenic leukocytes from NOD stocks congenic for various MHC haplotypes. NOD APC are clearly defective in functions normally necessary to mediate multiple immunoregulatory processes, including the negative selection of cell-autoreactive T cells (reviewed in Ref. 2). Therefore, it was not unexpected that NOD.Rag1^null.AI4 Tg T cells failed to be stimulated by APC from standard NOD mice (Fig. 5A). In contrast, NOD.Rag1^null.AI4 Tg T cells were strongly stimulated when exposed to APC resident among NOD.H2^nb1 splenocytes. However, NOD.Rag1^null.AI4 Tg T cells were not stimulated by splenic APC from NOD.H2^b or NOD.H2^b-Ab⁰ donors, which share MHC class I but not class II alleles with the NOD.H2^nb1 stock.

View larger version (27K): [in this window] [in a new window]   FIGURE 5. Signaling through the AI4 TCR is strongly stimulated by engagement of a H2nb1 class II variant or an H2q-encoded ligand. A, Triplicate aliquots of 5 x 105 splenic leukocytes from NOD.Rag1null.AI4 Tg mice were cocultured for 5 days with an equal number of irradiated (2000 rad) splenic leukocytes from NOD mice or the indicated NOD MHC congenic stocks. The extent of activation through the AI4 TCR was then assessed by ELISA measurements of IFN- levels in pooled supernatants from each culture. Data are presented as the mean IFN- concentration ± SEM of each sample assayed in triplicate. Similar results were obtained in two other experiments. B, IFN- production by NOD.Rag1null.AI4 Tg T cells stimulated as described above with irradiated NOD.H2nb1 splenocytes in the presence and absence of mAbs (10 µg/ml) previously shown to recognize Kb class I (28-13-3), Kd class I (SF1-1.1), or Ab,d,q Ed,k (M5/114) and Ed.k (14-4-4) MHC class II molecules.

NOD.Rag1^null.AI4 Tg T cells also secreted significant levels of IFN- when stimulated with NOD.H2^q splenocytes (Fig. 5A). As described earlier, AI4 T cells seed the periphery of (NOD.H2^q x NOD.AI4 Tg)F₁ mice but are functionally impaired due to diminished levels of CD8 expression. Hence, it is possible while the combined presence of H2^nb1 class I and II variants can initiate sufficient levels of signaling through the AI4 TCR to induce virtually complete negative selection, H2^q-encoded ligands provide a somewhat lower level of stimulation that only deletes those AI4 T cells expressing high levels of the CD8 coreceptor.

	Discussion

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Until now it has been unknown if similar to the case with MHC class II-restricted effectors, whether class I-restricted diabetogenic T cell responses could also be dominantly inhibited by the expression of particular MHC variants. The current study indicates this is indeed the case. Specifically, we show AI4 T cells that exert a class I-restricted cell-autoreactive effector function in NOD mice are negatively selected or functionally anergized by the expression of some MHC genes that confer dominant T1D resistance. Unexpectedly, we found that while AI4 T cells exert a class I-restricted diabetogenic effector function, the expression of particular MHC class II molecules on APC can contribute to the intrathymic negative selection of this pathogenic clonotype. Such class II molecules include a variant encoded within the H2^nb1 haplotype, most likely E^k, that is not produced by the closely related H2^b haplotype. The ability of T cells that mediate a class I-restricted effector function to be negatively selected by some class II variants is not unprecedented (44, 45, 46). However, ours is the first demonstration that some class II variants can regulate the development of T cells that contribute to autoimmunity in a class I-restricted fashion. This ability to contribute to the deletion of pathogenic CD8 as well as CD4 T cells (10, 11) adds to the mechanisms by which some MHC class II variants can mediate dominant resistance to T1D.

In addition to being deleted in the presence of some class II variants, we also found that AI4 T cells could be functionally anergized through interactions with class I molecules encoded by the H2^b but not the NOD H2^g7 haplotype. This process entailed a down-regulation of TCR expression. While the H2^nb1 and H2^b haplotypes do not share class II variants whose additional presence can trigger the deletion of AI4 T cells, they are class I matched. Hence, it appears that certain combinations of MHC class I and class II variants can interactively confer protection against T1D by mediating the negative selection of pathogenic CD4 and CD8 T cells and, at least in the case of the latter class of effectors, anergize those escaping this deletional process. While unable to assign the function to class I or class II variants, AI4 T cells maturing in the presence of H2^q MHC molecules were also functionally impaired due to greatly diminished levels of CD8 coreceptor expression. However, this may not represent true anergy, but might rather be a by product of an incomplete negative selection process that only spared AI4 T cells expressing low levels of CD8.

The fact it can engage class II as well as class I MHC structures indicates the AI4 TCR is quite promiscuous. Such promiscuity has been reported to be a feature of other autoreactive TCRs (10, 11, 12, 47). If expression of a promiscuous TCR is a common feature of autoreactive but not foreign Ag-reactive T cells, one consequence is that during their development the former would have a greatly increased probability of encountering negatively selecting ligands. Under this scenario, particular autoreactive T cells would only develop in individuals unfortunate enough to lack thymic expression of any of the multiple ligands that can normally contribute to negative selection of the clonotype in question. Of course, genetically controlled defects in APC function, such as those documented in NOD mice (reviewed in Ref. 2), could augment this process by limiting the tolerogenic presentation of the multiple ligands that might normally engage a promiscuous autoreactive TCR. While it may normally increase their probability of being negatively selected during thymic development, expression of a promiscuous TCR on any T cells that do reach the periphery could make them highly dangerous. Indeed, previous studies have indicated the promiscuity and pathogenicity of diabetogenic CD8 T cells in NOD mice may further increase with age due to TCR avidity maturation that results from peripheral RAG gene re-expression (47, 48, 49).

In conclusion, our results provide further insights to the identity of MHC variants that can dominantly inhibit T1D development and the mechanisms by which they do so. We verify that certain class II variants can provide protection against T1D, but unexpectedly do so by contributing to the negative selection of cell-autoreactive CD8 as well as CD4 T cells. Furthermore, while other studies indicate certain common MHC class I alleles can provide an increased risk for T1D development when expressed in the right genetic context, our results demonstrate other such variants also exert disease-protective effects. In the current study, we found that one mechanism of MHC class I-mediated T1D resistance is an ability to anergize cell-autoreactive CD8 T cells. However, it cannot be ruled out that some MHC class I variants also exert other T1D-protective effects. Our results also indicate some MHC haplotypes, such as H2^nb1, encode separate gene products that in an additive fashion can induce the negative selection or functional inactivation of autoreactive diabetogenic CD8 T cells. Establishing mixed hemopoietic engraftment of APC expressing allogeneic MHC genes blocks the development of cell-autoreactive T cells and T1D in NOD mice (34, 42, 50). Not surprisingly, resistance is more efficiently established when APC express multiple rather than a single MHC gene with T1D-protective effects. As safer preconditioning regimens are developed, it may be possible to consider the use of mixed hemopoietic chimerization to prevent T1D development in humans at future risk for this disease. Our current results indicate extended MHC haplotype analyses in humans should not only be used to identify highly pathogenic combinations of T1D susceptibility genes, but also strongly interactive resistance variants. Such information would aid identification of the hemopoietic donor cells that would provide most efficacious T1D protection in any chimerization protocols ultimately approved for clinical use.

	Footnotes

 
¹ This work was supported by Grants DK46266 (to D.V.S.), DK51090 (to D.V.S.), and DK064315 (to T.P.D.) from the National Institutes of Health, as well as by grants from the Juvenile Diabetes Research Foundation. T.P.D. is a member of the National Institutes of Health-supported Diabetes Research and Training Center at Albert Einstein College of Medicine.

² Address correspondence and reprint requests to Dr. David V. Serreze, Staff Scientist, The Jackson Laboratory, 600 Main Street, Bar Harbor, ME 04609. E-mail address: dvs{at}jax.org

³ Abbreviations used in this paper: T1D, type 1 diabetes; NOD, nonobese diabetic; Tg, transgenic; MFI, mean fluorescence intensity.

Received for publication May 13, 2003. Accepted for publication October 29, 2003.

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