Generation of T cells with lytic specificity for atypical antigens. I. A mitochondrial antigen in the rat

SCNT-derived ESCs with mismatched mitochondria trigger an immune response in allogeneic hosts

The generation of pluripotent stem cells by somatic cell nuclear transfer (SCNT) has recently been achieved in human cells and sparked new interest in this technology. The authors reporting this methodical breakthrough speculated that SCNT would allow the creation of patient-matched embryonic stem cells, even in patients with hereditary mitochondrial diseases. However, herein we show that mismatched mitochondria in nuclear-transfer-derived embryonic stem cells (NT-ESCs) possess alloantigenicity and are subject to immune rejection. In a murine transplantation setup, we demonstrate that allogeneic mitochondria in NT-ESCs, which are nucleus-identical to the recipient, may trigger an adaptive alloimmune response that impairs the survival of NT-ESC grafts. The immune response is adaptive, directed against mitochondrial content, and amenable for tolerance induction. Mitochondrial alloantigenicity should therefore be considered when developing therapeutic SCNT-based strategies.

Generation of histocompatible tissues using nuclear transplantation

Nuclear transplantation (therapeutic cloning) could theoretically provide a limitless source of cells for regenerative therapy. Although the cloned cells would carry the nuclear genome of the patient, the presence of mitochondria inherited from the recipient oocyte raises questions about the histocompatibility of the resulting cells. In this study, we created bioengineered tissues from cardiac, skeletal muscle, and renal cells cloned from adult bovine fibroblasts. Long-term viability was demonstrated after transplantation of the grafts into the nuclear donor animals. Reverse transcription-PCR (RT-PCR) and western blot analysis confirmed that the cloned tissues expressed tissue-specific mRNA and proteins while expressing a different mitochondrial DNA (mtDNA) haplotype. In addition to creating skeletal muscle and cardiac "patches", nuclear transplantation was used to generate functioning renal units that produced urinelike fluid and demonstrated unidirectional secretion and concentration of urea nitrogen and creatinine. Examination of the explanted renal devices revealed formation of organized glomeruli- and tubule-like structures. Delayed-type hypersensitivity (DTH) testing in vivo and Elispot analysis in vitro suggested that there was no rejection response to the cloned renal cells. The ability to generate histocompatible cells using cloning techniques addresses one of the major challenges in transplantation medicine.

Polymorphisms in the coding region of mtDNA and effects on clinical outcome of unrelated bone marrow transplantation

The entire protein-coding region was divided into 45 fragments, separately amplified and analyzed for polymorphism by the PCR-SSCP (single-strand conformation polymorphism) method. The effect of polymorphism mismatching on the clinical outcome of unrelated bone marrow transplantation was studied to clarify whether products from mtDNA become minor antigens. Variability in PCR-SSCP pattern combinations of the 45 fragments suggests that each individual has a different polymorphism combination in the protein-coding region if all the coding regions were compared at the nucleotide sequence level. Nonsynonymous polymorphisms were found at relatively high frequency in MTATP8 and MTND3. Both the polymorphisms with and without substitution matched the peptide-binding motifs of HLA-A*0201. The effects of the polymorphism matching were retrospectively analyzed in 340 recipients transplanted with HLA-A, -B, -DRB1 allele-matched bone marrow from unrelated donors. There were no effects of polymorphism matching on the incidence of acute GVHD and cumulative disease-free survival. These results suggest that polymorphisms which generate peptides, with and without substitutions, that bind the same HLA molecule hardly influence GVHD because the difference between the HLA-peptide complexes is minute.

Two different, highly exposed, bulged structures for an unusually long peptide bound to rat MHC class I RT1-Aa

The rat MHC class Ia molecule RT1-Aa has the unusual capacity to bind long peptides ending in arginine, such as MTF-E, a thirteen-residue, maternally transmitted minor histocompatibility antigen. The antigenic structure of MTF-E was unpredictable due to its extraordinary length and two arginines that could serve as potential anchor residues. The crystal structure of RT1-Aa-MTF-E at 2.55 A shows that both peptide termini are anchored, as in other class I molecules, but the central residues in two independent pMHC complexes adopt completely different bulged conformations based on local environment. The MTF-E epitope is fully exposed within the putative T cell receptor (TCR) footprint. The flexibility demonstrated by the MTF-E structures illustrates how different TCRs may be raised against chemically identical, but structurally dissimilar, pMHC complexes.

H2-M3, a full-service class Ib histocompatibility antigen

H2-M3 is an MHC class Ib molecule of the mouse with a unique preference for N-formylated peptides, which may come from the N-termini of endogenous, mitochondrial proteins or foreign, bacterial proteins. The crystal structure of M3 revealed a hydrophobic peptide-binding groove with an occluded A pocket and the peptide shifted one residue relative to class Ia structures. The formyl group is held by a novel hydrogen bonding network, involving His9 on the bottom of the groove, and the side chain of the P1 methionine is lodged in the B pocket. M3 is a full-service histocompatibility (H) antigen, i.e. self-M3 can present endogenous peptides as minor H antigens and foreign, bacterial antigens in a defensive immune response to infection; and foreign M3 complexed with endogenous self-peptides.

Evidence against an X-linked visual loss susceptibility locus in Leber hereditary optic neuropathy

Pedigree analysis of British families with Leber hereditary optic neuropathy (LHON) closely fits a model in which a pathogenic mtDNA mutation interacts with an X-linked visual loss susceptibility locus (VLSL). This model predicts that 60% of affected females will show marked skewing of X inactivation. Linkage analysis in British and Italian families with genetically proven LHON has excluded the presence of such a VLSL over 169 cM of the X chromosome both when all families were analyzed together and when only families with the bp 11778 mutation were studied. Further, there was no excess skewing of X inactivation in affected females. There was no evidence for close linkage to three markers in the pseudoautosomal region of the sex chromosomes. The mechanism of incomplete penetrance and male predominance in LHON remains unclear.

HLA class I genotypes in Leber's hereditary optic neuropathy

There is evidence that mitochondrial DNA (mtDNA) encoded peptides can restrict the immune response in rodents and that these peptides are presented by classical and 'neoclassical' class I major histocompatibility complex (MHC) molecules. We investigated the frequency of HLA-A and two HLA-B genotypes in index cases of 77 families with Leber's hereditary optic neuropathy (LHON), on the basis that there may be an autoimmune component to this disease. There was no association between LHON and any genotype. We conclude that the classical class I MHC loci are not major determinants of the development of blindness in LHON.

Sequence of the human homologue of a mitochondrially encoded murine transplantation antigen in patients with multiple sclerosis

There is some evidence that mitochondrial genes may contribute to susceptibility to multiple sclerosis (MS), and a mitochondrial DNA-encoded peptide, the N-terminal portion of NADH-dehydrogenase subunit 1, acts as a transplantation antigen in mice. We have analysed the DNA sequence of the corresponding region of human mitochondrial DNA in 87 patients with MS, 10 with Leber's hereditary optic neuropathy in association with an MS-like illness, and 31 control subjects. This sequence appears to be highly conserved. Only three base pair changes were identified, each being found once only in one control and two patients, and these are likely to be harmless polymorphisms. There is thus no evidence that polymorphism in this region contributes to genetic susceptibility in MS.

MtDNA-encoded histocompatibility antigens

Mitochondrially encoded H antigens are by-products of a system that has evolved in vertebrates to present peptides from intracellular pathogens on the cell surface for detection by CTLs, which can lyse the infected cell. CTL lines and clones with defined specificity against mitochondrial H antigens, which can be maintained in culture for long periods, offer a unique tool in mitochondrial genetics. Expression of polymorphic mitochondrial H antigens depends on both the presence and the activity of the corresponding mitochondrial genome, and CTLs can provide strong selection against cells displaying their cognate antigen.

The tum- antigens P91A and P198 derive from proteins located in the cytosolic compartment of cells

To characterize the proteins P91Ap and P198p, of which mutants generate the tum- antigens P91A and P198, respectively, rabbit antisera were raised with ovalbumin-coupled synthetic peptides that correspond to their respective C terminus. In immunoadsorption tests using immobilized protein A the antisera recognized the translation products synthesized by rabbit reticulocyte lysates programmed with the SP6 polymerase transcripts of the P91A and P198 cDNA. The presence of the two proteins was demonstrated by SDS-PAGE and immunoblotting in all the mouse cells and organs examined. P91Ap is a constituent of the cytosol; despite a remarkable homology to the Drosophila diphenol oxidase DOX-A2, it separates from murine catechol oxidase activity in rate zonal sedimentation analysis. P198p is a ribosomal constituent, or a factor firmly linked to both the free and membrane-bound ribosomes. These subcellular localizations strengthen other evidence that the antigens presented to T lymphocytes by class I products of the major histocompatibility complex derive from proteins of the cytosol, or in direct contact with it.

Protein export from the mitochondrial matrix

Assembly of a functional mitochondrion requires import of proteins from the cytosol and export of proteins from the matrix. Most previous studies have focused on the import pathway followed by nucleus-encoded proteins. However, it is now clear that proteins encoded in the nucleus as well as those encoded in the mitochondrion also move from the matrix into and across the inner membrane, a process defined here as export. These exported proteins are found in at least three cellular locations: the inner mitochondrial membrane, the intermembrane space and the cell surface. Here, we consider the pathways for export and the relationships between import and export.

Proteolysis, proteasomes and antigen presentation

Proteins presented to the immune system must first be cleaved to small peptides by intracellular proteinases. Proteasomes are proteolytic complexes that degrade cytosolic and nuclear proteins. These particles have been implicated in ATP-ubiquitin-dependent proteolysis and in the processing of intracellular antigens for cytolytic immune responses.

Generation of T cells with lytic specificity for atypical antigens. III. Priming F1 animals with antigen-bearing cells also having reactivity for host alloantigens allows for potent lytic T cell responses

Here, we explore the conditions required for generating two different highly potent F1 antiparental killer cell populations to unusual antigens in rats. The first, L/DA anti-DA, has lytic specificity for two antigen systems: MTA, a mitochondrial antigen expressed on DA and DA Lewis (L) target cells restricted by RT1A class I molecules; and H, an antigen that maps to the class I-like RT1C region and is present only on parental target cells from donors homozygous at the major histocompatibility complex. The second killer population is generated in the reciprocal DA/L anti-DA combination and has lytic specificity only for the H antigen system. We show that the killer cells are T cells, and that generation of these F1 cytotoxic T lymphocytes (CTL) requires an in vivo priming step in which it is essential that the inoculated parental cells bear the relevant target antigens and possess alloreactivity for F1 host antigens. The requirement for alloreactivity and antigen on the same priming cell population suggests that these potent lytic responses depend on a situation akin to a hapten-carrier effect that bypasses otherwise ineffective helper responses by the host to these unusual antigens. Restimulation of F1 lymphocytes in culture is also necessary, requiring the presence of antigen on irradiated lymphoblast stimulator cells, but alloreactivity to responder cell antigens is not necessary; normal, nonactivated lymph node cells are completely ineffective as stimulators. For effective lysis, the target cells need not possess the potential for alloreactivity to responder F1 CTL. We also demonstrate in a preliminary way additional antigen systems defined by killer populations raised with other F1 antiparental strain combinations.

Generation of T cells with lytic specificity for atypical antigens. II. A novel antigen system in the rat dependent on homozygous expression of major histocompatibility complex genes of the class I-like RT1C region

Lymphocytes from parental strain DA rats can induce potent killer cell responses to atypical antigen systems in F1 Lewis (L)/DA and DA/L recipients. Here, we describe an antigen system, H, present on homozygous parental target cells, but not on F1 cells. This antigen system is unusual in several respects: it does not involve class I RT1A gene products usually used by killer cell responses in the rat, it maps to the major histocompatibility complex (MHC) class I-like RT1C region, and it requires homozygous expression of RT1Cav1 alleles. This may be another example, this time involving the RT1C region, of an MHC gene product antigenically altered by an MHC-linked trans-activating modifier gene.