Molecular definition of a mitochondrially encoded mouse minor histocompatibility antigen

No immune responses by the expression of the yeast Ndi1 protein in rats

Background

The rotenone-insensitive internal NADH-quinone oxidoreductase from yeast, Ndi1, has been shown to work as a replacement molecule for complex I in the respiratory chain of mammalian mitochondria. In the so-called transkingdom gene therapy, one major concern is the fact that the yeast protein is foreign in mammals. Long term expression of Ndi1 observed in rodents with no apparent damage to the target tissue was indicative of no action by the host's immune system.

Methodology/Principal Findings

In the present study, we examined rat skeletal muscles expressing Ndi1 for possible signs of inflammatory or immune response. In parallel, we carried out delivery of the GFP gene using the same viral vector that was used for the NDI1 gene. The tissues were subjected to H&E staining and immunohistochemical analyses using antibodies specific for markers, CD11b, CD3, CD4, and CD8. The data showed no detectable signs of an immune response with the tissues expressing Ndi1. In contrast, mild but distinctive positive reactions were observed in the tissues expressing GFP. This clear difference most likely comes from the difference in the location of the expressed protein. Ndi1 was localized to the mitochondria whereas GFP was in the cytosol.

Conclusions/Significance

We demonstrated that Ndi1 expression did not trigger any inflammatory or immune response in rats. These results push forward the Ndi1-based molecular therapy and also expand the possibility of using foreign proteins that are directed to subcellular organelle such as mitochondria.

The H4b minor histocompatibility antigen is caused by a combination of genetically determined and posttranslational modifications

Minor histocompatibility (H) Ag disparities result in graft-vs-host disease and chronic solid allograft rejection in MHC-identical donor-recipient combinations. Minor H Ags are self protein-derived peptides presented by MHC class I molecules. Most arise as a consequence of allelic variation in the bound peptide (p) that results in TCR recognizing the p/MHC as foreign. We used a combinational peptide screening approach to identify the immune dominant H2K(b)-restricted epitope defining the mouse H4(b) minor H Ag. H4(b) is a consequence of a P3 threonine to isoleucine change in the MHC-bound peptide derived from epithelial membrane protein-3. This allelic variation also leads to phosphorylation of the H4(b) but not the H4(a) epitope. Further, ex vivo CD8(+) T lymphocytes bind phosphorylated Ag tetramers with high efficiency. Although we document the above process in the minor H Ag system, posttranslational modifications made possible by subtle amino acid changes could also contribute to immunogenicity and immune dominance in tumor immunotherapeutic settings.

Partially TAP-independent protection against Listeria monocytogenes by H2-M3-restricted CD8+ T cells

Effective protection against Listeria monocytogenes requires Ag-specific CD8(+) T cells. A substantial proportion of CD8(+) T cells activated during L. monocytogenes infection of C57BL/6 mice are restricted by the MHC class Ib molecule H2-M3. In this study, an H2-M3-restricted CD8(+) T cell clone specific for a known H2-M3 epitope (fMIGWII) was generated from L. monocytogenes-infected mice. The clone was cytotoxic, produced IFN-gamma, and could mediate strong protection against L. monocytogenes when transferred to infected mice. Macrophages pulsed with heat-killed LISTERIAE: presented Ag to the clone in a TAP-independent manner. Both TAP-independent and -dependent processing occurred in vivo, as TAP-deficient mice infected with L. monocytogenes were partially protected by adoptive transfer of the clone. This is the first example of CD8(+) T cell-mediated, TAP-independent protection against a pathogen in vivo, confirming the importance of alternative MHC class I processing pathways in the antibacterial immunity.

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.

A Listeria monocytogenes pentapeptide is presented to cytolytic T lymphocytes by the H2-M3 MHC class Ib molecule

Polymorphism of MHC class Ia molecules severely constrains vaccine development against intracellular pathogens. Antigen presentation by MHC class Ib molecules, which are generally conserved between different individuals, may circumvent this obstacle. Herein, we use tandem mass spectrometry to identify a Listeria monocytogenes pentapeptide antigen that is presented to T lymphocytes by the H2-M3 MHC class Ib molecule. The peptide contains N-formyl methionine at the N terminus and exclusively hydrophobic amino acids. Mice of the H-2 d, H-2 b,and H-2 k haplotypes respond to this peptide upon infection with Listeria monocytogenes. Identification of antigens presented by MHC class Ib molecules is feasible and may provide opportunities for relatively unrestricted vaccine development.

MHC evolution and development of a recognition system

The unrestricted viability of allophenic mice shows that MHC-different cell lines have no problem engaging in organogenesis together. Thus, outside of the immune system, mammals appear to have no self-non-self discrimination mechanism based on polymorphic and ubiquitously expressed class I MHC antigens. Here, it should be pointed out that even within the immune system, certain responses require no self-non-self discrimination, for example, antiphosphocholine response and certain antipolysaccharide responses that exploit differences between bacterial and host sugar transferases. Thus, the self-non-self discrimination via peptide fragments presented by ubiquitously expressed class I MHC antigens can be viewed as the late addition that enabled the adaptive immune system to cope with intracellular parasites that are primarily viruses. The preference for different types of peptide fragments suffices to explain extensive polymorphism as well as multiple gene loci for class I and possibly also class II MHC antigens. Yet, a too specialized class I MHC allele that presents a very unusual peptide fragment is of no use, for such a peptide fragment is not likely to be found among viral proteins. Effective MHC antigens are those that prefer common enough peptide fragments, so that at least one T epitope can be found in one out of every three viral proteins. Yet, such common peptide fragments are also likely to be present among multitudes of intracellular proteins that are the self. The immune system appears to have solved the above dilemma by mounting a vigorous cytotoxic T-cell response only when viruses are actively propagating by synthesizing a few of their own proteins in large amounts, thereby suppressing the host protein synthesis. To attack infected cells in which viruses are in the quiescent state of symbiosis with the host is the ultimate folly.

HMT, encoded by H-2M3, is a neoclassical major histocompatibility class I antigen

H-2M3 encodes HMT, the major histocompatibility complex (MHC) class I heavy chain of the maternally transmitted antigen (Mta). Like classical MHC class I genes, the expression of M3 can be stimulated by gamma-interferon and its message can be detected from mid-gestational embryos (day 8) through adulthood. HMTb, a nonimmunogenic allelic form of HMT, differs from the common HMTa molecule by four amino acids, of which only two (residues 31 and 95) are located in the alpha 1 and alpha 2 domains that form the peptide-binding groove. Recognition of site-directed mutants by Mta-specific cytotoxic T lymphocytes was hardly affected by the substitution of Met for Val31 but was abolished by the substitution of Gln for Leu95, which is located in the beta-sheet floor of the peptide-binding groove. Thus a single amino acid difference is responsible for the immunological silence of HMTb.

Cloning and expression of class I major histocompatibility complex genes of the rat

Little is known about the organization of class I genes in the rat although there is prima facie evidence that it is distinct from that of the mouse. We report the cloning of 61 nonclassical rat class I genes into cosmid clusters with a total mapped length of 1,264 kb. It is certain that the total number of class I genes in the rat must exceed this number. From restriction maps it is possible to identify substantial regions of duplication. By transfection of cosmids into mouse L cells, it has been possible to demonstrate at least seven different nonclassical rat class I genes that are expressible on the cell surface. Crossreaction of a single mouse monoclonal antibody with all of these class I molecules is consistent with sequence homogenization within the rat nonclassical system. Attempts to find rat homologues of the mouse Tla genes by crosshybridization of rat cosmids with a range of different TLa-specific probes were unsuccessful, suggesting that this large group of divergent class I genes is absent or nearly so from the rat. The large number of class I genes in the rat appears to have arisen by expansion of genes more closely related to the classical sequence.

Complexity in the major histocompatibility complex

The human major histocompatibility complex (MHC) is one of the most intensively studied regions of the human genome, containing over 70 known genes and spanning about 4 million base pairs (4 Mbp) of DNA on chromosome 6p21.3 (Klein, 1986). It can be divided up into three regions: the class I region (telomeric), the class II region (centromeric), and the class III region (between class I and II), which includes the complement component genes C2, C4, and Bf (Trowsdale & Campbell, 1988). The MHC has been mapped in detail using pulse field gel electrophoresis (PFGE) and by cloning in yeast artificial chromosome (YAC) and cosmid vectors, revealing long stretches of DNA between the regions as well as between individual class I and class II genes. Novel genes, that have no sequence relationships with class I, class II or complement components, have recently been found in these areas, and we will present an update on these after reviewing the more established loci.

A new human HLA class II-related locus, DM

HLA class II molecules have a crucial role in the immune response to antigens. We have isolated two new class II-like complementary DNA sequences, RING6 and RING7, which map between the HLA-DNA and -DOB loci. They are novel members of the immunoglobulin gene family which may have diverged before the duplications that gave rise to the main class II loci. The RING6 and RING7 genes seem to encode alpha- and beta-chains of a previously undiscovered class II-related protein.

Expression of a MHC non-classical class I gene, Q4, is similar to a classical class I gene, Dp

We have investigated the effects of the cell cycle on expression of Q4 mRNA. Q4, the gene encoding the Qb-1 antigen, is transcribed in a wide variety of tissues, unlike many other non-classical class I genes. We have compared the pattern of Q4 transcription in the cell cycle to classical class I, beta-2-microglobulin and actin. We found that the pattern of Q4 RNA levels resembles that of the classical class I genes, consistent with the similarity of the 5' sequences of Q4 and K/D. Thus, Q4 mRNA accumulates during the cell cycle along with the total RNA, but does not show specific transcriptional enhancement. This is consistent with a function for Q4 similar to the classical K/D gene products.

Dominant expression of a distinctive V14+ T-cell antigen receptor alpha chain in mice

A distinctive variable region 14-positive (V14+) alpha chain (V alpha 14+) of the T-cell antigen receptor is predominantly expressed in multiple mouse subspecies. The V alpha 14 family has two members, V alpha 14.1 and V alpha 14.2, which differ by only three amino acids at positions 50-52. Based on the EcoRI restriction fragment length polymorphism of the gene encoding V alpha 14, mice can be divided into three groups: type I with an 11.2-kilobase (kb) fragment, type II with a 2.0-kb fragment, and type III with the 2.0-kb and 11.2-kb fragments. Usage of V alpha 14-J alpha 281, where J alpha 281 is an alpha-chain joining segment, with a one-base N region dominates at the level of 0.02-1.5% of alpha chains in all laboratory strains, Mus musculus castaneus, and Mus musculus domesticus but not in Mus musculus molossinus, Mus musculus musculus, and Mus spicilegus samples. The preferential V alpha 14-J alpha 281 expression seems to be due to positive selection because the V-J junctional region is always glycine, despite the ability of the V alpha 14 gene to associate with J alpha other than J alpha 281. As V alpha 14-J alpha 281 expression is independent of known major histocompatibility complex antigens, including H-2, TLA, Qa, and HMT, the selecting ligand must be a monomorphic molecule of the mouse, expressed in a subspecies-specific manner. Additional observations, such as the expression of homogeneous V alpha 14-J alpha 281 in athymic mice, suggest that the positive selection of V alpha 14+ T cells occurs extrathymically.

Recognition of MHC TL gene products by gamma delta T cells

We have studied the ligand specificity of a gamma delta T-cell receptor (TCR) derived from a mouse T-cell hybridoma (KN6). KN6 cells reacted with syngeneic (C57BL/6) cells from various origins (splenocytes, thymocytes, peritoneal exudate cells, etc.) and cells from many different mouse strains. KN6 reactivity against cells from a panel of congenic and recombinant mouse strains demonstrated that the ligand recognized by KN6 is controlled by an MHC-linked gene that most probably maps in the TL region. We cloned this gene and formally proved that it does map in the TL region. This gene turned out to be a novel class I gene (designated T22b) belonging to a hitherto unidentified cluster of TL region genes in strain C57BL/6. This gene was expressed in many different tissues and cell types. We also examined the tissue expression of several other TL genes. One of these, the structural gene (T3b) encoding the thymus leukemia (TL) antigen from C57BL/6 mice, was specifically expressed in the epithelium of the small intestine. Since the intestinal epithelium of the mouse is known to be the homing site for a subset of gamma delta T cells (i-IEL) bearing diverse TCR with V7 rearranged gamma chains, we propose that the T3b gene product is part of the ligand recognized by some of the i-IEL. Our data support the idea that gamma delta T cells might be specific for non-classical class I or class I-like molecules and suggest that gamma delta TCR and non-classical MHC co-evolved for the recognition of a conserved set of endogenous or foreign peptides.

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.

Characterization of naturally occurring minor histocompatibility peptides including H-4 and H-Y

Minor histocompatibility (H) antigens can be peptides derived from cellular proteins that are presented on the cell surface by major histocompatibility complex (MHC) class I molecules. This is similar to viral antigens, because in both cases cytotoxic T lymphocytes (CTLs) recognize artificially produced peptides loaded on target cells. Naturally processed minor H peptides were found to be similar to those artificial CTL-epitopes, as far as size and hydrophobicity is concerned. The peptides studied were isolated from a transfectant that expressed a model CTL-defined antigen, beta-galactosidase, from male cells that express H-Y, which has been known operationally since 1955, and from cells that express H-4, known since 1961.