Maternally transmitted antigens are codominantly expressed by mouse cells containing two kinds of mitochondrial DNA

Mitochondrial DNA segregation in hematopoietic lineages does not depend on MHC presentation of mitochondrially encoded peptides

Mutations in mitochondrial DNA (mtDNA) are associated with a broad spectrum of clinical disorders. The segregation pattern of pathogenic mtDNAs is an important determinant of both the onset and the severity of the disease phenotype, but the mechanisms controlling mtDNA segregation remain poorly understood. To investigate this, we previously generated heteroplasmic mice containing two different mtDNA haplotypes and showed that BALB/c mtDNA was invariably selected over NZB mtDNA in blood and spleen. Here, we have characterized this process in hematopoietic tissues and tested whether it involves the presentation of mtDNA-encoded peptides by MHC class Ib molecules. Selection against NZB mtDNA was widespread across different hematopoietic cell lineages and proportional to heteroplasmy levels. Backcrossing heteroplasmic mice with CAST/Ei, a strain in which the MHC class Ib molecule H2-M3 is silent, completely abolished selection against NZB mtDNA in the spleen. To test whether this effect depended on an intact immune system, we generated heteroplasmic mice missing functional copies of Tap1, beta2m or Rag1 to impair presentation or recognition of mtDNA-encoded peptides. The kinetics of selection against NZB mtDNA were unaltered in these mice compared with their wild-type littermates. We conclude that mtDNA selection in hematopoietic tissues is not based on an immune mechanism, but likely involves metabolic signaling.

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.

Kinetics of killing by monoclonal cytotoxic T lymphocytes. I. Colorimetric detection of cryptic CTL determinants on adherent target cells and survivorship analysis

Some targets of cell-mediated cytolysis do not efficiently release 51Cr but manifestly are killed in direct viability assays. We characterize and validate an alternative and non-radioactive (colorimetric) method for measuring killing of adherent targets by monoclonal CTL. The method obviates concerns about the effects of trypsinization, is technically simple, quantitative and in some cases more sensitive than conventional 51Cr assays. Target loss obeyed first-order kinetics with respect both to [CTL] and time. These results are consistent with an exponential (Poisson) model of killing and support the use of a single kinetic parameter to describe the lytic activity of monoclonal CTL on adherent targets. When monoclonal CTL are used at appropriate effector:target ratios (less than or equal to 1:1), the residuals obtained after least squares linear regression are homoscedastic and normally distributed, justifying the use of commonly available statistical calculators or programs for the analysis of CTL data.

Specialized functions of MHC class I molecules. I. An N-formyl peptide receptor is required for construction of the class I antigen Mta

Maternally transmitted factor (Mtf) is a mitochondrial gene that controls the antigenic polymorphism of the MHC class I maternally transmitted antigen (Mta). Synthetic peptides from the NH2 terminus of the mitochondrially encoded NADH dehydrogenase subunit 1 (ND1) mimic Mtf peptide activity in an allele-specific manner. We show that the minimal ND1-alpha peptide length recognized by Mtaa-specific polyclonal CTLs was between 8 and 12 amino acids, while some Mtaa-specific CTL clones recognized a six amino acid peptide. The N-formyl group at the NH2 terminus of ND1 was essential for Mta activity. Competition experiments using N-substituted ND1-alpha peptides showed that an N-formyl peptide receptor on the target cell, which differs from the chemotactic peptide receptor, was required for Mta expression. The specificity of this receptor can account for the distinct immune restriction of Mta in which Mtf peptides are uniquely restricted by Hmt. It is possible that the Hmt gene product is the N-formyl peptide receptor itself and that it represents a class I antigen presentation molecule specialized for binding, transport, and immune presentation of N-formyl-peptide antigens of mitochondrial and prokaryotic origin.

Variants of Mta, the mouse maternally transmitted antigen, determined by three alleles of the extrachromosomal gene Mtf

Two new forms of Mta, the maternally transmitted antigen of the mouse, were defined by cytotoxic T lymphocytes. One is found in laboratory mice carrying the beta allele of the maternally transmitted factor, Mtf, which were previously thought to be Mta-negative. The other is associated with a new allele, Mtf gamma, and is found in wild mice from Toulouse. Like the common form of Mta, determined by the extrachromosomal Mtf alpha, the new forms are also dependent on the a allele of Hmt, a chromosomal gene linked to H-2, and are associated with beta 2-microglobulin. Killers could be raised in all six combinations of alpha, beta, and gamma mice, showing that each form of Mtf determines a distinct (set of) epitope(s). We propose that the product of the Hmt gene, a class I MHC antigen, presents a peptide derived from a mitochondrial gene product on the cell surface and thereby creates the target antigen, Mta.

Unglycosylated Mtaa expresses an Mtab-like determinant

Antigenic polymorphism of the class I-like maternally transmitted antigen (Mta) is controlled by a maternally transmitted factor (Mtf) thought to reside in mitochondria. However, the mechanisms by which Mtf generates antigenic polymorphism are not known. To address this issue, we investigated a possible role of posttranslational oligosaccharide addition in the formation of Mta determinants. We examined the expression of Mta on cytotoxic T lymphocyte (CTL) target cells cultured in tunicamycin (TM), a known inhibitor of asparagine(N)-linked glycosylation. Of 18 Mtab-specific CTL lines, 8 lysed TM-treated Mtaa targets. Furthermore, a subclone of one of these eight lines, 17D5.G2, lysed TM-treated targets from all Mtaa strains tested, regardless of H-2K/D haplotype. On the other hand, this CTL clone did not lyse TM-treated target cells from the Mta null, but H-2 expressing strain B10.CAS2. Therefore expression of this Mtab-like determinant is concordant with the expression of Mtaa and seems unlikely to represent a cross-reactive H-2K/D epitope. Our data suggest that an Mtab-like determinant is expressed on unglycosylated Mtaa molecules. Thus, N-linked oligosaccharides probably prevent the expression of an Mtab-like determinant on the Mtaa molecule. We discuss how Mtf may contribute to Mta polymorphism through glycosylation.