H2-M3, A FULL-SERVICE CLASS I b HISTOCOMPATIBILITY ANTIGEN

Self Cite

H2-M3 is a class Ib MHC molecule that binds a highly restricted pool of peptides, resulting in its intracellular retention under normal conditions. However, addition of exogenous M3 ligands induces its escape from the endoplasmic reticulum (ER) and, ultimately, its expression at the cell surface. These features of M3 make it a powerful and novel model system to study the potentially interrelated functions of the ER-resident class I chaperone tapasin. The functions ascribed to tapasin include: 1) ER retention of peptide-empty class I molecules, 2) TAP stabilization resulting in increased peptide transport, 3) direct facilitation of peptide binding by class I, and 4) peptide editing. We report in this study that M3 is associated with the peptide-loading complex and that incubation of live cells with M3 ligands dramatically decreased this association. Furthermore, high levels of open conformers of M3 were efficiently retained intracellularly in tapasin-deficient cells, and addition of exogenous M3 ligands resulted in substantial surface induction that was enhanced by coexpression of either membrane-bound or soluble tapasin. Thus, in the case of M3, tapasin directly facilitates intracellular peptide binding, but is not required for intracellular retention of open conformers. As an alternative approach to define unique aspects of M3 biosynthesis, M3 was expressed in human cell lines that lack an M3 ortholog, but support expression of murine class Ia molecules. Unexpectedly, peptide-induced surface expression of M3 was observed in only one of two cell lines. These results demonstrate that M3 expression is dependent on a unique factor compared with class Ia molecules.

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confidence: 99%

Tapasin Enhances Peptide-Induced Expression of H2-M3 Molecules, but Is Not Required for the Retention of Open Conformers

Lybarger

Chun

et al. 2001

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“…We chose this system because H2-M3 preferentially binds peptides that carry a formylated methionine (fM) in the N-terminal position (37). In the mouse, only 13 mitochondrial genes can give rise to fM peptides (38). This limited number of fM peptides enabled us to define a NADH dehydrogenase subunit 1 (ND1)-derived peptide as the physiological ligand of positive selection for the C10.4 TCR trans ϩ mouse (32).…”

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confidence: 99%

A Physiological Ligand of Positive Selection Is Recognized as a Weak Agonist

Berg

Irion

Kattman

et al. 2000

Positive selection is a process that ensures that peripheral T cells express TCR that are self-MHC restricted. This process occurs in the thymus and requires both self-MHC and self-peptides. We have recently established a TCR transgenic (TCRtrans+) mouse model using the C10.4 TCR restricted to the MHC class Ib molecule, H2-M3. Having defined H2-M3 as the positively selecting MHC molecule, the severely limited number of H2-M3 binding peptides allowed us to characterize a mitochondrial NADH dehydrogenase subunit 1-derived 9-mer peptide as the physiological ligand of positive selection. Here, we demonstrate that the NADH dehydrogenase subunit 1 self-peptide is seen by mature C10.4 TCRtrans+ T cells as a weak agonist and induces positive selection at a defined concentration range. We also found that the full-length cognate peptide, a strong agonist for mature C10.4 TCRtrans+ T cells, initiated positive selection, albeit at significantly lower concentrations. At increased peptide concentrations, and thus increased epitope densities, either peptide only induced the development of partially functional T cells. We conclude that successful positive selection only proceeded at a defined, yet fairly narrow window of avidity.

“…Like other class I-b genes, M3 is virtually monomorphic in Mus musculus (11). A minor allele, M3 b , has been considered null because it does not restrict lysis by known M3-specific CTL (11); null activity was mapped to a Leu 95 Gln substitution in the ␣2 domain (12).…”

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confidence: 99%

“…The mechanism of N-formyl specificity in M3 is well-studied (11). The crystal structure of M3 a (20) indicated 10 amino acids coordinated N-formyl specificity, with a key contribution from histidine in position 9 (His 9 ).…”

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confidence: 99%

Hyperconservation of theN-Formyl Peptide Binding Site ofM3: Evidence thatM3Is an Old Eutherian Molecule with Conserved Recognition of a Pathogen-Associated Molecular Pattern

Doyle

Davis

Cook

et al. 2003

The mouse MHC class I-b molecule H2-M3 has unique specificity for N-formyl peptides, derived from bacteria (and mitochondria), and is thus a pathogen-associated molecular pattern recognition receptor (PRR). To test whether M3 was selected for this PRR function, we studied M3 sequences from diverse murid species of murine genera Mus, Rattus, Apodemus, Diplothrix, Hybomys, Mastomys, and Tokudaia and of sigmodontine genera Sigmodon and Peromyscus. We found that M3 is highly conserved, and the 10 residues coordinating the N-formyl group are almost invariant. The ratio of nonsynonymous and synonymous substitution rates suggests the Ag recognition site of M3, unlike the Ag recognition site of class I-a molecules, is under strong negative (purifying) selection and has been for at least 50–65 million years. Consistent with this, M3 α1α2 domains from Rattus norvegicus and Sigmodon hispidus and from the “null” allele H2-M3b specifically bound N-formyl peptides. The pattern of nucleotide substitution in M3 suggests M3 arose rapidly from murid I-a precursors by an evolutionary leap (“saltation”), perhaps involving intense selective pressure from bacterial pathogens. Alternatively, M3 arose more slowly but prior to the radiation of eutherian (placental) mammals. Older dates for the emergence of M3, and the accepted antiquity of CD1, suggest that primordial class I MHC molecules could have evolved originally as monomorphic PRR, presenting pathogen-associated molecular patterns. Such MHC PRR molecules could have been preadaptations for the evolution of acquired immunity during the early vertebrate radiation.