Differences between the lateral organization of conventional and inositol phospholipid-anchored membrane proteins. A further definition of micrometer scale membrane domains

Plasma membranes of many cells appear to be divided into domains, areas whose composition and function differ from the average for an entire membrane. We have previously used fluorescence photo-bleaching and recovery to demonstrate one type of membrane domain, with dimensions of micrometers (Yechiel, E., and M. Edidin. 1987, J. Cell Biol. 105: 755- 760). The presence of membrane domains is inferred from the dependence of the apparent mobile fraction of labeled molecules on the size of the membrane area probed. We now find that by this definition classical class I MHC molecules, H-2Db, are concentrated in domains in the membranes of K78-2 hepatoma cells, while the nonclassical class I- related molecules, Qa-2, are free to pass the boundaries of these domains. The two proteins are highly homologous but differ in their mode of anchorage to the membrane lipid bilayer. H-2Db is anchored by a transmembrane peptide, while Qa-2 is anchored by a glycosylphosphatidylinositol (GPI) anchor. A mutant class I protein with its external portion derived from Qa-2 but with transmembrane and cytoplasmic sequences from a classical class I molecule shows a dependence of its mobile fraction on the area of membrane probed, while a mutant whose external portions are a mixture of classical and nonclassical class I sequences, GPI-linked to the bilayer, does not show this dependence and hence by our definition is not restricted to membrane domains

Regulation of Murine Class I Genes by Interferons is Controlled by Regions Located Both 5' and 3' to the Transcription Initiation Site

Interferons regulate the expression of a large number of mammalian genes, including the major histocompatibility antigen genes. To investigate the mechanisms involved in interferon action, we have analyzed the ability of murine H-2Ld and H-2Dd DNA sequences to control the responses to interferon. The results indicate that interferon regulation of class I gene expression is complex and involves at least two mechanisms that are dependent on class I sequences located upstream and downstream to the transcription initiation site. In transfected mouse L cells, both of these regions are required for full enhancement of class I gene expression, with the major portion of the response controlled by the sequences located 3' to the transcription initiation site. The fine-mapping analysis of the 5' region-encoded response also suggests that recombinant alpha and gamma interferons may exert their effects on class I gene expression by using different cis-acting regulatory sequences

An MHC class Ib–restricted CD8 T cell response confers antiviral immunity

Although immunity against intracellular pathogens is primarily provided by CD8 T lymphocytes that recognize pathogen-derived peptides presented by major histocompatibility complex (MHC) class Ia molecules, MHC class Ib–restricted CD8 T cells have been implicated in antiviral immunity. Using mouse polyoma virus (PyV), we found that MHC class Ia–deficient (Kb−/−Db−/−) mice efficiently control this persistently infecting mouse pathogen. CD8 T cell depletion mitigates clearance of PyV in Kb−/−Db−/− mice. We identified the ligand for PyV-specific CD8 T cells in Kb−/−Db−/− mice as a nonamer peptide from the VP2 capsid protein presented by Q9, a member of the β2 microglobulin–associated Qa-2 family. Using Q9-VP2 tetramers, we monitored delayed but progressive expansion of these antigen-specific CD8αβ T cells in Kb−/−Db−/− mice. Importantly, we demonstrate that Q9-VP2–specific CD8 T cells more effectively clear wild-type PyV than a VP2 epitopenull mutant PyV. Finally, we show that wild-type mice also generate Q9-restricted VP2 epitope–specific CD8 T cells to PyV infection. To our knowledge, this is the first evidence for a defined MHC class Ib–restricted antiviral CD8 T cell response that contributes to host defense. This study motivates efforts to uncover MHC class Ib–restricted CD8 T cell responses in other viral infections, and given the limited polymorphism of MHC class Ib molecules, it raises the possibility of developing peptide-based viral vaccines having broad coverage across MHC haplotypes

Negative and positive selection of antigen-specific cytotoxic T lymphocytes affected by the α3 domain of MHC I molecules

THE α1 and α2 domains of major histocompatibility complex (MHC) class I molecules function in the binding and presentation of foreign peptides to the T-cell antigen receptor and control both negative and positive selection of the T-cell repertoire. Although the α3 domain of class I is not involved in peptide binding, it does interact with the T-cell accessory molecule, CDS. CDS is important in the selection of T cells as anti-CDS antibody injected into perinatal mice interfers with this process. We previously used a hybrid class I molecule with the α1/α2 domains from L^d and the α3 domain from Q7^b and showed that this molecule binds an L^d-restricted peptide but does not interact with CD8-dependent cytotoxic T lymphocytes. Expression of this molecule in transgenic mice fails to negatively select a subpopulation of anti-L^d cytotoxic T lymphocytes. In addition, positive selection of virus-specific L^d-restricted cytotoxic T lymphocytes does not occur. We conclude that besides the α1/α2 domains of class I, the α3 domain plays an important part in both positive and negative selection of antigen-specific cells

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements