117 research outputs found
Perturbative quantum gravity with the Immirzi parameter
We study perturbative quantum gravity in the first-order tetrad formalism.
The lowest order action corresponds to Einstein-Cartan plus a parity-odd term,
and is known in the literature as the Holst action. The coupling constant of
the parity-odd term can be identified with the Immirzi parameter of loop
quantum gravity. We compute the quantum effective action in the one-loop
expansion. As in the metric second-order formulation, we find that in the case
of pure gravity the theory is on-shell finite, and the running of Newton's
constant and the Immirzi parameter is inessential. In the presence of fermions,
the situation changes in two fundamental aspects. First, non-renormalizable
logarithmic divergences appear, as usual. Second, the Immirzi parameter becomes
a priori observable, and we find that it is renormalized by a four-fermion
interaction generated by radiative corrections. We compute its beta function
and discuss possible implications. The sign of the beta function depends on
whether the Immirzi parameter is larger or smaller than one in absolute value,
and the values plus or minus one are UV fixed-points (we work in Euclidean
signature). Finally, we find that the Holst action is stable with respect to
radiative corrections in the case of minimal coupling, up to higher order
non-renormalizable interactions.Comment: v2 minor amendment
Stabilizing All Kahler Moduli in Type IIB Orientifolds
We describe a simple and robust mechanism that stabilizes all Kahler moduli
in Type IIB orientifold compactifications. This is shown to be possible with
just one non-perturbative contribution to the superpotential coming from either
a D3-instanton or D7-branes wrapped on an ample divisor. This
moduli-stabilization mechanism is similar to and motivated by the one used in
the fluxless G_2 compactifications of M-theory. After explaining the general
idea, explicit examples of Calabi-Yau orientifolds with one and three Kahler
moduli are worked out. We find that the stabilized volumes of all two- and
four-cycles as well as the volume of the Calabi-Yau manifold are controlled by
a single parameter, namely, the volume of the ample divisor. This feature would
dramatically constrain any realistic models of particle physics embedded into
such compactifications. Broad consequences for phenomenology are discussed, in
particular the dynamical solution to the strong CP-problem within the
framework.Comment: RevTeX, 24 pages, 2 tables, 1 figure
Co-Administration of a Plasmid DNA Encoding IL-15 Improves Long-Term Protection of a Genetic Vaccine against Trypanosoma cruzi
Background: Immunization of mice with the Trypanosoma cruzi trans-sialidase (TS) gene using plasmid DNA, adenoviral vector, and CpG-adjuvanted protein delivery has proven highly immunogenic and provides protection against acute lethal challenge. However, long-term protection induced by TS DNA vaccines has not been reported. the goal of the present work was to test whether the co-administration of a plasmid encoding IL-15 (pIL-15) could improve the duration of protection achieved through genetic vaccination with plasmid encoding TS (pTS) alone.Methodology: We immunized BALB/c mice with pTS in the presence or absence of pIL-15 and studied immune responses [with TS-specific IFN-gamma ELISPOT, serum IgG ELISAs, intracellular cytokine staining (IFN-gamma, TNF-alpha, and IL-2), tetramer staining, and CFSE dilution assays] and protection against lethal systemic challenge at 1 to 6 months post vaccination. Mice receiving pTS alone developed robust TS-specific IFN-gamma responses and survived a lethal challenge given within the first 3 months following immunization. the addition of pIL-15 to pTS vaccination did not significantly alter T cell responses or protection during this early post-vaccination period. However, mice vaccinated with both pTS and pIL-15 challenged 6 months post-vaccination were significantly more protected against lethal T. cruzi challenges than mice vaccinated with pTS alone (P6 months post immunization. Also, these TS-specific T cells were better able to expand after in vitro restimulation.Conclusion: Addition of pIL-15 during genetic vaccination greatly improved long-term T cell survival, memory T cell expansion, and long-term protection against the important human parasite, T. cruzi.National Institutes of HealthFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Millennium Institute for Gene TherapySt Louis Univ, Dept Internal Med, St Louis, MO 63103 USAUniversidade Federal de São Paulo, Ctr Terapia Celular & Mol, Escola Paulista Med, São Paulo, BrazilSt Louis Univ, Dept Mol Microbiol, St Louis, MO 63103 USAUniv Fed Minas Gerais, Inst Ciencias Biol, Dept Microbiol, Belo Horizonte, MG, BrazilUniversidade Federal de São Paulo, Ctr Terapia Celular & Mol, Escola Paulista Med, São Paulo, BrazilNational Institutes of Health: RO1 AI040196CNPq: 420067/2005-1Web of Scienc
Minimal Length Scale Scenarios for Quantum Gravity
We review the question of whether the fundamental laws of nature limit our
ability to probe arbitrarily short distances. First, we examine what insights
can be gained from thought experiments for probes of shortest distances, and
summarize what can be learned from different approaches to a theory of quantum
gravity. Then we discuss some models that have been developed to implement a
minimal length scale in quantum mechanics and quantum field theory. These
models have entered the literature as the generalized uncertainty principle or
the modified dispersion relation, and have allowed the study of the effects of
a minimal length scale in quantum mechanics, quantum electrodynamics,
thermodynamics, black-hole physics and cosmology. Finally, we touch upon the
question of ways to circumvent the manifestation of a minimal length scale in
short-distance physics.Comment: Published version available at
http://www.livingreviews.org/lrr-2013-
The Pore-Forming Toxin Listeriolysin O Mediates a Novel Entry Pathway of L. monocytogenes into Human Hepatocytes
Intracellular pathogens have evolved diverse strategies to invade and survive within host cells. Among the most studied facultative intracellular pathogens, Listeria monocytogenes is known to express two invasins-InlA and InlB-that induce bacterial internalization into nonphagocytic cells. The pore-forming toxin listeriolysin O (LLO) facilitates bacterial escape from the internalization vesicle into the cytoplasm, where bacteria divide and undergo cell-to-cell spreading via actin-based motility. In the present study we demonstrate that in addition to InlA and InlB, LLO is required for efficient internalization of L. monocytogenes into human hepatocytes (HepG2). Surprisingly, LLO is an invasion factor sufficient to induce the internalization of noninvasive Listeria innocua or polystyrene beads into host cells in a dose-dependent fashion and at the concentrations produced by L. monocytogenes. To elucidate the mechanisms underlying LLO-induced bacterial entry, we constructed novel LLO derivatives locked at different stages of the toxin assembly on host membranes. We found that LLO-induced bacterial or bead entry only occurs upon LLO pore formation. Scanning electron and fluorescence microscopy studies show that LLO-coated beads stimulate the formation of membrane extensions that ingest the beads into an early endosomal compartment. This LLO-induced internalization pathway is dynamin-and F-actin-dependent, and clathrin-independent. Interestingly, further linking pore formation to bacteria/bead uptake, LLO induces F-actin polymerization in a tyrosine kinase-and pore-dependent fashion. In conclusion, we demonstrate for the first time that a bacterial pathogen perforates the host cell plasma membrane as a strategy to activate the endocytic machinery and gain entry into the host cell
The origin of multicellularity in cyanobacteria
Background: Cyanobacteria are one of the oldest and morphologically most diverse prokaryotic phyla on our planet. The early development of an oxygen-containing atmosphere approximately 2.45 - 2.22 billion years ago is attributed to the photosynthetic activity of cyanobacteria. Furthermore, they are one of the few prokaryotic phyla where multicellularity has evolved. Understanding when and how multicellularity evolved in these ancient organisms would provide fundamental information on the early history of life and further our knowledge of complex life forms.
Results: We conducted and compared phylogenetic analyses of 16S rDNA sequences from a large sample of taxa representing the morphological and genetic diversity of cyanobacteria. We reconstructed ancestral character states on 10,000 phylogenetic trees. The results suggest that the majority of extant cyanobacteria descend from multicellular ancestors. Reversals to unicellularity occurred at least 5 times. Multicellularity was established again at least once within a single-celled clade. Comparison to the fossil record supports an early origin of multicellularity, possibly as early as the “Great Oxygenation Event” that occurred 2.45 - 2.22 billion years ago.
Conclusions: The results indicate that a multicellular morphotype evolved early in the cyanobacterial lineage and was regained at least once after a previous loss. Most of the morphological diversity exhibited in cyanobacteria today —including the majority of single-celled species— arose from ancient multicellular lineages. Multicellularity could have conferred a considerable advantage for exploring new niches and hence facilitated the diversification of new lineages
Firewalls and flat mirrors: An alternative to the AMPS experiment which evades the Harlow-Hayden obstacle
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