HIV-1 Nef Targets MHC-I and CD4 for Degradation Via a Final Common β-COP–Dependent Pathway in T Cells

To facilitate viral infection and spread, HIV-1 Nef disrupts the surface expression of the viral receptor (CD4) and molecules capable of presenting HIV antigens to the immune system (MHC-I). To accomplish this, Nef binds to the cytoplasmic tails of both molecules and then, by mechanisms that are not well understood, disrupts the trafficking of each molecule in different ways. Specifically, Nef promotes CD4 internalization after it has been transported to the cell surface, whereas Nef uses the clathrin adaptor, AP-1, to disrupt normal transport of MHC-I from the TGN to the cell surface. Despite these differences in initial intracellular trafficking, we demonstrate that MHC-I and CD4 are ultimately found in the same Rab7+ vesicles and are both targeted for degradation via the activity of the Nef-interacting protein, β-COP. Moreover, we demonstrate that Nef contains two separable β-COP binding sites. One site, an arginine (RXR) motif in the N-terminal α helical domain of Nef, is necessary for maximal MHC-I degradation. The second site, composed of a di-acidic motif located in the C-terminal loop domain of Nef, is needed for efficient CD4 degradation. The requirement for redundant motifs with distinct roles supports a model in which Nef exists in multiple conformational states that allow access to different motifs, depending upon which cellular target is bound by Nef

Positioning of APOBEC3G/F Mutational Hotspots in the Human Immunodeficiency Virus Genome Favors Reduced Recognition by CD8+ T Cells

Due to constitutive expression in cells targeted by human immunodeficiency virus (HIV), and immediate mode of viral restriction upon HIV entry into the host cell, APOBEC3G (A3G) and APOBEC3F (A3F) have been considered primarily as agents of innate immunity. Recent bioinformatic and mouse model studies hint at the possibility that mutation of the HIV genome by these enzymes may also affect adaptive immunity but whether this occurs in HIV-infected individuals has not been examined. We evaluated whether APOBEC-mediated mutations within common HIV CD8+ T cell epitopes can potentially enhance or diminish activation of HIV-specific CD8+ T cells from infected individuals. We compared ex vivo activation of CD8+ T lymphocytes from HIV-infected individuals by wild type HIV peptide epitopes and synthetic variants bearing simulated A3G/F-induced mutations by measuring interferon-c (IFN-c) production. We found that A3G/F-induced mutations consistently diminished HIV-specific CD8+ T cell responses against the common epitopes we tested. If this reflects a significant trend in vivo, then adaptation by HIV to enrich sequences that are favored for mutation by A3G/F (A3G/F hotspots) in portions of its genome that encode immunogenic CD8+ T cell epitopes would favor CTL escape. Indeed, we found the most frequently mutated A3G motif (CCC) is enriched up to 6-fold within viral genomic sequences encoding immunodominant CD8+ T cell epitopes in Gag, Pol and Nef. Within each gene, A3G/F hotspots are more abundant in sequences encoding epitopes that are commonly recognized due to their HLA restriction. Thus, in our system, mutations of the HIV genome, mimicking A3G/F activity, appeared to abrogate or severely reduce CTL recognition. We suggest that the physiological significance of this potential effect in facilitating CTL escape is echoed in the adaptation of the HIV genome to enrich A3G/F hotspots in sequences encoding CTL epitopes that are more immunogenic at the population level

Boltzmann's ultraviolet cutoff and Nekhoroshev's theorem on Arnold diffusion

The greatest difficulty with classical statistical mechanics may be the fact that some degrees of freedom do not seem to attain the energy expected from the equipartition principle. This is typical of the vibrational modes in molecules and of the high frequencies in the electromagnetic radiation (problem of the ultraviolet cutoff). Boltzmann1 had the intuition that an explanation might be provided if each degree of freedom were to have a characteristic relaxation time until equilibrium, and that such a time should greatly increase with frequency; he spoke in terms of relaxation times of days or centuries. This possibility was seriously considered by Rayleigh and Jeans2, but they could not produce a clear classical mechanism to explain the freezing of the high frequency degrees of freedom; so, the equilibrium concept of the ultraviolet cutoff provided by quantum mechanics was accepted, and Boltzmann's hypothesis was forgotten. Here we point out that a general framework for understanding the ultraviolet cutoff in Boltzmann's dynamical sense in a classical context seems to be provided by Nekhoroshev's3 theorem on Arnold diffusion

Wait, treat and see: echocardiographic monitoring of brain-dead potential donors with stunned heart

<p>Abstract</p> <p>Background</p> <p>Heart transplantation is limited by a severe donor organ shortage. Potential donors with brain death (BD) and left ventricular dysfunction due to neurogenic stunning are currently excluded from donation – although such abnormalities can be reversible with aggressive treatment including Hormonal Treatment (HT) and deferred organ retrieval.</p> <p>Aim</p> <p>To assess the recovery of left ventricular dysfunction in potential brain-dead donors with hemodynamic instability treated by aggressive treatment and HT.</p> <p>Methods</p> <p>In a single-center, observational study design, we evaluated 15 consecutive brain-dead potential donors (DBD) (8 males, age = 48 ± 15 years) with hemodynamic instability. All underwent standard hemodynamic monitoring and transthoracic 2-dimensional echo (2-DE) with assessment of Ejection Fraction (EF). Measurements were obtained before BD and after BD within 6 h, at 24 h and within 48 h. HT (with insulin, methylprednisolone, vasopressin and T3) was started as soon as possible to treat hemodynamic instability and avoid administration of norepinephrine (NE). Eligible potential heart donors underwent coronary angiography.</p> <p>Results</p> <p>After HT, we observed a normalization of hemodynamic conditions with improvement of mean arterial pressure (pre = 68 ± 8 mmHg vs post = 83 ± 13 mmHg, p < .01), cardiac index (pre = 2.4 ± 0.6 L/min/m² vs post 3.7 ± 1.2 L/min/m², p < .05), EF (pre = 48 ± 15 vs post = 59 ± 3%, p < .01) without administration of norepinephrine (NE) in 67% of cases. Five potential donors were excluded from donation (opposition, n = 3, tubercolosis n = 1, malignancy n = 1). At pre-harvesting angiography<b>,</b> coronary artery stenosis was present in 2 of the 10 consented donors. Eight hearts were uneventfully transplanted. No early graft failure occurred and all eight recipients were alive at 6-month follow-up.</p> <p>Conclusion</p> <p>In BD donors, intensive treatment including HT is associated with improvement of regional and global LV function and reverse remodeling detectable by transthoracic 2DE. Donor hearts with recovered LV function may be eligible for uneventful heart transplant. The wait (in brain death), treat (with HT) and see (with 2D echo) strategy can help rescue organs suitable for heart donation.</p