Combining Regulatory T Cell Depletion and Inhibitory Receptor Blockade Improves Reactivation of Exhausted Virus-Specific CD8+ T Cells and Efficiently Reduces Chronic Retroviral Loads

Chronic infections with human viruses, such as HIV and HCV, or mouse viruses, such as LCMV or Friend Virus (FV), result in functional exhaustion of CD8⁺ T cells. Two main mechanisms have been described that mediate this exhaustion: expression of inhibitory receptors on CD8⁺ T cells and expansion of regulatory T cells (Tregs) that suppress CD8⁺ T cell activity. Several studies show that blockage of one of these pathways results in reactivation of CD8⁺ T cells and partial reduction in chronic viral loads. Using blocking antibodies against PD-1 ligand and Tim-3 and transgenic mice in which Tregs can be selectively ablated, we compared these two treatment strategies and combined them for the first time in a model of chronic retrovirus infection. Blocking inhibitory receptors was more efficient than transient depletion of Tregs in reactivating exhausted CD8⁺ T cells and reducing viral set points. However, a combination therapy was superior to any single treatment and further augmented CD8⁺ T cell responses and resulted in a sustained reduction in chronic viral loads. These results demonstrate that Tregs and inhibitory receptors are non-overlapping factors in the maintenance of chronic viral infections and that immunotherapies targeting both pathways may be a promising strategy to treat chronic infectious diseases.     

Biologically-Directed Modeling Reflects Cytolytic Clearance of SIV-Infected Cells In Vivo in Macaques

The disappointing outcomes of cellular immune-based vaccines against HIV-1 despite strong evidence for the protective role of CD8⁺ T lymphocytes (CTLs) has prompted revisiting the mechanisms of cellular immunity. Prior data from experiments examining the kinetics of Simian Immunodeficiency Virus (SIV) clearance in infected macaques with or without in vivo CD8 depletion were interpreted as refuting the concept that CTLs suppress SIV/HIV by direct killing of infected cells. Here we briefly review the biological evidence for CTL cytolytic activity in viral infections, and utilize biologically-directed modeling to assess the possibility of a killing mechanism for the antiviral effect of CTLs, taking into account the generation, proliferation, and survival of activated CD4⁺ and CD8⁺ T lymphocytes, as well as the life cycle of the virus. Our analyses of the published macaque data using these models support a killing mechanism, when one considers T lymphocyte and HIV-1 lifecycles, and factors such as the eclipse period before release of virions by infected cells, an exponential pattern of virion production by infected cells, and a variable lifespan for acutely infected cells. We conclude that for SIV/HIV pathogenesis, CTLs deserve their reputation as being cytolytic.     

Potentiation of the Antiviral Activity of Interferon by Actinomycin D

INTERFERON induces cellular synthesis of the antiviral protein which is probably responsible for conferring antiviral activities. Although this antiviral protein has not been isolated, indirect evidence favours a two-step mechanism in the development of cellular resistance to viruses. Taylor¹ and subsequently Friedman and Sonnabend² and Lockart³ have shown that from 0 to 4 h after the treatment of the cells with interferon, the induction of the antiviral state requires the integrity of the cellular apparatus for protein synthesis. Cassingena et al.⁴ have shown that in somatic mouse-monkey hybrid cells, the genes which code for the production and action of interferon are located at different chromosomal sites.     

Interactions of FGFs with target cells

Growth factors play a key role in cellular communication, a necessary step for the development of pluricellular organisms. The fibroblast growth factors (FGF) are among these polypeptides and have seven known members: FGF 1 to FGF 7 which are also known as acidic FGF, basic FGF, translation products of oncogenes hst, int 2, FGF 5, FGF 6 and FGF 7 or keratinocyte growth factor (KGF) respectively[1]. ² The best known and the most abundant in normal adult tissues are acidic and basic FGFs, or FGF 1 and 2 respectively, which have been subjected to extensive studies both in vitro and in vivo. These two factors have almost ubiquitous distribution and a wide spectrum of biological activity including action on cellular proliferation and differentiation, as well as neurotrophic and angiogenic properties[1]. These different activities are induced by triggering specific receptors present at the surface of the target cell. Following this interaction, the FGF-receptor complexes are internalized and activate intracellular pathways. An important effort of investigations has been produced to characterize these receptors and intracellular pathways. It is the purpose of this review to present this work which will focus on FGFs 1 and 2. The existence of two classes of interactions has been reported as early as 1987 [52,53,54] suggesting the presence of high and low affinity receptors for FGFs.     

Unraveling STIM2 function

The discovery of molecular players in capacitative calcium (Ca²⁺) entry, also referred to as store-operated Ca²⁺ entry (SOCE), supposed a great advance in the knowledge of cellular mechanisms of Ca²⁺ entry, which are essential for a broad range of cellular functions. The identification of STIM1 and STIM2 proteins as the sensors of Ca²⁺ stored in the endoplasmic reticulum unraveled the mechanism by which depletion of intracellular Ca²⁺ stores is communicated to store-operated Ca²⁺ channels located in the plasma membrane, triggering the activation of SOCE and intracellular Ca²⁺-dependent signaling cascades. Initial studies suggested a dominant function of STIM1 in SOCE and SOCE-dependent cellular functions compared to STIM2, especially those that participate in immune responses. Consequently, most of the subsequent studies focused on STIM1. However, during the last years, STIM2 has been demonstrated to play a more relevant and complex function than initially reported, being even important to sustain normal life in mice. These studies have led to reconsider the role of STIM2 in SOCE and its relevance in cellular physiology. This review is intended to summarize and provide an overview of the current data available about this exciting isoform, STIM2, and its actual position together with STIM1 in the mechanism of SOCE.     

An enzymatic locus participating in cellular division of a yeast

Growing cells of a filamentous mutant of a yeast, Candida albicans, were found to accumulate and reduce tetrazolium dyes whereas cells of the parent strain, growing as a normally budding yeast, accumulated the dye but did not reduce it. In older cultures, in which rapidly metabolizable carbohydrate has been depleted, the parent strain characteristically produces filaments. These cells, growing in the absence of cellular division, also exhibit tetrazolium reduction. The filamentous mutant synthesizes cell mass at a rate almost equal to that of the parent strain and is not distinguished therefrom in fermentation ability, nutritional requirements for growth, rate of endogenous respiration, or polysaccharide composition. These facts, in conjunction with the striking differences in tetrazolium reduction, lead to the conclusion that the morphological mutant has an impairment to a cellular oxidation mechanism at a flavoprotein locus. This locus is, then, the site at which a reaction essential for cellular division, is coupled via an oxidation-reduction to cellular metabolism. Preliminary evidence is presented providing good indication that uncoupling of cellular division (by genetic block) in the mutant or in the parent (by substrate exhaustion) results from impairment to a dissociable metal chelate mechanism which normally couples a reaction essential to cellular division to flavoprotein oxidation.     

The centrosome and mitotic spindle apparatus in cancer and senescence

Altered cell division is associated with overproliferation and tumorigenesis, however, mitotic aberrations can also trigger antiproliferative responses leading to postmitotic cell cycle exit. Here, we focus on the role of the centrosome and in particular of centrosomal TACC (transforming acidic coiled coil) proteins in tumorigenesis and cellular senescence. We have compiled recent evidence that inhibition or depletion of various mitotic proteins which take over key roles in centrosome and kinetochore integrity and mitotic checkpoint function is sufficient to activate a p53-p21^WAF driven premature senescence phenotype. These findings have direct implications for proliferative tissue homeostasis as well as for cellular and organismal aging.     

Store-operated Ca-permeable non-selective cation channels in smooth muscle cells

Over twenty years ago it was shown that depletion of the intracellular Ca²⁺ store in smooth muscle triggered a Ca²⁺ influx mechanism. The purpose of this review it to describe recent electrophysiological data which indicate that Ca²⁺ influx occurs through discrete ion channels in the plasmalemma of smooth muscle cells. The effect of external Ca²⁺ on the amplitude and reversal potential of whole-cell and single channel currents suggests that there are at least two, and probably more, distinct store-operated channels (SOCs) which have markedly different permeabilities to Ca²⁺ ions. Two activation mechanisms have been identified which involve Ca²⁺ influx factor and protein kinase C (PKC) activation via diacylglycerol. In addition, in rabbit portal vein cells there is evidence that stimulation of α-adrenoceptors can stimulate SOC opening via PKC in a store-independent manner. There is at present little knowledge on the molecular identity of SOCs but it has been proposed that TRPC1 may be a component of the functional channel. We also summarise the data showing that SOCs may be involved in contraction and cell proliferation of smooth muscle. Finally, we highlight the similarities and differences of SOCs and receptor-operated cation channels that are present in native rabbit portal vein myocytes.     

Differential Regulation of Mitogen- and Stress-activated Protein Kinase-1 and -2 (MSK1 and MSK2) by CK2 following UV Radiation

Mitogen- and stress-activated protein kinases, MSK1 and the closely related isoform MSK2, are nuclear kinases that are activated following mitogen stimulation or cellular stress, including UV radiation, by the ERK1/2 and p38 MAPK signaling cascades, respectively. However, factors that differentially regulate MSK1 and MSK2 have not been well characterized. Here we report that the CK2 protein kinase, which contributes to NF-κB activation following UV radiation in a p38-dependent manner, physically interacts with MSK2 but not MSK1 and that CK2 inhibition specifically impairs UV-induced MSK2 kinase activation. A putative site of CK2 phosphorylation was mapped to MSK2 residue Ser³²⁴ and when substituted to alanine (S324A) also compromised MSK2 activity. RNA interference-mediated depletion of MSK2 in human MDA-MB-231 cells, but not MSK1 depletion, resulted in impaired UV-induced phosphorylation of NF-κB p65 at Ser²⁷⁶ in vivo, which was restored by the ectopic expression of MSK2 but not by MSK2-S324A. Furthermore, UV radiation led to the activation of NF-κB-responsive gene expression in MDA-MB-231 cells and induced p65 transactivation capacity that was dependent on MSK2, MSK2 residue Ser³²⁴, and p65-Ser²⁷⁶. These results suggest that MSK1 and MSK2 are differentially regulated by CK2 during the UV response and that MSK2 is the major protein kinase responsible for the UV-induced phosphorylation of p65 at Ser²⁷⁶ that positively regulates NF-κB activity in MDA-MB-231 cells.     

Cellular Effects of a Protozoan-produced Growth-stimulating Factor

RESEARCH on growth factors produced by various protozoa has been reviewed by Lilly and Stillwell¹ and by Lilly². Here, the effect of a growth factor (“probiotic”) from Colpidium campylum is reported. Its effects were tested on Glaucoma chattoni because a chemically defined growth medium with no heat labile substances or particulate turbidity has recently been developed³ for this organism, making it particularly suitable for research into basic cellular mechanisms.     

Attenuation of TORC1 signaling delays replicative and oncogenic RAS-induced senescence

Numerous stimuli, including oncogenic signaling, DNA damage or eroded telomeres trigger proliferative arrest, termed cellular senescence. Accumulating evidence suggests that cellular senescence is a potent barrier to tumorigenesis in vivo, however oncogene induced senescence can also promote cellular transformation.^1,2 Several oncogenes, whose overexpression results in cellular senescence, converge on the TOR (target of rapamycin) pathway. We therefore examined whether attenuation of TOR results in delay or reversal of cellular senescence. By using primary human fibroblasts undergoing either replicative or oncogenic RAS-induced senescence, we demonstrated that senescence can be delayed, and some aspects of senescence can be reversed by inhibition of TOR, using either the TOR inhibitor rapamycin or by depletion of TORC1 (TOR Complex 1). Depletion of TORC2 fails to affect the course of replicative or RAS-induced senescence. Overexpression of REDD1 (Regulated in DNA Damage Response and Development), a negative regulator of TORC1, delays the onset of replicative senescence. These results indicate that TORC1 is an integral component of the signaling pathway that mediates cellular senescence.     

FANCJ promotes DNA synthesis through G‐quadruplex structures

Our genome contains many G-rich sequences, which have the propensity to fold into stable secondary DNA structures called G4 or G-quadruplex structures. These structures have been implicated in cellular processes such as gene regulation and telomere maintenance. However, G4 sequences are prone to mutations particularly upon replication stress or in the absence of specific helicases. To investigate how G-quadruplex structures are resolved during DNA replication, we developed a model system using ssDNA templates and Xenopus egg extracts that recapitulates eukaryotic G4 replication. Here, we show that G-quadruplex structures form a barrier for DNA replication. Nascent strand synthesis is blocked at one or two nucleotides from the G4. After transient stalling, G-quadruplexes are efficiently unwound and replicated. In contrast, depletion of the FANCJ/BRIP1 helicase causes persistent replication stalling at G-quadruplex structures, demonstrating a vital role for this helicase in resolving these structures. FANCJ performs this function independently of the classical Fanconi anemia pathway. These data provide evidence that the G4 sequence instability in FANCJ^−/− cells and Fancj/dog1 deficient C. elegans is caused by replication stalling at G-quadruplexes.