Semliki Forest virus core protein fragmentation: its possible role in nucleocapsid disassembly

Oral mucosa immunity: ultimate strategy to stop spreading of pandemic viruses

Global pandemics are most likely initiated via zoonotic transmission to humans in which respiratory viruses infect airways with relevance to mucosal systems. Out of the known pandemics, five were initiated by respiratory viruses including current ongoing coronavirus disease 2019 (COVID-19). Striking progress in vaccine development and therapeutics has helped ameliorate the mortality and morbidity by infectious agents. Yet, organism replication and virus spread through mucosal tissues cannot be directly controlled by parenteral vaccines. A novel mitigation strategy is needed to elicit robust mucosal protection and broadly neutralizing activities to hamper virus entry mechanisms and inhibit transmission. This review focuses on the oral mucosa, which is a critical site of viral transmission and promising target to elicit sterile immunity. In addition to reviewing historic pandemics initiated by the zoonotic respiratory RNA viruses and the oral mucosal tissues, we discuss unique features of the oral immune responses. We address barriers and new prospects related to developing novel therapeutics to elicit protective immunity at the mucosal level to ultimately control transmission.

Expression and Immunogenicity of Recombinant African Swine Fever Virus Proteins Using the Semliki Forest Virus

African swine fever virus (ASFV) is a large DNA virus belonging to the Asfarviridae family that damages the immune system of pigs, resulting in the death or slaughter of millions of animals worldwide. Recent modern techniques in ASFV vaccination have highlighted the potential of viral replicon particles (RPs), which can efficiently express foreign proteins and induce robust cellular and humoral immune responses compared with the existing vaccines. In this study, we established a Semliki Forest virus (SFV) vector by producing replication-defective viral particles. This vector was used to deliver RPs expressing ASFV antigens. SFV-RPs expressing ASFV p32 (SFV-p32) and p54 (SFV-p54) were tested in baby hamster kidney (BHK-21) cells. Proteins expression was evaluated via western blotting and indirect immunofluorescence, while immunogenicity was evaluated in BALB/c mice. The resulting RPs exhibited high levels of protein expression and elicited robust humoral and cellular immune responses.

Beneficial Properties of Bromelain

Bromelain is a major sulfhydryl proteolytic enzyme found in pineapple plants, having multiple activities in many areas of medicine. Due to its low toxicity, high efficiency, high availability, and relative simplicity of acquisition, it is the object of inexhaustible interest of scientists. This review summarizes scientific reports concerning the possible application of bromelain in treating cardiovascular diseases, blood coagulation and fibrinolysis disorders, infectious diseases, inflammation-associated diseases, and many types of cancer. However, for the proper application of such multi-action activities of bromelain, further exploration of the mechanism of its action is needed. It is supposed that the anti-viral, anti-inflammatory, cardioprotective and anti-coagulatory activity of bromelain may become a complementary therapy for COVID-19 and post-COVID-19 patients. During the irrepressible spread of novel variants of the SARS-CoV-2 virus, such beneficial properties of this biomolecule might help prevent escalation and the progression of the COVID-19 disease.

The Combination of Bromelain and Acetylcysteine (BromAc) Synergistically Inactivates SARS-CoV-2

Severe acute respiratory syndrome coronavirus (SARS-CoV-2) infection is the cause of a worldwide pandemic, currently with limited therapeutic options. The spike glycoprotein and envelope protein of SARS-CoV-2, containing disulfide bridges for stabilization, represent an attractive target as they are essential for binding to the ACE2 receptor in host cells present in the nasal mucosa. Bromelain and Acetylcysteine (BromAc) has synergistic action against glycoproteins by breakage of glycosidic linkages and disulfide bonds. We sought to determine the effect of BromAc on the spike and envelope proteins and its potential to reduce infectivity in host cells. Recombinant spike and envelope SARS-CoV-2 proteins were disrupted by BromAc. Spike and envelope protein disulfide bonds were reduced by Acetylcysteine. In in vitro whole virus culture of both wild-type and spike mutants, SARS-CoV-2 demonstrated a concentration-dependent inactivation from BromAc treatment but not from single agents. Clinical testing through nasal administration in patients with early SARS-CoV-2 infection is imminent.

Alphavirus Replication: The Role of Cardiac Glycosides and Ion Concentration in Host Cells

Alphaviruses are arthropod-borne viruses that can cause fever, rash, arthralgias, and encephalitis. The mosquito species Aedes aegypti and Aedes albopictus are the most frequent transmitters of alphaviruses. There are no effective vaccines or specific antivirals available for the treatment of alphavirus-related infections. Interestingly, changes in ion concentration in host cells have been characterized as critical regulators of the alphavirus life cycle, including fusion with the host cell, glycoprotein trafficking, genome translation, and viral budding. Cardiac glycosides, which are classical inhibitors of the Na⁺ K⁺ ATPase (NKA), can inhibit alphavirus replication although their mechanisms of action are poorly understood. Nonetheless, results from multiple studies suggest that inhibition of NKA may be a suitable strategy for the development of alphavirus-specific antiviral treatments. This review is aimed at exploring the role of changes in ion concentration during alphavirus replication and at considering the possibility of NKA as a potential therapeutic target for antiviral drugs.

The regulation of disassembly of alphavirus cores

Alphaviruses are used as model viruses for structure determination and for analysis of virus entry. They are used also as vectors for protein expression and gene therapy. Virus particles are assembled by budding, using preformed cores and a modified cellular membrane. During entry, alphaviruses release the viral core into the cytoplasm. Cores are disassembled during virus entry and accumulate in the cytoplasm during virus multiplication. The regulation of core disassembly is the subject of this review. A working model compatible with all experimental data is formulated. This model comprises the following steps: (1) The incoming core is present in the cytoplasm in a metastable state, primed for disassembly. A core structure containing the so-called linker region of the core protein in an exposed position susceptible to proteolytic cleavage on the core surface might represent the primed state. (2) The primed core allows access of cellular proteins to the viral genome RNA, e.g. initiation factors of protein synthesis. (3) In a following step, ribosomal 60S subunits bind to the complex and lead to core disassembly with a concomitant transfer of core protein or of core protein fragments to the 28S rRNA. The linker region may be involved in this transfer. (4) During the later stages of virus multiplication, cellular components involved in step (2) and/or in step (3) are inactivated. This inactivation might involve the binding of newly synthesised core protein to 28S rRNA. (5) Unprimed cores, e.g. core particles containing the linker region in an unexposed position, are assembled during virus multiplication. Priming of cores and inactivation of host-cell factors each represent a complete mechanism of regulation of core disassembly. Future experiments will show whether or not both processes are actually used. Since alphaviruses, e.g. Chikungunya virus, Ross River virus, Semliki Forest virus, and Sindbis virus, are human pathogens, these experiments are of practical relevance, since they might identify targets for antiviral chemotherapy.

Lentiviral vectors for cancer immunotherapy: transforming infectious particles into therapeutics

Lentiviral vectors have emerged as promising tools for both gene therapy and immunotherapy purposes. They exhibit several advantages over other viral systems in that they are less immunogenic and are capable of transducing a wide range of different cell types, including dendritic cells (DC). DC transduced ex vivo with a whole range of different (tumor) antigens were capable of inducing strong antigen-specific T-cell responses, both in vitro and in vivo. Recently, the administration of lentiviral vectors in vivo has gained substantial interest as an alternative method for antigen-specific immunization. This method offers a number of advantages over DC vaccines as the same lentivirus can in principle be used for all patients resulting in a significantly reduced cost and requirement for considerably less expertise for the generation and administration of lentiviral vaccines. By selectively targeting lentiviral vectors to, or restricting transgene expression in certain cell types, selectivity, safety and efficacy can be further improved. This review will focus on the use of direct administration of lentiviral vectors encoding tumor-associated antigens (TAA) for the induction of tumor-specific immune responses in vivo, with a special focus on problems related to the generation of large amounts of highly purified virus and specific targeting of antigen-presenting cells (APC).

Virus membrane proteins and proteinaceous pores

Enveloped viruses penetrate the host cells by fusion of the viral envelope with a cellular target membrane. One of the best studied viruses with respect to its penetration and uncoating is the alphavirus Semliki Forest virus that is taken up by endocytosis. The alphavirus membrane glycoprotein E1 harbors a so-called fusion peptide, which is responsible for interaction with the endosomal membrane, leading to fusion. Besides this fusion process, cell infection by alphaviruses is accompanied by membrane permeability changes, thus implying some form of pore across the membrane. However, the ability of E1 protein to form ion pores has not been widely accepted. This review provides an overview of studies that confirm earlier results predicting the formation of a proteinaceous pore by the alphavirus spike proteins. Furthermore, different models to explain this pore formation during virus entry are discussed.

The fragmentation of incoming Semliki Forest virus nucleocapsids in mosquito (Aedes albopictus) cells might be coupled to virion uncoating

The fate of Semliki Forest virus (SFV) nucleocapsid, especially the capsid protein (C-protein), was investigated during the early stages of a productive infection in mosquito Aedes albopictus cells. Infection of the cells resulted in a time dependent accumulation of a C-protein derived fragment. This fragmentation of incoming viral nucleocapsid was prevented by NH4Cl, an agent generally used to elevate the pH in acidic intracellular compartments, suggesting that a low intravesicular pH is required for this process. Density gradient analysis of the postnuclear cell lysate demonstrated that the fragmentation was associated with a cellular compartment showing a density of 1.14 +/- 0.02 g/ml. This cellular compartment was devoid from a lysosomal marker enzyme and represented the timely preceding cellular fraction through which SFV passed before encountering a lysosomal fraction. Furthermore, the intracellular distribution of the viral, 3H-uridine-labeled RNA suggested that the same fraction might represent a key cellular compartment in which the separation of the viral RNA from the viral structural proteins is primed. In conclusion, these data lead to the suggestion that the fragmentation of incoming SFV nucleocapsids in Aedes albopictus cells might be the part of the mechanism leading to the release of viral RNA into the cytosol during early stages of productive infection.

Entry and uncoating of enveloped viruses

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