Mutant Virus Strains Research Articles

Advances in Antiviral Therapy for Subacute Sclerosing Panencephalitis

Subacute sclerosing panencephalitis (SSPE) is a late-onset, intractable, and fatal viral disease caused by persistent infection of the central nervous system by a mutant strain of the measles virus. Ribavirin intracerebroventricular therapy has already been administered to several SSPE patients in Japan based on fundamental and clinical research findings from our group, with positive therapeutic effects reported in some patients. However, the efficacy of this treatment approach has not been unequivocally established. Hence, development of more effective therapeutic methods using new antiviral agents is urgently needed. This review describes the current status of SSPE treatment and research, highlighting promising approaches to the development of more effective therapeutic methods.

From Cold to Killer: How SARS-CoV-2 Evolved without Hemagglutinin Esterase to Agglutinate, Then Clot Blood Cells in Pulmonary and Systemic Microvasculature

The role of vascular occlusion in the morbidities, pulmonary and systemic, of COVID-19 has received increasing focus. Histological studies of lung tissue from COVID-19 patients have found extensively damaged endothelium of capillaries adjoining relatively intact alveoli, corresponding to hypoxemia accompanying normal breathing mechanics in such patients. Advanced image analysis of lung CT scans of COVID-19 patients reveals redistribution of blood flow from smaller to larger diameter blood vessels, this effect correlated with the degree of breathing dysfunction.Essential to the study of vascular occlusion in COVID-19 are viral properties dating back to studies of Jonas Salk in the 1940s that have been positively established for SARS-CoV-2. First, SARS-CoV-2 binds to red blood cells (RBCs), in vitro and also clinically in COVID-19 patients. Second, although fusion and replication of SARS-CoV-2 occur via ACE2, such hemagglutinating viruses initially attach to infective targets and clump with blood cells via much more abundantly distributed sialic acid (SA) glycoconjugate binding sites. SARS-CoV-2, in particular, attaches to these SA sites. Third, certain enveloped viruses express an enzyme, hemagglutinin esterase (HE), that counteracts viral-RBC clumping. Notably, among betacoronaviruses, the common cold strains express HE while SARS-CoV-2, SARS-CoV-1 and MERS, the virulent strains, do not.These hemagglutinating properties of SARS-COV-2 establish a framework for “catch and clump” induction of microvascular occlusion proposed here. Ultramicroscopic studies of tissues from COVID-19 patients indicate a key role for hemagglutination early and mid-course in COVID-19, before such clumps harden into clots via the coagulation cascade. Hemagglutination may be reversed by two anti-COVID-19 therapeutics that each competitively bind to SARS-CoV-2 spike protein, blocking such viral attachments. One therapeutic is antiviral antibodies generated by vaccines, the anti-hemagglutination effect of which is exhibited in Jonas Salk’s hemagglutination inhibition assay. The other therapeutic is ivermectin (IVM), a drug of Nobel Prize honored distinction, distributed in 3.7 billion doses worldwide. In ten clinical trials, three with randomized controls, IVM yielded mortality reductions for COVID-19 of 90% at highest doses. IVM may limit virulence of SARS-CoV-2 by steric interference with multivalent spike protein attachments to SA binding sites, blocking hemagglutination, an effect likely to target mutant viral strains.

Bioisosterism-based design and enantiomeric profiling of chiral hydroxyl-substituted biphenyl-diarylpyrimidine nonnucleoside HIV-1 reverse transcriptase inhibitors

The single enantiomers of seven hydroxyl-substituted biphenyl-diarylpyrimidines were designed and synthesized by a bioisosterism strategy as novel HIV-1 non-nucleoside reverse transcriptase inhibitors (NNRTIs). The cellular and enzymatic assays indicated that the novel obtained compounds had significant activities and relatively low cytotoxicity. The supercritical fluid chromatography (SFC) enantioseparations of the racemic compounds and the enantiomeric profiling resulted that the (S) forms were generally more potent than the (R) counterparts. Among all the chiral derivatives, (S)-(−)-12a showed the best potency with the antiviral activities against wild-type (WT) and single mutant strains (L100I, K103 N, Y181C, E138K; especially Y188L), and RT enzyme in the low nanomolar concentration range. Toward double mutant virus strains (F227L + V106A, RES056), (S)-(−)-12a possessed submicromolar antiviral activities. In addition, (S)-(−)-12a showed a high cell-based selectivity index (SI WT = 5822) and no apparent toxicity was observed in the in vivo acute toxicity assay and electrocardiogram. The molecular docking studies predicted the binding modes of the compounds with RT and explained the activity differences for the enantiomers. Although the rat pharmacokinetic assay indicated a poor oral metabolism of the hydroxyl compound, the promising antiviral activity of the chiral hydroxyl-substituted biphenyl-diarylpyrimidines provided valuable lead compounds for further anti-HIV drug design.

A REVIEW ON THE STATE OF ZIKA VIRUS IN NIGERIA

Background: Zika virus, a flavivirus primarily transmitted through bites of mosquito (Aedes aegypti), blood transfusion, sexual contact and motherto-child was named after Zika forest in Uganda where it was first isolated from a sentinel monkey in 1947. It is a positively sensed single stranded RNA virus (+ssRNA) of approximately 11000Kb in length, encoding three (3) structural and seven (7) non-structural proteins. The virus has spread across several countries of the world between 1947 and today. Between March 2015 and November 2016, about 500,000 to 1.5 million people have been reported infected with Zika virus in Brazil alone, resulting in several cases of microcephaly, severe brain and ocular malformations, abortions, Guillain-Barre syndrome and even deaths. Cases of Zika virus infections have also been documented in some African countries with resulting isolation and sequencing of several strains. In Nigeria however, the first case of Zika virus was reported as far back as 1954 in the Eastern part of the country during a research on Yellow fever. Zika virus mutant strains might have evolved with corresponding effects on man due to continuous environmental changes and human factors worldwide. Aim: This study was aimed at reviewing the state of Zika virus in Nigeria: past and present. Methods: Original research articles and reviews were searched on the NCBI, Research gate, PubMed and Google scholar using queries such as; Zika, epidemiology of Zika, pathogenesis of Zika and molecular characterization of Zika. Results: High points on virology, epidemiology, pathogenesis, diagnosis, prevention and management were pooled together and discussed. Conclusion: The knowledge gained from this paper will help understand the current state of Zika virus in Nigeria. Keywords: Nigeria, Zika, flavivirus, RNA, microcephaly, epidemiology, genomics.

Stochastic Dynamics of the Latently Infected Cell Reservoir During HIV Infection.

The presence of cells latently infected with HIV is currently considered to be a major barrier to viral eradication within a patient. Here, we consider birth-death-immigration models for the latent cell population in a single patient, and present analytical results for the size of this population in the absence of treatment. We provide results both at steady state (viral set point), and during the non-equilibrium setting of early infection. We obtain semi-analytic results showing how latency-reversing drugs might be expected to affect the size of the latent pool over time. We also analyze the probability of rare mutant viral strains joining the latent cell population, allowing for steady-state and dynamic viral populations within the host.

Susceptibility of Brazilian influenza A(H1N1)pdm09 viruses to neuraminidase inhibitors in the 2014–2016 seasons: Identification of strains bearing mutations associated with reduced inhibition profile

Neuraminidase inhibitors (NAIs) are the main class of antivirals currently used for the treatment of influenza infections. As influenza viruses are constantly evolving, drug-resistance can emerge resulting in reduced effectiveness of treatment. This study evaluated the presence of molecular markers associated with NAI susceptibility in 724 influenza A(H1N1)pdm09 positive samples from Brazilian surveillance system from the 2014–2016 seasons, including 76 isolates tested for oseltamivir (OST) susceptibility and 23 isolates also tested for zanamivir, peramivir and laninamivir susceptibility. We identified the H275Y (n = 3) and I223K (n = 1) NA substitutions, associated with reduced inhibition (RI) by the NAIs. Noteworthy, no epidemiological links were identified among the patients infected with the mutant viruses. Phylogenetic analysis from NA and hemagglutinin genes showed that mutant viruses were not clustered. All mutant virus strains carried the permissive substitutions V241I and N369K, in addition to the N386K, which has been shown to destabilize the NA structure. Functional NA analysis of one virus containing the H275Y mutation confirmed its highly RI profile to OST and peramivir and demonstrated that it had decreased viral replication and NA thermostability compared to the wild type virus. The remaining tested isolates presented normal inhibition profile to the NAIs tested. In conclusion, the overall frequency of influenza A(H1N1)pdm09 viruses bearing mutations associated with NAI RI was 0.6%, similar to what has been observed in recent global studies.

Gene Therapy with CRISPR/Cas9 Coming to Age for HIV Cure

Gene Therapy with CRISPR/Cas9 Coming to Age for HIV Cure

Problems of the Current Vaccination Strategy against Hepatitis B

Accumulation of escape mutants of hepatitis B virus (HBV) occur in the many countries at the conditions of wide-scale vaccination against hepatitis B, especially among vaccinated children. The real prevalence of such mutants is underestimated and the capacity of current vaccines to provide cross-immunity against G145R mutant is almost completely absent. The potential for spread of mutant virus strains accumulates. Consequently, a question about the need to change the design of vaccines arises with the purpose of expansion of their action spectrum. An approach of inclusion pre-S epitopes into vaccines has own advantages in fight with escape mutants, but some researchers suggest that the inclusion of modificated a-determinant capable to induce neutralizing antibodies specific to escape mutants into vaccine composition is more preferable. Nevertheless, modification of antigen components of vaccines has certain difficulties related to evaluation of the accuracy of folding of recombinant proteins and their conformity to natural analogues. A recently proposed method for selecting of recombinant antigens possessing the necessary properties likely allows to overcome existing problems. However, apparently, due to the features of species immunity to HBsAg and the limitations of using the chimpanzee model for testing of hepatitis B vaccines, the final answer to this question can only be obtained after clinical studies.

Generation of a Live Attenuated Influenza Vaccine that Elicits Broad Protection in Mice and Ferrets

New influenza vaccines that provide effective and broad protection are desperately needed. Live attenuated viruses are attractive vaccine candidates because they can elicit both humoral and cellular immune responses. However, recent formulations of live attenuated influenza vaccines (LAIVs) have not been protective. We combined high-coverage transposon mutagenesis of influenza virus with a rapid high-throughput screening for attenuation to generate W7-791, a live attenuated mutant virus strain. W7-791 produced only a transient asymptomatic infection in adult and neonatal mice even at doses 100-fold higher than the LD50 of the parent strain. A single administration of W7-791 conferred full protection to mice against lethal challenge with H1N1, H3N2, and H5N1 strains, and improved viral clearance in ferrets. Adoptive transfer of Tcells from W7-791-immunized mice conferred heterologous protection, indicating a role for Tcell-mediated immunity. These studies present an LAIV development strategy to rapidly generate and screen entire libraries of viral clones.

Mutations in surface protein of swine flu: A major problem for H1N1 inhibitor

The emergence of new mutant strains of influenza virus like H1N1, H3N2, H1N9, H7N9, etc. in every season of flu is mainly due to frequent mutation in eight genes of flu virus. This cause influenza virus spreads worldwide and become pandemic in 2009. Every year 36000 peoples are infected from flu. Around 123397 people have been tested itself in India in 2010. Drug for influenza treatment now has been resistance due to mutation in receptor of neuraminidase. Mutation is the major problem for designing inhibitor against influenza virus. Number of strains produced due genetic reassortment and mutant property of virus. This makes the different strains of hemagglutinin (H) and neuraminidase (N). Total eight gene of influenza virus, out of these only few gene involve for mutation continuously in which may be some beneficial or may some insensitive to flu. Gene mutation causes specific changes in sequence of the amino acids, and changes the conformation of other concerned residues which causing the unfitting binding site for influenza inhibitor. Number of other mutation associates with specific mutation like H274Y, N294S, I223R, E119V, Q136K and S31N etc. are responsible for changing conformation of binding site. Some mutation of surface protein of virus may cause negative or positive effect on binding site. Some induced mutation also helps in antiviral activity. This review highlights the mutation which causes resistance and changing residues in the binding site of in neuraminidase, hemagglutinin, and M2- channel protein which would give the pathway for designing rational drug

Rapid In Vitro Evolution of Human Cytomegalovirus UL56 Mutations That Confer Letermovir Resistance.

Letermovir (LMV) is an experimental cytomegalovirus terminase inhibitor undergoing phase 3 clinical trials. Viral mutations have been described at UL56 codons 231 to 369 that confer widely variable levels of LMV resistance. In this study, 15 independent experiments propagating an exonuclease mutant viral strain in escalating LMV concentrations replicated 6 of the 7 published UL56 mutations and commonly elicited additional resistance-conferring mutations at UL56 codons 231, 236, 237, 244, 257, 261, 325, and 329. Mutations were first detected earlier in LMV (median, 3 passages) than in 8 parallel experiments with foscarnet (median, 15 passages). As LMV concentrations increased, the typical initial UL56 change F261L, which confers low-grade resistance, combined or was replaced with mutations conferring higher-grade resistance, eventually enabling normal viral growth in 30 μM LMV (>5,000-fold the 50% effective concentration [EC50] for the wild type). At high LMV concentrations, the UL56 changes C325F/R were commonly detected, as well as a combination of changes at codons 236, 257, 329, and/or 369. Recombinant viruses containing individual UL56 mutations and combinations were constructed to confirm their resistance phenotypes and normal growth in cell culture. Several double and triple mutants showed much higher LMV resistance than the respective single mutants, particularly those including changes at both codons 236 and 257. The multiplicity of pathways to high-grade LMV resistance with minimal viral growth impact suggests a low viral genetic barrier and the need for close monitoring during treatment of active infection.

Mathematical analysis of the effect of a pulse vaccination to an HBV mutation

It has been proven that vaccine can play an important role for eradication of hepatitis B infection. When the mutant strain of virus appears, it changes all treatments strategies. The current problem is to find the critical vaccine threshold which can stimulate the immune system for eradicate the virus, or to find conditions at which mutant strain of the virus can persist in the presence of a CTL vaccine. In this paper, the dynamical behavior of a new hepatitis B virus model with two strains of virus and CTL immune responses is studied. We compute the basic reproductive ratio of the model and show that the dynamic depend of this threshold. After that, we extend the model incorporating pulse vaccination and we find conditions for eradication of the disease. Our result indicates that if the vaccine is sufficiently strong, both strains are driven to extinction, assuming perfect adherence. Il a été démontré que le vaccin peut jouer un rôle très important dans le processus d’éradication de l’hépatite B. Lorsque la souche mutante apparait, elle modifie les stratégies de luttes. Le problème courant est celui de déterminer le seuil vaccinal capable de stimuler le système immunitaire pour éliminer le virus, ou de trouver les conditions pour lesquelles la souche mutante persistera en présence du vaccin. Dans ce travail, nous considérons un nouveau modèle d’hépatite B dans lequel nous prenons en compte une souche de virus sauvage et une souche mutante et leur interaction avec les cellules du système immunitaire. Nous calculons le taux de reproduction de base et montrons que la dynamique dépend de ce seuil. Une extension de ce modèle est faite afin d’inclure un schéma impulsionnel de vaccination et de déterminer les conditions pour éliminer la maladie. Nos résultats montrent que si le vaccin est suffisamment puissant, les deux souches pourront être éliminées.

Antiviral Activity of the Human Cathelicidin, LL-37, and Derived Peptides on Seasonal and Pandemic Influenza A Viruses.

Human LL-37, a cationic antimicrobial peptide, was recently shown to have antiviral activity against influenza A virus (IAV) strains in vitro and in vivo. In this study we compared the anti-influenza activity of LL-37 with that of several fragments derived from LL-37. We first tested the peptides against a seasonal H3N2 strain and the mouse adapted H1N1 strain, PR-8. The N-terminal fragment, LL-23, had slight neutralizing activity against these strains. In LL-23V9 serine 9 is substituted by valine creating a continuous hydrophobic surface. LL-23V9 has been shown to have increased anti-bacterial activity compared to LL-23 and we now show slightly increased antiviral activity compared to LL-23 as well. The short central fragments, FK-13 and KR-12, which have anti-bacterial activity did not inhibit IAV. In contrast, a longer 20 amino acid central fragment of LL-37 (GI-20) had neutralizing activity similar to LL-37. None of the peptides inhibited viral hemagglutination or neuraminidase activity. We next tested activity of the peptides against a strain of pandemic H1N1 of 2009 (A/California/04/09/H1N1 or “Cal09”). Unexpectedly, LL-37 had markedly reduced activity against Cal09 using several cell types and assays of antiviral activity. A mutant viral strain containing just the hemagglutinin (HA) of 2009 pandemic H1N1 was inhibited by LL-37, suggested that genes other than the HA are involved in the resistance of pH1N1. In contrast, GI-20 did inhibit Cal09. In conclusion, the central helix of LL-37 incorporated in GI-20 appears to be required for optimal antiviral activity. The finding that GI-20 inhibits Cal09 suggests that it may be possible to engineer derivatives of LL-37 with improved antiviral properties.

Anti-HIV diarylpyrimidine–quinolone hybrids and their mode of action

A molecular hybridization approach is a powerful tool in the design of new molecules with improved affinity and efficacy. In this context, a series of diarylpyrimidine–quinolone hybrids were synthesized and evaluated against both wt HIV-1 and mutant viral strains. The most active hybrid 5a displayed an EC50 value of 0.28±0.07μM against HIV-1 IIIB. A couple of enzyme-based assays clearly pinpoint a RT-targeted mechanism of action. Docking studies revealed that these hybrids could be well located in the NNIBP of HIV-1 RT despite the bulky and polar properties of a quinolone 3-carboxylic acid moiety in the molecules.

Genetic analyses of the FRNK motif function of Turnip mosaic virus uncover multiple and potentially interactive pathways of cross-protection.

Cross-protection triggered by a mild strain of virus acts as a prophylaxis to prevent subsequent infections by related viruses in plants; however, the underling mechanisms are not fully understood. Through mutagenesis, we isolated a mutant strain of Turnip mosaic virus (TuMV), named Tu-GK, that contains an Arg182Lys substitution in helper component-proteinase (HC-Pro(K)) that confers complete cross-protection against infection by a severe strain of TuMV in Nicotiana benthamiana, Arabidopsis thaliana Col-0, and the Arabidopsis dcl2-4/dcl4-1 double mutant defective in DICER-like ribonuclease (DCL)2/DCL4-mediated silencing. Our analyses showed that HC-Pro(K) loses the ability to interfere with microRNA pathways, although it retains a partial capability for RNA silencing suppression triggered by DCL. We further showed that Tu-GK infection triggers strong salicylic acid (SA)-dependent and SA-independent innate immunity responses. Our data suggest that DCL2/4-dependent and -independent RNA silencing pathways are involved, and may crosstalk with basal innate immunity pathways, in host defense and in cross-protection.

The fitness landscape of HIV-1 gag: advanced modeling approaches and validation of model predictions by in vitro testing.

Viral immune evasion by sequence variation is a major hindrance to HIV-1 vaccine design. To address this challenge, our group has developed a computational model, rooted in physics, that aims to predict the fitness landscape of HIV-1 proteins in order to design vaccine immunogens that lead to impaired viral fitness, thus blocking viable escape routes. Here, we advance the computational models to address previous limitations, and directly test model predictions against in vitro fitness measurements of HIV-1 strains containing multiple Gag mutations. We incorporated regularization into the model fitting procedure to address finite sampling. Further, we developed a model that accounts for the specific identity of mutant amino acids (Potts model), generalizing our previous approach (Ising model) that is unable to distinguish between different mutant amino acids. Gag mutation combinations (17 pairs, 1 triple and 25 single mutations within these) predicted to be either harmful to HIV-1 viability or fitness-neutral were introduced into HIV-1 NL4-3 by site-directed mutagenesis and replication capacities of these mutants were assayed in vitro. The predicted and measured fitness of the corresponding mutants for the original Ising model (r = −0.74, p = 3.6×10−6) are strongly correlated, and this was further strengthened in the regularized Ising model (r = −0.83, p = 3.7×10−12). Performance of the Potts model (r = −0.73, p = 9.7×10−9) was similar to that of the Ising model, indicating that the binary approximation is sufficient for capturing fitness effects of common mutants at sites of low amino acid diversity. However, we show that the Potts model is expected to improve predictive power for more variable proteins. Overall, our results support the ability of the computational models to robustly predict the relative fitness of mutant viral strains, and indicate the potential value of this approach for understanding viral immune evasion, and harnessing this knowledge for immunogen design.

Dynamics of gd T Cell Expansion Can Predict the Resolution of CMV Infection and the Emergence of a Mutant Viral Strain in Kidney Transplant Recipients.

Cytomegalovirus (CMV) infection in solid organ transplantation is associated with increased morbidity and mortality, particularly in the case of the emergence of a CMV mutant strain with anti-viral drug resistance. In this situation, immune monitoring assays should provide relevant clinical information on efficient CMV-specific T cells response. For many years, we have shown that gd T lymphocytes (gd-TL) are involved in the control of CMV infection. The purpose of this study was to assess if gd-TL kinetics can predict the resolution of CMV infection in high risk patients: D+ R- and R+ with anti-thymocyte globulin - ATG, or the emergence of a mutant strain. Methods gd-TL kinetics has been described in D+ R- or R+/ATG kidney transplant recipients between 2003 and 2011. This kinetics was compared between infected and non infected patients and between patients with and without mutation. gd-TL expansion was estimated by a linear mixed model, the time of expansion was defined as the date of occurrence of this expansion from the beginning of infection. Results 167 patients D+ R- (93 infected and 74 non-infected) and 104 R+/ATG (74 infected and 30 non-infected) patients were included. The gd-TL expansion following CMV infection was comparable in D+R- and R+/ATG patients. Time of gd-TL expansion was highly correlated with the duration of DNAemia (r=0.91, p < 0.0001). The delay of gd-TL expansion was closely associated with the presence of a mutant strain (p < 0.0007). This mutation was predicted by persistent DNAemia after 39 days of gd-TL expansion (AUC= 0.76). Conclusion In high risk patients, gd-TL expansion is related to the resolution of CMV infection. Conversely, the delay of expansion and the persistence of DNAemia after expansion are predictive of the occurrence of a mutant strain. Monitoring gd-TL could be useful during CMV infection in order to predict the resolution of the infection and the emergence of a mutant viral strain.

Rapid Screening of HIV Reverse Transcriptase and Integrase Inhibitors

Although a number of anti HIV drugs have been approved, there are still problems with toxicity and drug resistance. This demonstrates a need to identify new compounds that can inhibit infection by the common drug resistant HIV-1 strains with minimal toxicity. Here we describe an efficient assay that can be used to rapidly determine the cellular cytotoxicity and efficacy of a compound against WT and mutant viral strains. The desired target cell line is seeded in a 96-well plate and, after a 24 hr incubation, serially dilutions of the compounds to be tested are added. No further manipulations are necessary for cellular cytotoxicity assays; for anti HIV assays a predetermined amount of either a WT or drug resistant HIV-1 vector that expresses luciferase is added to the cells. Cytotoxicity is measured by using an ATP dependent luminescence assay and the impact of the compounds on infectivity is measured by determining the amount of luciferase in the presence or the absence of the putative inhibitors. This screening assay takes 4 days to complete and multiple compounds can be screened in parallel. Compounds are screened in triplicate and the data are normalized to the infectivity/ATP levels in absence of target compounds. This technique provides a quick and accurate measurement of the efficacy and toxicity of potential anti HIV compounds.

Rapid screening of HIV reverse transcriptase and integrase inhibitors.

Although a number of anti HIV drugs have been approved, there are still problems with toxicity and drug resistance. This demonstrates a need to identify new compounds that can inhibit infection by the common drug resistant HIV-1 strains with minimal toxicity. Here we describe an efficient assay that can be used to rapidly determine the cellular cytotoxicity and efficacy of a compound against WT and mutant viral strains.The desired target cell line is seeded in a 96-well plate and, after a 24 hr incubation, serially dilutions of the compounds to be tested are added. No further manipulations are necessary for cellular cytotoxicity assays; for anti HIV assays a predetermined amount of either a WT or drug resistant HIV-1 vector that expresses luciferase is added to the cells. Cytotoxicity is measured by using an ATP dependent luminescence assay and the impact of the compounds on infectivity is measured by determining the amount of luciferase in the presence or the absence of the putative inhibitors.This screening assay takes 4 days to complete and multiple compounds can be screened in parallel. Compounds are screened in triplicate and the data are normalized to the infectivity/ATP levels in absence of target compounds. This technique provides a quick and accurate measurement of the efficacy and toxicity of potential anti HIV compounds.

Antiherpetic Mechanism of a Sulfated Derivative of Agaricus brasiliensis Fruiting Bodies Polysaccharide

Objective: To study the anti-herpes simplex virus (HSV) activity of a (1→6)-(1→3)-β-<smlcap>D</smlcap>-glucan isolated from Agaricus brasiliensis fruiting bodies (FR) as well as its chemically sulfated derivative (FR-S). Methods: The antiherpetic activity and mechanism of action was studied by viral plaque assay applying different methodological strategies. Results: Although FR presented no in vitro antiherpetic action at 1 mg/ml, FR-S displayed promising anti-HSV-1 and anti-HSV-2 activities in both simultaneous and postinfection treatments, resulting in selectivity indices (CC₅₀/EC₅₀) higher than 393. FR-S had no virucidal effect, but significantly suppressed HSV-1 (EC₅₀ = 0.32 µg/ml) and HSV-2 (EC₅₀ = 0.10 µg/ml) adsorption. FR-S was less effective on adsorption inhibition of mutant virus strains devoid of gC (HSV-1 gC^-39 and HSV-2 gCneg1), indicating a possible interaction with this glycoprotein. The reduction of viral adsorption upon cell pretreatment with FR-S also suggests its interaction with cellular components. FR-S inhibited HSV-1 (EC₅₀ = 8.39 µg/ml) and HSV-2 (EC₅₀ = 2.86 µg/ml) penetration more efficiently than heparin. FR-S reduced HSV-1 and HSV-2 cell-to-cell spread. A synergic effect between FR-S and acyclovir was also detected. Conclusions: FR-S displays an interesting mechanism of antiviral action and represents a promising candidate for the treatment and/or prevention of herpetic infections, to be used as a single therapeutic agent or in combination with acyclovir.