Truncation and sequence shuffling of segment 6 generate replication-competent neuraminidase-negative influenza H5N1 viruses

Epitopes in the HA and NA of H5 and H7 avian influenza viruses that are important for antigenic drift

Avian influenza viruses evolve antigenically to evade host immunity. Two influenza A virus surface glycoproteins, the haemagglutinin and neuraminidase, are the major targets of host immunity and undergo antigenic drift in response to host pre-existing humoral and cellular immune responses. Specific sites have been identified as important epitopes in prominent subtypes such as H5 and H7, which are of animal and public health significance due to their panzootic and pandemic potential. The haemagglutinin is the immunodominant immunogen, it has been extensively studied, and the antigenic reactivity is closely monitored to ensure candidate vaccine viruses are protective. More recently, the neuraminidase has received increasing attention for its role as a protective immunogen. The neuraminidase is expressed at a lower abundance than the haemagglutinin on the virus surface but does elicit a robust antibody response. This review aims to compile the current information on haemagglutinin and neuraminidase epitopes and immune escape mutants of H5 and H7 highly pathogenic avian influenza viruses. Understanding the evolution of immune escape mutants and the location of epitopes is critical for identification of vaccine strains and development of broadly reactive vaccines that can be utilized in humans and animals.

Induction of humoral and cell-mediated immunity to the NS1 protein of TBEV with recombinant Influenza virus and MVA affords partial protection against lethal TBEV infection in mice

Introduction

Tick-borne encephalitis virus (TBEV) is one of the most relevant tick-transmitted neurotropic arboviruses in Europe and Asia and the causative agent of tick-borne encephalitis (TBE). Annually more than 10,000 TBE cases are reported despite having vaccines available. In Europe, the vaccines FSME-IMMUN® and Encepur® based on formaldehyde-inactivated whole viruses are licensed. However, demanding vaccination schedules contribute to sub-optimal vaccination uptake and breakthrough infections have been reported repeatedly. Due to its immunogenic properties as well as its role in viral replication and disease pathogenesis, the non-structural protein 1 (NS1) of flaviviruses has become of interest for non-virion based flavivirus vaccine candidates in recent years.

Methods

Therefore, immunogenicity and protective efficacy of TBEV NS1 expressed by neuraminidase (NA)-deficient Influenza A virus (IAV) or Modified Vaccinia virus Ankara (MVA) vectors were investigated in this study.

Results

With these recombinant viral vectors TBEV NS1-specific antibody and T cell responses were induced. Upon heterologous prime/boost regimens partial protection against lethal TBEV challenge infection was afforded in mice.

Discussion

This supports the inclusion of NS1 as a vaccine component in next generation TBEV vaccines.

Bat-Borne Influenza A Viruses: An Awakening

Influenza A viruses (IAVs) originating from aquatic waterfowl recurrently cross interspecies barriers, which is greatly facilitated by utilizing cell surface-exposed monosaccharide sialic acids located on vertebrate cells as a universal host cell receptor. These glycan structures are first bound by the viral hemagglutinin (HA) for cell entry and then cleaved by the viral neuraminidase (NA) for particle release. In contrast, viruses of the recently identified bat-borne IAV subtypes H17N10 and H18N11 encode HA and NA homologs unable to interact with sialic acid residues despite a high degree of structural homology with their conventional counterparts. However, the most recent findings show that bat IAV HAs make use of the major histocompatibility complex class II proteins of different vertebrate species to gain entry into host cells, potentially permitting a broader host tropism. This review recapitulates current progress in the field of bat IAV research including the first assessment of the spillover potential of these bat viruses into other mammals.

Establishment of Adequate Functional Cellular Immune Response in Chicks Is Age Dependent

The development of immunocompetence in chicks after hatching is not fully understood. However, detailed knowledge of immunocompetence and maturation processes in day-old chicks (DOCs) and juvenile chickens (Gallus gallus domesticus) is necessary to implement enhanced immunization strategies. For viral diseases, this especially includes the development of cellular immunity focusing on T-cell-dependent responses. In the current study, we investigated T-cell subsets in blood and lymphoid tissues of 1-to-21-day-old chickens concerning their cellular composition and localization. We detected an increase of T-cell frequencies in blood and spleen and a shift of the CD8α dimer expression toward a CD8αβ expression on the surface of T cells with increasing age. A relocalization of lymphocytes into antigen presentation structures within the spleen was affirmed. In addition, changes in basal messenger RNA (mRNA) level, with increasing IL2 and IFNγ mRNA levels at different ages were measured. These detected changes suggest an improved T-cell-dependent antiviral response with increasing age in chickens. To confirm this finding on a functional level, we conducted a transfer experiment: adult and, as a negative control, neonatal naïve lymphocytes were transferred into DOCs. Afterward, the protection induced by these transferred cells was verified by a sublethal infection by using a highly pathogenic avian influenza virus with neuraminidase deletion, H5N_del. Previous experiments have shown that adult animals survive infection with this virus strain, while naïve DOCs show severe symptoms or even die. As a result, the transfer of adult, but not neonatal lymphocytes, confers protection to DOCs against the infection, demonstrating functional differences in lymphocytes from chicks of different ages. Collectively, these data reveal the inability of chicks to mount an effective, cellular antiviral response in the first 3 wk of life. Therefore, we propose that the observed maturation of both the innate and the adaptive arms of the immune system early in development is mandatory for controlling influenza infection in chickens, as well as for an effective vaccination with replication-competent viral vaccine strains.

Bat influenza viruses transmit among bats but are poorly adapted to non-bat species

Major histocompatibility complex class II (MHC-II) molecules of multiple species function as cell-entry receptors for the haemagglutinin-like H18 protein of the bat H18N11 influenza A virus, enabling tropism of the viruses in a potentially broad range of vertebrates. However, the function of the neuraminidase-like N11 protein is unknown because it is dispensable for viral infection or the release of H18-pseudotyped viruses. Here, we show that infection of mammalian cells with wild-type H18N11 leads to the emergence of mutant viruses that lack the N11 ectodomain and acquired mutations in H18. An infectious clone of one such mutant virus, designated rP11, appeared to be genetically stable in mice and replicated to higher titres in mice and cell culture compared with wild-type H18N11. In ferrets, rP11 antigen and RNA were detected at low levels in various tissues, including the tonsils, whereas the wild-type virus was not. In Neotropical Jamaican fruit bats, wild-type H18N11 was found in intestinal Peyer's patches and was shed to high concentrations in rectal samples, resulting in viral transmission to naive contact bats. Notably, rP11 also replicated efficiently in bats; however, only restored full-length N11 viruses were transmissible. Our findings suggest that wild-type H18N11 replicates poorly in mice and ferrets and that N11 is a determinant for viral transmission in bats.

Recombinant influenza A viruses as vaccine vectors

INTRODUCTION

Various viruses, including poxviruses, adenoviruses and vesicular stomatitis virus, have been considered as vaccine vectors for the delivery of antigens of interest in the development of vaccines against newly emerging pathogens.

AREAS COVERED

Here, we review results that have been obtained with influenza A viruses (IAV) as vaccine vectors. With the advent of reverse genetics technology, IAV-based recombinant vaccine candidates have been constructed that induce protective immunity to a variety of different pathogens of interest, including West Nile virus, Plasmodium falciparum and respiratory syncytial virus. The various cloning strategies to produce effective and attenuated, safe to use IAV-based viral vectors are discussed.

EXPERT COMMENTARY

It was concluded that IAV-based vector system has several advantages and holds promise for further development.

Pathogenicity evaluation of neuraminidase-negative H5 and H7 viruses in day-old chicks and adult chicken

We evaluated the virulence of replication-competent NA-negative (no functional neuraminidase) avian influenza viruses in chicken of different ages. Interestingly, the virulence of a previously described NA-negative H5-virus (Kalthoff et al., J Virol 2013;87:13556) is very much age-dependent. Day-old-chicks succumb to a systemic infection, while chicken one week of age do not show any clinical signs at all. Therefore, the day-old-chick organism is most likely not able to restrict replication of the virus as older chicken do. In addition to virulence dependency on host age, the exchange of the H5 HA by an H7 HA for an H7NA-deleted HA reassortant remarkably led to a highly pathogenic phenotype even in adult chicken. This provides evidence, that the virulence of the H7-subtype is less dependent on the NA-protein than that of the H5-virus. These striking observations suggest that the HA/NA interplay might be governed by other mechanisms in HPAIV with the different serotypes H5 or H7.

RNA elements in open reading frames of the bluetongue virus genome are essential for virus replication

Members of the Reoviridae family are non-enveloped multi-layered viruses with a double stranded RNA genome consisting of 9 to 12 genome segments. Bluetongue virus is the prototype orbivirus (family Reoviridae, genus Orbivirus), causing disease in ruminants, and is spread by Culicoides biting midges. Obviously, several steps in the Reoviridae family replication cycle require virus specific as well as segment specific recognition by viral proteins, but detailed processes in these interactions are still barely understood. Recently, we have shown that expression of NS3 and NS3a proteins encoded by genome segment 10 of bluetongue virus is not essential for virus replication. This gave us the unique opportunity to investigate the role of RNA sequences in the segment 10 open reading frame in virus replication, independent of its protein products. Reverse genetics was used to generate virus mutants with deletions in the open reading frame of segment 10. Although virus with a deletion between both start codons was not viable, deletions throughout the rest of the open reading frame led to the rescue of replicating virus. However, all bluetongue virus deletion mutants without functional protein expression of segment 10 contained inserts of RNA sequences originating from several viral genome segments. Subsequent studies showed that these RNA inserts act as RNA elements, needed for rescue and replication of virus. Functionality of the inserts is orientation-dependent but is independent from the position in segment 10. This study clearly shows that RNA in the open reading frame of Reoviridae members does not only encode proteins, but is also essential for virus replication.