Immunization with baculovirus-expressed recombinant rotavirus proteins VP1, VP4, VP6, and VP7 induces CD8+ T lymphocytes that mediate clearance of chronic rotavirus infection in SCID mice

Impact of the Microbiota on Viral Infections

The mammalian gastrointestinal tract (GIT) hosts a diverse and highly active microbiota composed of bacteria, eukaryotes, archaea, and viruses. Studies of the GIT microbiota date back more than a century, although modern techniques, including mouse models, sequencing technology, and novel therapeutics in humans, have been foundational to our understanding of the roles of commensal microbes in health and disease. Here, we review the impacts of the GIT microbiota on viral infection, both within the GIT and systemically. GIT-associated microbes and their metabolites alter the course of viral infection through a variety of mechanisms, including direct interactions with virions, alteration of the GIT landscape, and extensive regulation of innate and adaptive immunity. Mechanistic understanding of the full breadth of interactions between the GIT microbiota and the host is still lacking in many ways but will be vital for the development of novel therapeutics for viral and nonviral diseases alike.

Interaction Between the Microbiota, Epithelia, and Immune Cells in the Intestine

The gastrointestinal tract harbors numerous commensal bacteria, referred to as the microbiota, that benefit host health by digesting dietary components and eliminating pathogens. The intestinal microbiota maintains epithelial barrier integrity and shapes the mucosal immune system, balancing host defense and oral tolerance with microbial metabolites, components, and attachment to host cells. To avoid aberrant immune responses, epithelial cells segregate the intestinal microbiota from immune cells by constructing chemical and physical barriers, leading to the establishment of host-commensal mutualism. Furthermore, intestinal immune cells participate in the maintenance of a healthy microbiota community and reinforce epithelial barrier functions. Perturbations of the microbiota composition are commonly observed in patients with autoimmune diseases and chronic inflammatory disorders. An understanding of the intimate interactions between the intestinal microbiota, epithelial cells, and immune cells that are crucial for the maintenance of intestinal homeostasis might promote advances in diagnostic and therapeutic approaches for various diseases.

Intestinal intraepithelial lymphocytes: Maintainers of intestinal immune tolerance and regulators of intestinal immunity

Intestinal immune tolerance is essential for the immune system, as it prevents abnormal immune responses to large quantities of antigens from the intestinal lumen, such as antigens from commensal microorganisms, and avoids self‐injury. Intestinal intraepithelial lymphocytes (IELs), a special group of mucosal T lymphocytes, play a significant role in intestinal immune tolerance. To accomplish this, IELs exhibit a high threshold of activation and low reactivity to most antigens from the intestinal lumen. In particular, CD8αα⁺TCRαβ⁺ IELs, TCRγδ⁺ IELs, and CD4⁺CD8αα⁺ IELs show great potential for maintaining intestinal immune tolerance and regulating intestinal immunity. However, if the intestinal microenvironment becomes abnormal or intestinal tolerance is broken, IELs may be activated abnormally and become pathogenic.

Commensal Bacteria-Dependent CD8αβ+ T Cells in the Intestinal Epithelium Produce Antimicrobial Peptides

The epithelium of the intestine functions as the primary “frontline” physical barrier for protection from enteric microbiota. Intraepithelial lymphocytes (IELs) distributed along the intestinal epithelium are predominantly CD8⁺ T cells, among which CD8αβ⁺ IELs are a large population. In this investigation, the proportion and absolute number of CD8αβ⁺ IELs decreased significantly in antibiotic-treated and germ-free mice. Moreover, the number of CD8αβ⁺ IELs was correlated closely with the load of commensal microbes, and induced by specific members of commensal bacteria. Microarray analysis revealed that CD8αβ⁺ IELs expressed a series of genes encoding potent antimicrobial peptides (AMPs), whereas CD8αβ⁺ splenocytes did not. The antimicrobial activity of CD8αβ⁺ IELs was confirmed by an antimicrobial-activity assay. In conclusion, microbicidal CD8αβ⁺ IELs are regulated by commensal bacteria which, in turn, secrete AMPs that have a vital role in maintaining the homeostasis of the small intestine.

Activation of PI3K, Akt, and ERK during early rotavirus infection leads to V-ATPase-dependent endosomal acidification required for uncoating

The cellular PI3K/Akt and/or MEK/ERK signaling pathways mediate the entry process or endosomal acidification during infection of many viruses. However, their roles in the early infection events of group A rotaviruses (RVAs) have remained elusive. Here, we show that late-penetration (L-P) human DS-1 and bovine NCDV RVA strains stimulate these signaling pathways very early in the infection. Inhibition of both signaling pathways significantly reduced production of viral progeny due to blockage of virus particles in the late endosome, indicating that neither of the two signaling pathways is involved in virus trafficking. However, immunoprecipitation assays using antibodies specific for pPI3K, pAkt, pERK and the subunit E of the V-ATPase co-immunoprecipitated the V-ATPase in complex with pPI3K, pAkt, and pERK. Moreover, Duolink proximity ligation assay revealed direct association of the subunit E of the V-ATPase with the molecules pPI3K, pAkt, and pERK, indicating that both signaling pathways are involved in V-ATPase-dependent endosomal acidification. Acidic replenishment of the medium restored uncoating of the RVA strains in cells pretreated with inhibitors specific for both signaling pathways, confirming the above results. Isolated components of the outer capsid proteins, expressed as VP4-VP8* and VP4-VP5* domains, and VP7, activated the PI3K/Akt and MEK/ERK pathways. Furthermore, psoralen-UV-inactivated RVA and CsCl-purified RVA triple-layered particles triggered activation of the PI3K/Akt and MEK/ERK pathways, confirming the above results. Our data demonstrate that multistep binding of outer capsid proteins of L-P RVA strains with cell surface receptors phosphorylates PI3K, Akt, and ERK, which in turn directly interact with the subunit E of the V-ATPase to acidify the late endosome for uncoating of RVAs. This study provides a better understanding of the RVA-host interaction during viral uncoating, which is of importance for the development of strategies aiming at controlling or preventing RVA infections.

Viral particles must transport their genome into the cytoplasm or the nucleus of host cells to initiate successful infection. Knowledge of how viruses may pirate host cell signaling cascades or molecules to promote their own replication can facilitate the development of antiviral drugs. Group A rotavirus (RVA) is a major etiological agent of acute gastroenteritis in young children and the young of various mammals. RVA enters cells by a complex multistep process. However, the cellular signaling cascades or molecules that facilitate these processes are incompletely understood. Here, we demonstrate that infection with late-penetration RVA strains results in phosphorylation of PI3K, Akt, and ERK signaling molecules at an early stage of infection, a process mediated by the multistep binding of RVAs outer capsid proteins. Specific inhibitors for PI3K/Akt and MEK/ERK signaling pathways trap the viral particles in late endosome, and acidic replenishment restores and releases them. Moreover, the RVA-induced phosphorylated PI3K, Akt, and ERK directly interact with the subunit E of the V-ATPase proton pump, required for endosomal acidification and RVA uncoating. Understanding how RVA-induced early activation of cellular signaling molecules mediates the V-ATPase-dependent endosomal acidification required for uncoating of viral particles opens up opportunities for targeted interventions against rotavirus entry.

Lineage re-commitment of CD4CD8αα intraepithelial lymphocytes in the gut

The gastrointestinal tract forms the largest surface in our body with constantly being exposed to various antigens, which provides unique microenvironment for the immune system in the intestine. Accordingly, the gut epithelium harbors the most T lymphocytes in the body as intraepithelial lymphocytes (IELs), which are phenotypically and functionally heterogeneous populations, distinct from the conventional mature T cells in the periphery. IELs arise either from pre-committed thymic precursors (natural IELs) or from conventional CD4 or CD8αβ T cells in response to peripheral antigens (induced IELs), both of which commonly express CD8α homodimers (CD8αα). Although lineage commitment to either conventional CD4 T helper (Th) or cytotoxic CD8αβ T cells as well as their respective co-receptor expression are mutually exclusive and irreversible process, CD4 T cells can be redirected to the CD8 IELs with high cytolytic activity upon migration to the gut epithelium. Recent reports show that master transcription factors for CD4 and CD8 T cells, ThPOK (Th-inducing BTB/POZ-Kruppel-like factor) and Runx3 (Runt related transcription factor 3), respectively, are the key regulators for re-programming of CD4 T cells to CD8 lineage in the intestinal epithelium. This review will focus on the unique differentiation process of IELs, particularly lineage re-commitment of CD4 IELs. [BMB Reports 2016; 49(1): 11-17]

B-Cell-Deficient and CD8 T-Cell-Depleted Gnotobiotic Pigs for the Study of Human Rotavirus Vaccine-Induced Protective Immune Responses

Genetically modified pigs have become available recently. In this study, we established the gnotobiotic pig model of human rotavirus (HRV) infection using cloned pigs with homozygous disruption in the gene encoding immunoglobulin heavy chain (HCKO), which totally impairs B-cell development. To clarify importance of B cells and cytotoxic T cells in rotavirus immunity, CD8 cells in a subset of the pigs were depleted by injecting antipig CD8 antibodies and the immune phenotypes of all pigs were examined. HCKO pigs, CD8 cell-depleted HCKO pigs, and wild-type (WT) pigs were vaccinated with an attenuated HRV vaccine and challenged with virulent HRV. Protection against HRV infection and diarrhea was assessed postchallenge and detailed T-cell subset responses were determined pre- and postchallenge. Significantly longer duration of virus shedding was seen in vaccinated HCKO pigs than in WT pigs, indicating the importance of B cells in vaccine-induced protective immunity. Vaccinated HCKO/CD8(-) pigs shed significantly higher number of infectious virus than WT pigs and non-CD8-depleted HCKO pigs, indicating the importance of CD8 T cells in controlling virus replication. Therefore, both B cells and CD8 T cells play an important role in the protection against rotavirus infection. HCKO and HCKO/CD8(-) pigs did not differ significantly in diarrhea and virus shedding postchallenge; increased CD4 and CD8(-) γδ T-cell responses probably compensated partially for the lack of CD8 T cells. This study demonstrated that HCKO pigs can serve as a valuable model for dissection of protective immune responses against viral infections and diseases.

Rotavirus VP6 preparations as a non-replicating vaccine candidates

Rotavirus (RV) structural proteins VP4 and VP7, located on the surface of viral particles, elicit neutralizing antibodies (Abs) and are therefore considered to be important components of RV vaccines. However, despite inducing neutralizing Abs, limits of cross-neutralizing activity and lack of full correlation with protection limit the usefulness of these proteins as protective agents against RV disease. VP6 protein, which forms the middle layer of RV particles, is discussed as an alternative vaccine candidate since it can induce cross-protective immune responses against different RV strains although the Ab raised is not neutralizing. This report reviews different functions of VP6 that can lead to considering it as an alternative vaccine against RV disease.

The light and dark sides of intestinal intraepithelial lymphocytes

The intraepithelial lymphocytes (IELs) that reside within the epithelium of the intestine form one of the main branches of the immune system. As IELs are located at this critical interface between the core of the body and the outside environment, they must balance protective immunity with an ability to safeguard the integrity of the epithelial barrier: failure to do so would compromise homeostasis of the organism. In this Review, we address how the unique development and functions of intestinal IELs allow them to achieve this balance.

Intestinal T cells: facing the mucosal immune dilemma with synergy and diversity

The epithelium of the gastrointestinal tract, which represents the greatest body surface area exposed to the outside environment, is confronted with a plethora of foreign and potentially harmful antigens. Consequently, the immune system of the gut faces the daunting task of distinguishing harmless dietary proteins and commensal bacteria from potentially dangerous pathogens, and of then responding accordingly. Mucosal T cells play a central role in maintaining barrier function and controlling the delicate balance between immune activation and immune tolerance. This review will focus on the unique features of mucosal T cell subsets that reside in the epithelium and lamina propria of the gut.

Synthesis and assembly of a cholera toxin B subunit-rotavirus VP7 fusion protein in transgenic potato

A gene encoding VP7, the outer capsid protein of simian rotavirus SA11, was fused to the carboxyl terminus of the cholera toxin B subunit gene. A plant expression vector containing the fusion gene under control of the mannopine synthase P2 promoter was introduced into Solanum tuberosum cells by Agrobacterium tumefaciens-mediated transformation. The CTB::VP7 fusion gene was detected in the genomic DNA of transformed potato leaf cells by polymerase chain reaction (PCR) amplification methods. Immunoblot analysis of transformed potato tuber tissue extracts showed that synthesis and assembly of the CTB::VP7 fusion protein into oligomers of pentameric size occurred in the transformed plant cells. The binding of CTB::VP7 fusion protein pentamers to sialo-sugar containing GM1 ganglioside receptors on the intestinal epithelial cell membrane was quantified by enzyme-linked immunosorbent assay (ELISA). The ELISA results showed that the CTB::VP7 fusion protein made up approx 0.01% of the total soluble tuber protein. Synthesis and assembly of CTB::VP7 monomers into biologically active pentamers in transformed potato tubers demonstrates the feasibility of using edible plants as a mucosal vaccine for the production and delivery system for rotavirus capsid protein antigens.

The role of serum antibodies in the protection against rotavirus disease: an overview

A critical observation in understanding immunity to rotavirus is that children infected with wild virus or vaccinated with oral live vaccines develop a humoral immune response and are protected against severe disease upon reinfection. Nevertheless, much controversy exists as to whether these serum antibodies are directly involved in protection or merely reflect recent infection, leaving the protective role to mucosal or cell-mediated immunity or to other as-yet-undefined mechanisms. We have reviewed data from a variety of studies in humans, including challenge experiments in adult volunteers, longitudinal studies of rotavirus infection in young children, and clinical trials of animal and animal-human reassortant rotavirus vaccines in infants. These data suggest that serum antibodies, if present at critical levels, are either protective themselves or are an important and powerful correlate of protection against rotavirus disease, even though other host effectors may play an important role as well.

CD4 T cells are the only lymphocytes needed to protect mice against rotavirus shedding after intranasal immunization with a chimeric VP6 protein and the adjuvant LT(R192G)

Intranasal immunization of mice with a chimeric VP6 protein and the mucosal adjuvant Escherichia coli heat labile toxin LT(R192G) induces nearly complete protection against murine rotavirus (strain EDIM [epizootic diarrhea of infant mice virus]) shedding for at least 1 year. The aim of this study was to identify the protective lymphocytes elicited by this new vaccine candidate. Immunization of mouse strains lacking one or more lymphocyte populations revealed that protection was dependent on alphabeta T cells but mice lacking gammadelta T cells and B cells remained fully protected. Furthermore, depletion of CD8 T cells in immunized B-cell-deficient mice before challenge resulted in no loss of protection, while depletion of CD4 T cells caused complete loss of protection. Therefore, alphabeta CD4 T cells appeared to be the only lymphocytes required for protection. As confirmation, purified splenic T cells from immunized mice were intraperitoneally injected into Rag-2 mice chronically infected with EDIM. Transfer of 2 x 10(6) CD8 T cells had no effect on shedding, while transfer of 2 x 10(5) CD4 T cells fully resolved shedding in 7 days. Interestingly, transfer of naive splenic CD4 T cells also resolved shedding but more time and cells were required. Together, these results establish CD4 T cells as effectors of protection against rotavirus after intranasal immunization of mice with VP6 and LT(R192G).

Nasal immunization of mice with virus-like particles protects offspring against rotavirus diarrhea

Rotavirus is the major cause of diarrhea among young infants in both humans and animals. Immune protection of newborns by vaccination is difficult to achieve since there is not enough time to mount an immune response before exposure to the virus. We have designed a vaccination strategy mediating transfer of neutralizing antibodies from the mother to the offspring during pregnancy and/or lactation. Adult female mice were nasally immunized with virus-like particles (VLPs) made of viral proteins VP2 and 6 (VLP2/6) or VP 2, 6, and 7 (VLP2/6/7) derived from the RF rotavirus strain in the presence or absence of cholera toxin. Both vaccines elicited serum and milk antibodies against the respective VPs. Four days after parturition, suckling pups were challenged orally with RF rotavirus. Pups from mothers immunized with VLP2/6/7 but not VLP2/6 were protected against rotavirus diarrhea, indicating that VP7 plays a key role in protection. Protection was mediated by milk rather than serum antibodies, and mucosal adjuvants were not required. In conclusion, VLPs containing VP7 administered nasally to mothers represent a promising vaccine candidate for the protection of suckling newborns against rotavirus-induced diarrhea, even in the absence of a mucosal adjuvant.

Mucosal immunity and tolerance: relevance to vaccine development

The mucosal immune system of mammals consists of an integrated network of lymphoid cells which work in concert with innate host factors to promote host defense. Major mucosal effector immune mechanisms include secretory antibodies, largely of immunoglobulin A (IgA) isotype, cytotoxic T cells, as well as cytokines, chemokines and their receptors. Immunologic unresponsiveness (tolerance) is a key feature of the mucosal immune system, and deliberate vaccination or natural immunization by a mucosal route can effectively induce immune suppression. The diverse compartments located in the aerodigestive and genitourinary tracts and exocrine glands communicate via preferential homing of lymphocytes and antigen-presenting cells. Mucosal administration of antigens may result in the concomitant expression of secretory immunoglobulin A (S-IgA) antibody responses in various mucosal tissues and secretions, and under certain conditions, in the suppression of immune responses. Thus, developing formulations based on efficient delivery of selected antigens/tolerogens, cytokines and adjuvants may impact on the design of future vaccines and of specific immunotherapeutic approaches against diseases associated with untoward immune responses, such as autoimmune disorders, allergic reactions, and tissue-damaging inflammatory reactions triggered by persistent microorganisms.

Murine model of rotavirus infection

The murine model of homologous rotavirus infection has been used to study the determinants of protection. The local IgA immune response appears to be the critical factor in generating protective immunity after natural infection. A series of knockout mice were used to evaluate the contribution of T cells and B cells to immunity and resolution from primary infection. Both arms of immune system played a role in the resolution of primary infection but antibody was much more important for prevention of reinfection.

DNA vaccines against rotavirus infections

Plasmid DNA vaccines encoding for murine rotaviral proteins VP4, VP6, and VP7 were tested in adult BALB/c mice for their ability to induce immune responses and provide protection against rotavirus challenge. Serum antibodies were measured by virus neutralization and by ELISA. Cellular immunity was assessed by measuring cytotoxic T cell (CTL) responses. The vaccines were administered by inoculation into cells of the epidermis with an Accell gene gun (Auragen, Inc., Middleton, WI, USA). Each of the three vaccines elicited rotavirus-specific serum antibodies as measured by ELISA. Virus neutralizing antibodies were detected in mice receiving DNA vaccines encoding for VP4 and VP7, but not in those which received the plasmid encoding for VP6. Vaccines encoding for VP4, VP6, or VP7 generated virus-specific CTL responses in recipient mice. Efficacy of the vaccines was determined by challenge with homotypic rotaviruses. Each of the three vaccines was effective in protecting mice against infection after rotavirus (100 ID50) challenge. Significant reductions (p < 0.0002, analysis of variance) in viral excretion measured over a 9 day period were seen in mice receiving the DNA vaccines compared with mice that received control plasmids.

Role of B cells and cytotoxic T lymphocytes in clearance of and immunity to rotavirus infection in mice

The immune mechanisms involved in clearance of and immunity to rotavirus infection are poorly understood. Although mice with severe combined immunodeficiency (SCID mice) become chronically infected, nude mice have been reported to clear rotavirus infection similarly to immunocompetent controls. To better characterize the role of cytotoxic T lymphocytes (CTLs) in clearance of and immunity to rotavirus infection, we infected naive or previously infected beta 2-microglobulin (beta 2m) knockout mice with murine rotavirus. Naive beta 2m knockout mice shed rotavirus antigen 2 days longer than did normal control mice but completely resolved primary infection. beta 2m knockout naive mice treated with depleting doses of an anti-CD8 monoclonal antibody before infection shed viral antigen for an additional day. Upon rechallenge, beta 2m knockout mice, either treated with the anti-CD8 antibody or not treated, were completely resistant to reinfection. Clearance of rotavirus infection in naive beta 2m knockout mice correlated with the development of intestinal rotavirus-specific immunoglobulin A. Before rechallenge, beta 2m knockout mice had high levels of intestinal rotavirus-specific immunoglobulin A. These findings suggest that CTLs mediate rotavirus clearance but are not required for this function and that CTLs are not necessary for development of immunity to rotavirus reinfection. To further characterize the effector mechanisms involved in clearance and prevention of rotavirus infection, similar studies were performed with B-cell-deficient JHD knockout mice. After primary infection, most naive JHD mice had similar virus-shedding clearance curves as did control mice and completely resolved primary infection. However, 2 of 29 became chronically infected. All JHD mice treated with anti-CD8 antibody became chronically infected with murine rotavirus. Upon rechallenge, JHD mice which had cleared primary infection were all susceptible to reinfection. These findings suggest that B cells also play a role in clearance of primary infection but are absolutely necessary for development of immunity against rotavirus reinfection.

From virus to vaccine: developments using the simple eukaryote, Dictyostelium discoideum

Mass vaccination compaigns against viral diseases, both human and anim al, depend on the availability of cheap viral antigens. The eukaryote Dictyostelium discoideum has simple growth requirements and rapid growth rates and forms stable cell lines. These features, together with the possibility of secreting recombinant (glyco)proteins into a defined buffer, make the D. discoideum expression system an attractive option for producing economical recombinant subunit vaccines.

Expression of the rotavirus SA11 protein VP7 in the simple eukaryote Dictyostelium discoideum

The outer capsid protein of rotavirus, VP7, is a major neutralization antigen and is considered a necessary component of any subunit vaccine developed against rotavirus infection. For this reason, significant effort has been directed towards producing recombinant VP7 that maintains the antigenic characteristics of the native molecule. Using a relatively new expression system, the simple eukaryote Dictyostelium discoideum, we have cloned the portion of simian rotavirus SA11 genome segment 9, encoding the mature VP7 protein, downstream of a native D. discoideum secretion signal sequence in a high-copy-number extrachromosomal vector. The majority of the recombinant VP7 expressed by transformants was intracellular and was detected by Western immunoblot following gel electrophoresis as two or three bands with an apparent molecular mass of 35.5 to 37.5 kDa. A small amount of VP7 having an apparent molecular mass of 37.5 kDa was secreted. Both the intracellular VP7 and the secreted VP7 were N glycosylated and sensitive to endoglycosidase H digestion. Under nonreducing electrophoresis conditions, over half the intracellular VP7 migrated as a monomer while the remainder migrated with an apparent molecular mass approximately twice that of the monomeric form. In an enzyme-linked immunosorbent assay, intracellular VP7 reacted with both nonneutralizing and neutralizing antibodies. The monoclonal antibody recognition pattern paralleled that found with VP7 expressed in either vaccinia virus or herpes simplex virus type 1 and confirms that D. discoideum-expressed VP7 is able to form the major neutralization domains present on viral VP7. Because D. discoideum cells are easy and cheap to grow, this expression system provides a valuable alternative for the large-scale production of recombinant VP7 protein.

Vaccinia virus expression of the VP7 protein of South African bluetongue virus serotype 4 and its use as an antigen in a capture ELISA

Recombinant vaccinia viruses expressing the VP7 core protein of South African bluetongue virus serotype 4 (SA-BTV4) were identified by polymerase chain reaction amplification. Expression of VP7 was verified by radio-immunoprecipitation and a F(ab')2-based ELISA. Antibodies to VP7 were detected in sera from sheep that had been infected with 20 different virulent BTV serotypes by using the vaccinia virus (VV) expressed VP7 as antigen in a capture ELISA. F(ab')2-immobilised VV-expressed SA-BTV4 VP7 cross-reacted with sera directed against all 9 African horsesickness virus serotypes and epizootic haemorrhagic disease virus serotype 2.

Attenuated poliovirus strain as a live vector: expression of regions of rotavirus outer capsid protein VP7 by using recombinant Sabin 3 viruses

The ability to express heterologous antigens from attenuated poliovirus strains suggests the potential for use as live vectored vaccines. Full- or partial-length sequences of the gene encoding rotavirus major outer capsid protein VP7 were cloned into the open reading frame of a full-length cDNA copy of poliovirus Sabin type 3. They were inserted either at the 5' end or immediately after the capsid protein coding region, at the junction between precursors P1 and P2. A protease cleavage site for 3C protease was introduced 3' to the foreign sequences to enable proteolytic processing of the antigen from the poliovirus polyprotein. Infectious viruses were generated from several of the DNA constructs, and the presence of the foreign gene sequences was confirmed by reverse transcription of the viral RNA and PCR amplification. Viruses with inserts of about 300 bases maintained the foreign sequences during passage in Vero cells. Viruses carrying larger sequences were unstable, and deletions were generated within the foreign sequences. Expression of the VP7 polypeptides was demonstrated by immunoprecipitation with specific antiserum of labeled proteins from cells infected with Sabin 3 recombinant viruses. Comparative studies of RNA synthesis showed similar kinetics for Sabin 3 and the Sabin 3/VP7 recombinants. One-step growth curves showed that production of recombinant viruses was slower than that of Sabin 3 and that the final titers were 1 to 1.5 logs lower. Accumulation of VP7-containing precursors in infected cells suggests that slow cleavage at the engineered 3C protease site may be a limiting step in the growth of these recombinant Sabin polioviruses and may influence the permissible size of foreign sequence to be inserted.

Rotaviruses: immunological determinants of protection against infection and disease

Although studies of rotavirus immunity in experimental animals and humans have often yielded conflicting data, a preponderance of evidence supports the following answers to the questions initially posed. 1. What is the importance of virus serotype in formulating an optimal vaccine? Both vp4 and vp7 induce virus-neutralizing antibodies after either natural infection or immunization; the capacity of vp4 to induce rotavirus-specific neutralizing antibodies is probably greater than that of vp7. However, protection against disease after immunization of infants and young children is induced by strains heterotypic to the challenge virus (e.g., immunization with WC3 induces protection against disease induced by serotypically distinct human G1 strains). In addition, oral inoculation of infants with primate or bovine reassortant rotaviruses containing genes that encode human vp7 has not consistently induced a higher level of protection against challenge than that induced by parent animal rotaviruses (see Table I). Therefore, although vp4 or vp7 or both are probably important in inducing protection against challenge, it has not been clearly demonstrated that inclusion of the epidemiologically important human (as distinct from animal) P or G type is important in protection against human disease. 2. Which immunological effector arm most likely protects against rotavirus disease? No immunological effector arm clearly explains protection against heterotypic challenge. Protection against disease is not predicted by rotavirus-specific neutralizing antibodies in serum. Rotavirus-specific, binding sIgA in feces [detected by enzyme-linked immunosorbent assay (ELISA)] induced after natural infection does correlate with protection against disease induced by subsequent infection. However, protection after immunization with WC3 may occur in the absence of a detectable fecal sIgA response. The relationship between rotavirus-binding sIgA and sIgA-mediated neutralizing activity directed against the challenge virus remains to be determined. Binding rotavirus-specific sIgA in feces detected by ELISA may only be a correlate of other events occurring at the intestinal mucosal surface. The presence of broadly cross-reactive, rotavirus-specific CTLs at the intestinal mucosal surface of mice acutely after infection is intriguing. It would be of interest to determine the degree to which the presence of cross-reactive, rotavirus-specific CTLs in the circulation is predictive of the presence of virus-specific CTLs among intestinal lymphocytes and protection against challenge. Unfortunately, studies of virus-specific CTLs are difficult to perform in children. 3. By what means is virus antigen best presented to the host to elicit a protective immune response? Oral inoculation may not be necessary to induce a protective, virus-specific immune response at the intestinal mucosal surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Protective immunity of Hantaan virus nucleocapsid and envelope protein studied using baculovirus-expressed proteins

Recombinant Hantaan virus nucleocapsid protein (rNP) and recombinant envelope (rEnv) proteins were prepared using a baculovirus expression system to examine the role of Hantaan virus structural proteins in protective immunity. Passive transfer of spleen cells from mice immunized with rNP conferred partial protection or prolongation of time to death from fatal Hantaan virus infection in suckling mice which were challenged with Hantaan virus at 40 LD50 (survival rate: 43%) or 4 LD50 (survival rate: 43%). The T cell-enriched fraction protected one mouse from lethal infection but the B cell-enriched fraction had no such effect on fatal HTN infection. The protective effects of the antibody against HTN challenge were examined by passive immunization. The monoclonal antibody ECO 2 directed to NP also conferred partial survival and significant difference in time to death. Although rEnv antigen failed to induce neutralizing antibody, both immune spleen cells and immune serum to rEnv conferred partial protection upon suckling mice. These results indicate that both nucleocapsid and envelope proteins of Hantaan virus were responsible for induction of cell mediated protective immunity. Vero E 6 cells infected with Hantaan virus expressed envelope protein on the surface, as determined by flow cytometry. However, there was only negligible expression of nucleocapsid protein.

The use of baculoviruses as expression vectors

The use of recombinant baculoviruses as high level expression systems is becoming more and more popular. This review aims to provide a summary of the impact of this expression system in biochemistry and biotechnology, highlighting important advances that have been made utilizing the system. The potential of newly developed multiple baculovirus expression systems to enable the reconstruction of complex biological molecules and processes is also reviewed.

Development of candidate rotavirus vaccines

Candidate rotavirus vaccines tested to date have been developed using a 'Jennerian' approach. Strains of bovine and simian rotaviruses that are naturally attenuated for humans have been assessed and found to confer immunity that is serotype specific in a varying proportion of recipients. The spectrum of protection has been widened by developing reassortants in which the bovine or simian gene coding for VP7 (the major outer capsid protein) has been replaced by the corresponding gene from human VP7 types 1, 2, 3 or 4. Once the protective antigen(s) are identified it may be possible to develop subunit vaccines that eliminate side effects sometimes observed with live vaccine candidates.

Rotavirus VP6 modified for expression on the plasma membrane forms arrays and exhibits enhanced immunogenicity

The major inner capsid protein of rotavirus is VP6, a 42-kDa polypeptide that forms the icosahedral surface of the rotavirus single-shelled particle. A chimeric form of VP6 (VP6sc) was constructed containing an upstream leader sequence derived from the influenza virus hemagglutinin and a downstream membrane-spanning (anchor) domain from a mouse immunoglobulin gene. When VP6sc was expressed in cells using a recombinant vaccinia virus, the protein was transported, glycosylated, and anchored in the plasma membrane as a trimer with the major domains of the protein orientated externally. Immunofluorescence and immunolabeling with colloidal gold indicated that VP6sc also localized in patches on the cell surface; electron microscopy revealed that the protein assembled into two-dimensional arrays which exhibited the same periodicity as the paracrystalline arrays formed by purified (viral) VP6. Mice inoculated with a recombinant vaccinia virus that expressed VP6sc produced rotavirus-specific antibodies at a titer 10 times higher than that achieved when wild-type, intracellular VP6 was delivered in the same way. Presentation at the cell surface therefore may represent a general method for enhancing the immunogenicity of rotavirus proteins.