Virus-like particle vaccines and adjuvants: the HPV paradigm

Vaccine adjuvants: current status, research and development, licensing, and future opportunities

Vaccines represent one of the most significant inventions in human history and have revolutionized global health. Generally, a vaccine functions by triggering the innate immune response and stimulating antigen-presenting cells, leading to a defensive adaptive immune response against a specific pathogen's antigen. As a key element, adjuvants are chemical materials often employed as additives to increase a vaccine's efficacy and immunogenicity. For over 90 years, adjuvants have been essential components in many human vaccines, improving their efficacy by enhancing, modulating, and prolonging the immune response. Here, we provide a timely and comprehensive review of the historical development and the current status of adjuvants, covering their classification, mechanisms of action, and roles in different vaccines. Additionally, we perform systematic analysis of the current licensing processes and highlights notable examples from clinical trials involving vaccine adjuvants. Looking ahead, we anticipate future trends in the field, including the development of new adjuvant formulations, the creation of innovative adjuvants, and their integration into the broader scope of systems vaccinology and vaccine delivery. The article posits that a deeper understanding of biochemistry, materials science, and vaccine immunology is crucial for advancing vaccine technology. Such advancements are expected to lead to the future development of more effective vaccines, capable of combating emerging infectious diseases and enhancing public health.

Identification and validation of viral antigens sharing sequence and structural homology with tumor-associated antigens (TAAs)

Background

The host’s immune system develops in equilibrium with both cellular self-antigens and non-self-antigens derived from microorganisms which enter the body during lifetime. In addition, during the years, a tumor may arise presenting to the immune system an additional pool of non-self-antigens, namely tumor antigens (tumor-associated antigens, TAAs; tumor-specific antigens, TSAs).

Methods

In the present study, we looked for homology between published TAAs and non-self-viral-derived epitopes. Bioinformatics analyses and ex vivo immunological validations have been performed.

Results

Surprisingly, several of such homologies have been found. Moreover, structural similarities between paired TAAs and viral peptides as well as comparable patterns of contact with HLA and T cell receptor (TCR) α and β chains have been observed. Therefore, the two classes of non-self-antigens (viral antigens and tumor antigens) may converge, eliciting cross-reacting CD8⁺ T cell responses which possibly drive the fate of cancer development and progression.

Conclusions

An established antiviral T cell memory may turn out to be an anticancer T cell memory, able to control the growth of a cancer developed during the lifetime if the expressed TAA is similar to the viral epitope. This may ultimately represent a relevant selective advantage for patients with cancer and may lead to a novel preventive anticancer vaccine strategy.

Development of meningococcal polysaccharide conjugate vaccine that can elicit long-lasting and strong cellular immune response with hepatitis B core antigen virus-like particles as a novel carrier protein

Neisseria meningitidis caused meningitis is life-threatening acute infection with high fatality and high frequency of severe sequelae. Meningococcal capsular polysaccharides can be used to prevent meningococcal disease; while conjugating the polysaccharides to carrier protein was found necessary to improve the immunogenicity and induce memory responses in infants and young children. Nevertheless, repeated administration of glycoconjugate vaccines might lead to carrier-induced epitope suppression due to limited number of carrier proteins. Here in this study, full-length hepatitis B core antigen virus-like particles (HBc VLPs) was used as a novel potential carrier protein for conjugation of meningococcal group C polysaccharides (CPS) with heterobifunctional polyethylene glycol (PEG) of different length (2, 5 and 10 kDa) as linkers. The physiochemical properties of the CPS-PEG-HBc conjugate vaccines were fully characterized. The TEM, DLS, native agarose gel electrophoresis, and HPLC analyses all confirmed the successful conjugation. As compared to plain CPS and the physical mixture of CPS and HBc, the immunization with the conjugate vaccines can generate about 10 times increase in CPS specific IgG titers with a significant boosting effect. HBc conjugation induced a shift to a Th1 cellular immune type response, as assessed by the increased IgG2a subclass production. In addition, vaccination of the conjugate vaccines elicited much enhanced avidity functional antibody and long-lasting immunological memory. IgG titers elicited by CPS-P2k-HBc, CPS-P5k-HBc and CPS-P10k-HBc at week 18 maintained 38.1%, 17.9% and 33.3% of their peak values. All these results demonstrated that HBc VLPs can be used as potential carrier protein to develop polysaccharide conjugate vaccines effective in eliciting long-lasting and strong cellular immune response.

Update on Head and Neck Cancer: Current Knowledge on Epidemiology, Risk Factors, Molecular Features and Novel Therapies

Tobacco use and alcohol consumption are the main risk factors associated with head and neck squamous cell carcinoma (SCC) development due to their cytotoxic and mutagenic effects on the exposed epithelia of the upper aerodigestive tract. Epstein-Barr virus (EBV) and high-risk human papillomaviruses (HPVs), both encoding viral oncoproteins able to interfere with cell cycle control, have been recognized as the etiological agents of nasopharynx carcinoma and a fraction of oropharyngeal carcinoma, respectively. Head and neck SCC is a deadly disease and despite innovative treatments represents a major challenge for patients. Recently, a number of genomic studies have highlighted the molecular heterogeneity of head and neck SCC based on methylation profiles, microRNA expression, mutated genes and new druggable pathways which may represent new targets for cancer-tailored therapies. To date, cetuximab is the only FDA-approved anti-epidermal growth factor receptor therapy for the treatment of head and neck SCC. In addition, a number of monoclonal antibodies targeting AKT, mTOR and PI3K pathways are under evaluation. Several therapeutic vaccines against HPV16 and EBV proteins are also under study. The purpose of this article is to review the epidemiology, pathogenesis and molecular features of head and neck SCC, with an emphasis on new therapies.

Enterovirus-71 virus-like particles induce the activation and maturation of human monocyte-derived dendritic cells through TLR4 signaling

Enterovirus 71 (EV71) causes seasonal epidemics of hand-foot-and-mouth disease and has a high mortality rate among young children. We recently demonstrated potent induction of the humoral and cell-mediated immune response in monkeys immunized with EV71 virus-like particles (VLPs), with a morphology resembling that of infectious EV71 virions but not containing a viral genome, which could potentially be safe as a vaccine for EV71. To elucidate the mechanisms through which EV71 VLPs induce cell-mediated immunity, we studied the immunomodulatory effects of EV71 VLPs on human monocyte-derived dendritic cells (DCs), which bind to and incorporate EV71 VLPs. DC treatment with EV71 VLPs enhanced the expression of CD80, CD86, CD83, CD40, CD54, and HLA-DR on the cell surface; increased the production of interleukin (IL)-12 p40, IL-12 p70, and IL-10 by DCs; and suppressed the capacity of DCs for endocytosis. Treatment with EV71 VLPs also enhanced the ability of DCs to stimulate naïve T cells and induced secretion of interferon (IFN)-γ by T cells and Th1 cell responses. Neutralization with antibodies against Toll-like receptor (TLR) 4 suppressed the capacity of EV71 VLPs to induce the production of IL-12 p40, IL-12 p70, and IL-10 by DCs and inhibited EV71 VLPs binding to DCs. Our study findings clarified the important role for TLR4 signaling in DCs in response to EV71 VLPs and showed that EV71 VLPs induced inhibitor of kappaB alpha (IκBα) degradation and nuclear factor of kappaB (NF-κB) activation.

VLPs and particle strategies for cancer vaccines

Effective delivery of tumor antigens to APCs is one of the key steps for eliciting a strong and durable immune response to tumors. Several cancer vaccines have been evaluated in clinical trials, based on soluble peptides, but results have not been fully satisfactory. To improve immunogenicity particles provide a valid strategy to display and/or incorporate epitopes which can be efficiently targeted to APCs for effective induction of adaptive immunity. In the present review, we report some leading technologies for developing particulate vaccines employed in cancer immunotherapy, highlighting the key parameters for a rational design to elicit both humoral and cellular responses.

Insect cells as a production platform of complex virus-like particles

Virus-like particles (VLPs) are multiprotein structures that resemble the conformation of native viruses but lack a viral genome, potentiating their application as safer and cheaper vaccines. The production of VLPs has been strongly linked with the use of insect cells and the baculovirus expression vector system, especially those particles composed of two or more structural viral proteins. In fact, this expression platform has been extensively improved over the years to address the challenges of coexpression of multiple proteins and their proper assembly into complexes in the same cell. In this article, the role of insect cell technology in the development and production of complex VLPs is overviewed; recent achievements, current bottlenecks and future trends are also highlighted.

Infections and cancer: debate about using vaccines as a cancer control tool

In 2012, Infectious Agents and Cancer commissioned a thematic series collection of articles on Prevention of HPV related cancer. The articles have attracted wide interest and stimulated debate, including about the utility of vaccines in cancer control. The application of vaccines to cancer control fulfills a promise envisioned at the turn of the 20^th century when remarkable experiments showed that some cancers were caused by infections. This suggested the possibility of applying infection-control strategies to cancer control. Vaccines represent the most practical cost-effective technology to prevent wide human suffering and death from many acute infectious diseases, such as small pox or polio. Hitherto applied to control of acute fatal infections, vaccines, if developed, might provide a potent way to control cancer. The articles in the HPV thematic series show success in developing and applying a vaccine against human papilloma virus (HPV). A vaccine is also available against hepatitis B virus (HBV), which causes liver cancer. These vaccines augment the tools available to control the associated cancers. Scientific endeavor continues for six other cancer-associated infections, mostly viruses. Not surprisingly, debate about the safety of vaccines targeting cancer has been triggered in the scientific community. Questions about safety have been raised for those populations where other means to control these cancers may be available. Although it is difficult to quantify risk from vaccines in individuals where other cancer control services exist, it is likely to be low. Vaccines are much safer today than before. Technological advancement in vaccine development and manufacture and improved regulatory review and efficient distribution have minimized substantially the risk for harm from vaccines. Formal and informal debate about the pros and cons of applying vaccines as a cancer control tools is ongoing in scientific journals and on the web. Infectious Agents and Cancer encourages evidence-based discussion to clarify understanding of the role of vaccines in cancer control. In a similar vein, the journal will not consider anecdotal reports and rhetorical arguments because they are unlikely to inform policy, regulation, or the public.

Construction of an artificial recombinant bicistronic plasmid DNA vaccine against porcine rotavirus

The attenuated Salmonella typhimurium χ4550 strain was used to harbour a reconstructed bicistronic DNA vaccine against porcine rotavirus, which carried the rotavirus nonstructural protein 4 (NSP4) and VP7 genes simultaneously. Using a balanced lethal system, the kanamycin resistance gene of expressing eukaryotic plasmids pVAX1 and pVAXD were replaced by the aspartate β-semialdehyde dehydrogenase (asd) gene. The NSP4 cleavage product (259-525) of rotavirus OSU strain and VP7 full-length genes were amplified by reverse transcription polymerase chain reaction and then inserted into the eukaryotic single-expression plasmid, pVAX1-asd, and the eukaryotic dual-expression plasmid, pVAXD-asd, respectively. The recombinant plasmids pVAX1-asd-NSP4, pVAX1-asd-VP7 and pVAXD-asd-NSP4-VP7 were transformed into the attenuated S. typhimurium χ4550 strain by electrotransformation. An indirect immunofluorescence assay of the expressed COS-7 cell suggested that the recombinant S. typhimurium χ4550 strain was constructed successfully. The recombinant S. typhimurium χ4550 strain was orally administered to BALB/c mice. The group immunised with dual- expression plasmids produced a significantly higher level of serum Immunoglobulin G (IgG) and intestinal Immunoglobulin A (IgA) than the group immunised with single-expression plasmids. These results indicated that eukaryotic bicistronic plasmid DNA vaccines could be successfully constructed to enhance humoural, mucosal and cellular immune response against rotavirus infection.

Developments in virus-like particle-based vaccines for HIV

Virus-like particles (VLPs) hold great promise for the development of effective and affordable vaccines. VLPs, indeed, are suitable for presentation and efficient delivery to antigen-presenting cells of linear as well as conformational antigens. This will ultimately result in a crosspresentation with both MHC class I and II molecules to prime CD4(+) T-helper and CD8(+) cytotoxic T cells. This review describes an update on the development and use of VLPs as vaccine approaches for HIV.

Developments in virus-like particle-based vaccines for infectious diseases and cancer

Virus-like particles hold great promise for the development of effective and affordable vaccines. Indeed, virus-like particles are suitable for presentation and efficient delivery of linear as well as conformational antigens to antigen-presenting cells. This will ultimately result in optimal B-cell activation and cross-presentation with both MHC class I and II molecules to prime CD4(+) T-helper as well as CD8(+) cytotoxic T cells. This article provides an update on the development and use of virus-like particles as vaccine approaches for infectious diseases and cancer.

Effects of adjuvants on IgG subclasses elicited by virus-like particles

BACKGROUND

Virus-Like Particles (VLPs) represent an efficient strategy to present and deliver conformational antigens to the immune system, inducing both arms of the adaptive immune response. Moreover, their particulate structure surrounded by cell membrane provides an adjuvanted effect to VLP-based immunizations. In the present study, the elicitation of different patterns of IgG subclasses by VLPs, administered in CpG ODN1826 or poly(I:C) adjuvants, has been evaluated in an animal model.

RESULTS

Adjuvanted VLPs elicited a higher titer of total specific IgG compared to VLPs alone. Furthermore, while VLPs alone induced a balanced TH2 pattern, VLPs formulated with either adjuvant elicited a TH1-biased IgG subclasses (IgG2a and IgG3), with poly(I:C) more potent than CpG ODN1826.

CONCLUSIONS

The results confirmed that adjuvants efficiently improve antigen immunogenicity and represent a suitable strategy to skew the adaptive immune response toward the differentiation of the desired T helper subset, also using VLPs as antigen.

Enterovirus type 71 neutralizing antibodies in the serum of macaque monkeys immunized with EV71 virus-like particles

Enterovirus type 71 (EV71) is a virulent form of enteroviruses causing hospitalizations for children less than three years of age. Currently there are no anti-viral therapies or vaccines available for EV71. Due to the high risk of poliomyelitis-like paralysis and fatal encephalitis, an effective vaccine to EV71 could potentially prevent virus-induced morbidity and mortality. In this study, we first tested a potential EV71 vaccine candidate based on virus-like particles (VLP). We vaccinated macaque monkeys to validate the immunogenicity of the VLP vaccine to EV71. We detected the VLP or EV71-specific antibodies, neutralization titers, ELISPOT, and T cell response to find their immune responses to EV71. When the VLP vaccine adjuvanted with alum was given to macaque monkeys, these monkeys developed both specific humoral and cellular immune responses to EV71. Despite lower neutralizing antibodies to EV71 were found in sera of VLP-immunized monkeys than monkeys vaccinated with inactivated EV71, VLP-based vaccine generated a memory immune response to EV71. Hence, VLP-based EV71 vaccine is a potential vaccine against EV71 infection.

Vaccination against type 1 diabetes

The clinical onset of type 1 diabetes or autoimmune diabetes occurs after a prodrome of islet autoimmunity. The warning signals for the ensuing loss of pancreatic islet beta cells are autoantibodies against insulin, GAD65, IA-2 and ZnT8, alone or in combinations. Autoantibodies against, for example, insulin alone have only a minor risk of type 1 diabetes. However, progression to clinical onset is increased by the induction of multiple islet autoantibodies. At the time of clinical onset, insulitis may be manifest, which seems to reduce the efficacy of immunosuppression. Autoantigen-specific immunotherapy with alum-formulated GAD65 (Diamyd(®)) shows promise to reduce the loss of beta-cell function after the clinical onset of type 1 diabetes. The mechanisms are unclear but may involve the induction of T regulatory cells, which may suppress islet autoantigen reactivity. Past and ongoing clinical trials have been safe. Future clinical trials, perhaps as combination autoantigen-specific immunotherapy, may increase the efficacy in preventing the clinical onset in subjects with islet autoantibodies or preserve residual beta-cell function in patients newly diagnosed with type 1 diabetes.

New adjuvants in evolving vaccine strategies

Adjuvants are becoming the key players of vaccine formulations to enhance the immunogenicity of subunit (peptides, proteins, virus-like particles (VLPs)) and DNA vaccines, as well as to reach the current new goals of preventing and/or treating chronic infectious diseases and cancers. Induction of humoral response, in particular neutralizing antibodies able to inhibit the binding of pathogens to their cellular receptors, remains a major goal of vaccines targeted to prevent acute lytic infections; induction/modulation of cellular immunity is, however, critical to fight latently/chronically infected cells as well as cancer cells. The new adjuvants, included in vaccine preparations, are currently able to modify the presentation of epitopes to the immune system with a specific T(H)1 versus T(H)2 polarization efficacy. A paradigm of the relevance of these new adjuvants is the immunological result obtained with the inclusion of monophosphoryl lipid A in the formulation of L1-based human papillomavirus (HPV)-naked VLPs. In the May issue of this journal, Garcon and colleagues describe the highly enhanced humoral and memory B cellular immunity of the AS04-adjuvanted HPV vaccine, which results in a long-lasting and broad spectrum immunity.

HIV-Gag VLPs presenting trimeric HIV-1 gp140 spikes constitutively expressed in stable double transfected insect cell line

We have previously described the establishment and characterization of a stably transfected insect cell line for the constitutive and efficient expression of Pr55 HIV Gag proteins, which auto-assemble into enveloped Virus-Like Particles (VLPs) released into the cell culture supernatant. Such HIV-Gag VLPs have been shown to elicit a specific systemic humoral response in vivo, proving the appropriate antigenic presentation of the HIV Gag protein to the immune system. Here we describe the establishment of a stable double transfected insect cell line for the constitutive and reproducible production of Pr55Gag-VLPs expressing on their surface trimeric forms of HIV-1 envelope glycoproteins. The persistence of HIV coding genes has been verified in clonal resistant insect cells, the protein expression and conformation has been verified by Western blot analysis. The resulting HIV-VLPs have been visualized by standard transmission electron microscopy and their immunogenicity has been evaluated in vivo. This represents, to our knowledge, the first example of stable double transfected insect cell line for the constitutive production of enveloped HIV-Gag VLPs presenting trimeric HIV-gp140 on their surface.

The role of human papillomavirus infection in breast cancer

Breast cancer is the leading female cancer and the third most common cause of cancer deaths worldwide. Many studies have suggested a possible link between breast cancer pathogenesis and viral infection, particularly mouse mammary tumour virus, simian virus 40, Epstein-Barr virus, and human papillomavirus (HPV). A significant number of recent studies have reported that approximately 29% of human breast cancer tissues were positive for high-risk HPV subtypes, especially HPV subtypes 16, 18, or 33. In contrast, several other investigations did not detect any HPV subtypes in either breast cancer tissue or normal breast tissue from patients diagnosed with breast cancer. Given these conflicting data and the established complexity of the association between HPV with other cancers, a definitive relationship between human breast cancer and HPV infection has not been determined. Recent advances in laboratory methodologies aim to overcome the inherent challenges in detecting HPV in breast cancer tissue. There is an urgent need to obtain additional evidence in order to assess the possibility of breast cancer prevention using HPV vaccines.

Engineered biological entities for drug delivery and gene therapy protein nanoparticles

The development of genetic engineering techniques has speeded up the growth of the biotechnological industry, resulting in a significant increase in the number of recombinant protein products on the market. The deep knowledge of protein function, structure, biological interactions, and the possibility to design new polypeptides with desired biological activities have been the main factors involved in the increase of intensive research and preclinical and clinical approaches. Consequently, new biological entities with added value for innovative medicines such as increased stability, improved targeting, and reduced toxicity, among others have been obtained. Proteins are complex nanoparticles with sizes ranging from a few nanometers to a few hundred nanometers when complex supramolecular interactions occur, as for example, in viral capsids. However, even though protein production is a delicate process that imposes the use of sophisticated analytical methods and negative secondary effects have been detected in some cases as immune and inflammatory reactions, the great potential of biodegradable and tunable protein nanoparticles indicates that protein-based biotechnological products are expected to increase in the years to come.

Plant-based anti-HIV-1 strategies: vaccine molecules and antiviral approaches

The introduction of highly active antiretroviral therapy has drastically changed HIV infection from an acute, very deadly, to a chronic, long-lasting, mild disease. However, this requires continuous care management, which is difficult to implement worldwide, especially in developing countries. Sky-rocketing costs of HIV-positive subjects and the limited success of preventive recommendations mean that a vaccine is urgently needed, which could be the only effective strategy for the real control of the AIDS pandemic. To be effective, vaccination will need to be accessible, affordable and directed against multiple antigens. Plant-based vaccines, which are easy to produce and administer, and require no cold chain for their heat stability are, in principle, suited to such a strategy. More recently, it has been shown that even highly immunogenic, enveloped plant-based vaccines can be produced at a competitive and more efficient rate than conventional strategies. The high variability of HIV epitopes and the need to stimulate both humoral neutralizing antibodies and cellular immunity suggest the importance of using the plant system: it offers a wide range of possible strategies, from single-epitope to multicomponent vaccines, modulators of the immune response (adjuvants) and preventive molecules (microbicides), either alone or in association with plant-derived monoclonal antibodies, besides the potential use of the latter as therapeutic agents. Furthermore, plant-based anti-HIV strategies can be administered not only parenterally but also by the more convenient and safer oral route, which is a more suitable approach for possible mass vaccination.

Chloroplasts as expression platforms for plant-produced vaccines

Production of recombinant subunit vaccines from genes incorporated in the plastid genome is advantageous because of the attainable expression level due to high transgene copy number and the absence of gene silencing; biocontainment as a consequence of maternal inheritance of plastids and no transgene presence in the pollen; and expression of multiple transgenes in prokaryotic-like operons. We discuss the core technology of plastid transformation in Chlamydomonas reinhardtii, a unicellular alga, and Nicotiana tabacum (tobacco), a flowering plant species, and demonstrate the utility of the technology for the production of recombinant vaccine antigens.

Virulence and immunomodulatory roles of bacterial outer membrane vesicles

Outer membrane (OM) vesicles are ubiquitously produced by Gram-negative bacteria during all stages of bacterial growth. OM vesicles are naturally secreted by both pathogenic and nonpathogenic bacteria. Strong experimental evidence exists to categorize OM vesicle production as a type of Gram-negative bacterial virulence factor. A growing body of data demonstrates an association of active virulence factors and toxins with vesicles, suggesting that they play a role in pathogenesis. One of the most popular and best-studied pathogenic functions for membrane vesicles is to serve as natural vehicles for the intercellular transport of virulence factors and other materials directly into host cells. The production of OM vesicles has been identified as an independent bacterial stress response pathway that is activated when bacteria encounter environmental stress, such as what might be experienced during the colonization of host tissues. Their detection in infected human tissues reinforces this theory. Various other virulence factors are also associated with OM vesicles, including adhesins and degradative enzymes. As a result, OM vesicles are heavily laden with pathogen-associated molecular patterns (PAMPs), virulence factors, and other OM components that can impact the course of infection by having toxigenic effects or by the activation of the innate immune response. However, infected hosts can also benefit from OM vesicle production by stimulating their ability to mount an effective defense. Vesicles display antigens and can elicit potent inflammatory and immune responses. In sum, OM vesicles are likely to play a significant role in the virulence of Gram-negative bacterial pathogens.

Lympho-geographical concepts in vaccine delivery

The key triggers and regulators of immune responses are antigens and their appearance in immune-privileged secondary lymphatic organs. Currently, the majority of vaccines are administered intramuscularly or subcutaneously, although neither the muscular tissue nor the subcutis is particularly rich in immuno-competent cells. Thus, introducing antigens at sites with a higher density of immune-competent cells, such as the dermis, lymph nodes, or afferent lymphatic conducts, with appropriate formulations and injection devices may induce more efficacious immune responses and protection. In this work, we first reviewed the geographical and functional map of the most important lymphatic elements that play a key role in the induction of a specific immune response, such as site of injection, choice of adjuvants and etc. In a first set of experiments, we demonstrated that short intervals of boosting (daily versus weekly) increase the production of IgG2a antibody against the injected model antigen, while increasing rather than constant booster doses increase the number of antigen-specific CD8(+) IFN-γ producing cells. Such antigen presentation patterns reflect the initially increasing amounts of antigen associated with natural infections by highly virulent and replicating pathogens. In a second set of experiments, we studied the importance of administration route (subcutaneous, intradermal, intramuscular, intralymphatic) for the induction of antigen-specific IgG2a, and of IFN-γ produced by antigen-specific lymphocytes when using PLGA microparticles for delivery of antigen. Interestingly, both IgG2a and IFN-γ production were significantly enhanced after intramuscular and intra-lymph node administration when compared to the other two routes. In conclusion, the results suggest that traditional vaccination schedules and administration routes should be reconsidered in vaccine development, particularly when using more advanced formulations and delivery systems such as micro- and nanoparticles or combinations of antigen and immune-response modifiers.

Rotavirus enterotoxin NSP4 has mucosal adjuvant properties

Rotavirus nonstructural protein 4 (NSP4) is a protein with pleiotropic properties. It functions in rotavirus morphogenesis, pathogenesis, and is the first described viral enterotoxin. Since many bacterial toxins function as potent mucosal adjuvants, we evaluated whether baculovirus-expressed recombinant simian rotavirus SA11 NSP4 possesses adjuvant activity by co-administering NSP4 with keyhole limpet hemocyanin (KLH), tetanus toxoid (TT) or ovalbumin (OVA) as model antigens in mice. Following intranasal immunization, NSP4 significantly enhanced both systemic and mucosal immune responses to model immunogens, as compared to the control group, in an antigen-specific manner. Both full-length and a cleavage product of SA11 NSP4 had adjuvant activity, localizing this activity to the C-terminus of the protein. NSP4 forms from virulent and avirulent porcine rotavirus OSU strain, and SA11 NSP4 localized within a 2/6-virus-like particle (VLP) also exhibited adjuvant effects. These studies suggest that the rotavirus enterotoxin NSP4 can function as an adjuvant to enhance immune responses for a co-administered antigen.