M cell-targeted DNA vaccination

DNA Vaccines: Their Formulations, Engineering and Delivery

The concept of DNA vaccination was introduced in the early 1990s. Since then, advancements in the augmentation of the immunogenicity of DNA vaccines have brought this technology to the market, especially in veterinary medicine, to prevent many diseases. Along with the successful COVID mRNA vaccines, the first DNA vaccine for human use, the Indian ZyCovD vaccine against SARS-CoV-2, was approved in 2021. In the current review, we first give an overview of the DNA vaccine focusing on the science, including adjuvants and delivery methods. We then cover some of the emerging science in the field of DNA vaccines, notably efforts to optimize delivery systems, better engineer delivery apparatuses, identify optimal delivery sites, personalize cancer immunotherapy through DNA vaccination, enhance adjuvant science through gene adjuvants, enhance off-target and heritable immunity through epigenetic modification, and predict epitopes with bioinformatic approaches. We also discuss the major limitations of DNA vaccines and we aim to address many theoretical concerns.

Intestinal Uptake and Tolerance to Food Antigens

Food allergy is a growing concern due to its increasing world-wide incidence. Strict avoidance of allergens is a passive treatment strategy. Since the mechanisms responsible for the occurrence and development of food allergy have not yet been fully elucidated, effective individualized treatment options are lacking. In this review, we summarize the pathways through which food antigens enter the intestine and review the proposed mechanisms describing how the intestine acquires and tolerates food antigens. When oral tolerance is not established, food allergy occurs. In addition, we also discuss the contribution of commensal bacteria of the gut in shaping tolerance to food antigens in the intestinal tract. Finally, we propose that elucidating the mechanisms of intestinal uptake and tolerance of food antigens will provide additional clues for potential treatment options for food allergy.

Intranasal vaccine: Factors to consider in research and development

Most existing vaccines for human use are administered by needle-based injection. Administering vaccines needle-free intranasally has numerous advantages over by needle-based injection, but there are only a few intranasal vaccines that are currently approved for human use, and all of them are live attenuated influenza virus vaccines. Clearly, there are immunological as well as non-immunological challenges that prevent vaccine developers from choosing the intranasal route of administration. We reviewed current approved intranasal vaccines and pipelines and described the target of intranasal vaccines, i.e. nose and lymphoid tissues in the nasal cavity. We then analyzed factors unique to intranasal vaccines that need to be considered when researching and developing new intranasal vaccines. We concluded that while the choice of vaccine formulations, mucoadhesives, mucosal and epithelial permeation enhancers, and ligands that target M-cells are important, safe and effective intranasal mucosal vaccine adjuvants are needed to successfully develop an intranasal vaccine that is not based on live-attenuated viruses or bacteria. Moreover, more effective intranasal vaccine application devices that can efficiently target a vaccine to lymphoid tissues in the nasal cavity as well as preclinical animal models that can better predict intranasal vaccine performance in clinical trials are needed to increase the success rate of intranasal vaccines in clinical trials.

Immunology-Guided Biomaterial Design for Mucosal Cancer Vaccines

Cancer of mucosal tissues is a major cause of worldwide mortality for which only palliative treatments are available for patients with late-stage disease. Engineered cancer vaccines offer a promising approach for inducing antitumor immunity. The route of vaccination plays a major role in dictating the migratory pattern of lymphocytes, and thus vaccine efficacy in mucosal tissues. Parenteral immunization, specifically subcutaneous and intramuscular, is the most common vaccination route. However, this induces marginal mucosal protection in the absence of tissue-specific imprinting signals. To circumvent this, the mucosal route can be utilized, however degradative mucosal barriers must be overcome. Hence, vaccine administration route and selection of materials able to surmount transport barriers are important considerations in mucosal cancer vaccine design. Here, an overview of mucosal immunity in the context of cancer and mucosal cancer clinical trials is provided. Key considerations are described regarding the design of biomaterial-based vaccines that will afford antitumor immune protection at mucosal surfaces, despite limited knowledge surrounding mucosal vaccination, particularly aided by biomaterials and mechanistic immune-material interactions. Finally, an outlook is given of how future biomaterial-based mucosal cancer vaccines will be shaped by new discoveries in mucosal vaccinology, tumor immunology, immuno-therapeutic screens, and material-immune system interplay.

Antigen Sampling CSF1R-Expressing Epithelial Cells Are the Functional Equivalents of Mammalian M Cells in the Avian Follicle-Associated Epithelium

The follicle-associated epithelium (FAE) is a specialized structure that samples luminal antigens and transports them into mucosa-associated lymphoid tissues (MALT). In mammals, transcytosis of antigens across the gut epithelium is performed by a subset of FAE cells known as M cells. Here we show that colony-stimulating factor 1 receptor (CSF1R) is expressed by a subset of cells in the avian bursa of Fabricius FAE. Expression was initially detected using a CSF1R-reporter transgene that also label subsets of bursal macrophages. Immunohistochemical detection using a specific monoclonal antibody confirmed abundant expression of CSF1R on the basolateral membrane of FAE cells. CSF1R-transgene expressing bursal FAE cells were enriched for expression of markers previously reported as putative M cell markers, including annexin A10 and CD44. They were further distinguished from a population of CSF1R-transgene negative epithelial cells within FAE by high apical F-actin expression and differential staining with the lectins jacalin, PHA-L and SNA. Bursal FAE cells that express the CSF1R-reporter transgene were responsible for the bulk of FAE transcytosis of labeled microparticles in the size range 0.02-0.1 μm. Unlike mammalian M cells, they did not readily take up larger bacterial sized microparticles (0.5 μm). Their role in uptake of bacteria was tested using Salmonella, which can enter via M cells in mammals. Labeled Salmonella enterica serovar Typhimurium entered bursal tissue via the FAE. Entry was partially dependent upon Type III secretion system-1. However, the majority of invading bacteria were localized to CSF1R-negative FAE cells and in resident phagocytes that express the phosphatidylserine receptor TIM4. CSF1R-expressing FAE cells in infected follicles showed evidence of cell death and shedding into the bursal lumen. In mammals, CSF1R expression in the gut is restricted to macrophages which only indirectly control M cell differentiation. The novel expression of CSF1R in birds suggests that these functional equivalents to mammalian M cells may have different ontological origins and their development and function are likely to be regulated by different growth factors.

M Cells: Intelligent Engineering of Mucosal Immune Surveillance

M cells are specialized intestinal epithelial cells that provide the main machinery for sampling luminal microbes for mucosal immune surveillance. M cells are usually found in the epithelium overlying organized mucosal lymphoid tissues, but studies have identified multiple distinct lineages of M cells that are produced under different conditions, including intestinal inflammation. Among these lineages there is a common morphology that helps explain the efficiency of M cells in capturing luminal bacteria and viruses; in addition, M cells recruit novel cellular mechanisms to transport the particles across the mucosal barrier into the lamina propria, a process known as transcytosis. These specializations used by M cells point to a novel engineering of cellular machinery to selectively capture and transport microbial particles of interest. Because of the ability of M cells to effectively violate the mucosal barrier, the circumstances of M cell induction have important consequences. Normal immune surveillance insures that transcytosed bacteria are captured by underlying myeloid/dendritic cells; in contrast, inflammation can induce development of new M cells not accompanied by organized lymphoid tissues, resulting in bacterial transcytosis with the potential to amplify inflammatory disease. In this review, we will discuss our own perspectives on the life history of M cells and also raise a few questions regarding unique aspects of their biology among epithelia.

Intranasal delivery of nanoparticle-based vaccines

Most pathogens gain access to the human body and initiate systemic infections through mucosal sites. A large number of currently marketed licensed vaccines are parenterally administered; they generate strong systemic immunity but not mucosal immunity. Nasal vaccination is an appealing strategy for the induction of mucosal-specific immunity; however, its development is mostly challenged by several factors, such as inefficient antigen uptake, its rapid mucociliary clearance, size-restricted permeation across epithelial barriers and absence of safe human mucosal adjuvants. Therefore, a safer mucosal-adjuvanting strategy or efficient mucosal delivery platform is much warranted. This review summarizes challenges and the rationale for nasal vaccine development with a special focus on the use of nanoparticles based on polymers and lipids for mucosal vaccine delivery.

Virus-Like Particle, Liposome, and Polymeric Particle-Based Vaccines against HIV-1

It is acknowledged that vaccines remain the best hope for eliminating the HIV-1 epidemic. However, the failure to produce effective vaccine immunogens and the inability of conventional delivery strategies to elicit the desired immune responses remains a central theme and has ultimately led to a significant roadblock in HIV vaccine development. Consequently, significant efforts have been applied to generate novel vaccine antigens and delivery agents, which mimic viral structures for optimal immune induction. Here, we review the latest developments that have occurred in the nanoparticle vaccine field, with special emphasis on strategies that are being utilized to attain highly immunogenic, systemic, and mucosal anti-HIV humoral and cellular immune responses. This includes the design of novel immunogens, the central role of antigen-presenting cells, delivery routes, and biodistribution of nanoparticles to lymph nodes. In particular, we will focus on virus-like-particle formulations and their preclinical uses within the HIV prophylactic vaccine setting.

Compartmentalization of immunosenescence: a deeper look at the mucosa

Developments in medical care and living conditions led to an astonishing increase in life-span perspective and subsequently a rise in the old population. This can be seen as a success for public health policies but it also challenges society to adapt, in order to cope with the potentially overwhelming cost for the healthcare system. A fast-growing number of older people lose their ability to live independently because of diseases and disabilities, frailty or cognitive impairment. Many require long-term care, including home-based nursing, communities and hospital-based care. Immunosenescence, an age-related deterioration in immune functions, is considered a major contributory factor for the higher prevalence and severity of infectious diseases and the poor efficacy of vaccination in the elderly. When compared with systemic immunosenescence, alterations in the mucosal immune system with age are less well understood. For this reason, this area deserves more extensive and intensive research and support. In this article, we provide an overview of age-associated changes occurring in systemic immunity and discuss the distinct features of mucosal immunosenescence.

Roles of M cells in infection and mucosal vaccines

The mucosal immune system plays a crucial part in the control of infection. Exposure of humans and animals to potential pathogens generally occurs through mucosal surfaces, thus, strategies that target the mucosa seem rational and efficient vaccination measures. Vaccination through the mucosal immune system can induce effective systemic immune responses simultaneously with mucosal immunity compared with parenteral vaccination. M cells are capable of transporting luminal antigens to the underlying lymphoid tissues and can be exploited by pathogens as an entry portal to invade the host. Therefore, targeting M-cell-specific molecules might enhance antigen entry, initiate the immune response, and induce protection against mucosal pathogens. Here, we outline our understanding of the distribution and function of M cells, and summarize the advances in mucosal vaccine strategies that target M cells.

Phenotype and function of nasal dendritic cells

Intranasal (i.n.) vaccination generates immunity across local, regional, and distant sites. However, nasal dendritic cells (DCs), pivotal for the induction of i.n. vaccine-induced immune responses, have not been studied in detail. Here, by using a variety of parameters, we define nasal DCs in mice and humans. Distinct subsets of "classical" DCs, dependent on the transcription factor zbtb46 were identified in the murine nose. The murine nasal DCs were Fms-related tyrosine 3 kinase ligand responsive and displayed unique phenotypic and functional characteristics, including the ability to present antigen, induce an allogeneic T-cell response, and migrate in response to lipopolysaccharide or live bacterial pathogens. Importantly, in a cohort of human volunteers, BDCA-1(+) DCs were observed to be the dominant nasal DC population at steady state. During chronic inflammation, the frequency of both BDCA-1(+) and BDCA-3(hi) DCs was reduced in the nasal tissue, associating the loss of these immune sentinels with chronic nasal inflammation. The present study is the first detailed description of the phenotypic, ontogenetic, and functional properties of nasal DCs, and will inform the design of preventative immunization strategies as well as therapeutic modalities against chronic rhinosinusitis.

Development of chitosan-pullulan composite nanoparticles for nasal delivery of vaccines: in vivo studies

Here, we aimed at developing chitosan/pullulan composite nanoparticles and testing their potential as novel systems for the nasal delivery of diphtheria toxoid (DT). All the chitosan derivatives [N-trimethyl (TMC), chloride and glutamate] and carboxymethyl pullulan (CMP) were synthesised and antigen-loaded composites were prepared by polyion complexation of chitosan and pullulan derivatives (particle size: 239-405 nm; surface charge: +18 and +27 mV). Their immunological effects after intranasal administration to mice were compared to intramuscular route. Composite nanoparticles induced higher levels of IgG responses than particles formed with chitosan derivative and antigen. Nasally administered TMC-pullulan composites showed higher DT serum IgG titre when compared with the other composites. Co-encapsulation of CpG ODN within TMC-CMP-DT nanoparticles resulted in a balanced Th1/Th2 response. TMC/pullulan composite nanoparticles also induced highest cytokine levels compared to those of chitosan salts. These findings demonstrated that TMC-CMP-DT composite nanoparticles are promising delivery system for nasal vaccination.

Nasal immunization using a mimovirus vaccine based on the Eppin B-cell epitope induced suppressed fertility in mice

To elicit potent humoral immunity and produce adequate neutralizing antibody especially in the genital tract and eventually to promote its immunogenicity, we designed an Eppin B-cell-dominant-epitope-based mimovirus vaccine with an RGD motif which can be nasally inoculated into male mice. Our results indicate that this immune strategy successfully generated a high antibody response with significantly higher anti-Eppin IgA in the genital tract, and eventually achieve significant inhibition of fertility without any interference with testis function and alteration in structural integrity. The fertility rate of the females mating with the vaccinated males declined and the progeny size was greatly reduced, but the contraceptive efficacy was still far from that of immunocontraceptives for human use. However, the research showed a new contraceptive vaccine construction and inoculation avenue, that is, mimovirus vaccine delivered nasally. Further investigation geared toward improving fertility inhibition efficacy using this inoculation strategy still remains to be explored.

M cell-derived vesicles suggest a unique pathway for trans-epithelial antigen delivery

M cells are a subset of mucosal epithelial cells with specialized capability to transport antigens across the mucosal barrier, but there is limited information on antigen transfer in the subepithelial zone due to the challenges in tracking microparticles and antigens that are transcytosed by this unique cell. Using transgenic reporter mice expressing dsRed in the cytoplasm of M cells and EGFP in myeloid cells, we observed that the M cell basolateral pocket hosts a close interaction between B lymphocytes and dendritic cells. Interestingly, we identified a population of previously undescribed M cell-derived vesicles (MCM) that are constitutively shed into the subepithelial space and readily taken up by CX3CR1(+)CD11b(+) CD11c(+) dendritic cells. These MCM are characterized by their cytoplasmic dsRed confirming their origin from the M cell cytoplasm. MCM showed preferential colocalization in dendritic cells with transcytosed bacteria but not transcytosed polystyrene beads, indicating a selective sorting of cargo fate in the subepithelial zone. The size and number of MCM were found to be upregulated by bacterial transcytosis and soluble toll-like receptor 2 (TLR2) agonist, further pointing to dynamic regulation of this mechanism. These results suggest that MCM provide a unique function by delivering to dendritic cells, various materials such as M cell-derived proteins, effector proteins, toxins, and particles found in the M cell cytoplasm during infection or surveillance.

Mucosal Immunosenescence in the Gastrointestinal Tract: A Mini-Review

It has been shown that pathogen-specific secretory IgA (SIgA) antibody (Ab) is the major player at mucosal surfaces for host defense. However, alterations in the mucosal immune system occur in advanced aging, which results in a failure of induction of SIgA Abs for the protection from infectious diseases. Signs of mucosal senescence first appear in the gut immune system. Further, changes in the intestinal microbiota most likely influence mucosal immunity. To overcome the immunological aging decline in mucosal immunity, several adjuvant systems including mucosal dendritic cell targeting have been shown to be attractive and effective immunological strategies. Similarly, microfold (M) cells involved in the antigen (Ag) uptake are ideal targets for facilitating Ag-specific mucosal immune responses. However, the numbers of M cells are reduced in aged mice. In this regard, Spi-B, an essential transcription factor for the functional and structural differentiation of M cells, could be a potent strategy for the induction of effective mucosal immunity in aging.

Intranasal DNA Vaccine for Protection against Respiratory Infectious Diseases: The Delivery Perspectives

Intranasal delivery of DNA vaccines has become a popular research area recently. It offers some distinguished advantages over parenteral and other routes of vaccine administration. Nasal mucosa as site of vaccine administration can stimulate respiratory mucosal immunity by interacting with the nasopharyngeal-associated lymphoid tissues (NALT). Different kinds of DNA vaccines are investigated to provide protection against respiratory infectious diseases including tuberculosis, coronavirus, influenza and respiratory syncytial virus (RSV) etc. DNA vaccines have several attractive development potential, such as producing cross-protection towards different virus subtypes, enabling the possibility of mass manufacture in a relatively short time and a better safety profile. The biggest obstacle to DNA vaccines is low immunogenicity. One of the approaches to enhance the efficacy of DNA vaccine is to improve DNA delivery efficiency. This review provides insight on the development of intranasal DNA vaccine for respiratory infections, with special attention paid to the strategies to improve the delivery of DNA vaccines using non-viral delivery agents.

Microfold (M) cells: important immunosurveillance posts in the intestinal epithelium

The transcytosis of antigens across the gut epithelium by microfold cells (M cells) is important for the induction of efficient immune responses to some mucosal antigens in Peyer's patches. Recently, substantial progress has been made in our understanding of the factors that influence the development and function of M cells. This review highlights these important advances, with particular emphasis on: the host genes which control the functional maturation of M cells; how this knowledge has led to the rapid advance in our understanding of M-cell biology in the steady state and during aging; molecules expressed on M cells which appear to be used as "immunosurveillance" receptors to sample pathogenic microorganisms in the gut; how certain pathogens appear to exploit M cells to infect the host; and finally how this knowledge has been used to specifically target antigens to M cells to attempt to improve the efficacy of mucosal vaccines.

Inside the mucosal immune system

An intricate network of innate and immune cells and their derived mediators function in unison to protect us from toxic elements and infectious microbial diseases that are encountered in our environment. This vast network operates efficiently by use of a single cell epithelium in, for example, the gastrointestinal (GI) and upper respiratory (UR) tracts, fortified by adjoining cells and lymphoid tissues that protect its integrity. Perturbations certainly occur, sometimes resulting in inflammatory diseases or infections that can be debilitating and life threatening. For example, allergies in the eyes, skin, nose, and the UR or digestive tracts are common. Likewise, genetic background and environmental microbial encounters can lead to inflammatory bowel diseases (IBDs). This mucosal immune system (MIS) in both health and disease is currently under intense investigation worldwide by scientists with diverse expertise and interests. Despite this activity, there are numerous questions remaining that will require detailed answers in order to use the MIS to our advantage. In this issue of PLOS Biology, a research article describes a multi-scale in vivo systems approach to determine precisely how the gut epithelium responds to an inflammatory cytokine, tumor necrosis factor-alpha (TNF-α), given by the intravenous route. This article reveals a previously unknown pathway in which several cell types and their secreted mediators work in unison to prevent epithelial cell death in the mouse small intestine. The results of this interesting study illustrate how in vivo systems biology approaches can be used to unravel the complex mechanisms used to protect the host from its environment.

Strategies for intranasal delivery of vaccines

The vast majority of human pathogens colonize and invade at the mucosal surfaces. Preventing infection at these sites via mucosally active vaccines is a promising and rational approach for vaccine development. However, it is only recently that the stimulation of local immunity at the mucosal surfaces has become a primary objective in addition to inducing systemic immunity. This review describes vaccine formulations designed for mucosal delivery to the nasal-associated lymphoid tissue, via intranasal administration. The association of antigens with mucosal adjuvants and delivery systems is emphasised.

Novel vaccine development strategies for inducing mucosal immunity

To develop protective immune responses against mucosal pathogens, the delivery route and adjuvants for vaccination are important. The host, however, strives to maintain mucosal homeostasis by responding to mucosal antigens with tolerance, instead of immune activation. Thus, induction of mucosal immunity through vaccination is a rather difficult task, and potent mucosal adjuvants, vectors or other special delivery systems are often used, especially in the elderly. By taking advantage of the common mucosal immune system, the targeting of mucosal dendritic cells and microfold epithelial cells may facilitate the induction of effective mucosal immunity. Thus, novel routes of immunization and antigen delivery systems also show great potential for the development of effective and safe mucosal vaccines against various pathogens. The purpose of this review is to introduce several recent approaches to induce mucosal immunity to vaccines, with an emphasis on mucosal tissue targeting, new immunization routes and delivery systems. Defining the mechanisms of mucosal vaccines is as important as their efficacy and safety, and in this article, examples of recent approaches, which will likely accelerate progress in mucosal vaccine development, are discussed.

Loss of sialic acid binding domain redirects protein σ1 to enhance M cell-directed vaccination

Ovalbumin (OVA) genetically fused to protein sigma 1 (pσ1) results in tolerance to both OVA and pσ1. Pσ1 binds in a multi-step fashion, involving both protein- and carbohydrate-based receptors. To assess the relative pσ1 components responsible for inducing tolerance and the importance of its sialic binding domain (SABD) for immunization, modified OVA-pσ1, termed OVA-pσ1(short), was deleted of its SABD, but with its M cell targeting moiety intact, and was found to be immunostimulatory and enhanced CD4⁺ and CD8⁺ T cell proliferation. When used to nasally immunize mice given with and without cholera toxin (CT) adjuvant, elevated SIgA and serum IgG responses were induced, and OVA-pσ1(s) was more efficient for immunization than native OVA+CT. The immune antibodies (Abs) were derived from elevated Ab-forming cells in the upper respiratory tissues and submaxillary glands and were supported by mixed Th cell responses. Thus, these studies show that pσ1(s) can be fused to vaccines to effectively elicit improved SIgA responses.

The mucosal immune system of the respiratory tract

Most viruses use host mucosal surfaces as their initial portals of infection. The respiratory tract has the body's second-largest mucosal surface area after the digestive tract. An understanding of the unique nature of the mucosal immune system of respiratory organs is therefore extremely important for the development of new-generation vaccines and novel methods of preventing and treating respiratory infectious diseases, including viral infections.

M cell-targeted mucosal vaccine strategies

Immune responses in the aerodigestive tract are characterized by production and transport of specific IgA antibodies across the epithelium to act as a first line of defense against pathogens in the external environment. To sample antigens on mucosal surfaces in the intestine and upper respiratory tract, the immune system relies on a close collaboration between specialized antigen-sampling epithelial M cells and lymphoid cells. Depending on various factors, local antigen presentation in the mucosal tissue leads to tolerance or initiation of an active immune response. Recently, molecules that could be used to target vaccine antigens to apical M cell surfaces have been identified. Here we review the M cell-targeted vaccine strategy, an approach that could be used to enhance uptake and efficacy of vaccines delivered in the nasal cavity or intestine.

Avian reovirus sigma C enhances the mucosal and systemic immune responses elicited by antigen-conjugated lactic acid bacteria

Mucosal surfaces are common sites of pathogen colonization/entry. Effective mucosal immunity by vaccination should provide protection at this primary infection site. Our aim was to develop a new vaccination strategy that elicits a mucosal immune response. A new strain of Enterococcus faecium, a non pathogenic lactic acid bacteria (LAB) with strong cell adhesion ability, was identified and used as a vaccine vector to deliver two model antigens. Specifically, sigma (σ) C protein of avian reovirus (ARV), a functional homolog of mammalian reovirus σ1 protein and responsible for M-cell targeting, was administered together with a subfragment of the spike protein of infectious bronchitis virus (IBV). Next, the effect of immunization route on the immune response was assessed by delivering the antigens via the LAB strain. Intranasal (IN) immunization induced stronger humoral responses than intragastic (IG) immunization. IN immunization produced antigen specific IgA both systemically and in the lungs. A higher IgA titer was induced by the LAB with ARV σC protein attached. Moreover, the serum of mice immunized with LAB displaying divalent antigens had much stronger immune reactivity against ARV σC protein compared to IBV-S1. Our results indicate that ARV σC protein delivered by LAB via the IN route elicits strong mucosal immunity. A needle-free delivery approach is a convenient and cost effective method of vaccine administration, especially for respiratory infections in economic animals. Furthermore, ARV σC, a strong immunogen of ARV, may be able to serve as an immunoenhancer for other vaccines, especially avian vaccines.

M cell targeting by a Claudin 4 targeting peptide can enhance mucosal IgA responses

Background

Mucosal immune surveillance is thought to be largely achieved through uptake by specialized epithelial M cells. We recently identified Claudin 4 as an M cell target receptor and developed a Claudin 4 targeting peptide (CPE) that can mediate uptake of nanoparticles through Nasal Associated Lymphoid Tissue (NALT) M cells.

Methods

Recombinant influenza hemagglutinin (HA) and a version with the CPE peptide at the C-terminal end was used to immunize mice by the intranasal route along with a single dose of cholera toxin as an adjuvant. Serum and mucosal IgG and IgA responses were tested for reactivity to HA.

Results

We found that the recombinant HA was immunogenic on intranasal administration, and inclusion of the CPE targeting peptide induced higher mucosal IgA responses. This mucosal administration also induced systemic serum IgG responses with Th2 skewing, but targeting did not enhance IgG responses, suggesting that the IgG response to mucosal immunization is independent of the effects of CPE M cell targeting.

Conclusions

M cell targeting mediated by a Claudin 4-specific targeting peptide can enhance mucosal IgA responses above the response to non-targeted mucosal antigen. Since Claudin 4 has also been found to be regulated in human Peyer's patch M cells, the CPE targeting peptide could be a reasonable platform delivery technology for mucosal vaccination.

The M cell-targeting ligand promotes antigen delivery and induces antigen-specific immune responses in mucosal vaccination

Oral mucosal immunization can induce protective immunity in both systemic compartments and the mucosa. Successful mucosal immunization depends on Ag delivery to the mucosal immune induction site. The high transcytotic activity of M cells within the mucosa makes these cells attractive targets for mucosal Ag delivery, although it remains unclear whether delivery of Ag to M cells only can guarantee the induction of effective immune responses. In this study, we evaluated the ability of an M cell-targeting ligand with adjuvant activity to induce immunity against ligand-fused Ag. We selected M cell-targeting ligands through biopanning of a phage display library against differentiated in vitro M-like cells and produced the recombinant Ags fused to the selected ligands using the model Ag. One of the selected peptide ligands, Co1, promoted the binding of ligand-fused Ag to mouse Peyer's patch M cells and human M-like cells that had been defined by binding with the M cell-specific and anti-GP2 Abs. In addition, Co1 ligand enhanced the uptake of fused Ag by immunogenic tissue in an ex vivo loop assay and in vivo oral administration experiments. After oral administration, the ligand-fused Ag enhanced immune responses against the fused Ag compared with those of the control Ag without ligand. In addition, this use of the ligand supported a skewed Th2-type immune response against the fused Ag. Collectively, these results suggest that the ligand selected through biopanning against cultured M-like cells could be used as an adjuvant for targeted Ag delivery into the mucosal immune system to enhance immune induction.

Intranasal M cell uptake of nanoparticles is independently influenced by targeting ligands and buffer ionic strength

In mucosal tissues, epithelial M cells capture and transport microbes across the barrier to underlying immune cells. Previous studies suggested that high affinity ligands targeting M cells may be used to deliver mucosal vaccines; here, we show that particle composition and dispersion buffer ionic strength can independently influence their uptake in vivo. First, addition of a poloxamer 188 to nanoparticle formulations increased uptake of intranasally administered nanoparticles in vivo, but the effect was dependent on the presence of the M cell-targeting ligand. Second, solvent ionic strength is known to effect electrostatic interactions; accordingly, reduced ionic strength increased the electrostatic potential between the epithelium and the particles. Interestingly, below a critical ionic strength, intranasal particle uptake in vivo significantly was increased even when controlled for osmolarity. Similar results were obtained for uptake of bacterial particles. Surprisingly, at low ionic strength, the specific enhancement effect by the targeting peptide was negligible. Modeling of the electrostatic forces predicted that the enhancing effects of the M cell-targeting ligand only are enabled at high ionic strength, as particle electrostatic forces are reduced through Debye screening. Thus, electrostatic forces can have a dramatic effect on the in vivo M cell particle uptake independent of the action of targeting ligands. Examination of these forces will be helpful to optimizing mucosal vaccine and drug delivery.

Nanotechnology solutions for mucosal immunization

The current prevalence of infectious diseases in many developing regions of the world is a serious burden, impacting both the general health as well as economic growth of these communities. Additionally, treatment with conventional medication becomes increasingly challenging due to emergence of new and drug resistant strains jeopardizing the progress made in recent years towards control and elimination of certain types of infectious diseases. Thus, from a public health perspective, prevention such as through immunization by vaccination, which has proven to be most effective, might be the best alternative to prevent and combat infectious diseases in these regions. To achieve this, development of wide-scale immunization programs become necessary including vaccines that can easily and widely be distributed, stored and administered. Mucosal vaccines offer great potential since they can be administered via oral or intranasal delivery route which does not require trained personnel, avoids the use of needles and improves overall patient compliance and acceptance. However, it necessitates the implementation of specific immunization strategies to improve their efficacy. Application of nanotechnology to design and create particle mediated delivery systems that can efficiently encapsulate vaccine components for protection of the sensitive payload, target the mucosal immune system and incorporate mucosal adjuvants maximizing immune response is key strategy to improve the effectiveness of mucosal vaccines.

Claudin 4-targeted protein incorporated into PLGA nanoparticles can mediate M cell targeted delivery

Polymer-based microparticles are in clinical use mainly for their ability to provide controlled release of peptides and compounds, but they are also being explored for their potential to deliver vaccines and drugs as suspensions directly into mucosal sites. It is generally assumed that uptake is mediated by epithelial M cells, but this is often not directly measured. To study the potential for optimizing M cell uptake of polymer microparticles in vivo, we produced sub-micron size PLGA particles incorporating a recombinant protein. This recombinant protein was produced with or without a c-terminal peptide previously shown to have high affinity binding to Claudin 4, a protein associated with M cell endocytosis. While the PLGA nanoparticles incorporate the protein throughout the matrix, much of the protein was also displayed on the surface, allowing us to take advantage of the binding activity of the targeting peptide. Accordingly, we found that instillation of these nanoparticles into the nasal passages or stomach of mice was found to significantly enhance their uptake by upper airway and intestinal M cells. Our results suggest that a reasonably simple nanoparticle manufacture method can provide insight into developing an effective needle-free delivery system.

Mucosal vaccines: novel advances in technology and delivery

Mucosal vaccines are considered the most suitable type of vaccines to combat emerging and re-emerging infectious diseases because of their ability to induce both mucosal and systemic immunity. Considerable advances have been made toward the development of mucosal vaccines against influenza virus and rotavirus. Many additional mucosal vaccines are in development, including vaccines against cholera, typhoid, traveler's diarrhea and respiratory infections. In addition to oral and nasal vaccines, transcutaneous (or skin patch) and sublingual immunizations are now part of a new generation of mucosal vaccines. Furthermore, a rice-based oral vaccine (MucoRice) has been receiving global attention as a new form of cold chain-free vaccine, because it is stable at room temperature for a prolonged period. This review describes recent developments in mucosal vaccines with promising preclinical and clinical results.

Targeted delivery of immunogen to primate m cells with tetragalloyl lysine dendrimer

Effective uptake of Ags by specialized M cells of gut-associated lymphoid tissues is an important step in inducing efficient immune responses after oral vaccination. Although stable nontoxic small molecule mimetics of lectins, such as synthetic multivalent polygalloyl derivatives, may have potential in murine M cell targeting, it remains unclear whether synthetic multivalent polygalloyl derivatives effectively target nonhuman and human M cells. In this study, we evaluated the ability of a tetragalloyl derivative, the tetragalloyl-D-lysine dendrimer (TGDK), to target M cells in both in vivo nonhuman primate and in vitro human M-like cell culture models. TGDK was efficiently transported from the lumen of the intestinal tract into rhesus Peyer's patches by M cells and then accumulated in germinal centers. Oral administration of rhesus CCR5-derived cyclopeptide conjugated with TGDK in rhesus macaque resulted in a statistically significant increase in stool IgA response against rhesus CCR5-derived cyclopeptide and induced a neutralizing activity against SIV infection. Furthermore, TGDK was specifically bound to human M-like cells and efficiently transcytosed from the apical side to the basolateral side in the M-like cell model. Thus, the TGDK-mediated vaccine delivery system represents a potential approach for enabling M cell-targeted mucosal vaccines in primates.

M cell-targeted delivery of vaccines and therapeutics

BACKGROUND

M (microfold or membranous) cells are specialised epithelial cells responsible for antigen sampling at the interface of mucosal surfaces and the environment. Their high transcytotic ability make M cells an attractive target for mucosally delivered vaccines and therapeutics.

OBJECTIVE

This brief review discusses the current state of M cell-targeted mucosal delivery systems and the potential of such delivery systems for the development of new vaccines and therapeutics against mucosal infectious and inflammatory diseases.

SCOPE

A variety of synthetic microparticles/nanoparticles have been developed and tested as vehicles for M cell-targeted mucosal drug and vaccine delivery. beta1 integrins, pathogen recognition receptors, specific carbohydrate residues and other M cell surface antigens have been exploited as potential targets for the delivery of mucosal vaccines and therapeutics.

CONCLUSION

Despite a considerable body of literature, much work still needs to be done before an effective M cell-targeted vaccine or therapeutic is developed.

Ovalbumin-protein sigma 1 M-cell targeting facilitates oral tolerance with reduction of antigen-specific CD4+ T cells

BACKGROUND & AIMS

The follicle-associated epithelium (FAE) plays key roles in antigen uptake and subsequent induction of mucosal immunity. In this study, we examined whether M-cell targeting using a protein antigen (Ag) delivery system would induce oral tolerance instead of enhancement of Ag-specific mucosal antibody (Ab) responses.

METHODS

Mice were fed different doses of a recombinant protein sigma 1 of reovirus genetically conjugated to ovalbumin (OVA-psigma1), psigma1 only, or phosphate-buffered saline (PBS) before oral challenge with OVA plus cholera toxin as mucosal adjuvant. OVA-specific Ab and CD4-positive (CD4(+)) T-cell responses were determined.

RESULTS

A low dose of OVA-psigma1 reduced anti-OVA Ab and CD4(+) T-cell responses in both mucosal and systemic lymphoid tissues. OVA/MHC I-A(d) tetramer staining showed that the numbers of OVA-specific CD4(+) T cells were significantly reduced in lamina propria of mice fed OVA-psigma1 than those fed psigma1 only or PBS only. In fact, Foxp3 expressing CD25(+) CD4(+) T cells were markedly increased in this tissue. Nonetheless, CD25(+) CD4(+) T cells from the spleen, mesenteric lymph nodes, and Peyer's patches of orally tolerized mice showed increased transforming growth factor beta1 (TGF-beta1) and interleukin-10 (IL-10) production compared with nontolerized mice.

CONCLUSIONS

These results show that an FAE M-cell targeting protein Ag delivery system facilitates oral tolerance induction because of a reduction in Ag-specific CD4(+) T cells and increased levels of TGF-beta1 and IL-10 producing, CD25(+) CD4(+) regulatory T cells in both systemic and mucosal lymphoid tissues.

Low-dose tolerance is mediated by the microfold cell ligand, reovirus protein sigma1

Mucosal tolerance induction generally requires multiple or large Ag doses. Because microfold (M) cells have been implicated as being important for mucosal tolerance induction and because reovirus attachment protein sigma1 (psigma1) is capable of binding M cells, we postulated that targeting a model Ag to M cells via psigma1 could induce a state of unresponsiveness. Accordingly, a genetic fusion between OVA and the M cell ligand, reovirus psigma1, termed OVA-psigma1, was developed to enhance tolerogen uptake. When applied nasally, not parenterally, as little as a single dose of OVA-psigma1 failed to induce OVA-specific Abs even in the presence of adjuvant. Moreover, the mice remained unresponsive to peripheral OVA challenge, unlike mice given multiple nasal OVA doses that rendered them responsive to OVA. The observed unresponsiveness to OVA-psigma1 could be adoptively transferred using cervical lymph node CD4(+) T cells, which failed to undergo proliferative or delayed-type hypersensitivity responses in recipients. To discern the cytokines responsible as a mechanism for this unresponsiveness, restimulation assays revealed increased production of regulatory cytokines, IL-4, IL-10, and TGF-beta1, with greatly reduced IL-17 and IFN-gamma. The induced IL-10 was derived predominantly from FoxP3(+)CD25(+)CD4(+) T cells. No FoxP3(+)CD25(+)CD4(+) T cells were induced in OVA-psigma1-dosed IL-10-deficient (IL-10(-/-)) mice, and despite showing increased TGF-beta1 synthesis, these mice were responsive to OVA. These data demonstrate the feasibility of using psigma1 as a mucosal delivery platform specifically for low-dose tolerance induction.

Expression of tight junction proteins in epithelium including Ck20-positive M-like cells of human adenoids in vivo and in vitro

The human adenoid epithelium forms a continuous barrier against a wide variety of exogenous antigens. In this study, to elucidate the structures of the epithelial barrier in the human adenoid, including M-cells, we identified M-cells using an anti-cytokeratin 20 (Ck20) antibody and investigated expression of tight junction proteins in human adenoid epithelium in vivo and in vitro. In human adenoid epithelium and primary cultures, mRNAs of occludin, junctional adhesion molecule-A, ZO-1, and claudin-1, -4, -7, and -8 were detected by reverse transcription-polymerase chain reaction, whereas claudin-2 and -9 were expressed in vitro. In the epithelium in vivo, some Ck20-positive cells were randomly observed and indicated pocket-like structures, whereas Ck7 was positive in almost cells. Transmission electron microscopy revealed that Ck20-associated gold particles could be identified in M-like cells which had short microvilli and harboured the lymphocyte in the pocket-like structure. In primary cultures in vitro, Ck20-positive cells were also detected and had a function to take up fluorescent microparticles. In Ck20-positive cells in vivo and in vitro, expression of occludin, ZO-1, claudin-1 and -7 were observed at cell borders. These results indicate that the epithelial barrier of the human adenoid is stably maintained by expression of tight junction proteins in the epithelium including Ck20-positive M-like cells.

M-cells: origin, morphology and role in mucosal immunity and microbial pathogenesis

M-cells are specialized cells found in the follicle-associated epithelium of intestinal Peyer's patches of gut-associated lymphoid tissue and in isolated lymphoid follicles, appendix and in mucosal-associated lymphoid tissue sites outside the gastrointestinal tract. In the gastrointestinal tract, M-cells play an important role in transport of antigen from the lumen of the small intestine to mucosal lymphoid tissues, where processing and initiation of immune responses occur. Thus, M-cells act as gateways to the mucosal immune system and this function has been exploited by many invading pathogens. Understanding the mechanism by which M-cells sample antigen will inform the design of oral vaccines with improved efficacy in priming mucosal and systemic immune responses. In this review, the origin and morphology of M-cells, and their role in mucosal immunity and pathogenesis of infections are discussed.