The kinetics of soluble and particulate antigen trafficking in the afferent lymph, and its modulation by aluminum-based adjuvant

Vaccine adjuvants for infectious disease in the clinic

Adjuvants, materials added to vaccines to enhance the resulting immune response, are important components of vaccination that are many times overlooked. While vaccines always include an antigen to tell the body what to vaccinate to, of equal importance the adjuvant provides the how, a significant factor in producing a complete response. The adjuvant space has been slow to develop with the first use of an adjuvant in a licensed vaccine occurring in the 1930s, and remaining the only adjuvant in licensed vaccines for the next 80 years. However, with vaccination at the forefront of protection against new and complex pathogens, it is important to consider all components when designing an effective vaccine. Here we summarize the adjuvant space in licensed vaccines as well as the novel adjuvant space in clinical trials with a specific focus on the materials utilized and their resulting impact on the immune response. We discuss five major categories of adjuvant materials: aluminum salts, nanoparticles, viral vectors, TLR agonists, and emulsions. For each category, we delve into the current clinical trials space, the impact of these materials on vaccination, as well as some of the ways in which they could be improved. Adjuvants present an exciting opportunity to improve vaccine responses and stability, this review will help inform about the current progress of this space.

Translational impact statement

In the aftermath of the COVID-19 pandemic, vaccines for infectious diseases have come into the spotlight. While antigens have always been an important focus of vaccine design, the adjuvant is a significant tool for enhancing the immune response to the vaccine that has been largely underdeveloped. This article provides a broad review of the history of adjuvants and, the current vaccine adjuvant space, and the progress seen in adjuvants in clinical trials. There is specific emphasis on the material landscape for adjuvants and their resulting mechanism of action. Looking ahead, while the novel vaccine adjuvant space features exciting new technologies and materials, there is still a need for more to meet the protective needs of new and complex pathogens.

Vaccines Induce Homeostatic Immunity, Generating Several Secondary Benefits

The optimal immune response eliminates invading pathogens, restoring immune equilibrium without inflicting undue harm to the host. However, when a cascade of immunological reactions is triggered, the immune response can sometimes go into overdrive, potentially leading to harmful long-term effects or even death. The immune system is triggered mostly by infections, allergens, or medical interventions such as vaccination. This review examines how these immune triggers differ and why certain infections may dysregulate immune homeostasis, leading to inflammatory or allergic pathology and exacerbation of pre-existing conditions. However, many vaccines generate an optimal immune response and protect against the consequences of pathogen-induced immunological aggressiveness, and from a small number of unrelated pathogens and autoimmune diseases. Here, we propose an "immuno-wave" model describing a vaccine-induced "Goldilocks immunity", which leaves fine imprints of both pro-inflammatory and anti-inflammatory milieus, derived from both the innate and the adaptive arms of the immune system, in the body. The resulting balanced, 'quiet alert' state of the immune system may provide a jump-start in the defense against pathogens and any associated pathological inflammatory or allergic responses, allowing vaccines to go above and beyond their call of duty. In closing, we recommend formally investigating and reaping many of the secondary benefits of vaccines with appropriate clinical studies.

Mycobacterium avium subsp. paratuberculosis antigens induce cellular immune responses in cattle without causing reactivity to tuberculin in the tuberculosis skin test

Mycobacterium avium subspecies paratuberculosis (MAP) causes chronic progressive granulomatous enteritis leading to diarrhea, weight-loss, and eventual death in ruminants. Commercially available vaccine provides only partial protection against MAP infection and can interfere with the use of current diagnostic tests for bovine tuberculosis in cattle. Here, we characterized immune responses in calves to vaccines containing four truncated MAP antigens as a fusion (Ag85A^202-347-SOD^1-72-Ag85B^173-330-74F^{1-148+669-786}), either displayed on protein particles, or expressed as a soluble recombinant MAP (rMAP) fusion protein as well as to commercially available Silirum^® vaccine. The rMAP fusion protein elicited the strongest antigen-specific antibody responses to both PPDA and recombinant antigen and strong and long-lasting T-cell immune responses to these antigens, as indicated by increased production of IFN-γ and IL-17A in antigen-stimulated whole blood cultures. The MAP fusion protein particle vaccine induced minimal antibody responses and weak IFN-γ responses but stimulated IL-17A responses to recombinant antigen. The immune response profile of Silirum^® vaccine was characterized by weak antibodies and strong IFN-γ and IL-17A responses to PPDA. Transcription analysis on antigen-stimulated leukocytes from cattle vaccinated with rMAP fusion protein showed differential expression of several immune response genes and genes involved in costimulatory signaling, TLR4, TLR2, PTX3, PTGS2, PD-L1, IL1B, IL2, IL6, IL12B, IL17A, IL22, IFNG, CD40, and CD86. Moreover, the expression of several genes of immune pathways correlated with cellular immune responses in the rMAP fusion protein vaccinated group. These genes have key roles in pathways of mycobacterial immunity, including autophagy, manipulation of macrophage-mediated killing, Th17- and regulatory T cells- (Treg) mediated responses. Calves vaccinated with either the rMAP fusion protein or MAP fusion protein particle vaccine did not induce reactivity to PPDA and PPDB in a comparative cervical skin test, whereas Silirum^® induced reactivity to these tuberculins in most of the vaccinated animals. Overall, our results suggest that a combination of recombinant MAP antigens in the form of a soluble fusion protein vaccine are capable of inducing strong antigen-specific humoral and a balanced Th1/Th17-cell immune response. These findings, together with the absence of reactivity to tuberculin, suggest this subunit vaccine could provide protective immunity against intracellular MAP infection in cattle without compromising the use of current bovine tuberculosis surveillance test.

Review: Unravelling the Role of DNA Sensing in Alum Adjuvant Activity

Public interest in vaccines is at an all-time high following the SARS-CoV-2 global pandemic. Currently, over 6 billion doses of various vaccines are administered globally each year. Most of these vaccines contain Aluminium-based adjuvants (alum), which have been known and used for almost 100 years to enhance vaccine immunogenicity. However, despite the historical use and importance of alum, we still do not have a complete understanding of how alum works to drive vaccine immunogenicity. In this article, we critically review studies investigating the mechanisms of action of alum adjuvants, highlighting some of the misconceptions and controversies within the area. Although we have emerged with a clearer understanding of how this ubiquitous adjuvant works, we have also highlighted some of the outstanding questions in the field. While these may seem mainly of academic interest, developing a more complete understanding of these mechanisms has the potential to rationally modify and improve the immune response generated by alum-adjuvanted vaccines.

Photoacoustic mediated multifunctional tumor antigen trapping nanoparticles inhibit the recurrence and metastasis of ovarian cancer by enhancing tumor immunogenicity

The hypoimmunogenicity of tumors is one of the main bottlenecks of cancer immunotherapy. Enhancing tumor immunogenicity can improve the efficacy of tumor immunotherapy by increasing antigen exposure and presentation, and establishing an inflammatory microenvironment. Here, a multifunctional antigen trapping nanoparticle with indocyanine green (ICG), aluminum hydroxide (Al(OH)3) and oxaliplatin (OXA) (PPIAO) has been developed for tumor photoacoustic/ultrasound dual-modality imaging and therapy. The combination of photothermal/photodynamic therapy and chemotherapy induced tumor antigen exposure and release through immunogenic death of tumor cells. A timely capture and storage of antigens by aluminum hydroxide enabled dendritic cells to recognize and present those antigens spatiotemporally. In an ovarian tumor model, the photoacoustic-mediated PPIAO NPs combination therapy achieved a transition from "cold tumor" to "hot tumor" that promoted more CD8+ T lymphocytes activation in vivo and intratumoral infiltration, and successfully inhibited the growth of primary and metastatic tumors. An in situ tumor vaccine effect was produced from the treated tumor tissue, assisting mice against the recurrence of tumor cells. This study provided a simple and effective personalized tumor vaccine strategy for better treatment of metastatic and recurrent tumors. The developed multifunctional tumor antigen trapping nanoparticles may be a promising nanoplatform for integrating multimodal imaging monitoring, tumor treatment, and tumor vaccine immunotherapy.

Improved Aluminum Adjuvants Eliciting Stronger Immune Response When Mixed with Hepatitis B Virus Surface Antigens

Adjuvants can regulate the immune response triggered by vaccines. Traditional aluminum adjuvants can induce humoral immunity, but they lack the ability to effectively induce Th1 cellular immunity, which is not conducive to the development of vaccines with improved protective effects. Aluminum adjuvants from different sources may have different physicochemical properties, and therefore, completely different immune responses can be triggered. This suggests that adjuvant recognition by the immune system and its responses are closely associated with the physicochemical properties of the adjuvant itself. To test this hypothesis, in this study, we developed a new method for preparing an aluminum adjuvant. This aluminum adjuvant has a pseudoboehmite structure, strong protein adsorption capacity, and excellent suspension stability. The adjuvant was tested using the hepatitis B virus surface antigen (HBsAg) as a model antigen for immunization; the results showed that this aluminum adjuvant effectively induced not only humoral immunity but also an outstanding cellular immune response. These results provide a reference for improving the efficacy of adjuvants.

Neutrophils in secondary lymphoid organs

Neutrophils are traditionally considered short‐lived, circulating innate immune cells that are rapidly recruited to sites of inflammation in response to infectious and inflammatory stimuli. Neutrophils efficiently internalize, kill or entrap pathogens, but their effector molecules may cause collateral tissue damage. More recently, it has been appreciated that neutrophils can also influence adaptive immunity. Lymph nodes (LNs) are immune cell‐rich secondary lymphoid organs that provide an ideal platform for cellular interaction and the integration of immunological information collected from local tissues. A variety of peripheral stimuli promote neutrophil migration to draining LNs via blood or lymphatics, utilizing differing molecular cues depending on the site of entry. Within LNs, neutrophils interact with other innate and adaptive cells. Crosstalk with subcapsular sinus macrophages contributes to the control of pathogen spread beyond the LN. Neutrophils can influence antigen presentation indirectly by interacting with DCs or directly by expressing major histocompatibility complex (MHC) and costimulatory molecules for antigen presentation. Interactions between neutrophils and adaptive lymphocytes can alter B‐cell antibody responses. Studies have shown conflicting results on whether neutrophils exert stimulatory or inhibitory effects on other LN immune cells, with stimulus‐specific and temporal differences in the outcome of these interactions. Furthermore, neutrophils have also been shown to traffick to LNs in homeostasis, with a potential role in immune surveillance, antigen capture and in shaping early adaptive responses in LNs. Understanding the mechanisms underpinning the effects of neutrophils on LN immune cells and adaptive immunity could facilitate the development of neutrophil‐targeted therapies in inflammatory diseases.

Staphylococcus aureus Vaccine Research and Development: The Past, Present and Future, Including Novel Therapeutic Strategies

Staphylococcus aureus is one of the most important human pathogens worldwide. Its high antibiotic resistance profile reinforces the need for new interventions like vaccines in addition to new antibiotics. Vaccine development efforts against S. aureus have failed so far however, the findings from these human clinical and non-clinical studies provide potential insight for such failures. Currently, research is focusing on identifying novel vaccine formulations able to elicit potent humoral and cellular immune responses. Translational science studies are attempting to discover correlates of protection using animal models as well as in vitro and ex vivo models assessing efficacy of vaccine candidates. Several new vaccine candidates are being tested in human clinical trials in a variety of target populations. In addition to vaccines, bacteriophages, monoclonal antibodies, centyrins and new classes of antibiotics are being developed. Some of these have been tested in humans with encouraging results. The complexity of the diseases and the range of the target populations affected by this pathogen will require a multipronged approach using different interventions, which will be discussed in this review.

Adsorption of protein antigen to the cationic liposome adjuvant CAF®01 is required for induction of Th1 and Th17 responses but not for antibody induction

The degree of antigen adsorption to adjuvants in subunit vaccines may significantly influence the immune responses they induce upon vaccination. Commonly used approaches for studying how the level of adsorption affects the induction of antigen-specific immune responses include (i) using adjuvants with different abilities to adsorb antigens, and (ii) comparing different antigens selected based on their ability to adsorb to the adjuvant. A weakness of these approaches is that not only the antigen adsorption level is varied, but also other important functional factors such as adjuvant composition and/or the B/T cell epitopes, which may affect immunogenicity. Hence, we investigated how changing the adsorption capabilities of a single antigen to an adjuvant influenced the vaccine-induced immune responses. The model antigen lysozyme, which displays a positive net charge at physiological pH due to an isoelectric point (pI) of 11, was succinylated to different extents, resulting in a reduction of the pI value to 4.4–5.9, depending on the degree of succinylation. A pronounced inverse correlation was found between the pI value of the succinylated lysozyme analogues and the degree of adsorption to a cationic liposomal adjuvant consisting of dimethyldioctadecylammonium bromide (DDA) and trehalose dibehenate (TDB) (CAF®01). Furthermore, increased adsorption to this adjuvant correlated directly with the magnitude of lysozyme-specific Th1/Th17 immune responses induced by the vaccine in mice, while there was an inverse correlation with antibody induction. However, high lysozyme-specific antibody titers were induced with an increased antigen dose, even upon vaccination with a strongly adsorbed succinylated lysozyme analogue. Hence, these data illustrate that the degree of lysozyme adsorption to CAF®01 strongly affects the quality of the resulting immune responses.

Injectable Hydrogels for Sustained Codelivery of Subunit Vaccines Enhance Humoral Immunity

Vaccines aim to elicit a robust, yet targeted, immune response. Failure of a vaccine to elicit such a response arises in part from inappropriate temporal control over antigen and adjuvant presentation to the immune system. In this work, we sought to exploit the immune system's natural response to extended pathogen exposure during infection by designing an easily administered slow-delivery vaccine platform. We utilized an injectable and self-healing polymer-nanoparticle (PNP) hydrogel platform to prolong the codelivery of vaccine components to the immune system. We demonstrated that these hydrogels exhibit unique delivery characteristics, whereby physicochemically distinct compounds (such as antigen and adjuvant) could be codelivered over the course of weeks. When administered in mice, hydrogel-based sustained vaccine exposure enhanced the magnitude, duration, and quality of the humoral immune response compared to standard PBS bolus administration of the same model vaccine. We report that the creation of a local inflammatory niche within the hydrogel, coupled with sustained exposure of vaccine cargo, enhanced the magnitude and duration of germinal center responses in the lymph nodes. This strengthened germinal center response promoted greater antibody affinity maturation, resulting in a more than 1000-fold increase in antigen-specific antibody affinity in comparison to bolus immunization. In summary, this work introduces a simple and effective vaccine delivery platform that increases the potency and durability of subunit vaccines.

Technological approaches to streamline vaccination schedules, progressing towards single-dose vaccines

Vaccines represent the most successful medical intervention in history, with billions of lives saved. Although multiple doses of the same vaccine are typically required to reach an adequate level of protection, it would be advantageous to develop vaccines that induce protective immunity with fewer doses, ideally just one. Single-dose vaccines would be ideal to maximize vaccination coverage, help stakeholders to greatly reduce the costs associated with vaccination, and improve patient convenience. Here we describe past attempts to develop potent single dose vaccines and explore the reasons they failed. Then, we review key immunological mechanisms of the vaccine-specific immune responses, and how innovative technologies and approaches are guiding the preclinical and clinical development of potent single-dose vaccines. By modulating the spatio-temporal delivery of the vaccine components, by providing the appropriate stimuli to the innate immunity, and by designing better antigens, the new technologies and approaches leverage our current knowledge of the immune system and may synergize to enable the rational design of next-generation vaccination strategies. This review provides a rational perspective on the possible development of future single-dose vaccines.

Controlling timing and location in vaccines

Vaccines are one of the most powerful technologies supporting public health. The adaptive immune response induced by immunization arises following appropriate activation and differentiation of T and B cells in lymph nodes. Among many parameters impacting the resulting immune response, the presence of antigen and inflammatory cues for an appropriate temporal duration within the lymph nodes, and further within appropriate subcompartments of the lymph nodes- the right timing and location- play a critical role in shaping cellular and humoral immunity. Here we review recent advances in our understanding of how vaccine kinetics and biodistribution impact adaptive immunity, and the underlying immunological mechanisms that govern these responses. We discuss emerging approaches to engineer these properties for future vaccines, with a focus on subunit vaccines.

Metalloimmunology: The metal ion-controlled immunity

Metals are essential components in all forms of life required for the function of nearly half of all enzymes and are critically involved in virtually all fundamental biological processes. Especially, the transition metals iron (Fe), zinc (Zn), manganese (Mn), nickel (Ni), copper (Cu) and cobalt (Co) are crucial micronutrients known to play vital roles in metabolism as well due to their unique redox properties. Metals carry out three major functions within metalloproteins: to provide structural support, to serve as enzymatic cofactors, and to mediate electron transportation. Metal ions are also involved in the immune system from metal allergies to nutritional immunity. Within the past decade, much attention has been drawn to the roles of metal ions in the immune system, since increasing evidence has mounted to suggest that metals are critically implicated in regulating both the innate immune sensing of and the host defense against invading pathogens. The importance of ions in immunity is also evidenced by the identification of various immunodeficiencies in patients with mutations in ion channels and transporters. In addition, cancer immunotherapy has recently been conclusively demonstrated to be effective and important for future tumor treatment, although only a small percentage of cancer patients respond to immunotherapy because of inadequate immune activation. Importantly, metal ion-activated immunotherapy is becoming an effective and potential way in tumor therapy for better clinical application. Nevertheless, we are still in a primary stage of discovering the diverse immunological functions of ions and mechanistically understanding the roles of these ions in immune regulation. This review summarizes recent advances in the understanding of metal-controlled immunity. Particular emphasis is put on the mechanisms of innate immune stimulation and T cell activation by the essential metal ions like calcium (Ca²⁺), zinc (Zn²⁺), manganese (Mn²⁺), iron (Fe²⁺/Fe³⁺), and potassium (K⁺), followed by a few unessential metals, in order to draw a general diagram of metalloimmunology.

Optimizing the utilization of aluminum adjuvants in vaccines: you might just get what you want

Aluminum-containing adjuvants have been used for over 90 years to enhance the immune response to vaccines. Recent work has significantly advanced our understanding of the physical, chemical, and biological properties of these adjuvants, offering key insights on underlying mechanisms. Given the long-term success of aluminum adjuvants, we believe that they should continue to represent the “gold standard” against which all new adjuvants should be compared. New vaccine candidates that require adjuvants to induce a protective immune responses should first be evaluated with aluminum adjuvants before other more experimental approaches are considered, since use of established adjuvants would facilitate both clinical development and the regulatory pathway. However, the continued use of aluminum adjuvants requires an appreciation of their complexities, in combination with access to the necessary expertise to optimize vaccine formulations. In this article, we will review the properties of aluminum adjuvants and highlight those elements that are critical to optimize vaccine performance. We will discuss how other components (excipients, TLR ligands, etc.) can affect the interaction between adjuvants and antigens, and impact the potency of vaccines. This review provides a resource and guide, which will ultimately contribute to the successful development of newer, more effective and safer vaccines.

Clay Nanoparticles Elicit Long-Term Immune Responses by Forming Biodegradable Depots for Sustained Antigen Stimulation

Nanomaterials have been widely tested as new generation vaccine adjuvants, but few evoke efficient immunoreactions. Clay nanoparticles, for example, layered double hydroxide (LDH) and hectorite (HEC) nanoparticles, have shown their potent adjuvanticity in generating effective and durable immune responses. However, the mechanism by which clay nanoadjuvants stimulate the immune system is not well understood. Here, it is demonstrated that LDH and HEC-antigen complexes form loose agglomerates in culture medium/serum. They also form nodules with loose structures in tissue after subcutaneous injection, where they act as a depot for up to 35 d. More importantly, clay nanoparticles actively and continuously recruit immune cells into the depot for up to one month, and stimulate stronger immune responses than FDA-approved adjuvants, Alum and QuilA. Sustained antigen release is also observed in clay nanoparticle depots, with 50-60% antigen released after 35 d. In contrast, Alum-antigen complexes show minimal antigen release from the depot. Importantly, LDH and HEC are more effective than QuilA and Alum in promoting memory T-cell proliferation. These findings suggest that both clay nanoadjuvants can serve as active vaccine platforms for sustained and potent immune responses.

Lymphatic cannulation models in sheep: Recent advances for immunological and biomedical research

Lymphatic cannulation models are useful tools for studying the immunobiology of the lymphatic system and the immunopathology of specific tissues in diseases. Sheep cannulations have been used extensively, as models for human physiology, fetal and neonatal development, human diseases, and for studies of ruminant pathobiology. The development of new and improved cannulation techniques in recent years has meant that difficult to access sites, such as mucosal associated tissues, are now more readily available to researchers. This review highlights the new approaches to cannulation and how these, in combination with advanced omics technologies, will direct future research using the sheep model.

Addition of 20-kDa PEG to Insulin Lispro Alters Absorption and Decreases Clearance in Animals

PURPOSE

Determine the pharmacokinetics of insulin peglispro (BIL) in 5/6-nephrectomized rats and study the absorption in lymph duct cannulated (LDC) sheep.

METHODS

BIL is insulin lispro modified with 20-kDa linear PEG at lysine B28 increasing the hydrodynamic size to 4-fold larger than insulin lispro. Pharmacokinetics of BIL and insulin lispro after IV administration were compared in 5/6-nephrectomized and sham rats. BIL was administered IV or SC into the interdigital space of the hind leg, and peripheral lymph and/or serum samples were collected from both LDC and non-LDC sheep to determine pharmacokinetics and absorption route of BIL.

RESULTS

The clearance of BIL was similar in 5/6-nephrectomized and sham rats, while the clearance of insulin lispro was 3.3-fold slower in 5/6-nephrectomized rats than in the sham rats. In non-LDC sheep, the terminal half-life after SC was about twice as long vs IV suggesting flip-flop pharmacokinetics. In LDC sheep, bioavailability decreased to <2%; most of the dose was absorbed via the lymphatic system, with 88% ± 19% of the dose collected in the lymph after SC administration.

CONCLUSION

This work demonstrates that increasing the hydrodynamic size of insulin lispro through PEGylation can impact both absorption and clearance to prolong drug action.

Transient Migration of Large Numbers of CD14(++) CD16(+) Monocytes to the Draining Lymph Node after Onset of Inflammation

The dynamics of skin-draining cells following infection or vaccination provide important insight into the initiation of immune responses. In this study, the local recruitment and activation of immune cells in draining lymph nodes (LNs) was studied in calves in an adjuvant-induced inflammation. A transient but remarkably strong recruitment of monocytes was demonstrated after onset of inflammation, constituting up to 41% of live cells in the draining LNs after 24 h. Numerous CD14(+) cells were visualized in subcutaneous tissues and draining LNs, and the majority of these cells did not express dendritic cell-associated markers CD205 and CD11c. In the LNs, recruited cells were predominately of a CD14(++) and CD16(+) phenotype, consistent with an intermediate monocyte subset characterized to possess a high inflammatory potential. Moreover, monocytes from the draining LN showed a high expression of genes coding for pro-inflammatory cytokines, including IL-1β, IL-6, TNFa, and TGFβ. Shortly after their appearance in the LN cortical areas, the monocytes had moved into the medulla followed by an increase in peripheral blood. In conclusion, this study provides novel information on in vivo monocyte recruitment and migration after onset of inflammation.

Preclinical safety study of a recombinant Streptococcus pyogenes vaccine formulated with aluminum adjuvant

A recombinant vaccine composed of a fusion protein formulated with aluminum hydroxide adjuvant is under development for protection against diseases caused by Streptococcus pyogenes. The safety and local reactogenicity of the vaccine was assessed by a comprehensive series of clinical, pathologic and immunologic tests in preclinical experiments. Outbred mice received three intramuscular injections of 1/5th of the human dose (0.1 ml) and rabbits received two injections of the full human dose. Control groups received adjuvant or protein antigen. The vaccine did not cause clinical evidence of systemic toxicity in mice or rabbits. There was a transient increase of peripheral blood neutrophils after the third vaccination of mice. In addition, the concentration of acute phase proteins serum amyloid A and haptoglobin was significantly increased 1 day after injection of the vaccine in mice. There was mild transient swelling and erythema of the injection site in both mice and rabbits. Treatment-related pathology was limited to inflammation at the injection site and accumulation of adjuvant-containing macrophages in the draining lymph nodes. In conclusion, the absence of clinical toxicity in two animal species suggest that the vaccine is safe for use in a phase I human clinical trial. Copyright © 2016 John Wiley & Sons, Ltd.

Beyond antigens and adjuvants: formulating future vaccines

The need to optimize vaccine potency while minimizing toxicity in healthy recipients has motivated studies of the formulation of vaccines to control how, when, and where antigens and adjuvants encounter immune cells and other cells/tissues following administration. An effective subunit vaccine must traffic to lymph nodes (LNs), activate both the innate and adaptive arms of the immune system, and persist for a sufficient time to promote a mature immune response. Here, we review approaches to tailor these three aspects of vaccine function through optimized formulations. Traditional vaccine adjuvants activate innate immune cells, promote cell-mediated transport of antigen to lymphoid tissues, and promote antigen retention in LNs. Recent studies using nanoparticles and other lymphatic-targeting strategies suggest that direct targeting of antigens and adjuvant compounds to LNs can also enhance vaccine potency without sacrificing safety. The use of formulations to regulate biodistribution and promote antigen and inflammatory cue co-uptake in immune cells may be important for next-generation molecular adjuvants. Finally, strategies to program vaccine kinetics through novel formulation and delivery strategies provide another means to enhance immune responses independent of the choice of adjuvant. These technologies offer the prospect of enhanced efficacy while maintaining high safety profiles necessary for successful vaccines.

Camellia sinensis Mediated Enhancement of Humoral Immunity to Particulate and Non-particulate Antigens

The most common drinking beverage in large portion of the world is Camellia sinensis (green tea). In the present study, we evaluated the adjuvant effect of green tea and tea polyphenols to particulate and non-particulate antigens. BALB/c mice were immunized with particulate and non-particulate antigens. Modulation of immunoglobulin-secreting splenocytes, IgG-mediated and IgM-mediated immunity, was evaluated by hemolytic plaque assay and enzyme-linked immunosorbent assay, respectively. Dose-dependent response of tea polyphenols was also assayed. Phenolic content was measured in crude preparations of green tea. We observed a stimulatory effect of green tea preparations on humoral immune response mediated by the increased number of antibody-secreted cells in spleen. A significant increase in IgM-mediated and IgG-mediated immune response to non-particulate antigen was also observed in green tea-treated animals. A dose-dependent adjuvant effect was seen in the case of tea polyphenols for a longer period of time compared with crude tea preparations. This study indicates polyphenols as major constituents responsible for the enhanced and sustained adjuvant activity of green tea. We suggest that tea polyphenols might be considered for real-life evaluation during adjuvant-mediated vaccination trial programs.

Microbe-dependent lymphatic migration of neutrophils modulates lymphocyte proliferation in lymph nodes

Neutrophil recruitment to the site of injury is an essential first step of an anti-bacterial response. However, little is known about the basis for and relevance of neutrophil migration from inflamed tissue into lymphoid organs. We established a photoconversion-based system to monitor the fate of neutrophils recruited to inflamed skin. While neutrophils are efficiently recruited to sites of both microbial and sterile lesions, subsequent re-localization to draining lymph nodes happens only when bacteria are present in the primary lesion. Skin egress of neutrophils occurs via lymphatic vessels and is dependent on CD11b and CXCR4 but not CCR7. Neutrophils are the predominant immune cell to migrate from inflamed skin into lymph nodes where they augment lymphocyte proliferation. Furthermore, inhibition of neutrophil migration from skin reduces T-cell proliferation in draining lymph nodes. Thus neutrophils mediate rapid cellular communication between the initial injury site and secondary lymphoid organs and modulate immune responsiveness.

Both sterile and microbial injuries lead to rapid neutrophil recruitment to the site of inflammation. Here the authors show that only neutrophils responding to microbial ligands leave inflamed skin and migrate to draining lymph nodes where they stimulate lymphocyte proliferation.

The mechanisms of action of vaccines containing aluminum adjuvants: an in vitro vs in vivo paradigm

Adjuvants such as the aluminum compounds (alum) have been dominantly used in many vaccines due to their immunopotentiation and safety records since 1920s. However, how these mineral agents influence the immune response to vaccination remains elusive. Many hypotheses exist as to the mode of action of these adjuvants, such as depot formation, antigen (Ag) targeting, and the induction of inflammation. These hypotheses are based on many in vitro and few in vivo studies. Understanding how cells interact with adjuvants in vivo will be crucial to fully understanding the mechanisms of action of these adjuvants. Interestingly, how alum influences the target cell at both the cellular and molecular level, and the consequent innate and adaptive responses, will be critical in the rational design of effective vaccines against many diseases. Thus, in this review, mechanisms of action of alum have been discussed based on available in vitro vs in vivo evidences to date.

Transcriptional profile in afferent lymph cells following vaccination with liposomes incorporating CpG

Vaccine formulations incorporating innate immune stimulants are highly immunogenic; however, the biological signals that originate in the peripheral tissues at the site of injection and are transmitted to the local lymph node to induce immunity remain unclear. By directly cannulating the ovine afferent lymphatic vessels, we have previously shown that it takes 72 hr for mature antigen-loaded dendritic cells and monocytes to appear within afferent lymph following injection of a liposomal formulation containing the Toll-like receptor ligand CpG. In this present study, we characterize the global transcriptional signatures at this time-point in ovine afferent lymph cells as they migrate from the injection site into the lymphatics following vaccination with a liposome antigen formulation incorporating CpG. We show that at 72 hr post vaccination, liposomes alone induce no changes in gene expression and inflammatory profiles within afferent lymph; however, the incorporation of CpG drives interferon, antiviral and cytotoxic gene programmes. This study also measures the expression of key genes within individual cell types in afferent lymph. Antiviral gene signatures are most prominent in lymphocytes, which may play a significant and unexpected role in sustaining the immune response to vaccination at the site of injection. These findings provide a comprehensive analysis of the in vivo immunological pathways that connect the injection site with the local draining lymph node following vaccination.

The innate response to peanut extract in ovine afferent lymph and its correlation with allergen sensitisation

The innate response generated after initial allergen exposure is crucial for polarising adaptive immunity, but little is known about how it drives an atopic or type-2 immune response. The present study characterises the response of skin-draining afferent lymph in sheep following injection with peanut (PN) extract in the presence or absence of aluminium hydroxide (AlOH) adjuvant. Lymph was collected and innate cell populations characterised over an 84 h time period. The innate response to PN extract in afferent lymph displayed an early increase in neutrophils and monocytes without any changes in the dendritic cell (DC) population. PN antigen was transported by neutrophils and monocytes for the first 36 h, after which time DCs were the major antigen trafficking cells. AlOH adjuvant gradually increased antigen uptake by DCs at the later time points. Following lymphatic characterisation, sheep were sensitised with PN extract by three subcutaneous injections of PN in AlOH, and the level of PN-specific immunoglobulin E (IgE) was determined. Sheep with higher levels of steady-state DCs in afferent lymph showed increased monocytic recruitment in afferent lymph and reduced PN-specific IgE following sensitisation. In addition, DCs from afferent lymph that had ingested PN antigen increased the expression of monocyte chemoattractant mRNA. The results of this study show that the innate response to PN extract involves a dynamic change in cell populations in the afferent lymph over time. In addition, DCs may determine the strength of the initial inflammatory cell response, which in turn may determine the nature of the antigen-specific adaptive response.

Vaccination with liposomal poly(I:C) induces discordant maturation of migratory dendritic cell subsets and anti-viral gene signatures in afferent lymph cells

Vaccine formulations administered in the periphery must activate naive immune cells within the lymph node. In this study, we have directly cannulated the ovine lymphatic vessels to investigate the cellular and molecular mechanisms that transfer information from the periphery into the local draining lymph node via the afferent lymph. Inclusion of poly(I:C) into a liposomal vaccine formulation enhances the neutrophil-associated inflammatory immune response in afferent lymph and increases antigen uptake by migratory dendritic cells (DCs). Interestingly, antigen positive migratory DCs undergo discordant maturation, with peak expression of CD86 at 4 h and CD80 at 48-72 h post vaccination. Afferent lymph monocytes up-regulate expression of genes related to inflammatory and anti-viral immune phenotypes following vaccination however show no differentiation into APCs prior to their migration to the local lymph node as measured by surface MHC II expression. Finally, this study reveals the addition of poly(I:C) increases systemic antigen-specific humoral immunity. These findings provide a detailed understanding of the real time in vivo immune response induced by liposomes incorporating the innate immune agonist poly(I:C) utilising a vaccination setting comparable to that administered in humans.

Systematic review of potential health risks posed by pharmaceutical, occupational and consumer exposures to metallic and nanoscale aluminum, aluminum oxides, aluminum hydroxide and its soluble salts

Abstract Aluminum (Al) is a ubiquitous substance encountered both naturally (as the third most abundant element) and intentionally (used in water, foods, pharmaceuticals, and vaccines); it is also present in ambient and occupational airborne particulates. Existing data underscore the importance of Al physical and chemical forms in relation to its uptake, accumulation, and systemic bioavailability. The present review represents a systematic examination of the peer-reviewed literature on the adverse health effects of Al materials published since a previous critical evaluation compiled by Krewski et al. (2007) . Challenges encountered in carrying out the present review reflected the experimental use of different physical and chemical Al forms, different routes of administration, and different target organs in relation to the magnitude, frequency, and duration of exposure. Wide variations in diet can result in Al intakes that are often higher than the World Health Organization provisional tolerable weekly intake (PTWI), which is based on studies with Al citrate. Comparing daily dietary Al exposures on the basis of "total Al"assumes that gastrointestinal bioavailability for all dietary Al forms is equivalent to that for Al citrate, an approach that requires validation. Current occupational exposure limits (OELs) for identical Al substances vary as much as 15-fold. The toxicity of different Al forms depends in large measure on their physical behavior and relative solubility in water. The toxicity of soluble Al forms depends upon the delivered dose of Al(+3) to target tissues. Trivalent Al reacts with water to produce bidentate superoxide coordination spheres [Al(O2)(H2O4)(+2) and Al(H2O)6 (+3)] that after complexation with O2(•-), generate Al superoxides [Al(O2(•))](H2O5)](+2). Semireduced AlO2(•) radicals deplete mitochondrial Fe and promote generation of H2O2, O2 (•-) and OH(•). Thus, it is the Al(+3)-induced formation of oxygen radicals that accounts for the oxidative damage that leads to intrinsic apoptosis. In contrast, the toxicity of the insoluble Al oxides depends primarily on their behavior as particulates. Aluminum has been held responsible for human morbidity and mortality, but there is no consistent and convincing evidence to associate the Al found in food and drinking water at the doses and chemical forms presently consumed by people living in North America and Western Europe with increased risk for Alzheimer's disease (AD). Neither is there clear evidence to show use of Al-containing underarm antiperspirants or cosmetics increases the risk of AD or breast cancer. Metallic Al, its oxides, and common Al salts have not been shown to be either genotoxic or carcinogenic. Aluminum exposures during neonatal and pediatric parenteral nutrition (PN) can impair bone mineralization and delay neurological development. Adverse effects to vaccines with Al adjuvants have occurred; however, recent controlled trials found that the immunologic response to certain vaccines with Al adjuvants was no greater, and in some cases less than, that after identical vaccination without Al adjuvants. The scientific literature on the adverse health effects of Al is extensive. Health risk assessments for Al must take into account individual co-factors (e.g., age, renal function, diet, gastric pH). Conclusions from the current review point to the need for refinement of the PTWI, reduction of Al contamination in PN solutions, justification for routine addition of Al to vaccines, and harmonization of OELs for Al substances.

Incorporation of CpG into a liposomal vaccine formulation increases the maturation of antigen-loaded dendritic cells and monocytes to improve local and systemic immunity

Liposomal vaccine formulations incorporating stimulants that target innate immune receptors have been shown to significantly increase vaccine immunity. Following vaccination, innate cell populations respond to immune stimuli, phagocytose and process Ag, and migrate from the injection site, via the afferent lymphatic vessels, into the local lymph node. In this study, the signals received in the periphery promote and sculpt the adaptive immune response. Effector lymphocytes then leave the lymph node via the efferent lymphatic vessel to perform their systemic function. We have directly cannulated the ovine lymphatic vessels to detail the in vivo innate and adaptive immune responses occurring in the local draining lymphatic network following vaccination with a liposome-based delivery system incorporating CpG. We show that CpG induces the rapid recruitment of neutrophils, enhances dendritic cell-associated Ag transport, and influences the maturation of innate cells entering the afferent lymph. This translated into an extended period of lymph node shutdown, the induction of IFN-γ-positive T cells, and enhanced production of Ag-specific Abs. Taken together, the results of this study quantify the real-time in vivo kinetics of the immune response in a large animal model after vaccination of a dose comparable to that administered to humans. This study details enhancement of numerous immune mechanisms that provide an explanation for the immunogenic function of CpG when employed as an adjuvant within vaccines.

The effect of different adjuvants on immune parameters and protection following vaccination of sheep with a larval-specific antigen of the gastrointestinal nematode, Haemonchus contortus

It has recently been recognised that vaccine adjuvants play a critical role in directing the nature of a vaccine induced effector response. In the present study, several adjuvants were evaluated for their ability to protect sheep after field vaccination with the larval-specific Haemonchus contortus antigen, HcsL3. Using a suboptimal antigen dose, aluminium adjuvant was shown to reduce the cumulative faecal egg counts (cFEC) and worm burden by 23% and 25% respectively, in agreement with a previous study. The addition of Quil A to the aluminium-adjuvanted vaccine brought cFEC back to control levels. Vaccination with the adjuvant DEAE-dextran almost doubled the protection compared to the aluminium-adjuvanted vaccine resulting in 40% and 41% reduction in cFEC and worm counts compared to controls. Examination of skin responses following i.d. injection of exsheathed L3, revealed that cFEC was negatively correlated with wheal size and tissue eosinophils for the DEAE-dextran and aluminium-adjuvanted groups respectively. These studies have for the first time shown the potential of DEAE-dextran adjuvant for helminth vaccines, and discovered significant cellular correlates of vaccine-induced protection.

Innate immune pathways in afferent lymph following vaccination with poly(I:C)-containing liposomes

Many modern vaccines use defined adjuvants to stimulate the innate immune system and shape the adaptive immune response. The exact nature of these innate signals and whether immune differentiation can originate within the periphery is not known. Here we used an ovine lymphatic cannulation model to characterise the cellular and transcriptomic profile of the afferent lymph following injection of a liposomal vaccine formulation incorporating diphtheria toxoid and the innate stimulator poly(I:C) over a 78-h period. The response to this vaccine featured an early activation of broad pro-inflammatory pathways (e.g. TLR signalling and inflammasome pathways) and the transient recruitment of granulocytes into the lymph. At 24 h a more monocytic cellular profile arose coinciding with a transition to a specific antiviral response characterised by the up-regulation of genes associated with the receptors typical for the viral mimic, poly(I:C) (e.g. TLR3, RIG-I and MDA5). At the latest time points the up-regulation of IL-17A and IL-17F suggested that Th17 cells may participate in the earliest adaptive response to this vaccine. These data provide the most comprehensive picture of the cellular and molecular mechanisms that link the periphery to the draining lymph node following vaccination, and indicate that the immune response is capable of specialising within the periphery.

An approach to local immunotoxicity induced by adjuvanted vaccines

The occurrence of injection site reactions following immunization is the most frequently reported toxicity manifestation of vaccines; however, the different types of local reactions and the different mechanisms involved are still unclear. Here, the current advances in adjuvants and the role that adjuvants play in local reactions are reviewed. The role of adjuvants in the formation of the loco-regional complex (LRC), which consists of the injection site, draining lymphatic vessels and regional lymph nodes, is also discussed. Finally, strategies and recommendations for the rational design of adjuvanted vaccines are discussed, with a particular interest in the reduction of local inflammation.

Kinetics of the inflammatory response following intramuscular injection of aluminum adjuvant

Aluminum-containing adjuvants are widely used in human and veterinary vaccines, but their mechanism of action is not well understood. Recent evidence suggests an important role for inflammation in the immune response to aluminum-adjuvanted vaccines. To better understand this process, vaccines with aluminum adjuvant were injected into naïve or previously immunized mice and the injection sites were characterized for the corresponding primary and secondary inflammatory response at different time points after immunization. Inflammatory cells appeared at the injection site between 2h and 6h after vaccination, dominated by neutrophils at first, followed by macrophages, and later eosinophils and MHCII(+) cells. The number of cells at the injection site increased over time, except neutrophils, which decreased in number after day 2. There was extensive phagocytosis of aluminum adjuvant particles by macrophages. In secondary immunized mice, a faster and more robust recruitment of eosinophils, macrophages, and antigen presenting cells was observed at the injection site. The enhanced recruitment of inflammatory cells in previously immunized mice coincided with increased expression of relevant chemokines at the injection site. Since neutrophils accumulated first in response to aluminum-adjuvanted vaccines, their role was evaluated by depleting them prior to vaccination. Neutrophil depletion transiently reduced the recruitment of macrophages but it did not change the recruitment of eosinophils and MHCII(+) cells or the quality and magnitude of the antibody response.

Stronger T cell immunogenicity of ovalbumin expressed intracellularly in Gram-negative than in Gram-positive bacteria

This study aimed to clarify whether Gram-positive (G+) and Gram-negative (G−) bacteria affect antigen-presenting cells differently and thereby influence the immunogenicity of proteins they express. Lactobacilli, lactococci and Escherichia coli strains were transformed with plasmids conferring intracellular ovalbumin (OVA) production. Murine splenic antigen presenting cells (APCs) were pulsed with washed and UV-inactivated OVA-producing bacteria, control bacteria, or soluble OVA. The ability of the APCs to activate OVA-specific DO11.10 CD4⁺ T cells was assessed by measurments of T cell proliferation and cytokine (IFN-γ, IL-13, IL-17, IL-10) production. OVA expressed within E. coli was strongly immunogenic, since 500 times higher concentrations of soluble OVA were needed to achieve a similar level of OVA-specific T cell proliferation. Furthermore, T cells responding to soluble OVA produced mainly IL-13, while T cells responding to E. coli-expressed OVA produced high levels of both IFN-γ and IL-13. Compared to E. coli, G+ lactobacilli and lactococci were poor inducers of OVA-specific T cell proliferation and cytokine production, despite efficient intracellular expression and production of OVA and despite being efficiently phagocytosed. These results demonstrate a pronounced difference in immunogenicity of intracellular antigens in G+ and G− bacteria and may be relevant for the use of bacterial carriers in vaccine development.

Mechanism of immunopotentiation and safety of aluminum adjuvants

Aluminum-containing adjuvants are widely used in preventive vaccines against infectious diseases and in preparations for allergy immunotherapy. The mechanism by which they enhance the immune response remains poorly understood. Aluminum adjuvants selectively stimulate a Th2 immune response upon injection of mice and a mixed response in human beings. They support activation of CD8 T cells, but these cells do not undergo terminal differentiation to cytotoxic T cells. Adsorption of antigens to aluminum adjuvants enhances the immune response by facilitating phagocytosis and slowing the diffusion of antigens from the injection site which allows time for inflammatory cells to accumulate. The adsorptive strength is important as high affinity interactions interfere with the immune response. Adsorption can also affect the physical and chemical stability of antigens. Aluminum adjuvants activate dendritic cells via direct and indirect mechanisms. Phagocytosis of aluminum adjuvants followed by disruption of the phagolysosome activates NLRP3-inflammasomes resulting in the release of active IL-1β and IL-18. Aluminum adjuvants also activate dendritic cells by binding to membrane lipid rafts. Injection of aluminum-adjuvanted vaccines causes the release of uric acid, DNA, and ATP from damaged cells which in turn activate dendritic cells. The use of aluminum adjuvant is limited by weak stimulation of cell-mediated immunity. This can be enhanced by addition of other immunomodulatory molecules. Adsorption of these molecules is determined by the same mechanisms that control adsorption of antigens and can affect the efficacy of such combination adjuvants. The widespread use of aluminum adjuvants can be attributed in part to the excellent safety record based on a 70-year history of use. They cause local inflammation at the injection site, but also reduce the severity of systemic and local reactions by binding biologically active molecules in vaccines.

Cell recruitment and antigen trafficking in afferent lymph after injection of antigen and poly(I:C) containing liposomes, in aqueous or oil-based formulations

After vaccination, innate cell populations transport antigen from the tissue, via the afferent lymphatic vessels, into the local lymph node where they provide critical signals for the generation of an adaptive immune response. The present study uses a unique lymphatic cannulation model to examine, in real time, changes in afferent lymph after injection of a liposome-based delivery system, incorporating diptheria toxoid (DT) and the innate stimulator, poly(I:C). There was a dramatic but temporal recruitment of innate cell populations over time, with neutrophils and monocytes peaking at 6h and 28h post vaccination respectively. The number of dendritic cells (DC) did not increase over the 198h time period, while lymphocytes were slightly elevated at the latest times, indicating the start of an adaptive response. Monocytes and neutrophils were the predominant cell types transporting antigen at the early time points while DC were the most dominant antigen-carrying cells after 78h, predominantly the Sirp-α(high) DC subtype. Resuspending liposomes in oil instead of aqueous solutions has recently been shown to dramatically increase the level and persistence of an immune response and forms the basis of the novel adjuvant formulations, Vaccimax© and Depovax©. In the present study, formulation of the DT and poly(I:C) containing liposomes in an oil carrier dramatically reduced antigen transport to the draining lymph nodes. Examination of the injection site revealed the creation of an ectopic lymphoid tissue with prominent antigen foci and organized lymphoid cells, providing a possible mechanism for the persistence of an immune response in liposome-in-oil adjuvant formulation. Together, the present studies demonstrate the real-time innate in vivo response to vaccination of two novel liposome-based adjuvant systems and the dramatic effect of different carrier formulations.

The immunogenic characteristics associated with multivalent display of Vi polysaccharide antigen using biodegradable polymer particles

Polysaccharides in their great majority are thymus-independent (TI) antigens. Anti-polysaccharide antibody responses are generally weak and characterized by lack of memory, isotype restriction and delayed ontogeny. We report here the generation of protective memory antibody response by multivalent display of polysaccharide antigens on biodegradable polymeric particles. Single dose immunization using polylactide (PLA) polymer particles entrapping Vi capsular polysaccharide antigen from Salmonella typhi promoted isotype switching and induced polysaccharide-specific memory antibody response in experimental animals. PLA nanoparticles as well as microparticles entrapping Vi polysaccharides elicited high IgG titer in comparison to soluble Vi immunization. Immunizations with particles co-entrapping both Vi polysaccharide and tetanus toxoid did not improve the anti-polysaccharide antibody responses. Lower antibody response from co-entrapped formulation was mostly due to inhibition of particle phagocytosis by the macrophages. Immunization using polylactide particles entrapping only Vi polysaccharide with higher density on surface elicited highest secondary antibody response as well as promoted isotype switching. The vaccination potential of particle based immunizations was further confirmed by the generation of quick memory antibody responses while challenging the immunized animals with live S. typhi. This approach provides a multivalent display of polysaccharide antigen using polymer particles and elicits protective memory antibody response without conjugation to a carrier protein.

Capacities of migrating CD1b+ lymph dendritic cells to present Salmonella antigens to naive T cells

Dendritic cells (DCs) are well known as professional antigen-presenting cells (APC) able to initiate specific T-cell responses to pathogens in lymph nodes (LN) draining the site of infection. However, the respective contribution of migratory and LN-resident DCs in this process remains unclear. As DC subsets represent important targets for vaccination strategies, more precise knowledge of DC subsets able to present vaccine antigens to T cells efficiently is required. To investigate the capacities of DCs migrating in the lymph (L-DCs) to initiate a specific T-cell response, we used physiologically generated DCs collected from a pseudoafferent lymphatic cannulation model in sheep. The CD1b⁺ L-DCs were assessed for presenting antigens from the vaccine attenuated strain of Salmonella enterica serovar Abortusovis. CD1b⁺ L-DCs were able to phagocytose, process and to present efficiently Salmonella antigens to effector/memory T cells in vitro. They were shown to be efficient APC for the priming of allogeneic naive T cells associated with inducing both IFN-γ and IL-4 responses. They were also efficient in presenting Salmonella antigens to autologous naive T cells associated with inducing both IFN-γ and IL-10 responses. The capacities of L-DCs to process and present Salmonella antigens to T cells were investigated in vivo after conjunctival inoculation of Salmonella. The CD1b⁺ L-DCs collected after inoculation were able to induce the proliferative response of CD4⁺ T cells suggesting the in vivo capture of Salmonella antigens by the CD1b⁺ L-DCs, and their potential to present them directly to CD4⁺ T cells. In this study, CD1b⁺ L-DCs present potential characteristics of APC to initiate by themselves T cell priming in the LN. They could be used as target cells for driving immune activation in vaccinal strategies.

Exploiting mucosal surfaces for the development of mucosal vaccines

Mucosal immunity covers a variety of mucosal surfaces susceptible to different pathogens. This review highlights the diversity of mucosal tissues and the unique microenvironments in which an immune response is generated. It argues that tissue-specific factors present throughout mucosal tissues and lymph nodes determine the differentiation into IgA-producing B cells, which in turn determines their migration patterns. Mucosal immunity can therefore be induced when antigen is delivered at any mucosal tissue without the need for specific 'mucosal adjuvants' or targeting to specialised lymphoid structures. Non-oral vaccination strategies directed at alternative and more accessible mucosal tissue sites, may provide new avenues for both mucosal and systemic immunization, and will be greatly facilitated by the use of large animal models.