Mucosal HIV vaccines: A holy grail or a dud?

Engineering intravaginal vaccines to overcome mucosal and epithelial barriers

The mucosal surface of the vagina is a primary human immunodeficiency virus (HIV) entry portal, making it an attractive site for HIV vaccination. However, HIV vaccines based on recombinant adenovirus (rAd) do not efficiently cross the mucus layers or underlying epithelium of the vagina. Here we designed nanocomplexes of rAd particles coated with (1) the polyethylene glycol derivative APS to provide a hydrophilic surface that would prevent entrapment in the hydrophobic mucus, and (2) the cell-penetrating peptide TAT to improve transduction efficiency. The optimized rAd-TAT-APS nanocomplexes could achieve the balance of effective mucus-penetrating and cellular transduction. Intravaginal delivery of rAd-TAT-APS encoding HIVgag p24 into mice strongly enhanced HIVgag-specific systemic and mucosal immune responses. This rAd-TAT-APS system may allow effective vaginal delivery of vaccines against HIV and other infectious agents.

Mechanisms Underlying the Immune Response Generated by an Oral Vibrio cholerae Vaccine

Mechanistic details underlying the resulting protective immune response generated by mucosal vaccines remain largely unknown. We investigated the involvement of Toll-like receptor signaling in the induction of humoral immune responses following oral immunization with Dukoral, comparing wild type mice with TLR-2-, TLR-4-, MyD88- and Trif-deficient mice. Although all groups generated similar levels of IgG antibodies, the proliferation of CD4+ T-cells in response to V. cholerae was shown to be mediated via MyD88/TLR signaling, and independently of Trif signaling. The results demonstrate differential requirements for generation of immune responses. These results also suggest that TLR pathways may be modulators of the quality of immune response elicited by the Dukoral vaccine. Determining the critical signaling pathways involved in the induction of immune response to this vaccine would be beneficial, and could contribute to more precisely-designed versions of other oral vaccines in the future.

Toll-like receptor 5 is not essential for the promotion of secretory immunoglobulin A antibody responses to flagellated bacteria

Toll-like receptor 5 recognizes bacterial flagellin, plays a critical role in innate immunity, and contributes to flagellin-specific humoral immunity. Further, TLR5-expressing dendritic cells play an important role in IgA synthesis in the intestine; however, the contribution of TLR5 to antigen (Ag)-specific mucosal immunity remains unclear. Thus, whether TLR5 is essential for the induction of intestinal secretory (S)IgA antibody (Ab) responses against flagellin and bacterial Ags attached to the bacterial surface in response to an oral flagellated bacterium, Salmonella, was explored in this study. Our results indicate that when TLR5 knockout (TLR5(-/-)) mice are orally immunized with recombinant Salmonella expressing fragment C of tetanus toxin (rSalmonella-Tox C), tetanus toxoid (TT)- and flagellin (FliC)-specific systemic IgG and intestinal SIgA Abs are elicited. The numbers of TT-specific IgG Ab-forming cells (AFCs) in the spleen and IgA AFCs in the lamina propria (LP) of TLR5(-/-) mice were comparable to those in wild-type mice. rSalmonella-Tox C was equally disseminated in TLR5(-/-) mice, TLR5(-/-) mice lacking Peyer's patches (PPs), and wild-type mice. In contrast, TLR5(-/-) PP-null mice failed to induce TT- and FliC-specific SIgA Abs in the intestine and showed significantly reduced numbers of TT-specific IgA AFCs in the LP. These results suggest that TLR5 is dispensable for the induction of flagellin and surface Ag-specific systemic and mucosal immunity against oral flagellated bacteria. Rather, pathogen recognition, which occurs in PPs, is a prerequisite for the induction of mucosal immunity against flagellated bacteria.

Delivery strategies to enhance oral vaccination against enteric infections

While the majority of human pathogens infect the body through mucosal sites, most licensed vaccines are injectable. In fact the only mucosal vaccine that has been widely used globally for infant and childhood vaccination programs is the oral polio vaccine (OPV) developed by Albert Sabin in the 1950s. While oral vaccines against Cholera, rotavirus and Salmonella typhi have also been licensed, the development of additional non-living oral vaccines against these and other enteric pathogens has been slow and challenging. Mucosal vaccines can elicit protective immunity at the gut mucosa, in part via antigen-specific secretory immunoglobulin A (SIgA). However, despite their advantages over the injectable route, oral vaccines face many hurdles. A key challenge lies in design of delivery strategies that can protect antigens from degradation in the stomach and intestine, incorporate appropriate immune-stimulatory adjuvants and control release at the appropriate gastrointestinal site. A number of systems including micro and nanoparticles, lipid-based strategies and enteric capsules have significant potential either alone or in advanced combined formulations to enhance intestinal immune responses. In this review we will outline the opportunities, challenges and potential delivery solutions to facilitate the development of improved oral vaccines for infectious enteric diseases.

Advances and challenges in mucosal adjuvant technology

Adjuvants play attractive roles in enhancement of immune response during vaccination; however, due to several challenges, only a limited number of adjuvants are licensed by health authorities. The lack of an effective mucosal adjuvant is even more significant as none of the licensed adjuvants revealed a strong enhancement in immune system after mucosal administration. Over the past two decades, several mucosal adjuvants have been developed to deliver antigens to the target cells in the mucosal immune system and increase specific immune responses. However, the safety and efficacy of these adjuvants for testing in human trials is still an important issue, requiring further study. In this article, we briefly review the challenges associated with most common mucosal adjuvants and discuss potential strategies for targeting the mucosal immune system.

The influence of delivery vectors on HIV vaccine efficacy

Development of an effective HIV/AIDS vaccine remains a big challenge, largely due to the enormous HIV diversity which propels immune escape. Thus novel vaccine strategies are targeting multiple variants of conserved antibody and T cell epitopic regions which would incur a huge fitness cost to the virus in the event of mutational escape. Besides immunogen design, the delivery modality is critical for vaccine potency and efficacy, and should be carefully selected in order to not only maximize transgene expression, but to also enhance the immuno-stimulatory potential to activate innate and adaptive immune systems. To date, five HIV vaccine candidates have been evaluated for efficacy and protection from acquisition was only achieved in a small proportion of vaccinees in the RV144 study which used a canarypox vector for delivery. Conversely, in the STEP study (HVTN 502) where human adenovirus serotype 5 (Ad5) was used, strong immune responses were induced but vaccination was more associated with increased risk of HIV acquisition than protection in vaccinees with pre-existing Ad5 immunity. The possibility that pre-existing immunity to a highly promising delivery vector may alter the natural course of HIV to increase acquisition risk is quite worrisome and a huge setback for HIV vaccine development. Thus, HIV vaccine development efforts are now geared toward delivery platforms which attain superior immunogenicity while concurrently limiting potential catastrophic effects likely to arise from pre-existing immunity or vector-related immuno-modulation. However, it still remains unclear whether it is poor immunogenicity of HIV antigens or substandard immunological potency of the safer delivery vectors that has limited the success of HIV vaccines. This article discusses some of the promising delivery vectors to be harnessed for improved HIV vaccine efficacy.

Challenges in mucosal HIV vaccine development: lessons from non-human primate models

An efficacious HIV vaccine is urgently needed to curb the AIDS pandemic. The modest protection elicited in the phase III clinical vaccine trial in Thailand provided hope that this goal might be achieved. However, new approaches are necessary for further advances. As HIV is transmitted primarily across mucosal surfaces, development of immunity at these sites is critical, but few clinical vaccine trials have targeted these sites or assessed vaccine-elicited mucosal immune responses. Pre-clinical studies in non-human primate models have facilitated progress in mucosal vaccine development by evaluating candidate vaccine approaches, developing methodologies for collecting and assessing mucosal samples, and providing clues to immune correlates of protective immunity for further investigation. In this review we have focused on non-human primate studies which have provided important information for future design of vaccine strategies, targeting of mucosal inductive sites, and assessment of mucosal immunity. Knowledge gained in these studies will inform mucosal vaccine design and evaluation in human clinical trials.

Recent progress in HIV vaccines inducing mucosal immune responses

In spite of several attempts over many years at developing a HIV vaccine based on classical strategies, none has convincingly succeeded to date. As HIV is transmitted primarily by the mucosal route, particularly through sexual intercourse, understanding antiviral immunity at mucosal sites is of major importance. An ideal vaccine should elicit HIV-specific antibodies and mucosal CD8⁺ cytotoxic T-lymphocyte (CTL) as a first line of defense at a very early stage of HIV infection, before the virus can disseminate into the secondary lymphoid organs in mucosal and systemic tissues. A primary focus of HIV preventive vaccine research is therefore the induction of protective immune responses in these crucial early stages of HIV infection. Numerous approaches are being studied in the field, including building upon the recent RV144 clinical trial. In this article, we will review current strategies and briefly discuss the use of adjuvants in designing HIV vaccines that induce mucosal immune responses.

Induction of Potent and Long-Lived Antibody and Cellular Immune Responses in the Genitorectal Mucosa Could be the Critical Determinant of HIV Vaccine Efficacy

The field of HIV prevention has indeed progressed in leaps and bounds, but with major limitations of the current prevention and treatment options, the world remains desperate for an HIV vaccine. Sadly, this continues to be elusive, because more than 30 years since its discovery there is no licensed HIV vaccine. Research aiming to define immunological biomarkers to accurately predict vaccine efficacy have focused mainly on systemic immune responses, and as such, studies defining correlates of protection in the genitorectal mucosa, the primary target site for HIV entry and seeding are sparse. Clearly, difficulties in sampling and analysis of mucosal specimens, as well as their limited size have been a major deterrent in characterizing the type (mucosal antibodies, cytokines, chemokines, or CTL), threshold (magnitude, depth, and breadth) and viral inhibitory capacity of HIV-1-specific immune responses in the genitorectal mucosa, where they are needed to immediately block HIV acquisition and arrest subsequent virus dissemination. Nevertheless, a few studies document the existence of HIV-specific immune responses in the genitorectal mucosa of HIV-infected aviremic and viremic controllers, as well as in highly exposed persistently seronegative (HEPS) individuals with natural resistance to HIV-1. Some of these responses strongly correlate with protection from HIV acquisition and/or disease progression, thus providing significant clues of the ideal components of an efficacious HIV vaccine. In this study, we provide an overview of the key features of protective immune responses found in HEPS, elite and viremic controllers, and discuss how these can be achieved through mucosal immunization. Inevitably, HIV vaccine development research will have to consider strategies that elicit potent antibody and cellular immune responses within the genitorectal mucosa or induction of systemic immune cells with an inherent potential to home and persist at mucosal sites of HIV entry.

Mucosal application of gp140 encoding DNA polyplexes to different tissues results in altered immunological outcomes in mice

Increasing evidence suggests that mucosally targeted vaccines will enhance local humoral and cellular responses whilst still eliciting systemic immunity. We therefore investigated the capacity of nasal, sublingual or vaginal delivery of DNA-PEI polyplexes to prime immune responses prior to mucosal protein boost vaccination. Using a plasmid expressing the model antigen HIV CN54gp140 we show that each of these mucosal surfaces were permissive for DNA priming and production of antigen-specific antibody responses. The elicitation of systemic immune responses using nasally delivered polyplexed DNA followed by recombinant protein boost vaccination was equivalent to a systemic prime-boost regimen, but the mucosally applied modality had the advantage in that significant levels of antigen-specific IgA were detected in vaginal mucosal secretions. Moreover, mucosal vaccination elicited both local and systemic antigen-specific IgG(+) and IgA(+) antibody secreting cells. Finally, using an Influenza challenge model we found that a nasal or sublingual, but not vaginal, DNA prime/protein boost regimen protected against infectious challenge. These data demonstrate that mucosally applied plasmid DNA complexed to PEI followed by a mucosal protein boost generates sufficient antigen-specific humoral antibody production to protect from mucosal viral challenge.

Mucosal vaccine design and delivery

Mucosal surfaces are a major portal of entry for many human pathogens that are the cause of infectious diseases worldwide. Vaccines capable of eliciting mucosal immune responses can fortify defenses at mucosal front lines and protect against infection. However, most licensed vaccines are administered parenterally and fail to elicit protective mucosal immunity. Immunization by mucosal routes may be more effective at inducing protective immunity against mucosal pathogens at their sites of entry. Recent advances in our understanding of mucosal immunity and identification of correlates of protective immunity against specific mucosal pathogens have renewed interest in the development of mucosal vaccines. Efforts have focused on efficient delivery of vaccine antigens to mucosal sites that facilitate uptake by local antigen-presenting cells to generate protective mucosal immune responses. Discovery of safe and effective mucosal adjuvants are also being sought to enhance the magnitude and quality of the protective immune response.

New insights in mucosal vaccine development

Mucosal surfaces are the major entrance for infectious pathogens and therefore mucosal immune responses serve as a first line of defence. Most current immunization procedures are obtained by parenteral injection and only few vaccines are administered by mucosal route, because of its low efficiency. However, targeting of mucosal compartments to induce protective immunity at both mucosal sites and systemic level represents a great challenge. Major efforts are made to develop new mucosal candidate vaccines by selecting appropriate antigens with high immunogenicity, designing new mucosal routes of administration and selecting immune-stimulatory adjuvant molecules. The aim of mucosal vaccines is to induce broad potent protective immunity by specific neutralizing antibodies at mucosal surfaces and by induction of cellular immunity. Moreover, an efficient mucosal vaccine would make immunization procedures easier and be better suited for mass administration. This review focuses on contemporary developments of mucosal vaccination approaches using different routes of administration.

Human immunodeficiency virus (HIV) immunopathogenesis and vaccine development: a review

The development of a safe, effective and globally affordable HIV vaccine offers the best hope for the future control of the HIV-1 pandemic. Since 1987, scores of candidate HIV-1 vaccines have been developed which elicited varying degrees of protective responses in nonhuman primate models, including DNA vaccines, subunit vaccines, live vectored recombinant vaccines and various prime-boost combinations. Four of these candidate vaccines have been tested for efficacy in human volunteers, but, to the exception of the recent RV144 Phase III trial in Thailand, which elicited a modest but statistically significant level of protection against infection, none has shown efficacy in preventing HIV-1 infection or in controlling virus replication and delaying progression of disease in humans. Protection against infection was observed in the RV144 trial, but intensive research is needed to try to understand the protective immune mechanisms at stake. Building-up on the results of the RV144 trial and deciphering what possibly are the immune correlates of protection are the top research priorities of the moment, which will certainly accelerate the development of an highly effective vaccine that could be used in conjunction with other HIV prevention and treatment strategies. This article reviews the state of the art of HIV vaccine development and discusses the formidable scientific challenges met in this endeavor, in the context of a better understanding of the immunopathogenesis of the disease.

Methods for evaluation of humoral immune responses in human genital tract secretions

The compilation of epidemiological, virological, and immunological data clearly indicates that HIV-1 infection must be considered primarily as a disease of the mucosal immune system. The earliest and most dramatic alterations of the immune system occur in the mucosal compartment. However, the mucosal immune systems of the genital and intestinal tracts display remarkable immunological differences that must be considered in the evaluation of humoral immune responses in HIV-1-infected individuals or in volunteers immunized with experimental HIV vaccines. In this regard, marked differences in the dominant Ig isotypes, molecular forms of HIV-1-specific antibodies, and their distinct effector functions in the genital versus intestinal tracts must be carefully evaluated and considered in the measurement and interpretation of humoral immune responses. Appropriate controls and alternative immunochemical assays should be used to complement and confirm results generated by ELISA, which are prone to false positivity. Special precautions and rigorous controls must be used in the evaluation of antibody-mediated virus neutralization in external secretions of the genital and intestinal tracts.

Immune-based approaches to the prevention of mother-to-child transmission of HIV-1: active and passive immunization

Despite more than 2 decades of research, an effective vaccine that can prevent HIV-1 infection in populations exposed to the virus remains elusive. In the pursuit of an HIV-1 vaccine, does prevention of exposure to maternal HIV-1 in utero, at birth or in early life through breast milk require special consideration? This article reviews what is known about the immune mechanisms of susceptibility and resistance to mother-to-child transmission (MTCT) of HIV-1 and summarizes studies that have used passive or active immunization strategies to interrupt MTCT of HIV-1. Potentially modifiable infectious cofactors that may enhance transmission and/or disease progression (especially in the developing world) are described. An effective prophylactic vaccine against HIV-1 infection needs to be deployed as part of the Extended Program of Immunization recommended by the World Health Organization for use in developing countries, so it is important to understand how the infant immune system responds to HIV-1 antigens, both in natural infection and presented by candidate vaccines.

Enhancing oral vaccine potency by targeting intestinal M cells

The immune system in the gastrointestinal tract plays a crucial role in the control of infection, as it constitutes the first line of defense against mucosal pathogens. The attractive features of oral immunization have led to the exploration of a variety of oral delivery systems. However, none of these oral delivery systems have been applied to existing commercial vaccines. To overcome this, a new generation of oral vaccine delivery systems that target antigens to gut-associated lymphoid tissue is required. One promising approach is to exploit the potential of microfold (M) cells by mimicking the entry of pathogens into these cells. Targeting specific receptors on the apical surface of M cells might enhance the entry of antigens, initiating the immune response and consequently leading to protection against mucosal pathogens. In this article, we briefly review the challenges associated with current oral vaccine delivery systems and discuss strategies that might potentially target mouse and human intestinal M cells.