Mucosal immunity and protection against HIV/SIV infection: strategies and challenges for vaccine design

Vaccine with bacterium-like particles displaying HIV-1 gp120 trimer elicits specific mucosal responses and neutralizing antibodies in rhesus macaques

Preclinical studies have shown that the induction of secretory IgA (sIgA) in mucosa and neutralizing antibodies (NAbs) in sera is essential for designing vaccines that can effectively block the transmission of HIV-1. We previously showed that a vaccine consisting of bacterium-like particles (BLPs) displaying Protan-gp120AE-MTQ (PAM) could induce mucosal immune responses through intranasal (IN) immunization in mice and NAbs through intramuscular (IM) immunization in guinea pigs. Here, we evaluated the ability of this vaccine BLP-PAM to elicit HIV-1-specific mucosal and systemic immune responses through IN and IM immunization combination strategies in rhesus macaques. First, the morphology, antigenicity and epitope accessibility of the vaccine were analysed by transmission electron microscopy, bio-layer interferometry and ELISA. In BLP-PAM-immunized macaques, HIV-1-specific sIgA were rapidly induced through IN immunization in situ and distant mucosal sites, although the immune responses are relatively weak. Furthermore, the HIV-1-specific IgG and IgA antibody levels in mucosal secretions were enhanced and maintained, while production of serum NAbs against heterologous HIV-1 tier 1 and 2 pseudoviruses was elicited after IM boost. Additionally, situ mucosal responses and systemic T cell immune responses were improved by rAd2-gp120AE boost immunization via the IN and IM routes. These results suggested that BLP-based delivery in combination with the IN and IM immunization approach represents a potential vaccine strategy against HIV-1.

Cooperation Between Systemic and Mucosal Antibodies Induced by Virosomal Vaccines Targeting HIV-1 Env: Protection of Indian Rhesus Macaques Against Low-Dose Intravaginal SHIV Challenges

A virosomal vaccine inducing systemic/mucosal anti-HIV-1 gp41 IgG/IgA had previously protected Chinese-origin rhesus macaques (RMs) against vaginal SHIV_SF162P3 challenges. Here, we assessed its efficacy in Indian-origin RMs by intramuscular priming/intranasal boosting (n=12/group). Group K received virosome-P1-peptide alone (harboring the Membrane Proximal External Region), Group L combined virosome-rgp41 plus virosome-P1, and Group M placebo virosomes. Vaccination induced plasma binding but no neutralizing antibodies. Five weeks after boosting, all RMs were challenged intravaginally with low-dose SHIV_SF162P3 until persistent systemic infection developed. After SHIV challenge #7, six controls were persistently infected versus only one Group L animal (vaccine efficacy 87%; P=0.0319); Group K was not protected. After a 50% SHIV dose increase starting with challenge #8, protection in Group L was lost. Plasmas/sera were analyzed for IgG phenotypes and effector functions; the former revealed that protection in Group L was significantly associated with increased binding to FcγR2/3(A/B) across several time-points, as were some IgG measurements. Vaginal washes contained low-level anti-gp41 IgGs and IgAs, representing a 1-to-5-fold excess over the SHIV inoculum's gp41 content, possibly explaining loss of protection after the increase in challenge-virus dose. Virosomal gp41-vaccine efficacy was confirmed during the initial seven SHIV challenges in Indian-origin RMs when the SHIV inoculum had at least 100-fold more HIV RNA than acutely infected men's semen. Vaccine protection by virosome-induced IgG and IgA parallels the cooperation between systemically administered IgG1 and mucosally applied dimeric IgA2 monoclonal antibodies that as single-agents provided no/low protection - but when combined, prevented mucosal SHIV transmission in all passively immunized RMs.

Protection of Newborn Macaques by Plant-Derived HIV Broadly Neutralizing Antibodies: a Model for Passive Immunotherapy during Breastfeeding

Preventing human immunodeficiency virus (HIV) infection in newborns by vertical transmission remains an important unmet medical need in resource-poor areas where antiretroviral therapy (ART) is not available and mothers and infants cannot be treated prepartum or during the breastfeeding period. In the present study, the protective efficacy of the potent HIV-neutralizing antibodies PGT121 and VRC07-523, both produced in plants, were assessed in a multiple-SHIV (simian-human immunodeficiency virus)-challenge breastfeeding macaque model. Newborn macaques received either six weekly subcutaneous injections with PGT121 alone or as a cocktail of PGT121-LS plus VRC07-523-LS injected three times every 2 weeks. Viral challenge with SHIVSF162P3 was twice weekly over 5.5 weeks using 11 exposures. Despite the transient presence of plasma viral RNA either immediately after the first challenge or as single-point blips, the antibodies prevented a productive infection in all babies with no sustained plasma viremia, compared to viral loads ranging from 103 to 5 × 108 virions/ml in four untreated controls. No virus was detected in peripheral blood mononuclear cells (PBMCs), and only 3 of 159 tissue samples were weakly positive in the treated babies. Newborn macaques proved to be immunocompetent, producing transient anti-Env antibodies and anti-drug antibody (ADA), which were maintained in the circulation after passive broadly neutralizing antibody clearance. ADA responses were directed to the IgG1 Fc CH2-CH3 domains, which has not been observed to date in adult monkeys passively treated with PGT121 or VRC01. In addition, high levels of VRC07-523 anti-idiotypic antibodies in the circulation of one newborn was concomitant with the rapid elimination of VRC07. Plant-expressed antibodies show promise as passive immunoprophylaxis in a breastfeeding model in newborns. IMPORTANCE Plant-produced human neutralizing antibody prophylaxis is highly effective in preventing infection in newborn monkeys during repeated oral exposure, modeling virus in breastmilk, and offers advantages in cost of production and safety. These findings raise the possibility that anti-Env antibodies may contribute to the control of viral replication in this newborn model and that the observed immune responsiveness may be driven by the long-lived presence of immune complexes.

Oral Vaccination Approaches for Anti-SHIV Immunity

We modified a Sabin Oral Poliovirus Vaccine (OPV) vector to permit secretion of the antigens of interest with the goal of improving anti-HIV Env humoral responses in a SHIV mucosal immunization composed of DNA and recombinant OPVs. We evaluated stimulation of systemic and mucosal cell-mediated and humoral immunity in Rhesus macaques by two regimens, both involving a prime with a SHIVBG505 DNA construct producing non-infectious particles formulated in lipid nanoparticles, administered in the oral cavity, and two different viral vector boostings, administered in the oral cavity and intestinally. Group 1 was boosted with rMVA-SHIVBG505, expressing SIV Gag/Pol and HIVBG505 Env. Group 2 was boosted with a SHIVBG505-OPV vaccine including a non-secreting SIVmac239CA-p6-OPV, expressing Gag CA, NC and p6 proteins, and a HIVBG505C1-V2-OPV, secreting the C1-V2 fragment of HIV EnvBG505, recognized by the broadly neutralizing antibody PG16. A time course analysis of anti-SHIV Gag and Env CD4+ and CD8+ T-cell responses in PBMC and in lymph node, rectal, and vaginal MNC was carried out. Both regimens stimulated significant cell-mediated responses in all compartments, with SHIVBG505-OPV immunization stimulating more significant levels of responses than rMVA- SHIVBG505. Boolean analysis of these responses revealed predominantly monofunctional responses with multifunctional responses also present in all tissues. Stimulation of antibody responses was disappointing in both groups with negative anti-SHIV IgG in plasma, and IgA in salivary, rectal and vaginal secretions being restricted to a few animals. After repeated rectal challenge with SHIVBG505, two Group 1 animals remained uninfected at challenge termination. No significant differences were observed in post-infection viral loads between groups. After the acute phase decline, CD4+ T cell percentages returned to normal levels in vaccinated as well as control animals. However, when compared to controls, vaccinate groups had more significant preservation of PBMC and rectal MNC Th17/Treg ratios, considered the strongest surrogate marker of progression to AIDS. We conclude that the vaccine platforms used in this study are insufficient to stimulate significant humoral immunity at the tested doses and schedule but sufficient to stimulate significant mucosal and systemic cell-mediated immunity, impacting the preservation of key Th17 CD4+ T cells in blood and rectal mucosa.

Mucosal Priming with a Recombinant Influenza A Virus-Vectored Vaccine Elicits T-Cell and Antibody Responses to HIV-1 in Mice

Recombinant influenza A viral (IAV) vectors are potential to stimulate systemic and mucosal immunity, but the packaging capacity is limited and only one or a few epitopes can be carried. Here, we report the generation of a replication-competent IAV vector that carries a full-length HIV-1 p24 gene linked to the 5'-terminal coding region of the neuraminidase segment via a protease cleavage sequence (IAV-p24). IAV-p24 was successfully rescued and stably propagated, and P24 protein was efficiently expressed in infected mammalian cells. In BALB/c mice, IAV-p24 showed attenuated pathogenicity compared to that of the parental A/PR/8/34 (H1N1) virus. An intranasal inoculation with IAV-p24 elicited moderate HIV-specific cell-mediated immune (CMI) responses in the airway and vaginal tracts and in the spleen, and an intranasal boost with a replication-incompetent adenovirus type 2 vector expressing the HIV-1 gag gene (Ad2-gag) greatly improved these responses. Importantly, compared to an Ad2-gag prime plus IAV-p24 boost regimen, the IAV-p24 prime plus Ad2-gag boost regimen had a greater efficacy in eliciting HIV-specific CMI responses. P24-specific CD8⁺ T cells and antibodies were robustly provoked both systemically and in mucosal sites and showed long-term durability, revealing that IAV-p24 may be used as a mucosa-targeted priming vaccine. Our results illustrate that IAV-p24 is able to prime systemic and mucosal immunity against HIV-1 and warrants further evaluation in nonhuman primates.IMPORTANCE An effective HIV-1 vaccine remains elusive despite nearly 40 years of research. CD8⁺ T cells and protective antibodies may both be desirable for preventing HIV-1 infection in susceptible mucosal sites. Recombinant influenza A virus (IAV) vector has the potential to stimulate these immune responses, but the packaging capacity is extremely limited. Here, we describe a replication-competent IAV vector expressing the HIV-1 p24 gene (IAV-p24). Unlike most other IAV vectors that carried one or several antigenic epitopes, IAV-p24 stably expressed the full-length P24 protein, which contains multiple epitopes and is highly conserved among all known HIV-1 sequences. Compared to the parental A/PR/8/34 (H1N1) virus, IAV-p24 showed an attenuated pathogenicity in BALB/c mice. When combined with an adenovirus vector expressing the HIV-1 gag gene, IAV-p24 was able to prime P24-specific systemic and mucosal immune responses. IAV-p24 as an alternative priming vaccine against HIV-1 warrants further evaluation in nonhuman primates.

Parainfluenza Virus 5 Priming Followed by SIV/HIV Virus-Like-Particle Boosting Induces Potent and Durable Immune Responses in Nonhuman Primates

The search for a preventive vaccine against HIV infection remains an ongoing challenge, indicating the need for novel approaches. Parainfluenza virus 5 (PIV5) is a paramyxovirus replicating in the upper airways that is not associated with any animal or human pathology. In animal models, PIV5-vectored vaccines have shown protection against influenza, RSV, and other human pathogens. Here, we generated PIV5 vaccines expressing HIV envelope (Env) and SIV Gag and administered them intranasally to macaques, followed by boosting with virus-like particles (VLPs) containing trimeric HIV Env. Moreover, we compared the immune responses generated by PIV5-SHIV prime/VLPs boost regimen in naïve vs a control group in which pre-existing immunity to the PIV5 vector was established. We demonstrate for the first time that intranasal administration of PIV5-based HIV vaccines is safe, well-tolerated and immunogenic, and that boosting with adjuvanted trimeric Env VLPs enhances humoral and cellular immune responses. The PIV5 prime/VLPs boost regimen induced robust and durable systemic and mucosal Env-specific antibody titers with functional activities including ADCC and neutralization. This regimen also induced highly polyfunctional antigen-specific T cell responses. Importantly, we show that diminished responses due to PIV5 pre-existing immunity can be overcome in part with VLP protein boosts. Overall, these results establish that PIV5-based HIV vaccine candidates are promising and warrant further investigation including moving on to primate challenge studies.

TFH Cells Induced by Vaccination and Following SIV Challenge Support Env-Specific Humoral Immunity in the Rectal-Genital Tract and Circulation of Female Rhesus Macaques

T follicular helper (T_FH) cells are pivotal in lymph node (LN) germinal center (GC) B cell affinity maturation. Circulating CXCR5⁺ CD4⁺ T (cT_FH) cells have supported memory B cell activation and broadly neutralizing antibodies in HIV controllers. We investigated the contribution of LN SIV-specific T_FH and cT_FH cells to Env-specific humoral immunity in female rhesus macaques following a mucosal Ad5hr-SIV recombinant priming and SIV gp120 intramuscular boosting vaccine regimen and following SIV vaginal challenge. T_FH and B cells were characterized by flow cytometry. B cell help was evaluated in T_FH-B cell co-cultures and by real-time PCR. Vaccination induced Env-specific T_FH and Env-specific memory (ESM) B cells in LNs. LN Env-specific T_FH cells post-priming and GC ESM B cells post-boosting correlated with rectal Env-specific IgA titers, and GC B cells at the same timepoints correlated with vaginal Env-specific IgG titers. Vaccination expanded cT_FH cell responses, including CD25⁺ Env-specific cT_FH cells that correlated negatively with vaginal Env-specific IgG titers but positively with rectal Env-specific IgA titers. Although cT_FH cells post-2^nd boost positively correlated with viral-loads following SIV challenge, cT_FH cells of SIV-infected and protected macaques supported maturation of circulating B cells into plasma cells and IgA release in co-culture. Additionally, cT_FH cells of naïve macaques promoted upregulation of genes associated with B cell proliferation, BCR engagement, plasma cell maturation, and antibody production, highlighting the role of cT_FH cells in blood B cell maturation. Vaccine-induced LN T_FH and GC B cells supported anti-viral mucosal immunity while cT_FH cells provided B cell help in the periphery during immunization and after SIV challenge. Induction of T_FH responses in blood and secondary lymphoid organs is likely desirable for protective efficacy of HIV vaccines.

Retargeting adenoviruses for therapeutic applications and vaccines

Adenoviruses (Ads) are robust vectors for therapeutic applications and vaccines, but their use can be limited by differences in their in vitro and in vivo pharmacologies. This review emphasizes that there is not just one Ad, but a whole virome of diverse viruses that can be used as therapeutics. It discusses that true vector targeting involves not only retargeting viruses, but importantly also detargeting the viruses from off-target cells.

Genetic Adjuvants in Replicating Single-Cycle Adenovirus Vectors Amplify Systemic and Mucosal Immune Responses against HIV-1 Envelope

Most infections occur at mucosal surfaces. Providing a barrier of protection at these surfaces may be a useful strategy to combat the earliest events in infection when there are relatively few pathogens to address. The majority of vaccines are delivered systemically by the intramuscular (IM) route. While IM vaccination can drive mucosal immune responses, mucosal immunization at intranasal (IN) or oral sites can lead to better immune responses at mucosal sites of viral entry. In macaques, IN immunization with replicating single-cycle adenovirus (SC-Ads) and protein boosts generated favorable mucosal immune responses. However, there was an apparent "distance effect" in generating mucosal immune responses. IN immunization generated antibodies against HIV envelope (env) nearby in the saliva, but weaker responses in samples collected from the distant vaginal samples. To improve on this, we tested here if SC-Ads expressing genetic adjuvants could be used to amplify antibody responses in distant vaginal samples when they are codelivered with SC-Ads expressing clade C HIV env immunogen. SC-Ads env 1157 was coadministered with SC-Ads expressing 4-1BBL, granulocyte macrophage colony-stimulating factor (GMCSF), IL-21, or Clostridoides difficile (C. diff.) toxin fragments by IN or IM routes. These data show that vaginal antibody responses were markedly amplified after a single immunization by the IN or IM routes, with SC-Ad expressing HIV env if this vaccine is complemented with SC-Ads expressing genetic adjuvants. Furthermore, the site and combination of adjuvants appear to "tune" these antibody responses towards an IgA or IgG isotype bias. Boosting these priming SC-Ad responses with another SC-Ad or with SOSIP native-like env proteins markedly amplifies env antibody levels in vaginal washes. Together, this data may be useful in informing the choice of route of delivery adenovirus and peptide vaccines against HIV-1.

Oral immunization of mice with Lactococcus lactis expressing Shiga toxin truncate confers enhanced protection against Shiga toxins of Escherichia coli O157:H7 and Shigella dysenteriae

Regardless of the communal impact of Shiga toxins, till today neither a specific treatment nor licensed vaccine is available. Lactococcus lactis (L. lactis), generally regarded as safe organism, is well known to provide a valuable approach regarding the oral delivery of vaccines. This study was undertaken to evaluate the protective efficacy of Stx2a1 expressed in nisin-inducible L. lactis, against Shiga toxins (Stx1, Stx2) in mouse model. Oral immunization of BALB/c mice with LL-Stx2a1 elicited significant serum antibody titer with elevated fecal and serum IgA, along with minimized intestinal and kidney damage resulting in survival of immunized animals at 84% and 100% when challenged with 10 × LD₅₀ of Escherichia coli O157 and Shigella dysenteriae toxins, respectively. HeLa cells incubated with immune sera and toxin mixture revealed high neutralizing capacity with 90% cell survivability against both the toxins. Mice immunized passively with both toxins and antibody mixture survived the observation period of 15 days, and the controls administered with sham sera and toxins were succumbed to death within 3 days. Our results revealed protective efficacy and toxin neutralization ability of LL-Stx2a1, proposing it as an oral vaccine candidate against Shiga toxicity mediated by E. coli O157 and S. dysenteriae.

Optimized Mucosal Modified Vaccinia Virus Ankara Prime/Soluble gp120 Boost HIV Vaccination Regimen Induces Antibody Responses Similar to Those of an Intramuscular Regimen

The benefits of mucosal vaccines over injected vaccines are difficult to ascertain, since mucosally administered vaccines often induce serum antibody responses of lower magnitude than those induced by injected vaccines. This study aimed to determine if mucosal vaccination using a modified vaccinia virus Ankara expressing human immunodeficiency virus type 1 (HIV-1) gp120 (MVAgp120) prime and a HIV-1 gp120 protein boost could be optimized to induce serum antibody responses similar to those induced by an intramuscularly (i.m.) administered MVAgp120 prime/gp120 boost to allow comparison of an i.m. immunization regimen to a mucosal vaccination regimen for the ability to protect against a low-dose rectal simian-human immunodeficiency virus (SHIV) challenge. A 3-fold higher antigen dose was required for intranasal (i.n.) immunization with gp120 to induce serum anti-gp120 IgG responses not significantly different than those induced by i.m. immunization. gp120 fused to the adenovirus type 2 fiber binding domain (gp120-Ad2F), a mucosal targeting ligand, exhibited enhanced i.n. immunogenicity compared to gp120. MVAgp120 was more immunogenic after i.n. delivery than after gastric or rectal delivery. Using these optimized vaccines, an i.n. MVAgp120 prime/combined i.m. (gp120) and i.n. (gp120-Ad2F) boost regimen (i.n./i.m.-plus-i.n.) induced serum anti-gp120 antibody titers similar to those induced by the intramuscular prime/boost regimen (i.m./i.m.) in rabbits and nonhuman primates. Despite the induction of similar systemic anti-HIV-1 antibody responses, neither the i.m./i.m. nor the i.n./i.m.-plus-i.n. regimen protected against a repeated low-dose rectal SHIV challenge. These results demonstrate that immunization regimens utilizing the i.n. route are able to induce serum antigen-specific antibody responses similar to those induced by systemic immunization.IMPORTANCE Mucosal vaccination is proposed as a method of immunization able to induce protection against mucosal pathogens that is superior to protection provided by parenteral immunization. However, mucosal vaccination often induces serum antigen-specific immune responses of lower magnitude than those induced by parenteral immunization, making the comparison of mucosal and parenteral immunization difficult. We identified vaccine parameters that allowed an immunization regimen consisting of an i.n. prime followed by boosters administered by both i.n. and i.m. routes to induce serum antibody responses similar to those induced by i.m. prime/boost vaccination. Additional studies are needed to determine the potential benefit of mucosal immunization for HIV-1 and other mucosally transmitted pathogens.

Divergent HIV-1-Directed Immune Responses Generated by Systemic and Mucosal Immunization with Replicating Single-Cycle Adenoviruses in Rhesus Macaques

Most human immunodeficiency virus type 1 (HIV-1) infections begin at mucosal surfaces. Providing a barrier of protection at these may assist in combating the earliest events in infection. Systemic immunization by intramuscular (i.m.) injection can drive mucosal immune responses, but there are data suggesting that mucosal immunization can better educate these mucosal immune responses. To test this, rhesus macaques were immunized with replicating single-cycle adenovirus (SC-Ad) vaccines expressing clade B HIV-1 gp160 by the intranasal (i.n.) and i.m. routes to compare mucosal and systemic routes of vaccination. SC-Ad vaccines generated significant circulating antibody titers against Env after a single i.m. immunization. Switching the route of second immunization with the same SC-Ad serotype allowed a significant boost in these antibody levels. When these animals were boosted with envelope protein, envelope-binding antibodies were amplified 100-fold, but qualitatively different immune responses were generated. Animals immunized by only the i.m. route had high peripheral T follicular helper (pTfh) cell counts in blood but low Tfh cell counts in lymph nodes. Conversely, animals immunized by the i.n. route had high Tfh cell counts in lymph nodes but low pTfh cell counts in the blood. Animals immunized by only the i.m. route had lower antibody-dependent cellular cytotoxicity (ADCC) antibody activity, whereas animals immunized by the mucosal i.n. route had higher ADCC antibody activity. When these Env-immunized animals were challenged rectally with simian-human immunodeficiency virus (SHIV) strain SF162P3 (SHIVSF162P3), they all became infected. However, mucosally SC-Ad-immunized animals had lower viral loads in their gastrointestinal tracts. These data suggest that there may be benefits in educating the immune system at mucosal sites during HIV vaccination.IMPORTANCE HIV-1 infections usually start at a mucosal surface after sexual contact. Creating a barrier of protection at these mucosal sites may be a good strategy for to protect against HIV-1 infections. While HIV-1 enters at mucosa, most vaccines are not delivered here. Most are instead injected into the muscle, a site well distant and functionally different than mucosal tissues. This study tested if delivering HIV vaccines at mucosa or in the muscle makes a difference in the quality, quantity, and location of immune responses against the virus. These data suggest that there are indeed advantages to educating the immune system at mucosal sites with an HIV-1 vaccine.

Replicating Adenovirus-SIV Immunization of Rhesus Macaques Induces Mucosal Dendritic Cell Activation and Function Leading to Rectal Immune Responses

Inducing strong mucosal immune responses by vaccination is important for providing protection against simian immunodeficiency virus (SIV). A replicating adenovirus type 5 host range mutant vector (Ad5hr) expressing SIV proteins induced mucosal immune responses in rectal tissue associated with delayed SIV acquisition in female rhesus macaques, but the initial mechanisms leading to the induced immunity have not been elucidated. As dendritic cells (DCs) are known to orchestrate both innate and adaptive effector immune cell responses, we investigated their role here. Rhesus macaques were immunized twice mucosally with a replicating Ad5hr expressing SIV Env, Gag, and Nef (Ad-SIV) or empty Ad5hr vector (Ad-Empty). DC subsets and their activation were examined in rectal tissue, blood, and LNs at 3 timepoints after each immunization. Plasmacytoid DCs, myeloid DCs, and Langerhans cells were significantly increased in the rectal mucosa, but only myeloid DCs were significantly increased in blood post-immunizations. All rectal DC subsets showed increased frequencies of cells expressing activation markers and cytokines post-immunization, blood DCs showed mixed results, and LN DCs showed few changes. Rectal DCs responded strongly to the vector rather than expressed SIV antigens, but rectal DC frequencies positively correlated with induced rectal antigen-specific memory T and B cells. These correlations were confirmed by in vitro co-cultures showing that rectal Ad-SIV DCs induced proliferation and antigen-specific cytokine production by autologous naïve T cells. Our results highlight the rapid response of DCs to Ad immunization and their role in mucosal immune activation and identify initial cellular mechanisms of the replicating Ad-SIV vaccine in the rhesus macaque model.

Mucosal Vaccine Approaches for Prevention of HIV and SIV Transmission

Optimal protective immunity to HIV will likely require that plasma cells, memory B cells and memory T cells be stationed in mucosal tissues at portals of viral entry. Mucosal vaccine administration is more effective than parenteral vaccine delivery for this purpose. The challenge has been to achieve efficient vaccine uptake at mucosal surfaces, and to identify safe and effective adjuvants, especially for mucosally administered HIV envelope protein immunogens. Here, we discuss strategies used to deliver potential HIV vaccine candidates in the intestine, respiratory tract, and male and female genital tract of humans and nonhuman primates. We also review mucosal adjuvants, including Toll-like receptor agonists, which may adjuvant both mucosal humoral and cellular immune responses to HIV protein immunogens.

Comparison of systemic and mucosal immunization with replicating Single cycle Adenoviruses

HIV-1 infections occur during sexual contact at mucosal surfaces. Vaccines need to provide mucosal barrier protection and stimulate systemic immune responses to control HIV spread. Most vaccines are delivered by systemic immunization via intramuscular (IM) injection route. While this can drive systemic and mucosal immune responses, there are data show that mucosal immunization may be superior at driving responses at mucosal barriers. To explore this question, we immunized mice with replicating single-cycle adenovirus (SC Ad) vaccines expressing clade B HIV-1 envelope (Env) by intramuscular (IM), intranasal (IN), or intravaginal (IVAG) routes to compare vaccine responses. SC-Ads generated significant antibodies against Env after only a single immunization by the IN route, but not the other routes. These animals were boosted by the same route or by the mucosal IVAG routes. IM and IN primed animals generated strong antibody responses regardless of the boosting route. In contrast, IVAG primed animals failed to generate robust antibodies whether they were boosted by the IVAG or IM routes. These data suggest there may be benefits in first educating the immune system at mucosal sites during HIV vaccination. IN and IM prime-boost were then compared in Syrian hamsters which support SC-Ad DNA replication. In this case, IN immunization again was the only route that generated significant Env antibodies after a single immunization. Following a boost by IN or IM routes, IN primed animals had significantly higher antibody responses than the IM primed animals. Env antibodies could still be detected one year after immunization, but only in animals that received at least one mucosal IN immunization. These data suggest that there is merit in vaccination by mucosal routes.

Single-cycle adenovirus vectors in the current vaccine landscape

INTRODUCTION

Traditional inactivated and protein vaccines generate strong antibodies, but struggle to generate T cell responses. Attenuated pathogen vaccines generate both, but risk causing the disease they aim to prevent. Newer gene-based vaccines drive both responses and avoid the risk of infection. While these replication-defective (RD) vaccines work well in small animals, they can be weak in humans because they do not replicate antigen genes like more potent replication-competent (RC) vaccines. RC vaccines generate substantially stronger immune responses, but also risk causing their own infections. To circumvent these problems, we developed single-cycle adenovirus (SC-Ad) vectors that amplify vaccine genes, but that avoid the risk of infection. This review will discuss these vectors and their prospects for use as vaccines.

AREAS COVERED

This review provides a background of different types of vaccines. The benefits of gene-based vaccines and their ability to replicate antigen genes are described. Adenovirus vectors are discussed and compared to other vaccine types. Replication-defective, single-cycle, and replication-competent Ad vaccines are compared.

EXPERT COMMENTARY

The potential utility of these vaccines are discussed when used against infectious diseases and as cancer vaccines. We propose a move away from replication-defective vaccines towards more robust replication-competent or single-cycle vaccines.

Immunogenicity and efficacy of DNA/MVA HIV vaccines in rhesus macaque models

INTRODUCTION

Despite 30 years of research on HIV, a vaccine to prevent infection and limit disease progression remains elusive. The RV144 trial showed moderate, but significant protection in humans and highlighted the contribution of antibody responses directed against HIV envelope as an important immune correlate for protection. Efforts to further build upon the progress include the use of a heterologous prime-boost regimen using DNA as the priming agent and the attenuated vaccinia virus, Modified Vaccinia Ankara (MVA), as a boosting vector for generating protective HIV-specific immunity. Areas covered: In this review, we summarize the immunogenicity of DNA/MVA vaccines in non-human primate models and describe the efficacy seen in SIV infection models. We discuss immunological correlates of protection determined by these studies and potential approaches for improving the protective immunity. Additionally, we describe the current progress of DNA/MVA vaccines in human trials. Expert commentary: Efforts over the past decade have provided the opportunity to better understand the dynamics of vaccine-induced immune responses and immune correlates of protection against HIV. Based on what we have learned, we outline multiple areas where the field will likely focus on in the next five years.

First-in-Human Evaluation of the Safety and Immunogenicity of an Intranasally Administered Replication-Competent Sendai Virus-Vectored HIV Type 1 Gag Vaccine: Induction of Potent T-Cell or Antibody Responses in Prime-Boost Regimens

Background. We report the first-in-human safety and immunogenicity assessment of a prototype intranasally administered, replication-competent Sendai virus (SeV)–vectored, human immunodeficiency virus type 1 (HIV-1) vaccine.

Methods. Sixty-five HIV-1–uninfected adults in Kenya, Rwanda, and the United Kingdom were assigned to receive 1 of 4 prime-boost regimens (administered at 0 and 4 months, respectively; ratio of vaccine to placebo recipients, 12:4): priming with a lower-dose SeV-Gag given intranasally, followed by boosting with an adenovirus 35–vectored vaccine encoding HIV-1 Gag, reverse transcriptase, integrase, and Nef (Ad35-GRIN) given intramuscularly (S_LA); priming with a higher-dose SeV-Gag given intranasally, followed by boosting with Ad35-GRIN given intramuscularly (S_HA); priming with Ad35-GRIN given intramuscularly, followed by boosting with a higher-dose SeV-Gag given intranasally (AS_H); and priming and boosting with a higher-dose SeV-Gag given intranasally (S_HS_H).

Results. All vaccine regimens were well tolerated. Gag-specific IFN-γ enzyme-linked immunospot–determined response rates and geometric mean responses were higher (96% and 248 spot-forming units, respectively) in groups primed with SeV-Gag and boosted with Ad35-GRIN (S_LA and S_HA) than those after a single dose of Ad35-GRIN (56% and 54 spot-forming units, respectively) or SeV-Gag (55% and 59 spot-forming units, respectively); responses persisted for ≥8 months after completion of the prime-boost regimen. Functional CD8⁺ T-cell responses with greater breadth, magnitude, and frequency in a viral inhibition assay were also seen in the S_LA and S_HA groups after Ad35-GRIN boost, compared with those who received either vaccine alone. SeV-Gag did not boost T-cell counts in the AS_H group. In contrast, the highest Gag-specific antibody titers were seen in the AS_H group. Mucosal antibody responses were sporadic.

Conclusions. SeV-Gag primed functional, durable HIV-specific T-cell responses and boosted antibody responses. The prime-boost sequence appears to determine which arm of the immune response is stimulated.

Clinical Trials Registration. NCT01705990.

Early short-term treatment with neutralizing human monoclonal antibodies halts SHIV infection in infant macaques

Prevention of mother to child transmission (MTCT) of HIV remains a major objective where antenatal care is not readily accessible. We tested anti-HIV-1 human neutralizing monoclonal antibodies (NmAb) as post-exposure therapy in an infant macaque model for intrapartum MTCT. One-month-old rhesus macaques were inoculated orally with SHIV_SF162P3. On days 1, 4, 7, and 10 after virus exposure, we injected animals subcutaneously with NmAbs and quantified systemic distribution of NmAbs in multiple tissues within 24 h following administration. Replicating virus was found in multiple tissues by day 1 in animals without treatment. All NmAb-treated macaques were free of virus in blood and tissues at 6 months post-exposure. We detected no anti-SHIV T cell responses in blood or tissues at necropsy, and no virus emerged following CD8⁺ T cell depletion. These results suggest early passive immunotherapy can eliminate early viral foci and thereby prevent the establishment of viral reservoirs.

Oral Immunization with a Recombinant Lactococcus lactis-Expressing HIV-1 Antigen on Group A Streptococcus Pilus Induces Strong Mucosal Immunity in the Gut

The induction of a potent humoral and cellular immune response in mucosal tissue is important for the development of an effective HIV vaccine. Most of the current HIV vaccines under development use the i.m. route for immunization, which is relatively poor in generating potent and long-lived mucosal immune responses. In this article, we explore the ability of an oral vaccination with a probiotic organism, Lactococcus lactis, to elicit HIV-specific immune responses in the mucosal and systemic compartments of BALB/c mice. We expressed the HIV-1 Gag-p24 on the tip of the T3 pilus of Streptococcus pyogenes as a fusion to the Cpa protein (LL-Gag). After four monthly LL-Gag oral immunizations, we observed strong Gag-specific IgG and IgA responses in serum, feces, and vaginal secretions. However, the Gag-specific CD8 T cell responses in the blood were at or below our detection limit. After an i.m. modified vaccinia Ankara/Gag boost, we observed robust Gag-specific CD8 T cell responses both in systemic and in mucosal tissues, including intraepithelial and lamina propria lymphocytes of the small intestine, Peyer's patches, and mesenteric lymph nodes. Consistent with strong immunogenicity, the LL-Gag induced activation of CD11c(+) CD11b(+) dendritic cells in the Peyer's patches after oral immunization. Our results demonstrate that oral immunization with L. lactis expressing an Ag on the tip of the group A Streptococcus pilus serves as an excellent vaccine platform to induce strong mucosal humoral and cellular immunity against HIV.

Vaginal gene therapy

In the last years, vaginal gene therapy has gained increasing attention mainly for the treatment and control of sexually transmitted infections. DNA delivery has been also suggested to improve reproductive outcomes for women with deficiencies in the female reproductive tract. Although no product has reached clinical phase, preclinical investigations reveal the potential of the vaginal tract as an effective administration route for gene delivery. This review focuses on the main advantages and challenges of vaginal gene therapy, and on the most used nucleic acid delivery systems, including viral and non-viral vectors. Additionally, the advances in the application of vaginal gene therapy for the treatment and/or prevention of infectious diseases such as the human immunodeficiency virus (HIV), the human papillomavirus (HPV) or the herpes simplex virus (HSV) are presented.

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.

IIIa deleted adenovirus as a single-cycle genome replicating vector

Replication competent adenovirus (RC-Ad) vectors mediate robust transgene expression by virtue of amplifying transgenes by replication but also put patients at a risk of frank adenovirus infection. In contrast, E1-deleted replication defective Ad (RD-Ad) vectors are safer but produce substantially less transgene product. To generate a robust, but safer adenoviral vector, we created a "single cycle" adenovirus (SC-Ad) vector that replicates its genome and transgene, but that does not cause adenovirus infections by deleting the capsid cement protein IIIa in low seroprevalence adenovirus serotype 6. Ad6-ΔIIIa can be produced in IIIa-expressing cell lines. In normal cells, Ad6-ΔIIIa replicates its genome and transgene but fails to package its DNA or form mature virus. SC-Ad and RC-Ad expressed transgenes hundreds of times higher than RD-Ad in human and mouse cells in vitro and in vivo in mice. These data suggest that SC-Ads may be safer amplifying vectors for vaccine and therapeutic applications.

Humoral immune responses to HIV in the mucosal secretions and sera of HIV-infected women

Although sera and all external secretions contain antibodies to human immunodeficiency virus (HIV), their levels, specificity, isotypes, and relevant effector functions display a great degree of variability. Antibodies that bind HIV antigens and neutralize the virus are predominantly associated with the IgG isotype in sera and in all external secretions, even where total levels of IgG are much lower than those of IgA. Rectal fluid that contains high IgA, but low IgG levels, displayed low neutralizing activity independent of antibodies. Therefore, external secretions should be evaluated before and after selective depletion of Ig. At the systemic level, HIV-specific IgA may interfere with the effector functions of IgG, as suggested by recent studies of individuals systemically immunized with an experimental HIV vaccine. Although HIV-specific IgG and IgA antibodies may exhibit their protective activities at mucosal surfaces through interference with viral entry and local neutralization at the systemic level, such antibodies may display discordant effector functions.

Phenotypes and distribution of mucosal memory B-cell populations in the SIV/SHIV rhesus macaque model

As vaccine-elicited antibodies have now been associated with HIV protective efficacy, a thorough understanding of mucosal and systemic B-cell development and maturation is needed. We phenotyped mucosal memory B-cells, investigated isotype expression and homing patterns, and defined plasmablasts and plasma cells at three mucosal sites (duodenum, jejunum and rectum) in rhesus macaques, the commonly used animal model for pre-clinical vaccine studies. Unlike humans, macaque mucosal memory B-cells lacked CD27 expression; only two sub-populations were present: naïve (CD21(+)CD27(-)) and tissue-like (CD21(-)CD27(-)) memory. Similar to humans, IgA was the dominant isotype expressed. The homing markers CXCR4, CCR6, CCR9 and α4β7 were differentially expressed between naïve and tissue-like memory B-cells. Mucosal plasmablasts were identified as CD19(+)CD20(+/-)HLA-DR(+)Ki-67(+)IRF4(+)CD138(+/-) and mucosal plasma cells as CD19(+)CD20(-)HLA-DR(-)Ki-67(-)IRF4(+)CD138(+). Both populations were CD39(+/-)CD27(-). Plasma cell phenotype was confirmed by spontaneous IgA secretion by ELISpot of positively-selected cells and J-chain expression by real-time PCR. Duodenal, jejunal and rectal samples were similar in B-cell memory phenotype, isotype expression, homing receptors and plasmablast/plasma cell distribution among the three tissues. Thus rectal biopsies adequately monitor B-cell dynamics in the gut mucosa, and provide a critical view of mucosal B-cell events associated with development of vaccine-elicited protective immune responses and SIV/SHIV pathogenesis and disease control.

Rhesus macaque rectal and duodenal tissues exhibit B-cell sub-populations distinct from peripheral blood that continuously secrete antigen-specific IgA in short-term explant cultures

It is becoming increasingly obvious that evaluation of a vaccine aimed at preventing HIV infection should include assessment of induced immunity at mucosal sites of viral entry. Among the most salient immune responses are viral-specific antibodies. A recent report on IgA-secreting plasma cells in human duodenal explants prompted us to examine similar duodenal and rectal biopsies of rhesus macaques, a key animal model for pre-clinical HIV/SIV vaccine studies, and characterize the local resident B-cells. Here we report that non-human primate rectal explants possess similar levels of B-cells as duodenal explants. We characterize the antibody isotype expression on mucosal memory B-cells and show for the first time that the B-cell memory subsets of the duodenum and rectum are distinct from those of PBMC, not only by essentially lacking CD27(+) cells, as previously reported for uninfected macaques (Titanji et al., 2010), but also in being mostly IgD(-). SIV- and SHIV-infected macaques had fewer total IgA-secreting cells in rectal tissue compared to naïve macaques. As expected, the fractions of B-cells with surface expression of IgA were dominant in the rectal and duodenal explants whereas in PBMC IgG surface expression was dominant among IgD(-) B-cells. Mucosal antibody secreting cells were found to be predominantly plasma cells/plasma blasts based on their lack of response to stimulation. Importantly, short-term culture of rectal explants of SIV- and SHIV-positive animals led to secretion of Env-specific IgA into the culture supernatant which could be easily measured by ELISA. Collection of such culture supernatant over several days allows for accumulation of mucosal antibody in amounts that should enable antibody purification, characterization, and use in functional assays. Rectal explants can be readily obtained and unequivocally identify the mucosal tissue as the source of antibody. Overall they facilitate evaluation of mucosal vaccines.

Chimeric rhinoviruses displaying MPER epitopes elicit anti-HIV neutralizing responses

Background

The development of an effective AIDS vaccine has been a formidable task, but remains a critical necessity. The well conserved membrane-proximal external region (MPER) of the HIV-1 gp41 glycoprotein is one of the crucial targets for AIDS vaccine development, as it has the necessary attribute of being able to elicit antibodies capable of neutralizing diverse isolates of HIV.

Methodology/Principle Findings

Guided by X-ray crystallography, molecular modeling, combinatorial chemistry, and powerful selection techniques, we designed and produced six combinatorial libraries of chimeric human rhinoviruses (HRV) displaying the MPER epitopes corresponding to mAbs 2F5, 4E10, and/or Z13e1, connected to an immunogenic surface loop of HRV via linkers of varying lengths and sequences. Not all libraries led to viable chimeric viruses with the desired sequences, but the combinatorial approach allowed us to examine large numbers of MPER-displaying chimeras. Among the chimeras were five that elicited antibodies capable of significantly neutralizing HIV-1 pseudoviruses from at least three subtypes, in one case leading to neutralization of 10 pseudoviruses from all six subtypes tested.

Conclusions

Optimization of these chimeras or closely related chimeras could conceivably lead to useful components of an effective AIDS vaccine. While the MPER of HIV may not be immunodominant in natural infection by HIV-1, its presence in a vaccine cocktail could provide critical breadth of protection.

A review of nanotechnological approaches for the prophylaxis of HIV/AIDS

Successful treatment and control of HIV/AIDS is one of the biggest challenges of 21st century. More than 33 million individuals are infected with HIV worldwide and more than 2 million new cases of HIV infection have been reported. The situation demands development of effective prevention strategies to control the pandemic of AIDS. Due to lack of availability of an effective HIV vaccine, antiretroviral drugs and nucleic acid therapeutics like siRNA have been explored for HIV prophylaxis. Clinical trials shave shown that antiretroviral drugs, tenofovir and emtricitabine can offer some degree of HIV prevention. However, complete prevention of HIV infection has not been achieved yet. Nanotechnology has brought a paradigm shift in the diagnosis, treatment and prevention of many diseases. The current review discusses potential of various nanocarriers such as dendrimers, polymeric nanoparticles, liposomes, lipid nanocarriers, drug nanocrystals, inorganic nanocarriers and nanofibers in improving efficacy of various modalities available for HIV prophylaxis.

Immunogenicity of a vaccine regimen composed of simian immunodeficiency virus DNA, rMVA, and viral particles administered to female rhesus macaques via four different mucosal routes

A comparative evaluation of the immunity stimulated with a vaccine regimen that includes simian immunodeficiency virus (SIV), interleukin 2 (IL-2), and IL-15 DNAs, recombinant modified vaccinia virus Ankara (rMVA), and inactivated SIVmac239 particles administered into the oral and nasal cavities, small intestine, and vagina was carried out in female rhesus macaques to determine the best route to induce diverse anti-SIV immunity that may be critical to protection from SIV infection and disease. All four immunizations generated mucosal SIV-specific IgA. Oral immunization was as effective as vaginal immunization in inducing SIV-specific IgA in vaginal secretions and generated greater IgA responses in rectal secretions and saliva samples compared to the other immunization routes. All four immunizations stimulated systemic T-cell responses against Gag and Env, albeit to a different extent, with oral immunization providing greater magnitude and nasal immunization providing wider functional heterogeneity. SIV-specific T cells producing gamma interferon (IFN-γ) dominated these responses. Limited levels of SIV-specific IgG antibodies were detected in plasma samples, and no SIV-specific IgG antibodies were detected in secretions. Vaccination also induced CD4(+) and CD8(+) T-cell responses in the rectal and vaginal mucosa with greater functional heterogeneity than in blood samples. Rectal T-cell responses were significantly greater in the orally vaccinated animals than in the other animals. The most balanced, diverse, and higher-magnitude vaginal T-cell responses were observed after intestinal vaccination. Significantly higher CD8(+) granzyme B-positive T-cell responses were observed systemically after intestinal vaccination and in rectal cells after oral immunization. The majority of SIV-specific T cells that produced granzyme B did not produce cytokines. Of the immunization routes tested, oral vaccination provided the most diverse and significant response to the vaccine.

An effective vaccination approach augments anti-HIV systemic and vaginal immunity in mice with decreased HIV-1 susceptible α4β7high CD4+ T cells

HIV-1 preferentially infects activated CD4(+) T cells expressing α4β7 integrin and conventional vaccination approaches non-selectively induce immune responses including α4β7(high) CD4(+) T cells, suggesting that current candidate AIDS vaccines may produce more target cells for HIV-1 and paradoxically enhance HIV-1 infection. Thus it remains a challenge to selectively induce robust anti-HIV immunity without the unwanted HIV-1 susceptible α4β77(high) CD4(+)+ T cells. Here we describe a vaccination strategy that targets ALDH1a2, a retinoic acid producing enzyme in dendritic cells (DCs). Silencing ALDH1a2 in DCs enhanced the maturation and production of proinflammatory cytokines of DCs and promoted Th1/Th2 differentiation while suppressing Treg. ALDH1a2-silenced DCs effectively downregulated the expression of guthoming receptors α4β77 and CCR9 on activated T and B lymphocytes. Consequently, intranasal immunization of a lentiviral vaccine encoding ALDH1a2 shRNA and HIV-1 gp140 redirected gp140-specific mucosal T cell and antibody responses from the gut to the vaginal tract, while dramatically enhancing systemic gp140-specific immune responses. We further demonstrated that silencing ALDH1a2 in human DCs resulted in downregulation of β7 expression on activated autologous CD4(+) T cells. Hence this study provides a unique and effective strategy to induce α4β7(low) anti-HIV immune responses.

Controlling the HIV/AIDS epidemic: current status and global challenges

This review provides an overview of the current status of the global HIV pandemic and strategies to bring it under control. It updates numerous preventive approaches including behavioral interventions, male circumcision (MC), pre- and post-exposure prophylaxis (PREP and PEP), vaccines, and microbicides. The manuscript summarizes current anti-retroviral treatment options, their impact in the western world, and difficulties faced by emerging and resource-limited nations in providing and maintaining appropriate treatment regimens. Current clinical and pre-clinical approaches toward a cure for HIV are described, including new drug compounds that target viral reservoirs and gene therapy approaches aimed at altering susceptibility to HIV infection. Recent progress in vaccine development is summarized, including novel approaches and new discoveries.

Functions of IL-15 in anti-viral immunity: multiplicity and variety

An effective immune response to an invading viral pathogen requires the combined actions of both innate and adaptive immune cells. For example, NK cells and cytotoxic CD8 T cells are capable of the direct engagement of infected cells and the mediation of antiviral responses. Both NK and CD8 T cells depend on common gamma chain (γc) cytokine signals for their development and homeostasis. The γc cytokine IL-15 is very well characterized for its role in promoting the development and homeostasis of NK cells and CD8 T cells, but emerging literature suggests that IL-15 mediates the anti-viral responses of these cell populations during an active immune response. Both NK cells and CD8 T cells must become activated, migrate to sites of infection, survive at those sites, and expand in order to maximally exert effector functions, and IL-15 can modulate each of these processes. This review focuses on the functions of IL-15 in the regulation of multiple aspects of NK and CD8 T cell biology, investigates the mechanisms by which IL-15 may exert such diverse functions, and discusses how these different facets of IL-15 biology may be therapeutically exploited to combat viral diseases.

Thymic stromal lymphopoietin (TSLP) acts as a potent mucosal adjuvant for HIV-1 gp140 vaccination in mice

The development of a successful vaccine against HIV is likely to require the induction of strong and long-lasting humoral immune responses at the mucosal portal of virus entry. Hence, the design of a vaccine strategy able to induce mucosal antibodies and in particular specific IgA, may be crucial to providing immune protection. Nasal immunisation is known to induce specific IgG and IgA responses in the cervicovaginal mucosa; however, there is an urgent need for the development of safe, effective and accessible mucosal adjuvants for nasal application in humans. To reduce the potential for adverse events associated with some nasal adjuvants, we have assessed whether the B-cell-activating cytokines APRIL, BAFF and TSLP enhance humoral immune responses to HIV-1 gp140. Following intranasal immunisation, TSLP but not APRIL or BAFF induced strong humoral responses both in serum and mucosa. The adjuvant effect of TSLP on humoral responses was similar to that of cholera toxin (CT). The use of TSLP as an adjuvant skewed both the cellular and humoral immune responses towards Th2 cells. This is the first time that TSLP has been demonstrated to have a positive effect as a mucosal adjuvant, and specifically to promote mucosal and systemic responses to HIV gp140.

CCL28 induces mucosal homing of HIV-1-specific IgA-secreting plasma cells in mice immunized with HIV-1 virus-like particles

Mucosae-associated epithelial chemokine (MEC or CCL28) binds to CCR3 and CCR10 and recruits IgA-secreting plasma cells (IgA-ASCs) in the mucosal lamina propria. The ability of this chemokine to enhance migration of IgA-ASCs to mucosal sites was assessed in a mouse immunization model using HIV-1(IIIB) Virus-like particles (VLPs). Mice receiving either HIV-1(IIIB) VLPs alone, CCL28 alone, or the irrelevant CCL19 chemokine were used as controls. Results showed a significantly increased CCR3 and CCR10 expression on CD19(+) splenocytes of HIV-1(IIIB) VPL-CCL28-treated mice. HIV-1 Env-specific IFN-γ, IL-4 and IL-5 production, total IgA, anti-Env IgA as well as gastro-intestinal mucosal IgA-secreting plasma cells were also significantly augmented in these mice. Notably, sera and vaginal secretions from HIV-1(IIIB) VLP-CCL28-treated mice exhibited an enhanced neutralizing activity against both a HIV-1/B-subtype laboratory strain and a heterologous HIV-1/C-subtype primary isolate. These data suggest that CCL28 could be useful in enhancing the IgA immune response that will likely play a pivotal role in prophylactic HIV vaccines.

Characterization of an effective CTL response against HIV and SIV infections

A vaccine inducing protective immunity in mucosal tissues and secretions may stop or limit HIV infection. Although cytotoxic T lymphocytes (CTLs) are clearly associated with control of viral replication in HIV and simian immunodeficiency virus (SIV) infections, there are examples of uncontrolled viral replication in the face of strong CD8⁺ T-cell responses. The number of functions, breadth, avidity, and magnitude of CTL response are likely to be important factors in the effectiveness of anti-HIV T-cell response, but the location and persistence of effector CD8⁺ T cells are also critical factors. Although the only HIV vaccine clinical trial targeting cellular immunity to prevent HIV infection failed, vaccine strategies using persistent agents against pathogenic mucosal challenge in macaque models are showing unique success. Thus, the key to control the initial focus of viral replication at the portal of entry may rely on the continuous generation of effector CTL responses at mucosal level.

A neonatal Fc receptor-targeted mucosal vaccine strategy effectively induces HIV-1 antigen-specific immunity to genital infection

Strategies to prevent the sexual transmission of HIV include vaccines that elicit durable, protective mucosal immune responses. A key to effective mucosal immunity is the capacity for antigens administered locally to cross epithelial barriers. Given the role of neonatal Fc receptor (FcRn) in transferring IgG across polarized epithelial cells which line mucosal surfaces, FcRn might be useful for delivering HIV vaccine antigens across mucosal epithelial barriers to the underlying antigen-presenting cells. Chimeric proteins composed of HIV Gag (p24) fused to the Fc region of IgG (Gag-Fc) bind efficiently to airway mucosa and are transported across this epithelial surface. Mice immunized intranasally with Gag-Fc plus CpG adjuvant developed local and systemic immunity, including durable B and T cell memory. Gag-specific immunity was sufficiently potent to protect against an intravaginal challenge with recombinant vaccinia virus expressing the HIV Gag protein. Intranasal administration of a Gag-Fc/CpG vaccine protected at a distal mucosal site. Our data suggest that targeting of FcRn with chimeric immunogens may be an important strategy for mucosal immunization and should be considered a new approach for preventive HIV vaccines.

Broad-spectrum antiviral therapeutics

Currently there are relatively few antiviral therapeutics, and most which do exist are highly pathogen-specific or have other disadvantages. We have developed a new broad-spectrum antiviral approach, dubbed Double-stranded RNA (dsRNA) Activated Caspase Oligomerizer (DRACO) that selectively induces apoptosis in cells containing viral dsRNA, rapidly killing infected cells without harming uninfected cells. We have created DRACOs and shown that they are nontoxic in 11 mammalian cell types and effective against 15 different viruses, including dengue flavivirus, Amapari and Tacaribe arenaviruses, Guama bunyavirus, and H1N1 influenza. We have also demonstrated that DRACOs can rescue mice challenged with H1N1 influenza. DRACOs have the potential to be effective therapeutics or prophylactics for numerous clinical and priority viruses, due to the broad-spectrum sensitivity of the dsRNA detection domain, the potent activity of the apoptosis induction domain, and the novel direct linkage between the two which viruses have never encountered.

Immunology of gut mucosal vaccines

Understanding the mechanisms underlying the induction of immunity in the gastrointestinal mucosa following oral immunization and the cross-talk between mucosal and systemic immunity should expedite the development of vaccines to diminish the global burden caused by enteric pathogens. Identifying an immunological correlate of protection in the course of field trials of efficacy, animal models (when available), or human challenge studies is also invaluable. In industrialized country populations, live attenuated vaccines (e.g. polio, typhoid, and rotavirus) mimic natural infection and generate robust protective immune responses. In contrast, a major challenge is to understand and overcome the barriers responsible for the diminished immunogenicity and efficacy of the same enteric vaccines in underprivileged populations in developing countries. Success in developing vaccines against some enteric pathogens has heretofore been elusive (e.g. Shigella). Different types of oral vaccines can selectively or inclusively elicit mucosal secretory immunoglobulin A and serum immunoglobulin G antibodies and a variety of cell-mediated immune responses. Areas of research that require acceleration include interaction between the gut innate immune system and the stimulation of adaptive immunity, development of safe yet effective mucosal adjuvants, better understanding of homing to the mucosa of immunologically relevant cells, and elicitation of mucosal immunologic memory. This review dissects the immune responses elicited in humans by enteric vaccines.

Prime-boost vaccination with heterologous live vectors encoding SIV gag and multimeric HIV-1 gp160 protein: efficacy against repeated mucosal R5 clade C SHIV challenges

We sought to induce primate immunodeficiency virus-specific cellular and neutralizing antibody (nAb) responses in rhesus macaques (RM) through a bimodal vaccine approach. RM were immunized intragastrically (i.g.) with the live-attenuated Listeria monocytogenes (Lm) vector Lmdd-BdopSIVgag encoding SIVmac239 gag. SIV Gag-specific cellular responses were boosted by intranasal and intratracheal administration of replication-competent adenovirus (Ad5hr-SIVgag) encoding the same gag. To broaden antiviral immunity, the RM were immunized with multimeric HIV clade C (HIV-C) gp160 and HIV Tat. SIV Gag-specific cellular immune responses and HIV-1 nAb developed in some RM. The animals were challenged intrarectally with five low doses of R5 SHIV-1157ipEL-p, encoding a heterologous HIV-C Env (22.1% divergent to the Env immunogen). All five controls became viremic. One out of ten vaccinees was completely protected and another had low peak viremia. Sera from the completely and partially protected RM neutralized the challenge virus > 90%; these RM also had strong SIV Gag-specific proliferation of CD8⁺ T cells. Peak and area under the curve of plasma viremia (during acute phase) among vaccinees was lower than for controls, but did not attain significance. The completely protected RM showed persistently low numbers of the α4β7-expressing CD4⁺ T cells; the latter have been implicated as preferential virus targets in vivo. Thus, vaccine-induced immune responses and relatively lower numbers of potential target cells were associated with protection.

Immunization with HIV-1 gp41 subunit virosomes induces mucosal antibodies protecting nonhuman primates against vaginal SHIV challenges

Human immunodeficiency virus (HIV)-1 is mainly transmitted mucosally during sexual intercourse. We therefore evaluated the protective efficacy of a vaccine active at mucosal sites. Macaca mulatta monkeys were immunized via both the intramuscular and intranasal routes with an HIV-1 vaccine made of gp41-subunit antigens grafted on virosomes, a safe delivery carrier approved in humans with self-adjuvant properties. Six months after 13 vaginal challenges with simian-HIV (SHIV)-SF162P3, four out of five vaccinated animals remained virus-negative, and the fifth was only transiently infected. None of the five animals seroconverted to p27gag-SIV. In contrast, all 6 placebo-vaccinated animals became infected and seroconverted. All protected animals showed gp41-specific vaginal IgAs with HIV-1 transcytosis-blocking properties and vaginal IgGs with neutralizing and/or antibody-dependent cellular-cytotoxicity activities. In contrast, plasma IgGs totally lacked virus-neutralizing activity. The protection observed challenges the paradigm whereby circulating antiviral antibodies are required for protection against HIV-1 infection and may serve in designing a human vaccine against HIV-1-AIDS.

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.

Is developing an HIV-1 vaccine possible?

PURPOSE OF REVIEW

This review discusses select recent data that suggest that indeed it is possible to make a clinically useful preventive vaccine for HIV-1 and outlines some of the remaining obstacles that stand in the way of success.

RECENT FINDINGS

Passive protection studies, with broad neutralizing antibodies for mucosal simian-HIV challenges, in nonhuman primates have suggested that lower doses of neutralizing antibodies than previously thought may be effective in preventing HIV-1 infection. The use of recombinant antibody technology coupled with the ability to culture single memory B cells has yielded new broad neutralizing antibodies and new targets for vaccine design. The success of the RV144 Thai HIV-1 efficacy trials with a replication-defective recombinant canarypox vector (ALVAC)/gp120 prime, clade B/E recombinant gp120 protein boost showing 31% efficacy has given hope that indeed a protective HIV-1 vaccine can be made.

SUMMARY

Recent data in the last year have provided new hope that a clinically useful preventive HIV-1 vaccine can potentially be made. The path forward will require development of improved immunogens, understanding the correlates of protection to HIV-1, and development of immunogens to induce antibodies that can prevent the initial stages of HIV-1 infection at mucosal sites, in order to improve on the RV144 trial results.

Evaluation of a quantitative real-time PCR assay to measure HIV-specific mucosal CD8+ T cell responses in the cervix

Several candidate HIV vaccines aim to induce virus-specific cellular immunity particularly in the genital tract, typically the initial site of HIV acquisition. However, standardized and sensitive methods for evaluating HIV-specific immune responses at the genital level are lacking. Therefore we evaluated real-time quantitative PCR (qPCR) as a potential platform to measure these responses. β-Actin and GAPDH were identified as the most stable housekeeping reference genes in peripheral blood mononuclear cells (PBMCs) and cervical mononuclear cells (CMCs) respectively and were used for normalizing transcript mRNA expression. HIV-specific cellular T cell immune responses to a pool of optimized CD8+ HIV epitopes (HIV epitope pool) and Staphylococcal enterotoxin B (SEB) superantigen control were assayed in HIV infected PBMC by qPCR, with parallel assessment of cytokine protein production. Peak HIV-specific mRNA expression of IFNγ, IL-2 and TNFα occurred after 3, 5 and 12 hours respectively. PBMCs were titrated to cervical appropriate cell numbers to determine minimum required assay input cell numbers; qPCR retained sensitivity with input of at least 2.5×10⁴ PBMCs. This optimized qPCR assay was then used to assess HIV-specific cellular T cell responses in cytobrush-derived cervical T cells from HIV positive individuals. SEB induced IFNγ mRNA transcription was detected in CMCs and correlated positively with IFNγ protein production. However, qPCR was unable to detect HIV-induced cytokine mRNA production in the cervix of HIV-infected women despite robust detection of gene induction in PBMCs. In conclusion, although qPCR can be used to measure ex vivo cellular immune responses to HIV in blood, HIV-specific responses in the cervix may fall below the threshold of qPCR detection. Nonetheless, this platform may have a potential role in measuring mitogen-induced immune responses in the genital tract.

The route of inoculation determines the tissue tropism of modified vaccinia Tiantan expressing the spike glycoprotein of SARS-CoV in mice

The live replication‐competent modified vaccinia virus Tiantan (MVTT) is an attractive vaccine vector, yet little is known about its tissue tropism and pathology in vivo. Recently, we demonstrated that a recombinant MVTT expressing the spike glycoprotein of SARS‐CoV (namely MVTT‐S) is superior to the non‐replicating modified vaccinia Ankara (MVA‐S) for inducing high level of neutralizing antibodies through mucosal vaccination. In this study, we further determined the tissue tropism and safety of MVTT‐S after the vaccine was administrated through various routes including: intramuscular (i.m.), intranasal (i.n.), and intravaginal (i.vag.) inoculations, respectively. Using real‐time PCR, nested PCR, immunohistochemistry and in situ hybridization assays, we found that MVTT‐S was able to produce a transient infection in all cases within 48 hr post‐inoculation, yet the major site of viral replication in various tissues or organs was dependent on the route of viral administration. We demonstrated that i.m. injection of MVTT‐S primarily targeted draining inguinal lymph nodes, whereas mucosal inoculation had broader range of tissue infections. i.n. inoculation involved infections in lungs, kidneys, spleens and cervix lymph nodes while i.vag. administration targeted uteruses, ovaries, kidneys and spleens. Critically, the infection did not cause severe pathogenic consequences in infected tissues, which was consistent to the attenuated phenotype of MVTT‐S. Our findings have implications for the optimization of vaccination route and for studies on the correlation between the magnitude of immune responses and the extent of tissue involvement in vivo. J. Med. Virol. 82: 727–734, 2010. © 2010 Wiley‐Liss, Inc.

GM-CSF increases mucosal and systemic immunogenicity of an H1N1 influenza DNA vaccine administered into the epidermis of non-human primates

Background

The recent H5N1 avian and H1N1 swine-origin influenza virus outbreaks reaffirm that the threat of a world-wide influenza pandemic is both real and ever-present. Vaccination is still considered the best strategy for protection against influenza virus infection but a significant challenge is to identify new vaccine approaches that offer accelerated production, broader protection against drifted and shifted strains, and the capacity to elicit anti-viral immune responses in the respiratory tract at the site of viral entry. As a safe alternative to live attenuated vaccines, the mucosal and systemic immunogenicity of an H1N1 influenza (A/New Caledonia/20/99) HA DNA vaccine administered by particle-mediated epidermal delivery (PMED or gene gun) was analyzed in rhesus macaques.

Methodology/Principal Findings

Macaques were immunized at weeks 0, 8, and 16 using a disposable single-shot particle-mediated delivery device designed for clinical use that delivers plasmid DNA directly into cells of the epidermis. Significant levels of hemagglutination inhibiting (HI) antibodies and cytokine-secreting HA-specific T cells were observed in the periphery of macaques following 1–3 doses of the PMED HA DNA vaccine. In addition, HA DNA vaccination induced detectable levels of HA-specific mucosal antibodies and T cells in the lung and gut-associated lymphoid tissues of vaccinated macaques. Importantly, co-delivery of a DNA encoding the rhesus macaque GM-CSF gene was found to significantly enhance both the systemic and mucosal immunogenicity of the HA DNA vaccine.

Conclusions/Significance

These results provide strong support for the development of a particle-mediated epidermal DNA vaccine for protection against respiratory pathogens such as influenza and demonstrate, for the first time, the ability of skin-delivered GM-CSF to serve as an effective mucosal adjuvant for vaccine induction of immune responses in the gut and respiratory tract.

Towards developing standardized protocols for evaluation of cellular mucosal immune responses - Recommendations from a DAIDS/NIH workshop, June 15-16, 2009

Although 80% of HIV infections occur through mucosal routes and vaccine strategies need to be designed for inducing protective immune responses at the site of the viral entry, it has proven to be very challenging to measure these responses. A 2-day workshop was convened by Division of AIDS, National Institutes of Health on June 15-16, 2009 to address the challenges encountered in the evaluation of mucosal T cell immune responses. The goal of the workshop was to obtain recommendations/consensus for developing standardized protocols for the assessment of mucosal immunity. This report summarizes the areas of consensus and recommendations that should assist in developing standardized methodologies for the evaluation of mucosal immune responses.

Elevation of intact and proteolytic fragments of acute phase proteins constitutes the earliest systemic antiviral response in HIV-1 infection

The earliest immune responses activated in acute human immunodeficiency virus type 1 infection (AHI) exert a critical influence on subsequent virus spread or containment. During this time frame, components of the innate immune system such as macrophages and DCs, NK cells, beta-defensins, complement and other anti-microbial factors, which have all been implicated in modulating HIV infection, may play particularly important roles. A proteomics-based screen was performed on a cohort from whom samples were available at time points prior to the earliest positive HIV detection. The ability of selected factors found to be elevated in the plasma during AHI to inhibit HIV-1 replication was analyzed using in vitro PBMC and DC infection models. Analysis of unique plasma donor panels spanning the eclipse and viral expansion phases revealed very early alterations in plasma proteins in AHI. Induction of acute phase protein serum amyloid A (A-SAA) occurred as early as 5-7 days prior to the first detection of plasma viral RNA, considerably prior to any elevation in systemic cytokine levels. Furthermore, a proteolytic fragment of alpha-1-antitrypsin (AAT), termed virus inhibitory peptide (VIRIP), was observed in plasma coincident with viremia. Both A-SAA and VIRIP have anti-viral activity in vitro and quantitation of their plasma levels indicated that circulating concentrations are likely to be within the range of their inhibitory activity. Our results provide evidence for a first wave of host anti-viral defense occurring in the eclipse phase of AHI prior to systemic activation of other immune responses. Insights gained into the mechanism of action of acute-phase reactants and other innate molecules against HIV and how they are induced could be exploited for the future development of more efficient prophylactic vaccine strategies.