Parsimonious Determination of the Optimal Infectious Dose of a Pathogen for Nonhuman Primate Models

Bridging the gaps to overcome major hurdles in the development of next-generation tuberculosis vaccines

Although tuberculosis (TB) remains one of the leading causes of death from an infectious disease worldwide, the development of vaccines more effective than bacille Calmette-Guérin (BCG), the only licensed TB vaccine, has progressed slowly even in the context of the tremendous global impact of TB. Most vaccine candidates have been developed to strongly induce interferon-γ (IFN-γ)-producing T-helper type 1 (Th1) cell responses; however, accumulating evidence has suggested that other immune factors are required for optimal protection against Mycobacterium tuberculosis (Mtb) infection. In this review, we briefly describe the five hurdles that must be overcome to develop more effective TB vaccines, including those with various purposes and tested in recent promising clinical trials. In addition, we discuss the current knowledge gaps between preclinical experiments and clinical studies regarding peripheral versus tissue-specific immune responses, different underlying conditions of individuals, and newly emerging immune correlates of protection. Moreover, we propose how recently discovered TB risk or susceptibility factors can be better utilized as novel biomarkers for the evaluation of vaccine-induced protection to suggest more practical ways to develop advanced TB vaccines. Vaccines are the most effective tools for reducing mortality and morbidity from infectious diseases, and more advanced technologies and a greater understanding of host-pathogen interactions will provide feasibility and rationale for novel vaccine design and development.

Broad host susceptibility of North American amphibian species to Batrachochytrium salamandrivorans suggests high invasion potential and biodiversity risk

Batrachochytrium salamandrivorans (Bsal) is a fungal pathogen of amphibians that is emerging in Europe and could be introduced to North America through international trade or other pathways. To evaluate the risk of Bsal invasion to amphibian biodiversity, we performed dose-response experiments on 35 North American species from 10 families, including larvae from five species. We discovered that Bsal caused infection in 74% and mortality in 35% of species tested. Both salamanders and frogs became infected and developed Bsal chytridiomycosis. Based on our host susceptibility results, environmental suitability conditions for Bsal, and geographic ranges of salamanders in the United States, predicted biodiversity loss is expected to be greatest in the Appalachian Region and along the West Coast. Indices of infection and disease susceptibility suggest that North American amphibian species span a spectrum of vulnerability to Bsal chytridiomycosis and most amphibian communities will include an assemblage of resistant, carrier, and amplification species. Predicted salamander losses could exceed 80 species in the United States and 140 species in North America.

Infectious diseases in free-range compared to conventional poultry production

Commercial chicken production for the supply of meat and eggs has expanded in the scale and variety of production systems in use to facilitate the supply of a range of cost-effective products in response to consumer demand. In the egg sector, traditional cage systems were required to be replaced in the EU by 2012, and most of this production capacity has been replaced by colony cage systems, which provide more space, perching, and environmental enrichment. In some countries there is significant production of "cage-free" or "barn" eggs in which the birds remain housed, but have access to litter. In the UK most egg production is now free-range. Meat poultry is produced in "barn" systems with various enrichments and some free-range. The rise in free-range production has seen the return of some diseases. Biosecurity is a critical tool for disease control in all production systems, particularly when disease challenge is high or farms are in a high poultry density area. An infectious dose of a pathogen is important in determining outcomes in terms of health and productivity, so biosecurity, hygiene, and good management are important across many diseases. Infections which are notifiable, of zoonotic importance and those which are sporadic or endemic in commercial poultry production are reviewed. General principles of preventative medicine applicable to the control of infectious diseases in different poultry production systems are proposed in areas such as biosecurity, system design and maintenance, range, and vaccination, though good management is always important.

Aerosolized pH1N1 influenza infection induces less systemic and local immune activation in the lung than combined intrabronchial, nasal and oral exposure in cynomolgus macaques

Non-human primates form an important animal model for the evaluation of immunogenicity and efficacy of novel 'universal' vaccine candidates against influenza virus. However, in most studies a combination of intra-tracheal or intra-bronchial, oral and nasal virus inoculation is used with a standard virus dose of between 1 and 10 million tissue culture infective doses, which differs from typical modes of virus exposure in humans. This paper studies the systemic and local inflammatory and immune effects of aerosolized versus combined-route exposure to pandemic H1N1 influenza virus. In agreement with a previous study, both combined-route and aerosol exposure resulted in similar levels of virus replication in nose, throat and lung lavages. However, the acute release of pro-inflammatory cytokines and chemokines, acute monocyte activation in peripheral blood as well as increased cytokine production and T-cell proliferation in the lungs were only observed after combined-route infection and not after aerosol exposure. Longitudinal evaluation by computed tomography demonstrated persistence of lung lesions after resolution of the infection and a tendency for more lesions in the lower lung lobes after combined-route exposure versus upper and middle lung lobes after aerosol exposure. Computed tomography scores were observed to correlate with fever. In conclusion, influenza virus infection by aerosol exposure is accompanied by less immune-activation and inflammation in comparison with direct virus installation, despite similar levels of virus replication and development of lesions in the lungs.

Adjuvanted HIV-1 vaccine promotes antibody-dependent phagocytic responses and protects against heterologous SHIV challenge

To augment HIV-1 pox-protein vaccine immunogenicity using a next generation adjuvant, a prime-boost strategy of recombinant modified vaccinia virus Ankara and multimeric Env gp145 was evaluated in macaques with either aluminum (alum) or a novel liposomal monophosphoryl lipid A (MPLA) formulation adsorbed to alum, ALFA. Binding antibody responses were robust and comparable between arms, while antibody-dependent neutrophil and monocyte phagocytotic responses were greatly enhanced by ALFA. Per-exposure vaccine efficacy against heterologous tier 2 SHIV mucosal challenge was 90% in ALFA-adjuvanted males (P = 0.002), while alum conferred no protection. Half of the ALFA-adjuvanted males remained uninfected after the full challenge series, which spanned seven months after the last vaccination. Antibody-dependent monocyte and neutrophil phagocytic responses both strongly correlated with protection. Significant sex differences in infection risk were observed, with much lower infection rates in females than males. In humans, MPLA-liposome-alum adjuvanted gp120 also increased HIV-1-specific phagocytic responses relative to alum. Thus, next-generation liposome-based adjuvants can drive vaccine elicited antibody effector activity towards potent phagocytic responses in both macaques and humans and these responses correlate with protection. Future protein vaccination strategies aiming to improve functional humoral responses may benefit from such adjuvants.

Evaluation of the 50% Infectious Dose of Human Norovirus Cin-2 in Gnotobiotic Pigs: A Comparison of Classical and Contemporary Methods for Endpoint Estimation

Human noroviruses (HuNoVs) are the leading causative agents of epidemic and sporadic acute gastroenteritis that affect people of all ages worldwide. However, very few dose–response studies have been carried out to determine the median infectious dose of HuNoVs. In this study, we evaluated the median infectious dose (ID₅₀) and diarrhea dose (DD₅₀) of the GII.4/2003 variant of HuNoV (Cin-2) in the gnotobiotic pig model of HuNoV infection and disease. Using various mathematical approaches (Reed–Muench, Dragstedt–Behrens, Spearman–Karber, logistic regression, and exponential and approximate beta-Poisson dose–response models), we estimated the ID₅₀ and DD₅₀ to be between 2400–3400 RNA copies, and 21,000–38,000 RNA copies, respectively. Contemporary dose–response models offer greater flexibility and accuracy in estimating ID₅₀. In contrast to classical methods of endpoint estimation, dose–response modelling allows seamless analyses of data that may include inconsistent dilution factors between doses or numbers of subjects per dose group, or small numbers of subjects. Although this investigation is consistent with state-of-the-art ID₅₀ determinations and offers an advancement in clinical data analysis, it is important to underscore that such analyses remain confounded by pathogen aggregation. Regardless, challenging virus strain ID₅₀ determination is crucial for identifying the true infectiousness of HuNoVs and for the accurate evaluation of protective efficacies in pre-clinical studies of therapeutics, vaccines and other prophylactics using this reliable animal model.

Single-Dose Mucosal Immunotherapy With Chimpanzee Adenovirus-Based Vaccine Accelerates Tuberculosis Disease Control and Limits Its Rebound After Antibiotic Cessation

BACKGROUND

The development of strategies to accelerate disease resolution and shorten antibiotic therapy is imperative in curbing the global tuberculosis epidemic. Therapeutic application of novel vaccines adjunct to antibiotics represents such a strategy.

METHODS

By using a murine model of pulmonary tuberculosis (TB), we have investigated whether a single respiratory mucosal therapeutic delivery of a novel chimpanzee adenovirus-vectored vaccine expressing Ag85A (AdCh68Ag85A) accelerates TB disease control in conjunction with antibiotics and restricts pulmonary disease rebound after premature (nonsterilizing) antibiotic cessation.

RESULTS

We find that immunotherapy via the respiratory mucosal, but not parenteral, route significantly accelerates pulmonary mycobacterial clearance, limits lung pathology, and restricts disease rebound after premature antibiotic cessation. We further show that vaccine-activated antigen-specific T cells, particularly CD8 T cells, in the lung play an important role in immunotherapeutic effects.

CONCLUSIONS

Our results indicate that a single-dose respiratory mucosal immunotherapy with AdCh68Ag85A adjunct to antibiotic therapy has the potential to significantly accelerate disease control and shorten the duration of conventional treatment. Our study provides the proof of principle to support therapeutic applications of viral-vectored vaccines via the respiratory route.

Disparate Tuberculosis Disease Development in Macaque Species Is Associated With Innate Immunity

While tuberculosis continues to afflict mankind, the immunological mechanisms underlying TB disease development are still incompletely understood. Advanced preclinical models for TB research include both rhesus and cynomolgus macaques (Macaca mulatta and Macaca fascicularis, respectively), with rhesus typically being more susceptible to acute progressive TB disease than cynomolgus macaques. To determine which immune mechanisms are responsible for this dissimilar disease development, we profiled a broad range of innate and adaptive responses, both local and peripheral, following experimental pulmonary Mycobacterium tuberculosis (Mtb) infection of both species. While T-cell and antibody responses appeared indistinguishable, we identified anti-inflammatory skewing of peripheral monocytes in rhesus and a more prominent local pro-inflammatory cytokine release profile in cynomolgus macaques associated with divergent TB disease outcome. Importantly, these differences were detectable both before and early after infection. This work shows that inflammatory and innate immune status prior to and at early stages after infection, critically affects outcome of TB infection.

Selective breeding for high natural antibody level increases resistance to avian pathogenic Escherichia coli (APEC) in chickens

Keyhole limpet hemocyanin (KLH)-binding natural antibody (NAb) titers in chickens are heritable, and higher levels have previously been associated with a higher survival. This suggests that selective breeding for higher NAb levels might increase survival by means of improved general disease resistance. Chickens were divergently selected and bred for total NAb levels binding KLH at 16 weeks of age for six generations, resulting in a High NAb selection line and a Low NAb selection line. To for test differences in disease resistance, chickens were challenged with avian pathogenic Escherichia coli (APEC) in two separate experiments. Chickens at 8 days of age received one of four intratracheal inoculations of 0.2 mL phosphate buffered saline (PBS): 1) mock inoculate, 2) with 0.2 mL PBS containing 10^8.20 colony-forming units (CFU)/mL APEC, 3) with 0.2 mL PBS containing 10^6.64 CFU/mL APEC, and 4) with 0.2 mL PBS containing 10^7.55 CFU/mL APEC. Mortality was recorded during 7 days post inoculation. Overall, 50-60% reduced mortality was observed in the High line compared to the Low line for all APEC doses. In addition, morbidity was determined of the surviving chickens at 15 days of age. The High line had lower morbidity scores compared to the Low line. We conclude that selective breeding for high KLH-binding NAb levels at 16 weeks of age increase APEC resistance in early life. This study and previous studies support the hypothesis that KLH-binding NAb might be used as an indicator trait for to selective breed for general disease resistance in an antigen non-specific fashion.

Prevention of tuberculosis infection and disease by local BCG in repeatedly exposed rhesus macaques

Tuberculosis (TB) remains the deadliest infectious disease¹, and the widely used Bacillus Calmette-Guérin (BCG) vaccine fails to curb the epidemic. An improved vaccination strategy could provide a cost-effective intervention to break the transmission cycle and prevent antimicrobial resistance^2,3. Limited knowledge of the host responses critically involved in protective immunity hampers the development of improved TB vaccination regimens. Therefore, assessment of new strategies in preclinical models to select the best candidate vaccines before clinical vaccine testing remains indispensable. We have previously established in rhesus macaques (Macaca mulatta) that pulmonary mucosal BCG delivery reduces TB disease where standard intradermal injection fails^4,5. Here, we show that pulmonary BCG prevents infection by using a repeated limiting-dose Mycobacterium tuberculosis challenge model and identify polyfunctional T-helper type 17 (T_H17) cells, interleukin-10 and immunoglobulin A as correlates of local protective immunity. These findings warrant further research into mucosal immunization strategies and their translation to clinical application to more effectively prevent the spread of TB.

Vectored delivery of anti-SIV envelope targeting mAb via AAV8 protects rhesus macaques from repeated limiting dose intrarectal swarm SIVsmE660 challenge

Gene based delivery of immunoglobulins promises to safely and durably provide protective immunity to individuals at risk of acquiring infectious diseases such as HIV. We used a rhesus macaque animal model to optimize delivery of naturally-arising, autologous anti-SIV neutralizing antibodies expressed by Adeno-Associated Virus 8 (AAV8) vectors. Vectored transgene expression was confirmed by quantitation of target antibody abundance in serum and mucosal surfaces. We tested the expression achieved at varying doses and numbers of injections. Expression of the transgene reached a saturation at about 2 x 10¹² AAV8 genome copies (gc) per needle-injection, a physical limitation that may not scale clinically into human trials. In contrast, expression increased proportionately with the number of injections. In terms of anti-drug immunity, anti-vector antibody responses were universally strong, while those directed against the natural transgene mAb were detected in only 20% of animals. An anti-transgene antibody response was invariably associated with loss of detectable plasma expression of the antibody. Despite having atypical glycosylation profiles, transgenes derived from AAV-directed muscle cell expression retained full functional activity, including mucosal accumulation, in vitro neutralization, and protection against repeated limiting dose SIVsmE660 swarm challenge. Our findings demonstrate feasibility of a gene therapy-based passive immunization strategy against infectious disease, and illustrate the potential for the nonhuman primate model to inform clinical AAV-based approaches to passive immunization.

Antibodies are the humoral component of an immune response against an invading pathogen or vaccine immunogen. For challenging vaccine targets, as an alternative to active vaccination to induce the immune system to generate antibodies, current research is exploring the delivery of these proteins to populations at high risk of infection as prophylactics against infectious diseases, like HIV, RSV, and Ebola, amongst others. Passive vaccination via purified protein will require periodic reinjection to retain protective levels in subjects, adding a barrier to large scale coverage. Alternatively, delivery of antibodies using gene therapy may provide a one-time passive vaccination alternative. This strategy comes with its own hurdles, including anti-vector immunity, anti-drug immunity, physical limitations of vector uptake and the need to confirm antibody functionality. To date, many passive vaccinations strategies remain untested in humans. Non-human primate models of infection are frequently useful for predicting the success of vaccine candidates or concepts. Here, we characterize and optimize a rhesus macaque model for the delivery of anti-viral antibodies via the gene therapy vector adeno-associated virus. Lastly, we demonstrate the ability of the mAbs to protect against viral challenge. Our work demonstrates the feasibility and utility of vectored delivery of antibody transgenes in rhesus macaques. We hope this model of antibody delivery may be applied to various disease models in non-human primates and will inform clinical trial design of passive vaccination against infectious diseases.