Models for Testing Vaccines – DEV Immunopaedia

Tuberculosis

The current tuberculosis (TB) vaccine, Bacillus Calmette–Guérin (BCG) is not effective in preventing TB infection in adolescents and adults and therefore a new, more effective and long-lasting TB vaccine is needed. There are a variety of animal models used to test TB vaccines. The efficacy of these candidate vaccines is usually compared to that of BCG – the standard TB vaccine given to infants.

Murine models

Murine mouse models are amongst animal models used for testing TB vaccines. Mice are extensively used in vaccine research because they are relatively inexpensive, easy to maintain and standardising models between different laboratories is easily achievable.
There are various mouse models to test the efficacy of preventative vaccines such as the low dose aerosol and the intratracheal models. There is also a murine latent TB model for testing the efficacy of therapeutic vaccines based on passive immunization.
C57Bl/6 mice are the most common mice used in TB vaccine development research. However, they are tolerant to TB infection and therefore exhibit a limited pathological profile which is very different from the human response to TB infection.
Another disadvantage of this model is that the pathology of mouse lungs is substantially different from that of the human lungs, which may lead to misleading results.

Guinea Pigs

Guinea pigs are also used to test TB vaccine efficacy.
Interestingly, guinea pigs have an immunological response similar to that observed in human lesions following TB infection although in guinea pigs eosinophils dominate this response.
BCG is very effective in preventing the establishment of TB infection in this animal model.
The similarities between the immune response in guinea pigs and humans makes it the most common model for testing TB vaccine efficacy after initial screening in mice.
One disadvantage of this model is that immunological reagents for these animals are not readily available as compared to mouse model reagents.

Non-human primates

Non-human primates (NHPs) are attractive models for evaluating TB vaccine candidates because of their close evolutionary link to humans. Rhesus and cynomolgus NHP models resemble the full spectrum of human TB-related lesions.
However, there are many challenges with NHP models. They are expensive to maintain and require specialized facilities. Only a small number of NHPs can be used per experiment which may limit the amount of information that can be obtained from these studies.
The lungs of NHPs are much smaller than human lungs and lack interlobular septae which may affect the control of pulmonary lesions.

Large mammals

Large mammals have the advantage of possessing lungs which contain interlobular septae like human lungs.
Some examples of large mammals used in candidate TB vaccine testing include cows, goats and pigs.
One of the disadvantages of using cows to test TB vaccine efficacy is that their strain of the TB-causing bacterium, Mycobacterium bovis, has much higher virulence as compared to the human Mycobacterium tuberculosis bacterium.

HIV

There is still no effective HIV vaccine which makes the testing of candidate vaccines in a relevant animal model essential. One area scientists are investigating is the elicitation of broadly neutralising antibody (bnAbs) by an immunogen. Focus for vaccine development has been drawn to neutralising antibodies because many existing licensed vaccines seem to work by eliciting neutralising antibodies.

Non-human primates

NHPs have been widely used to test the efficacy of HIV vaccine candidates with varied success. A well-known trial, the STEP or Phambili trial commenced in 2004 and used a trivalent adenoviral vector vaccine which encoded for HIV gag, pol and nef and aimed to induce high titre T-cell responses. Although this vaccine candidate had previously been found to control Simian/Human Immunodeficiency virus (SHIV) infections in NHPs, the vaccine not only had low efficacy in humans but also increased the risk of acquiring HIV in vaccinated volunteers who had pre-existing antibodies to adenovirus and/or volunteers who were uncircumcised.
The RV144 vaccine consisted of a canarypox prime with a recombinant gp120 boost. The biggest immunologic force exerted in this vaccine was the non-neutralising IgG antibody response targeted at the V1/V2 loop of the HIV envelope. The binding of IgA antibodies to envelope positively correlated with HIV infection. This finding was surprising as primate models had previously shown non-neutralising antibodies to be ineffective in preventing SHIV transmission.

Recombinant Envelope trimer immunization studies

Ongoing research uses HIV recombinant Envelope (Env) trimer proteins as immunogens to elicit bnAb responses.
These trimers have been tested in rabbits, guinea pigs, NHPs including macaques and recently in cows for immunogenicity (how well they can elicit an immune response in these animals).
NHPs are closely related to humans making them good models to test HIV vaccines and the engineering of SHIV strains also makes the model closer to resembling the human virus.
Cows are also good models to test immunogen responses although their antibody repertoire is different from that of humans which may result in misleading findings.

Models for HIV passive immunization

There have been numerous efforts, both in animal models and humans, to test the efficacy of administering purified monoclonal antibodies as a therapeutic measure to reduce HIV viraemia to undetectable levels at least temporarily and to use this method for a therapeutic vaccine.
Studies have been conducted in humans where HIV-infected individuals were treated with monoclonal antibodies with and without anti-retroviral therapy combination with ART. Some studies have been successful showing viral rebound; however, in most cases the virus mutates and becomes resistant to the therapeutic antibodies given.
Studies have also used humanized mice in antibody passive transfer studies. Humanized mice are a good model for human disease but have limitations. For example, the dynamics of HIV replication may differ in humanized mice as compared to humans so viraemic suppression by monoclonal antibodies may be easier to achieve in mice than it would be in nonhuman primates or humans. Furthermore, the tissue in humanized mice may not completely replicate that of humans and therefore these mice most likely have less viral reservoirs than humans which could influence the results.
Studies using passive immunization strategies have also been performed on NHPs such as rhesus monkeys. The studies show potential for the use of monoclonal antibodies to control the virus in HIV-1 infected individuals.

Malaria

Human trials

Malaria vaccine testing has a big advantage over TB and HIV because malaria challenge trials can be conducted in humans.
Since controlled malaria infections can be easily cured in adults with few to no long-term effects, study participants can be vaccinated and then inoculated with the parasite. Vaccine immunogenicity, safety and efficacy testing can therefore be conducted in humans.
Phase Ib trials which test efficacy of malaria vaccines can be conducted through exposing participants to infected mosquito bites; a process known as sporozoite challenge. Blood-stage challenge occurs when participants are inoculated with blood-stage parasites.
Human malaria sporozoite infections are conducted under strict clinical conditions where the participants are exposed to a certain number of bites from infected mosquitos. These exposures provide preliminary information of the efficacy of the candidate pre-erythrocytic stage malaria vaccine.
Some Phase IIb human infection trials are carried out in non-endemic areas. One of the concerns of this is that the response of previously malaria-unexposed individuals may differ from the response to the vaccine by people in malaria-endemic areas.

Avian models

Several avian (bird) models have been used in early malaria vaccine studies to screen for candidate vaccines. However, avian models did not fully represent mammalian malaria infection and therefore are rarely used now.

Mouse models

Mouse models are used extensively in the study of malaria vaccine responses. Mice are infected with Plasmodium species such as berghei and P. chabaudi.
Many immunological pathways related to malaria infection have been established using mouse models. For example, T cells have been shown to play a critical role in the immune response against liver stage malaria infection. CD8 T cells specifically have been found to have a protective role against rodent malaria infection and CD4 T cells play a protective role specifically in pre-erythrocytic stages of infection.
However, there is no consensus that the same immunological processes occur during human malaria infection and therefore mice may not be the best model for malaria vaccine development research.

Non-human primates

Since NHPs are so closely related to humans they are extensively used in malaria vaccine development.
Rhesus monkeys are naturally susceptible to the primate malaria species knowlesi and have been established as a good experimental malaria model.
The Aotus genus of NHPs is used in malaria research because it can be infected with falciparum and P. vivax – the main parasites causing the malaria disease burden in humans.