Preclinical Animal Models
Current Animal Models for Selection of New Vaccine Candidates in the Pipeline
The current animal models most used for vaccine evaluation are mouse, guinea pig and non-human primates (NHP) (macaques).[116,117] SCID mice, which are highly immunocompromised as they lack T- and B-lymphocyte, have emerged as the reference model for safety consented among TB vaccine researchers and regulatory bodies.[116,117] When it comes to protective efficacy evaluation, there are many models but all present limitations. The mouse model is the most readily used as it is cheaper and murine immunological reagents for TB assessment are readily available. Nevertheless, differently to human TB, commonly used mice strains (C57/BL6 and BALB/c) are highly resistant to MTB infection and do not form caseous granuloma in lungs. Moreover, human and mouse macrophages have been shown to use distinct mechanisms to kill intracellular MTB through mammalian Toll-like receptors (TLR). Moreover, human macrophages use a vitamin D-dependent mechanism to produce antimicrobial peptides not found in mice. Unlike the widely employed mouse models, guinea pigs are highly susceptible to TB infection and as such are considered a more stringent model to evaluate new TB vaccines, which usually show promising results in mice.[121,122] Histopathology of guinea pigs is more similar to human TB disease, with formation of characteristic caseous granuloma. However, availability of guinea pig immunological reagents is quite limited and this animal model is mainly employed for protective efficacy evaluation of new TB vaccines. Macaque models have quite similar course of disease pathology to humans, and events of latency and reactivation have been described; their use is highly limited. Normally, NHP are used with advanced candidates close to clinical trials. Other animals such as rabbits, cows, pigs or mini pigs are also explored as models. Lack of standardized vaccine evaluation methods present a real challenge in the development of new TB vaccine candidates.[60,124]
Protective Efficacy Readouts Used in Preclinical Studies
Although different readouts, such as weight gain, survival or lung histopathology, are used to compare vaccine protective efficacy, the parameter most utilized is reduction of replication of MTB usually following aerosol challenge in vaccinated animals. Significant higher reduction of bacterial burden in lungs (>0.5 logs) induced by new candidates in comparison with BCG is a criterion currently used for new TB candidates to move to clinical development.[116,117] Nevertheless, considering that BCG is able to significantly reduce MTB challenge replication when compared with unvaccinated animals, we should reckon how predictable protective efficacy of >0.5 logs afforded by new candidates is for human TB disease in clinic (as compared with BCG).
New Models for TB Transmission & Latency
Eradication of TB in 2050 could be possible, if new vaccines target prevention of TB transmission.[60,125] Moreover, since TB is transmitted only during the active form of the disease, prevention of TB reactivation would have a dramatic impact on TB pandemia.[60,125] Nevertheless, animal models for transmission and reactivation are quite limited. In the case of mice, the model most used for vaccines study, the different lung architecture and the absence of cavitation in infected lungs make difficult the establishment of a murine transmission model. Even though some works propose a transmission model in mouse, it is not clear whether this transmission occurs aerogenically. Natural infection of guinea pigs exposed to air ventilated from hospital TB wards has been tested. Efforts are currently underway to re-establish this model for evaluating vaccines and immune responses that may prevent transmission.
Regarding models for latency and reactivation, the cynomolgus macaque model results highly advantageous with respect to other animals. Following low-dose aerosol challenge, this model presents human-like episodes of TB disease: primary disease, latent infection and reactivation. Previous studies show that cohorts of macaques infected with a low-dose challenge demonstrated presence of infection (tuberculin skin test positivity), but only a part of the animals developed active TB.[123,127] This model has been expanded to include PET/CT imaging allowing visualization of the infection dynamics during different stages of infection.
Expert Rev Vaccines. 2013;12(12):1431-1448. © 2013 Expert Reviews Ltd.