Correlates of protection against Mycobacterium tuberculosis infection

Tuberculosis is an increasing health problem because HIV increases individuals? susceptibility to the disease. Furthermore the bacteria are now often resistant to antibiotics, making treatment difficult. An improved vaccine would therefore be a major contribution to world health. Because Bacille Calmette-Guerin (BCG) the current vaccine, provides partial protection it is likely that any new vaccine will be used to boost the protection afforded by BCG. Several candidate booster vaccines are currently entering clinical trials to test their safety and ability to boost the immune response to tuberculosis. However, proving that a booster increases protection will require a very large, expensive and long clinical trial. A test that could indicate which vaccine candidates can boost protection, would allow a rational selection from among the available candidates and save a great deal of time, money and effort.
In our project we will study the lymphocytes from mice given BCG and then booster vaccines that are already known to increase protection or have no effect. This will allow us to compare the properties of protective and non-protective lymphocytes and identify a ?signature? of protection. We shall then use this in cows immunised against bovine tuberculosis, a disease that is very similar to human tuberculosis, to predict whether different booster immunisations are protective. Finally we shall study human volunteers given a candidate booster vaccine and predict whether this is a good candidate to go on to efficacy trials.

A number of vaccine candidates aimed at boosting protective immunity to M. tuberculosis (Mtb) above the level afforded by BCG, have entered or are close to, early phase clinical trials. Thus a test that would allow identification of effective booster vaccines without the need for efficacy trials would be extremely valuable. We have established an aerosol challenge mouse model in which BCG priming and intra-nasal boosting with a recombinant adenovirus expressing Mtb antigen 85A (Ad85A), induces strong protection against Mtb. We will study in detail the properties of T lymphocytes that are present in the lungs after this immunisation regime. Our hypothesis is that these cells are responsible for increased protection, are part of an endothoracic recirculating pool of memory and effector T cells and exhibit a distinct lung homing and functional signature, which will provide a correlate of increased protection. We will use multi-colour flow cytometry to analyse in detail the phenotype of lung resident 85A-specific T cells, identify their functional properties and perform gene expression analysis using microarrays. These properties will be compared to 85A-specific cells in the spleen induced by intra-nasal Ad85A or splenic 85A-specific cells induced by non-protective immunisation regimes, to identify a signature for lung homing protective cells. We shall then show that the protective signature can be detected in the blood following protective but not non-protective immunisation in mice. We shall go on to examine pre-challenge blood samples from cows immunised with BCG or prime boost regimes that provide increased protection over BCG alone. Finally we shall examine blood samples from human volunteers immunised with BCG followed by modified vaccinia Ankara containing antigen 85A. This project will identify a correlate of increased protection above that afforded by BCG alone in mice and show whether this can be used to provide predictive information in cows and humans.