Elucidating the T-cell epitopes and T-cells responses of two B. pseudomallei vaccine antigens
Led by Dr Julen Tomás Cortázar (University College Dublin, Ireland), with Prof Susanna Dunachie (University of Oxford, UK), Asst Prof Louise Gourlay (University of Milan, Italy), Prof Giorgio Colombo (University of Pavia, Italy), and Dr Siobhán McClean (University College Dublin, Ireland)
Melioidosis is a potential lethal infection that is quite common in South East Asia and is on the increase in other tropical countries such as Brazil. It can kill up to 40% of people and is particularly dangerous for people with diabetes. The bacteria that cause melioidosis are extremely resistant to antibiotics and there is no vaccine available to protect people from this infection. We want to develop a vaccine that will be safe, effective and cheap to produce. As a result of several years studying how bacteria attach to human cells we have discovered some molecules (called antigens) that are likely to be good vaccines as they protect mice from this infection. More recently we have discovered two more antigens that we would like to study in this proposal to see if they would be effective in protecting people from melioidosis. There are two arms to the immune system: one requiring antibodies for effect and the other relying on T cells for effect. In the case of melioidosis the T-cell response is essential in successfully fighting this infection. In this proposal we will try to pinpoint the parts of the antigen molecules that are most likely to stimulate the arm of immune system that is most important to protect people from melioidosis infection. We will identify the parts of the antigen that stimulate the T-cell response. We will also look at the level of T cell response in mice and compare that to the T-cell responses in human blood.
The in silico prediction of T cell immunogenic peptides from the PDB structure of PA26 antigen revealed four different epitopes, located on the surface of the protein and thus, also accessible to antibodies. Unfortunately, one predicted epitope was highly hydrophobic and could not be synthesized in free peptide form. Immunization of mice with PA26 resulted in a weaker antibody response relative to that induced by the protective BpOmpW antigen. T cell responses against PA26 were moderate, and lower in comparison with protective antigen BpOmpW. Nevertheless, we observed elevated protective IFN-y responses relative to the control group. Insulin resistance altered the immune response against PA26, impairing Th2 response and regulatory T cells. Although high-resolution X-ray data were collected for the LP20 antigen, the crystal structure could not be solved, therefore we could not carry out epitope predictions on this antigen. Immunization of mice with LP20 antigen produced a robust antibody response, but poor T cell responses. In contrast, it elicited different Foxp3+ regulatory cell types.
Human sera from melioidosis survivors recognised BpOmpW but did not recognize any of the antigens, indicating that they do not participate in the immune response to melioidosis.