Novel IL-17 producing memory cells are key to vaccine-based immunity in the lung

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Project summary: Tuberculosis (TB) kills more than 2 million people every year worldwide. The only TB vaccine available, Bacille Calmette Guerin (BCG) has variable efficacy and this has prompted the search for more effective vaccines. Although significant progress has been made in identification of Mycobacterium tuberculosis (Mtb) antigen candidates, a significant hurdle to successful vaccine design remains our poor understanding of how the memory response mediates protection in the lung. The long-term goal of this proposal is to define the basic requirements for induction of protective memory immunity in the lung against pulmonary pathogens such as Mtb. The protective recall response to TB is associated with the appearance of CD4+ T cells that produce interferon (IFN)-y and many studies have focused on inducing these cells in the hope of improving vaccination. Unfortunately improvement over that mediated by BCG has not occurred. It is possible therefore that other factors play a role in protective memory and that determining what these factors are and how they can be modulated will lead to substantial improvement in vaccine efficacy. In this regard we have recently found that interleukin (IU-17-producina memory cells precede the IFN-Y memory cell response in the lung, and that in the absence of these cells the IFN-v memory response does not occur. Based on preliminary data we propose a three-phase model of vaccine-induced protection in TB. Firstly vaccination induces both IFN-y and IL-17-producing cells but only IL-17-producing cells populate the noninflamed lung. Upon challenge with Mtb, the lung resident memory cells produce IL-17 and trigger local expression of chemokines (phase 1). The chemokine gradient attracts IFN-y memory cells from the lymphoid pool (phase 2). In turn, these IFN-y cells activate myeloid cells in the lung to halt Mtb growth (phase 3). The absence of an effective IL-17 response (phase 1) ablates the ability of vaccinated mice to generate a protective recall immune response to Mtb challenge (phases 2 and 3). The finding that a lung-resident IL-17- producing population of CD4+ memory cells is generated by vaccination and that these cells are a critical component of vaccine-induced protection against TB is entirely novel. Determining the specific factors that are required for the persistence and survival of these cells in the lung following vaccination is crucial. We propose two aims. In Aim One the factors required for the survival and maintenance of IL-17-producing memory cells in the lung will be investigated. In Aim two, the location of the cells and the factors that impact the establishment of this population in the lung will be determined. The relevance of this work to public health is that it will promote rational development of vaccine strategies and will therefore have the potential to reduce the incidence of TB.