Mycobacterium tuberculosis evasion of CD4+ T cells in vivo

描述（由申请人提供）：开发有效结核病疫苗的最大障碍是结核分枝杆菌能够持续存在并引起进行性感染，尽管已经产生了适应性免疫反应。虽然部分有效的疫苗可能来自抗原、佐剂和递送系统的最佳选择，但结核病防治的重大进展将需要基于克服或绕过结核分枝杆菌用来逃避适应性免疫反应的机制来增强耐药性的新方法。为了识别和表征这些机制，我们开发了新的工具，用于小鼠对结核分枝杆菌的免疫模型，以发现和表征限制对结核分枝杆菌适应性免疫反应功效的机制。我们开发了一种使用单克隆抗体识别特定肽的系统：MHC II复合物和几种重组结核分枝杆菌菌株，以验证结核分枝杆菌在体内抑制MHC II抗原呈递的假设，并验证结核分枝杆菌将抗原转移到未感染细胞作为免疫逃避策略的假设。我们还开发了一种系统，使用来自一种具有结核分枝杆菌Ag85B特异性转基因T细胞抗原受体的独特小鼠系的CD4+ T细胞，并发现在结核分枝杆菌感染的慢性阶段，编码Ag85B基因的下调伴随着体内CD4+ T细胞对Ag85B的反应减弱。综上所述，这些结果表明结核分枝杆菌可能通过调节抗原基因的表达来避免CD4+效应T细胞的识别和清除。我们还开发了一种肺中结核分枝杆菌感染细胞和CD4+ T细胞的可视化和定位方法，并发现肺中少数结核分枝杆菌感染细胞与CD4+ T细胞直接接触，这支持了CD4+ T细胞在感染部位识别结核分枝杆菌感染细胞较差的假设。在这个应用中，我们提出实验来检验结核分枝杆菌感染细胞和肺中抗原特异性CD4+ T细胞之间的相互作用是有缺陷的，这有助于感染的持续存在的一般假设。我们将检验结核分枝杆菌感染细胞与CD4+ T细胞在慢性感染期间相互作用缺陷的具体假设，这是由于以下因素的结合：1)结核分枝杆菌抑制受感染细胞的抗原呈递；2)慢性感染阶段结核分枝杆菌抗原表达降低；3)将结核分枝杆菌抗原从感染细胞转移到肉芽肿中邻近的未感染细胞，使未感染细胞充当诱饵，在远离感染细胞的地方激活T细胞。我们提出的研究将为结核分枝杆菌逃避哺乳动物适应性免疫应答消除的机制提供前所未有的见解，并将指导未来努力开发提高人类对结核病耐药性的手段。

DESCRIPTION (provided by applicant): The greatest barrier to development of efficacious vaccines against tuberculosis is the ability of Mycobacterium tuberculosis to persist and to cause progressive infection despite development of an adaptive immune response. While partially-efficacious vaccines may result from optimal selection of antigens, adjuvants, and delivery systems, major progress against tuberculosis will require novel approaches to enhancing resistance, based on overcoming or bypassing the mechanisms used by M. tuberculosis to evade adaptive immune responses. To identify and characterize those mechanisms, we have developed novel tools for use in a mouse model of immunity to M. tuberculosis to discover and characterize mechanisms that limit the efficacy of the adaptive immune response to M. tuberculosis. We have developed a system using a monoclonal antibody that recognizes a specific peptide:MHC II complex and several recombinant strains of M. tuberculosis, to test the hypothesis that M. tuberculosis inhibits MHC II antigen presentation in vivo, and to test the hypothesis that M. tuberculosis transfers antigens to uninfected cells as an immune evasion strategy. We have also developed a system using CD4+ T cells from a unique line of mice with a transgenic T cell antigen receptor specific for M. tuberculosis Ag85B, and have discovered that downregulation of the gene encoding Ag85B during the chronic stage of M. tuberculosis infection is accompanied by diminished in vivo CD4+ T cell responses to Ag85B. Taken together, these results suggest that M. tuberculosis may modulate expression of antigen genes as a means of avoiding recognition and elimination by CD4+ effector T cells. We also developed a method for visualization and localization of M. tuberculosis-infected cells and CD4+ T cells in the lungs, and have found that a minority of M. tuberculosis-infected cells in the lungs are in direct contact with CD4+ T cells, which supports the hypothesis that CD4+ T cells recognize M. tuberculosis-infected cells poorly at the site of infection. In this application, we propose experiments to test the general hypothesis that interactions between M. tuberculosis-infected cells and antigen-specific CD4+ T cells in the lungs are defective, and that this contributes to persistence of the infection. We will test the specific hypothesis that defective interactions between M. tuberculosis-infected cells and CD4+ T cells during chronic infection are due to a combination of: 1) M. tuberculosis inhibition of antigen presentation by infected cells; 2) decreased expression of M. tuberculosis antigens during the chronic stage of infection; and 3) transfer of M. tuberculosis antigens from infected cells to neighboring uninfected cells in granulomas, allowing uninfected cells to act as decoys to activate T cells at a distance from infected cells. Our proposed studies will provide unprecedented insight into the mechanisms used by M. tuberculosis to evade elimination by the mammalian adaptive immune response, and will guide future efforts to develop the means to increase human resistance to tuberculosis. PUBLIC HEALTH RELEVANCE: One of the major problems that has prevented development of a successful TB vaccine is that the bacteria that cause TB are not eliminated by normal immune responses. In this project, we will identify and characterize the mechanisms that TB bacteria use to avoid recognition and elimination by immune cells. We expect that understanding these mechanisms will allow us and others to develop novel ways to increase resistance of people to tuberculosis.