Mycobacterium tuberculosis evasion of CD4+ T cells in vivo

描述(由申请人提供)：开发有效的结核病疫苗的最大障碍是结核分枝杆菌持续存在并导致渐进性感染的能力，尽管产生了适应性免疫反应。虽然部分有效的疫苗可能来自对抗原、佐剂和递送系统的最佳选择，但针对结核病的重大进展将需要基于克服或绕过结核分枝杆菌用于逃避适应性免疫反应的机制的新方法来增强耐药性。为了识别和表征这些机制，我们开发了新的工具用于结核分枝杆菌免疫的小鼠模型，以发现和表征限制结核分枝杆菌获得性免疫反应有效性的机制。我们已经开发了一种系统，使用一种识别特定多肽的单抗：MHC II复合体和几种重组的结核分枝杆菌菌株，以检验结核分枝杆菌在体内抑制MHC II抗原递呈的假设，并测试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，使用来自具有针对结核分枝杆菌Ag85B的转基因T细胞抗原受体的独特小鼠的CD4+T细胞，并发现在结核分枝杆菌感染的慢性期，编码Ag85B的基因下调伴随着体内对Ag85B的CD4+T细胞反应减弱。综上所述，这些结果提示，结核分枝杆菌可能通过调节抗原基因的表达来避免被CD4+效应T细胞识别和消除。我们还开发了一种可视化和定位肺中结核分枝杆菌感染细胞和CD4+T细胞的方法，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细胞在感染部位对结核分枝杆菌感染细胞的识别能力较差的假设。在这一应用中，我们提出了实验来检验一般假设，即结核分枝杆菌感染细胞与肺部抗原特异性CD4+T细胞之间的相互作用是有缺陷的，这有助于感染的持久性。我们将测试特定的假设，即在慢性感染期间结核分枝杆菌感染细胞与CD4+T细胞之间的缺陷相互作用是由于以下几种因素的综合作用：1)结核分枝杆菌感染细胞抑制了感染细胞的抗原提呈；2)在慢性感染阶段结核分枝杆菌抗原表达减少；3)在肉芽肿中，结核分枝杆菌抗原从感染细胞转移到邻近的未感染细胞，允许未感染细胞充当诱饵，在远离感染细胞的地方激活T细胞。我们提出的研究将对结核分枝杆菌用于逃避哺乳动物适应性免疫反应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的抵抗力的方法。 与公共卫生相关：阻碍研制成功的结核病疫苗的主要问题之一是，引起结核病的细菌不能通过正常的免疫反应来消除。在这个项目中，我们将识别和描述结核病细菌用来避免免疫细胞识别和消除的机制。我们希望，了解这些机制将使我们和其他人能够开发新的方法来提高人们对结核病的抵抗力。

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.