An adaptive immune response is initiated when naive T cells encounter specific antigen on the surface of an antigen-presenting cell in T-cell zones of secondary lymphoid tissues. In most cases, the antigen-presenting cells responsible for activating naive T cells, and inducing their clonal expansion, are conventional dendritic cells that express the co-stimulatory molecules B7.1 and B7.2. Conventional dendritic cells not only reside in lymphoid tissues, but they also survey the periphery, where they encounter pathogens, take up antigen at sites of infection, become activated through innate recognition, and migrate to local lymphoid tissue. The dendritic cell may become a potent direct activator of naive T cells, or it may transfer antigen to dendritic cells resident in secondary lymphoid organs for cross-presentation to naive CD8 T cells. Plasmacytoid dendritic cells contribute to rapid responses against viruses by producing type I interferons. Activated T cells produce IL-2, which is important in modulating early proliferation and differentiation of T cells; various other signals drive the differentiation of several types of effector T cells, which primarily act by releasing mediators directly onto their target cells. This triggering of effector T cells by peptide:MHC complexes occurs independently of co-stimulation, so that any infected target cell can be activated or destroyed by an effector T cell. CD8 cytotoxic T cells kill target cells infected with cytosolic pathogens, thus removing sites of pathogen replication. CD4 T cells can become specialized effectors that in turn promote distinct arms of the immune response by targeting different innate and adaptive immune cells for enhanced effector function: macrophages (TH1); eosinophils, basophils, and mast cells (TH2); neutrophils (TH17); or B cells (TFH). Thus, effector T cells control virtually all known effector mechanisms of the adaptive and innate immune response. In addition, subsets of CD4 regulatory T cells are produced that help control and limit immune responses by suppressing T-cell activity.
Fig. 9.47 Cytotoxic T cells kill target cells bearing specific antigen while sparing neighboring uninfected cells. all the cells in a tissue are susceptible to killing by the cytotoxic proteins of armed effector Cd8 T cells, but only infected cells are killed. specific recognition by the T-cell receptor identifies which target cell to kill, and the polarized release of the cytotoxic granules (not shown) ensures that neighboring cells are spared.
T cell recognizes infected cell Infected cell is programmed for death Neighboring uninfected cells are not killed
Questions.
9.1 Multiple Choice: Which of the following statements is true?
A. development of the arterial and venous system is regulated by the homeobox transcription factor prox1.
B. arteries deliver lymphotoxin to the non-hematopoietic stromal LTi cell to induce lymph-node development.
C. Lymphotoxin-α3 signaling represses nFκB to induce chemokines such as CXCL13.
D. Lymphotoxin-α3 binds TnFr-I and supports development of the cervical and mesenteric lymph nodes.