Effector T cells of the CD4+ lineage function by secreted cytokines and cell surface molecules to activate other cells to eliminate microbes. CD4+ T cells also participate indirectly in host defense by helping B lymphocytes to produce high-affinity antibodies against extracellular microbes and by promoting the development of fully functional CTLs that combat intracellular microbes such as viruses. The roles of helper T cells in antibody responses are described in Chapter 11 and in CTL responses in Chapter 9. Here our focus is on the roles of CD4+ T cells as the effector cells of cell-mediated immunity.
The functions of CD4+ effector cells in cell-mediated immunity can be divided into several steps (Fig. 10-4):
• Recruitment of other leukocytes. The recruitment of neutrophils, monocytes, and eosinophils to the site of the reaction is mediated by chemokines produced by the T cells themselves and by other cells in response to cytokines secreted by the T cells. As we have mentioned previously and will discuss in more detail later, different subsets of CD4+ effector cells recruit different types of leukocytes into the reaction.
• Activation of the recruited leukocytes. The mechanisms by which CD4+ T cells activate other leukocytes involve T cell expression of the surface protein CD40 ligand (CD40L) and secretion of cytokines. The CD40L-mediated pathway is best defined for TH1-mediated activation of macrophages and is described in this context later. The roles of cytokines in activating different leukocyte populations are also described later for each subset of effector T cells.
• Amplification of the response. As in all adaptive immune responses, there are several positive feedback loops that serve to amplify the response. For instance, cytokines produced by T cells activate macrophages to produce other cytokines that in turn act on the T cells and increase their responses.
• Downregulation of the response. Because effector T cells are typically short-lived, they die after performing their function. As the antigen is eliminated, the stimuli for propagating the response are lost, and the response declines over time. Special control mechanisms may also operate to limit effector responses. For instance, both TH1 cells and activated macrophages produce the cytokine IL-10, which functions mainly to inhibit further TH1 differentiation and macrophage activation. Additional inhibitory mechanisms, such as other anti-inflammatory cytokines and receptors that turn off T cell activation, may also be involved in controlling T cell-mediated responses.
With this overview, we proceed to a discussion of the functions of the three major subsets of effector CD4+ T cells in cell-mediated immunity.
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