Geppert T D, Lipsky P E
Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas 75235.
Reg Immunol. 1989 Jan-Feb;2(1):60-71.
T cells do not see antigen directly, but rather recognize antigen only when displayed by an antigen-presenting cell (APC). APC take up, internalize, and degrade antigen and present the relevant antigenic fragment in association with class II major histocompatibility complex (MHC) molecules to antigen-specific T cells. In addition to presenting the antigenic fragment-MHC complex to T cells, APC must also provide a variety of antigen-nonspecific influences to T cells for antigen recognition to result in T-cell activation. These influences include the effects of various cytokines and signals transmitted by a number of nonspecific receptor-ligand interactions. Traditionally, cells of bone marrow origin that express class II MHC molecules, such as mononuclear phagocytes and B lymphocytes, have been regarded as the only APC. Recently, however, it has been suggested that under various physiologic and pathologic conditions, other tissue cells may develop the capacity to express class II MHC molecules and function as APC. The capacity of these cells to function as APC, however, varies widely, depending on the cell type examined. One determinant of the capacity of a cell to function as an APC appears to be its ability to provide antigen-nonspecific signals. Cells with limited abilities to provide these signals have limited APC function. Indeed, it has been suggested that such cells may induce antigen-specific tolerance. In contrast, cells with the capacity to deliver these signals function as fully competent APC. The functional phenotype of the responding T cell may also vary, depending on the nonspecific signals provided by the APC. Some APC may preferentially stimulate antigen-specific T cells that promote the immune response, whereas others promote the activation of suppressor T cells, thereby limiting the response. Thus, it is likely that cells that are not of bone marrow origin play a role in normal immune responses. Moreover, they may also be important in the development of certain forms of autoimmunity.
T细胞不能直接识别抗原，而是只有当抗原由抗原呈递细胞（APC）呈递时才能识别。APC摄取、内化并降解抗原，然后将相关的抗原片段与II类主要组织相容性复合体（MHC）分子结合，呈递给抗原特异性T细胞。除了将抗原片段-MHC复合体呈递给T细胞外，APC还必须为T细胞提供多种抗原非特异性影响，以使抗原识别导致T细胞活化。这些影响包括各种细胞因子的作用以及许多非特异性受体-配体相互作用传递的信号。传统上，表达II类MHC分子的骨髓来源细胞，如单核吞噬细胞和B淋巴细胞，一直被视为唯一的APC。然而，最近有人提出，在各种生理和病理条件下，其他组织细胞可能会发展出表达II类MHC分子并作为APC发挥作用的能力。然而，这些细胞作为APC发挥作用的能力差异很大，这取决于所检测的细胞类型。细胞作为APC发挥作用的能力的一个决定因素似乎是其提供抗原非特异性信号的能力。提供这些信号能力有限的细胞，其APC功能也有限。事实上，有人提出这样的细胞可能会诱导抗原特异性耐受。相反，能够传递这些信号的细胞则作为完全有功能的APC发挥作用。反应性T细胞的功能表型也可能因APC提供的非特异性信号而异。一些APC可能优先刺激促进免疫反应的抗原特异性T细胞，而另一些则促进抑制性T细胞的活化，从而限制反应。因此，非骨髓来源的细胞很可能在正常免疫反应中发挥作用。此外，它们在某些自身免疫形式的发展中可能也很重要。
