Shen L, Potter T A, Kane K P
Department of Medical Microbiology and Immunology, Faculty of Medicine, University of Alberta, Edmonton, Canada.
J Exp Med. 1996 Nov 1;184(5):1671-83. doi: 10.1084/jem.184.5.1671.
Cytotoxic T lymphocyte (CTL) activation requires specific T cell receptor (TCR)-class I major histocompatibility complex (MHC) antigen complex interactions as well as the participation of coreceptor or accessory molecules on the surface of CTL. CD8 can serve as a coreceptor in that it binds to the same MHC class I molecules as the TCR to facilitate efficient TCR signaling. In addition, CD8 can be "activated" by TCR stimulation to bind to class I molecules with high avidity, including class I not recognized by the TCR as antigenic complexes (non-antigen [Ag] class I), to augment CTL responses and thus serve an accessory molecule function. A Glu/Asp227-->Lys substitution in the class I alpha 3 domain acidic loop abrogates lysis of target cells expressing these mutant molecules by alloreactive CD8-dependent CTL. Lack of response is attributed to the destruction of the CD8 binding site in the alpha 3 domain which is likely to disrupt CD8 coreceptor function. The relative importance of the class I alpha 3 domain acidic loop Glu227 in coreceptor as opposed to accessory functions of CD8 is unclear. To address this issue, we examined CTL adhesion and degranulation in response to immobilized class I-peptide complexes formed in vitro from antigenic peptides and purified class I molecules containing wild-type or Glu227-->Lys substituted alpha 3 domains. The alpha 3 domain mutant class I-peptide complexes were bound by CTL and triggered degranulation, however to much lower levels than wild-type class I-peptide complexes. In further experiments, it is directly demonstrated that the alpha 3 domain mutant class I molecules, which lack the Glu227 CD8 binding site, still serve as TCR-activated, avidity-enhanced CD8 accessory ligands. However, mutant class I peptide Ag complexes failed to effectively serve as CD8 coreceptor ligands to initiate TCR-dependent signals required to induce avidity-enhanced CD8 binding to coimmobilized non-Ag class I molecules. Thus the Glu227-->Lys mutation effectively distinguishes CD8 coreceptor and avidity-enhanced CD8 accessory functions.
细胞毒性T淋巴细胞（CTL）的激活需要特异性T细胞受体（TCR）与I类主要组织相容性复合体（MHC）抗原复合物相互作用，以及CTL表面共受体或辅助分子的参与。CD8可作为共受体，因为它与TCR结合相同的I类MHC分子，以促进有效的TCR信号传导。此外，CD8可被TCR刺激“激活”，以高亲和力结合I类分子，包括未被TCR识别为抗原复合物的I类分子（非抗原[Ag]I类），从而增强CTL反应，因此发挥辅助分子功能。I类α3结构域酸性环中的Glu/Asp227→Lys取代消除了同种异体反应性CD8依赖性CTL对表达这些突变分子的靶细胞的裂解作用。反应缺失归因于α3结构域中CD8结合位点的破坏，这可能会破坏CD8共受体功能。I类α3结构域酸性环中的Glu227在CD8的共受体功能与辅助功能方面的相对重要性尚不清楚。为了解决这个问题，我们检测了CTL对体外由抗原肽和含有野生型或Glu227→Lys取代α3结构域的纯化I类分子形成的固定化I类-肽复合物的粘附和脱颗粒反应。α3结构域突变的I类-肽复合物被CTL结合并触发脱颗粒，但水平远低于野生型I类-肽复合物。在进一步的实验中，直接证明了缺乏Glu227 CD8结合位点的α3结构域突变I类分子仍然作为TCR激活的、亲和力增强的CD8辅助配体。然而，突变的I类肽Ag复合物未能有效地作为CD8共受体配体，启动诱导亲和力增强的CD8与共固定的非Ag I类分子结合所需的TCR依赖性信号。因此，Glu227→Lys突变有效地区分了CD8共受体功能和亲和力增强的CD8辅助功能。
