A signaling role for the cytoplasmic segment of the CD8 alpha chain detected under limiting stimulatory conditions

Abstract

To test for the functional importance of the cytoplasmic segment of the CD8 molecule, a mouse T-cell hybridoma expressing a T-cell receptor specific for the class I major histocompatibility complex product H-2Kb was transfected with a set of CD8 alpha-chain (Ly-2) and/or beta-chain (Ly-3) genes encoding polypeptides with carboxyl-terminal truncations or substitutions. When challenged with Kb-positive splenocytes, transfectants expressing Ly-2 homodimers that lacked cytoplasmic tails responded nearly as effectively as wild-type Ly-2 transfectants. However in marked contrast to the wild-type Ly-2 transfectants, tailless Ly-2 transfectants were greatly impaired in their ability to respond to Kb-transfected L cells. Coexpression of the Ly-3 gene did not restore this impaired response. The unique functional property of the Ly-2 alpha cytoplasmic segment was further supported by the analysis of a chimeric Ly-3 subunit in which the cytoplasmic segment was replaced by the one from the Ly-2 alpha subunit. When associated with a soluble Ly-2 subunit lacking a transmembrane segment, the chimeric Ly-3 was indeed sufficient to restore the response to Kb-transfected L cells. Since the lateral mobility of the tailless Ly-2 molecules on the cell surface was nearly identical to that of the wild-type Ly-2 molecules, their partially impaired function may indicate that they have lost their cis-acting signaling properties but retained their ability to bind class I products of the major histocompatibility complex.