Surface expression of CD3 in the absence of T cell receptor (TcR): evidence for sorting of partial TcR/CD3 complexes in a post-endoplasmic reticulum compartment

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Abstract
The T cell receptor (TcR) for antigen, on the majority of T cells, is a disulfide‐linked heterodimer composed of the α and β chains, noncovalently associated with the CD3 complex of polypeptides (γ, δ, ϵ and ζ). In this report, two murine thymoma cell lines are described which synthesized incomplete TcR/CD3 complexes and expressed low levels of CD3 on their surface in the absence of the TcR chains. The partial TcR/CD3 complexes were composed primarily of the inherently metabolically stable CD3 γ and ϵ subunits. These results were in contrast to previous studies, which suggested that synthesis of all of the component chains of the TcR/CD3 complex is required for the successful transport of any of the chains to the cell surface. The efficiency of transport of the partial TcR/CD3 complexes from the endoplasmic reticulum (ER) to medial Golgi in the two thymomas was similar to complete complexes. However, the transport of the incomplete receptors was impaired at some point between the medial Golgi and the plasma membrane. Taken together with previous studies, these results suggested that T cells have mechanisms to retain partial TcR/CD3 complexes intracellularly both in the ER and in an undefined post‐ER compartment. However, the transport of low levels of partial TcR/CD3 complexes to the cell surface in some T cell lines implied that the retention mechanisms may not always be completely efficient. Cross‐linking of the surface, partial TcR/CD3 complexes with anti‐CD3 ϵ antibodies did not stimulate interleukin 2 (IL 2) production. It is possible, however, that the partial TcR/CD3 complexes have some function which is unrelated to the stimulation of IL 2 production.