Abnormal expression of various molecular forms and distribution of T cell receptor ? chain in patients with systemic lupus erythematosus

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Abstract
Objective T cells from the majority of patients with systemic lupus erythematosus (SLE) display antigen receptor‐mediated signaling aberrations associated with defective T cell receptor (TCR) ζ chain expression. The TCR ζ chain, a critical signaling molecule, exists in multiple molecular forms and membrane fractions with distinct functions in antigen‐mediated signaling processes. This study was undertaken to investigate the complete spectrum of expression of the different forms and distribution of the TCR ζ chain in SLE T cells. Methods T cells were isolated from 48 SLE patients and 21 healthy subjects. The expression of various forms of the TCR ζ chain was investigated by immunoblotting with specific antibodies. The lipid raft‐associated form of the ζ chain was determined by quantitating the solubilized ζ chain after disruption of the lipid rafts by cholesterol depletion using methyl‐β‐cyclodextrin. The distribution of the ζ chain was investigated by fluorescence microscopy. Results The phosphorylated 21‐ and 23‐kd forms and the detergent‐insoluble membrane‐associated form of the TCR ζ chain and alternatively spliced ζ chain were significantly decreased in SLE T cells. In contrast, major ubiquitinated forms of the ζ chain were increased in these cells. We also identified up‐regulation of a novel 14‐kd form of the ζ chain in SLE T cells. Resting SLE T cell membranes had an increased percentage of the residual membrane‐bound ζ chain in the lipid rafts. Fluorescence microscopy findings indicated that the residual ζ chain is more clustered on the cell membranes of SLE T cells. Conclusion These results suggest that, in addition to the 16‐kd form, expression of other molecular forms and fractions of the TCR ζ chain as well as its membrane distribution are abnormal in SLE T cells. Increased lipid raft association and surface clustering of the ζ chain may explain the molecular mechanisms underlying the signaling abnormalities in these cells.