We have demonstrated that T-cell receptor xi (xi) mRNA with a 562-bp deleted alternatively spliced 3'-untranslated region (3'UTR) observed in T cells of patients with systemic lupus erythematosus (SLE) can lead to a reduction in xi and TCR/CD3 (J. Immunol., 2003 & 2005). To determine the region in xi mRNA 3'UTR for the regulation of xi,xi mRNA with 3'UTR truncations ligated into pDON-A1 was used to infect murine T-cell hybridoma MA5.8 cells, which do not contain As a Western blot analysis demonstrated the importance of the regions from +871 to +950, containing conservative sequence 1 (CS1), and +1070 to +1136, containing CS2, for the production of, we constructed MA5.8 mutants carrying mRNA 3'UTR with deletions of these regions (Delta CS1 and Delta CS2 mutants). Western blot and FACS analyses showed significant reduction in the cell surface xi and TCR/CD3 in both these mutants, and IL-2 production was decreased, compared with MA5.8 cells transfected with wild-type xi mRNA. Furthermore, real-time PCR demonstrated the instability of xi mRNA with 3'UTR deletions in these MA5.8 mutants. In conclusion, CS1 and CS2 may be responsible for the regulation of and TCR/CD3 through the stability of xi mRNA in SLE T cells. (c) 2007 Elsevier Inc. All rights reserved.