Murine lymphotactin: gene structure, post-translational modification and inhibition of expression by CD28 costimulation.
The production of lymphokines by T cells is dependent on TCR signalling and is enhanced by costimulatory signals through CD28. In IL-2 producing T cells clones (Th1 cells), TCR signalling in the absence of costimulatory signals renders these cells unable to product IL-2 in response to subsequent stimulation through the TCR and CD28. Using a differential screening approach we independently cloned murine lymphotactin from a cDNA library produced from an unresponsive Th1 cell. Resting Th1 clones and freshly isolated CD8+ T cells produced lymphotactin mRNA maximally in response to TCR stimulation alone and expression was surprisingly inhibited by CD28 costimulation, at least early after activation. In addition, Th1 cells made unresponsive by prior TCR stimulation, produced lymphotactin but not IL-2 mRNA when restimulated through the TCR and CD28. Sequence analysis of the lymphotactin gene and mapping of the transcription initiation sites have delineated the promoter region and the boundaries of exons and introns. Studies on post-translational modification of lymphotactin demonstrate the cleavage of the signal peptide containing the first two cysteine residues in the predicted amino acid sequence and suggests that the secreted lymphotactin is an O-glycosylated protein.