The phosphorylation of L-plastin relies on T-cell costimulation 8, 9, which R788 mouse means it is dependent on signals from the TCR/CD3 receptor complex as well as from signals that origin from accessory receptor. The inhibition of L-plastin phosphorylation by dexamethasone could be

reverted by the synthetic steroid mifepristone, which shows a glucocorticoid receptor dependency 36. Thus, effects of dexamethasone on L-plastin phosphorylation are most likely due to gene expression, suggesting an interference with the signaling pathway upstream of L-plastin phosphorylation. It is known that dexamethasone inhibits proximal signals induced by TCR triggering 37–40. In addition, dexamethasone could interfere with CD28-mediated signals. PI3K activity was shown to be involved in CD28-mediated costimulation 41–43 GSK-3 phosphorylation and its inhibition interferes with L-plastin phosphorylation in immune complex-stimulated

PMN 44. Dexamethasone inhibits PI3K in mast cells 45, which suggests PI3K and its inhibition might be involved in L-plastin phosphorylation upon T-cell costimulation. However, the relevance of dexamethasone for CD28-mediated PI3K activation in primary human T cells remains to be determined. One function of costimulation is the receptor movement to the immunological synapse 7, 12. Consequently, interference with L-plastin expression 5 or phosphorylation (this study) disturbed LFA-1 accumulation in the immune synapse. Interestingly, the effects on the accumulation of CD3 were much weaker and not significant in 5A-LPL-expressing T cells. It was therefore tempting to speculate that L-plastin phosphorylation Ureohydrolase plays a role in peripheral SMAC, but not in central SMAC formation. The fact that 5E-LPL expression rescued only the LFA-1, but not the CD3 enrichment in dexamethasone-treated T cells strengthened that assumption. Interestingly, migration

of the TCR/CD3 complex toward the central SMAC depends on the actin cytoskeleton, as shown by the application of mycotoxins (e.g. cytochalasin D) 2. However, although 5A-LPL expression led to a lower F-actin content in stimulated T cells, the CD3 accumulation was not significantly disturbed. This might be due to the mode of inhibition of the actin cytoskeleton. Thus, in contrast to 5A-LPL expression, the application of mycotoxins to inhibit the actin cytoskeleton does not take into account the complex and spatio-temporal regulation of the actin cytoskeleton. In contrast to 5A-LPL expression, dexamethasone inhibits both the enrichment of the central SMAC-marker CD3 and the peripheral SMAC-marker LFA-1 in the immune synapse significantly. The difference between 5A-LPL expression and dexamethasone treatment on the CD3 enrichment in the immune synapse could be due to additional effects of dexamethasone on the actin cytoskeleton or signaling cascades.