A unique mode of CD28 augmentation of IL-2 expression in lamina propria T-cells: mRNA stabilization without transactivation of the CD28 response element (CD28RE)

Abstract

The pathways leading to T cell activation in lamina propria (LP) T cells are different from those of peripheral T cells. LP T cells have enhanced IL-2 secretion through activation of the CD2 pathway. Coligation of the CD28 molecule leads to a synergistic effect further enhancing IL-2 secretion and cell proliferation (J. Immunol. 1995, 154:664-675). Previous studies have characterized CD28 mediated augmentation of TCR signaling in PBL through transcriptional activation of an IL-2 promoter CD28 response element (CD28RE), as well as by enhanced mRNA stabilization. The aim of our study was to characterize the molecular basis involved in CD28 costimulation of IL-2 production in LP T cells. LP T cells were highly responsive to CD28 costimulation exhibiting a 7 to 10 fold increase in IL-2 secretion as compared to cells activated through the CD2 pathway alone. Increased IL-2 production was paralleled with upregulation of IL-2 mRNA, as detected in Northern blot analysis. This increase in mRNA was due in part to increased IL-2 mRNA stability as determined following actinomycin D treatment. In contrast to CD28 mediated transcriptional activation in PBL, electrumobility shift assays revealed that CD28 coligation of LP T cells does not result in increased binding of transacting factors to the CD28RE. Likewise, Western blots failed to detect any changes in IKBcc or IKB13 levels following CD28 coligation. Moreover, although CD2 enhanced IL-2 secretion is reflected by upregulation of activity of IL-2 promoter reporter constructs transfected into LP T cells, there was no enhanced CD28 mediated promoter activation following coligation. Our results indicate that unique molecular mechanisms are involved in CD28 cosignaling and regulation of IL-2 secretion in lamina propria T cells which differ from those seen in peripheral T cells. These observations suggest a biological significance for different mechanisms of 1L-2 gene activation in initiating and maintaining cytokine production in the mucosa. This work was supported by USPHS Grant DK43211.