Selective induction of p38 mitogen-activated protein kinase activity following A6H co-stimulation in primary human CD4(+) T cells

We have recently described the novel A6H antigen expressed on human periphe ral blood T cells and on renal cell carcinoma cells. Cross-linking of the A 6H antigen results in co-stimulation of human CD4(+) T cells, characterized by induction of the transcription factor activator protein-1 (AP-1), proli feration and prominent IFN-gamma production, but low levels of IL-2. The pr oximal signaling events associated with A6H ligation include protein tyrosi ne kinase phosphorylation and association of p56 Lck, ZAP-70 and the TCR ze ta chain. In this study we show that A6H co-stimulation selectively induced activation of the p38 mitogen-activated protein kinase (MAPK) pathway, whe reas no significant c-Jun N-terminal kinases (JNK) activity was observed. I n contrast, CD28 co-stimulation resulted in both p38 and JNK MAPK activitie s. Human CD4(+) T cells co-stimulated with A6H up-regulated AP-1 binding pr oteins reactive with a proximal AP-1 binding site in the human IFN-gamma pr omoter and a consensus AP-1 binding site. Moreover, preincubation of the T cells with the specific p38 MAPK inhibitor SB203580 resulted in decreased A P-1 binding following A6H or CD28 co-stimulation, This suggests that the p3 8 MAPK pathway is required for induction of full AP-1 binding activity in h uman CD4(+) T cells co-stimulated with A6H or CD28. Blocking the p38 MAPK p athway by SB203580 completely inhibited IFN-gamma production from A6H co-st imulated T cells and radically reduced IFN-gamma production from T cells co -stimulated with anti-CD28. In contrast, no significant inhibition of IL-2 production was seen after blocking of the p38 MAPK in either A6H or CD28 co -stimulated T cells. Since the p38 MAPK recently has been shown to be criti cally involved in regulation of IFN-gamma production from T(h)1 cells, we p ropose that A6H co-stimulation induces a specific pathway, mediated via p38 and AP-1 activation, for induction of a T(h)1 profile in human CD4(+) T ce lls.