Role of increased guanosine triphosphate cyclohydrolase-1 expression and tetrahydrobiopterin levels upon T cell activation

Tetrahydrobiopterin (BH₄) is an essential co-factor for the nitric-oxide (NO) synthases, and in its absence these enzymes. produce superoxide (O^-̇₂) rather than NO. The rate-limiting enzyme for BH4 production is guanosine triphosphate cyclohydrolase-1 (GTPCH-1). Because endogenously produced NO affects T cell function, we sought to determine whether antigen stimulation affected T cell GTPCH-1 expression and ultimately BH₄ levels. Resting T cells had minimal expression of inducible NOS (NOS2), endothelial NOS (NOS3), and GTPCH-1 protein and nearly undetectable levels of BH₄. Anti-CD3 stimulation of T cells robustly stimulated the coordinated expression of NOS2, NOS3, and GTPCH-1 and markedly increased both GTPCH-1 activity and T cell BH4 levels. The newly expressed GTPCH-1 was phosphorylated on serine 72 and pharmacological inhibition of casein kinase II reduced GTPCH-1 phosphorylation and blunted the increase in T cell BH ₄. Inhibition of GTPCH-1 with diaminohydroxypyrimidine(1mmol/liter) prevented T cell BH₄ accumulation, reduced NO production, and increased T cell O^-̇₂ production, due to both NOS2 and NOS3 uncoupling. GTPCH-1 inhibition also promoted TH2 polarization in memory CD4 cells. Ovalbumin immunization of mice transgenic for an ovalbumin receptor (OT-II mice) confirmed a marked increase in T cell BH₄in vivo. These studies identify a previously unidentified consequence of T cell activation, promoting BH₄ levels, NO production, and modulating T cell cytokine production.