CD4 and neurodegenerative disease: Activation of BH4 can stimulate inflammatory response,51 with elevated levels of BH4 pathway metabolites augmenting CD4+ and CD8+ T-cell responses, while its suppression abrogates T-cell–mediated immunity.52 Increased SPR expression was demonstrated in the brain Parkinson's disease patients.53 Interestingly, neopterin, one of the metabolites of BH4 synthesis, is a well-known marker of immune cells activity in neurodegenerative diseases, including multiple sclerosis.54