Increased neopterin and interferon-gamma secretion and lower availability of L-tryptophan in major depression: Further evidence for an immune response

There is now some evidence that major depression may be accompanied by an immune response. The latter condition is suggested by elevated secretion of neopterin and interferon-γ (IFNγ) and by lower L-tryptophan (L-TRP) plasma levels. This study investigated the plasma levels of neopterin, L-TRP, and the L-TRP/competing amino acids (CAA) ratio in 30 normal control subjects and 47 depressed subjects (16 minor depressed, 13 simple major depressed, and 18 melancholic subjects), and IFNγ secretion by mitogen-stimulated peripheral blood mononuclear cells in 7 normal control subjects and 13 major depressed subjects. Plasma neopterin levels were significantly higher in depressed subjects than in normal controls; 61% of melancholic patients had increased neopterin levels (≥ 7 nmol/1) with a specificity of 90%. Patients with major depression had significantly lower L-TRP and L-TRP/CAA values compared with normal control subjects. The amino acid values were significantly and negatively correlated with plasma neopterin levels. Major depressed subjects exhibited significantly higher IFNγ secretion than did normal control subjects. The results further support the hypothesis that major depression is accompanied by an immune response and that the lower L-TRP availability in that illness may be an epiphenomenon of immune activation.