Neopterin

Neopterin is a product of pteridine metabolism, released by activated macrophages and dendritic cells in response to interferon-γ (IFN-γ). It is a sensitive marker of the activation of the cellular immune system. Raised neopterin shows: macrophages are being activated via interferon-γ from T cells and natural killer cells. Raised values occur, for example, with viral infections, autoimmune diseases and persistent inflammation. Because interferon-γ also activates the enzyme IDO, neopterin and tryptophan breakdown via the kynurenine pathway often rise together.

How neopterin is formed

Neopterin is formed from GTP, a building block of genetic material and an energy carrier of the cell. The enzyme GTP cyclohydrolase I turns it into a precursor from which most body cells make tetrahydrobiopterin (BH4) – the cofactor for producing serotonin, dopamine and nitric oxide. Human macrophages and dendritic cells have only little enzyme for the next step. When they are activated by interferon-γ, which type 1 T helper cells and natural killer cells release in the defence against viruses and pathogens inside cells, they greatly increase GTP cyclohydrolase and release the precursor as 7,8-dihydroneopterin and neopterin. Neopterin is stable in blood and urine and is excreted via the kidneys. Because interferon-γ also switches on the enzyme IDO, neopterin and tryptophan breakdown often rise together.

Low neopterin – what does it mean?

A low value is the normal case: macrophages are currently hardly being stimulated via interferon-γ, and cellular defence is running at baseline level. The cells then use their GTP cyclohydrolase products mainly for making BH4. With medications that dampen the immune system, the value can be particularly low.

High neopterin – what happens in the body?

A raised value shows that T cells and natural killer cells are releasing interferon-γ and macrophages are activated. The macrophages then form not only neopterin but also reactive oxygen and nitrogen species with which they kill pathogens; neopterin itself can enhance these oxidative processes. Via the IDO that is activated at the same time, more tryptophan is broken down into kynurenine. Raised values occur with viral infections, with infections by pathogens that live inside cells, with autoimmune diseases and persistent inflammation, and with rejection reactions after transplants. Because neopterin is excreted via the kidneys, it also rises with reduced kidney function.

The interpretation is given in your report.