Cerebrospinal fluid, neuroinflammation, and electrospray ionization mass spectrometry

Abstract

Electrospray ionization tandem mass spectrometry (eSI MS/MS) offers a particularly sensitive and specific technique for the identification and quantitation of previously unrecognized biomarkers of disease (untargeted metabolomic analysis). Importantly, it may also simplify and improve the measurement of existing biomarkers, and allow combinations of tests to be consolidated into a single diagnostic workflow. Cross validation of techniques is also valuable, as an untargeted metabolomic approach should detect existing well validated biomarkers, if the sensitivity of the method allows. This is a good test of this type of study. Likewise, new biomarkers suggested by knowledgebased approaches should be picked up by untargeted methods, if they have diagnostic utility. The paper by Yan et al. exemplifies these points. The authors set out to find new biomarkers of acute inflammation in a retrospective cohort of patients and controls where stored cerebrospinal fluid (CSF) samples were available, using an accurate mass eSI MS/MS untargeted metabolomic platform. They found that markers of the kynurenine pathway are perturbed, indicating that indoleamine-2,3-dioxygenase (IDO) is activated in encephalitis. IDO activation in inflammation, in the central nervous system (CNS) as well as peripherally, is not new. Most publications have measured kynurenine, kynurenic acid, or the kynurenine:tryptophan ratio as measures of IDO activation. The advantage of the new methodology is that many metabolites can be measured simultaneously. Yan et al. found nine components of the tryptophan kynurenine pathway were significantly changed. The clear identification of known (but difficult to measure) biomarkers by their untargeted approach provides evidence of the strength of the method. They also found an increase in neopterin, another known biomarker of inflammation and in the ADMA:arginine ratio. Increased levels of ADMA are implicated in the encephalitis of liver disease, and dysfunction of the NOS pathway is considered to be a driver in the encephalopathy of hyperammonaemia. The authors show some data (CSF:serum albumin ratio) to indicate blood–brain barrier disruption in their patient group, and, as they acknowledge, this may mean that some of the changes described indicate peripheral as well as CNS changes. They suggested the calculation of ratios to improve specificity for CNS changes, and a prospective follow-up study measuring paired peripheral blood samples. If peripheral blood markers provide the same information as the relatively invasive CSF, it would be preferable to use them from a practical point of view, particularly for tracking disease progression and treatment efficacy, where sequential samples over time would be required. In conclusion, the study provides a useful validation of the untargeted approach in that known biomarkers of encephalitis are clearly identified by the analysis, and suggests that combinations of those biomarkers, measured using eSI MS/MS, may offer improved diagnostic power and speed of analysis.