Exploitation of the hepatic stellate cell Raman signature for their detection in native tissue samples

Hepatic stellate cells (HSCs) surround liver sinusoids and store retinol while they are quiescent. During fibrotic liver diseases and acute-on-chronic liver failure they change to the activated state in which they proliferate, lose their retinol content and deposit extracellular matrix molecules. The process of HSC activation is of utmost interest, but so far only insufficiently understood, because there is a lack of techniques to address the function of single HSCs in the tissue context. In this contribution, the potential of Raman micro-spectroscopy for the label-free detection of HSCs in mouse liver samples is demonstrated. First, culture-induced activation of primary mouse HSCs is followed in vitro and characterized by means of Raman spectroscopy. The HSC activation state is confirmed by immunofluorescence labeling of glial fibrillary acidic protein (GFAP) and α-smooth muscle actin (ASMA). As expected, the unique Raman spectrum of retinol in quiescent HSCs is lost during activation. Nevertheless, successful discrimination of HSCs from primary hepatocytes is possible during all states of activation. A classification model based on principal component analysis followed by linear discriminant analysis (PCA-LDA) of the lipid droplet Raman data yields a prediction accuracy of 99%. The in vitro results are transferred to fresh liver slices and freshly sampled livers. Quiescent HSCs and a HSC transforming from quiescent to activated state are identified based on their Raman signature. This provides valuable information on HSC activation state in the liver.