Purification and identification of natural CXCL11 isoforms. Conditioned medium from fibroblasts was purified by heparin-affinity and cation-exchange chromatography (not shown) and finally loaded on a C8 RP-HPLC column. Proteins were eluted from the C8 column in an acetonitrile gradient (dotted line) and detected at 214 nm (full line; A). The CXCL11 concentration in the column fractions was determined by ELISA (histograms). Panel B shows the averaged MS spectrum (mass/charge [m/z] versus abundance) for the proteins that eluted between 28 and 29 minutes from the RP-HPLC column. The charges of the detected ions (+ 5 to + 8) are indicated on the averaged spectrum, and the deconvoluted spectrum that was calculated from these charged ions is given as an inset in panel B.