Antigen discovery in chronic human inflammatory central nervous system disease: Panning phage-displayed antigen libraries identifies the targets of central nervous system-derived IgG in subacute sclerosing panencephalitis

The presence of increased IgG in the brains of humans with infectious and i nflammatory CNS diseases of unknown etiology such as multiple sclerosis may be a clue to the cause of disease. For example, the intrathecally synthesi zed oligoclonal bands in diseases such as subacute sclerosing panencephalit is (SSPE) or cryptococcal meningitis have been shown to represent Ab direct ed against the causative agents, measles virus (MV), or Cryptococcus neofor mans, respectively. Using SSPE as a model system, we developed a strategy t o identify the antigenic targets of the intrathecal disease-relevant IgG in chronic human inflammatory and demyelinating diseases of the CNS. Librarie s of cDNA Ags were displayed on the surface of T7Select bacteriophage and b iopanned on IgG extracted from the brain of an SSPE patient, or on a monosp ecific recombinant Fab identified from SSPE brain. After three or six round s of biopanning on either Ab, positive phage-displayed Ags reacting with Ig G were enriched to 35-77% of all panned clones. Sequence analysis of the po sitive clones identified fragments of the nucleocapsid protein of MV, the c ause of SSPE. The sensitivity of the system was determined by diluting the positive clones from this SSPE phage-displayed library at a ratio of 10(-6) into another phage-displayed library that did not contain any detectable M V Ags; after six rounds of panning, the positive clones comprised 34% of al l phage and were also shown to be NW nucleocapsid specific. This strategy w ill be useful to identify potentially rare Ags in diseases of unknown cause .