IgG oligoclonal bands (OCB) are detected in the cerebrospinal fluid (CSF) of more than 90% of multiple sclerosis (MS) patients and are considered the immunological hallmark that supports MS diagnosis. OCB are not unique to MS but are also observed in chronic CNS infections, where they target their causative agents (1–3). However, the target specificities of the IgG within OCB in MS have remained a mystery. Identification of those antigens recognized by OCB antibodies is thought to hold fundamental clues to the pathogenesis of MS. In a recent PNAS publication, Brändle et al.(4) provide evidence that OCB in MS target ubiquitous intracellular antigens released in cellular debris. In 1942, Kabat et al.(5) established the diagnostic value of quantitative determinations of CSF gamma globulins in clinical neurology, and particularly in MS. They observed that changes in CSF IgG were independent of those in serum, suggesting that this Ig production was compartmentalized to the CNS. Advances in CSF analytics and gel electrophoresis led to the identification of OCB in 1959 (6). CSF OCB in MS patients are persistent, which is thought to be a reflection of both ongoing CNS inflammation and immunologic memory. Understanding the specificity of OCB has since captivated the interest of clinical neurologists and scientists alike. It has been assumed that the OCB target antigens are relevant to MS pathogenesis. The most popular theory contends that IgG within OCB target myelin autoantigens and/or viruses that may elicit CNS damage directly or indirectly via molecular mimicry. Some earlier studies that evaluated whole CSF IgG from MS patients identified antibodies to several different viruses, such as measles, varicella zoster, human T-lymphocytic virus 1, and human hepatitis virus 6 (7), whereas other investigations found antibodies targeting major myelin proteins, myelin basic protein (MBP) and myelin oligodencrocyte glycoprotein (MOG)(8, 9) as well as glycolipids, fatty acids, and neurofilament proteins (10). Similarly, more recent investigations that have applied single-cell PCR cloning to individual CSF B cells in MS have detected antibodies to certain viruses or myelin proteins (11–13). However, it has been impossible to match specificity of antibodies identified in CSF to OCB by studying whole CSF IgG or recombinant antibodies constructed from rearranged Ig heavy-and light-chain genes in individual B cells.

Dornmair and coworkers used a combination of new biochemical, proteomic, and transcriptomic approaches (4, 14) to examine the specificity of antibodies in MS OCB. Disulfide-linked IgG heavy-(IgH) and IgG light-(IgL) chain complexes were purified from single OCB spots using affinity chromatography and two-dimensional gel electrophoresis. Those antibody (IgH2IgL2) complexes were then analyzed by mass spectrometry to generate patient-specific Ig peptidomes. In parallel, IgH and IgL genes, including the unique complementarity-determining region 3, from CSF B cells isolated from the corresponding patient were sequenced to generate patient-specific IgH and IgL transcriptomes. Alignment of patient-specific Ig peptidomes to the corresponding patient-specific Ig transcriptomes produced full-length sequences of matching IgG heavy and light chains, therefore representing distinct antibody species originating from one of the OCB. Using an expression system, Dornmair and coworkers produced recombinant OCB antibodies for target antigen characterization using a protein microarray that displayed over 9,400 full-length recombinant human proteins. As validation of this methodology, these investigators demonstrated that a recombinant OCB …