Detection of B-cell populations with monotypic light chain expression in cerebrospinal fluid specimens from patients with multiple sclerosis by polychromatic flow cytometry.

Background: Flow cytometric immunophenotyping is an established method for the detection of occult leptomeningeal disease in patients with aggressive B-cell non-Hodgkin lymphoma, and is increasingly being used in the evaluation of patients without an established diagnosis of lymphoma who present with signs and/or symptoms referable to the central nervous system. However, the specificity of flow cytometric immunophenotyping in the identification of monoclonal B-cell populations in cerebrospinal fluid (CSF) has not been systematically evaluated.

Methods: We searched a consecutive series of CSF specimens submitted to our laboratory for polychromatic (8-color) flow cytometric immunophenotyping between June, 2010 and December, 2012 for cases in which a B-cell population with monotypic immunoglobulin light chain expression was detected in patients without clinical or radiographic evidence of lymphoma.

Results: A B-cell population with monotypic light chain expression was identified in CSF specimens from three patients whose subsequent clinical and radiologic evaluation provided no evidence for lymphoma. In all three patients, a diagnosis of multiple sclerosis was ultimately rendered upon completion of the clinical evaluation.

Conclusions: We conclude that the detection of a B-cell population with monotypic light chain expression in CSF by polychromatic flow cytometry is not diagnostic of occult leptomeningeal involvement by a B-cell non-Hodgkin lymphoma. Moreover, these findings suggest that monotypic B-cell populations detectable by polychromatic flow cytometry may be prevalent in patients with multiple sclerosis, and highlight the importance of clinicopathologic correlation in this application of polychromatic flow cytometry.