Aberrant Humoral Immune Responses in Neurosyphilis: CXCL13/CXCR5 Play a Pivotal Role for B-Cell Recruitment to the Cerebrospinal Fluid

Background: Previous studies documented that humoral immune responses participated in neurological damage in neurosyphilis patients. However, the mechanisms that trigger and maintain humoral immunity involved in neurosyphilis remain unknown. Methods: Using flow cytometry, expression of B cells was measured in neurosyphilis and non-neurosyphilis. Expression of immunoglobulin indices and chemokine ligand CXCL13 was detected by enzyme-linked immunosorbent assay. The migration and inhibition assays were evaluated by modified chamber assays. The presence of CXCL13+ cells, cluster of differentiation (CD)20+ B cells, CD3+ T cells, CD138+ plasma cells and CD35+ follicular dendritic cells was studied by immunohistochemistry. Results: Enrichment of B cells was observed and activated in the cerebrospinal fluid (CSF) of neurosyphilis patients. Immunoglobulin indices were increased and associated with the progress to neurosyphilis. High expression of CSF CXCL13 mediated B cell migration both in vitro and in vivo. There was a positive correlation among the CSF B cells, immunoglobulin indices, and CSF CXCL13 levels. Ectopic germinal centers (EGCs), important structures for humoral immunity, were observed in the intracranial syphilitic gumma. Conclusions: CXCL13/CXCR5 mediated the aggregation of B cells, that directed the aberrant humoral immune responses via the formation of EGCs, which suggests a molecular mechanism of neurological damage in neurosyphilis.