Multiple sclerosis is a chronic inflammatory and demyelinating disorder of the 
CNS with an unknown aetiology. Although intrathecal immunoglobulin G (IgG) 
synthesis is a key feature of the disease, little is still known about the B 
cell response in the CNS of multiple sclerosis patients. We analysed the 
phenotype and kinetics of different B cell subsets in patients with multiple 
sclerosis, infectious disease (IND) and non-inflammatory neurological disease 
(NIND). B cells were detected in the CSF of multiple sclerosis and IND patients, 
but were largely absent in NIND patients. In the CSF, the majority of B cells 
had a phenotype of memory B cells and short-lived plasma blasts (PB); plasma 
cells were absent from the compartment. The proportion of PB was highest in 
multiple sclerosis patients and patients with acute CNS infection. While PB 
disappeared rapidly from the CSF after resolution of infection in IND patients, 
these cells were present at high numbers throughout the disease course in 
multiple sclerosis patients. CSF PB numbers in multiple sclerosis patients 
strongly correlated with intrathecal IgG synthesis and inflammatory parenchymal 
disease activity as disclosed by MRI. This study identifies short-lived plasma 
blasts as the main effector B cell population involved in ongoing active 
inflammation in multiple sclerosis patients.