Secretory products of multiple sclerosis B cells are cytotoxic to oligodendroglia in vitro

B cells are important in the pathogenesis of multiple sclerosis (MS) and some of
the effects are not dependent on maturation of B cells into immunoglobulin (Ig)
producing plasmablasts and plasma cells. B cells present antigen, activate T
cells, and are involved in immunoregulation and cytokine secretion. To determine
if B cells from MS patients secrete products that have deleterious effects on
glial cells not mediated by Ig, and to compare effects with secretory products of
normal controls (NC), we isolated B cells from 7 patients with relapsing
remitting MS (RRMS) and 4 NC. B cells were cultured alone or after stimulation
with CD40 ligand (CD40L), CD40L+cross-linking of the B cell antigen receptor
(xBCR) and CD40L+xBCR+stimulation of toll like receptor 9 (TLR9). Supernatants
were harvested and incubated with mixed central nervous system (CNS) neonatal rat
glial cells. Supernatants from unstimulated NC B cells induced on average death
of 7% (range 0-24%) of differentiated oligodendrocytes (OL); in contrast,
supernatants from unstimulated B cells from RRMS patients induced death of 57%
(range 35-74%) of OL. Supernatants of stimulated B cells from NC did not increase
the minimal OL death whereas stimulation of B cells from RRMS had variable
results compared to unstimulated B cells. Supernatants from both NC and RRMS
induced microglial enlargement and loss of normal resting bipolar morphology. OL
death did not correlate with levels of tumor necrosis alpha (TNF-α), lymphotoxin
alpha (LT-α), interleukin 6 (IL-6), IL-10, transforming growth factor beta 1
(TGF-β1) or any combination or ratio of these cytokines. Analysis of 26
supernatants from NC and RRMS patients failed to detect IgM. There were very low
levels of IgG in 8 of the 26 supernatants, and no correlation between of OL death
and presence or absence of IgG. Sera used in both the B cell and glial cell
cultures were heated, which inactivates complement. The effects of B cell
supernatants on OL could be direct and/or indirect involving either microglia
and/or astrocytes. The identity of the toxic factor(s) is as yet unknown. Thus we
have demonstrated that B cells from patients with RRMS but not NC secrete one or
more factors toxic to OL. It is possible that such factors produced by peripheral
blood B cells when within the CNS could contribute to demyelination in MS
patients.