The increasing evidence supporting a role for B cells in the pathogenesis of multiple sclerosis prompted us to investigate the influence of known susceptibility variants on the surface expression of co-stimulatory molecules in these cells. Using flow cytometry we measured surface expression of CD40 and CD86 in B cells from 68 patients and 162 healthy controls that were genotyped for the multiple sclerosis associated single nucleotide polymorphisms (SNP) rs4810485, which maps within the CD40 gene, and rs9282641, which maps within the CD86 gene. We found that carrying the risk allele at rs4810485T lowers the cell-surface expression of CD40 in all tested B cells sub-types (in total B cells p ≤ 5.10 x 10-5 in patients and ≤ 4.09 x 10-6 in controls), while carrying the risk allele at rs9282641G increased the expression of CD86, with this effect primarily seen in the naïve B cell subset (p = 0.048 in patients and 5.38 x 10-5 in controls). In concordance with these results analysis of RNA expression demonstrated that the risk allele of rs4810485T resulted in lower total CD40 expression (p = 0.057) but with an increased proportion of alternative splice-forms leading to decoy receptors (p = 4.00 x 10-7). Finally, we also observed that the risk allele of rs4810485T is associated with decreased levels of interleukin-10 (p = 0.020), which is considered to have an immunoregulatory function downstream of CD40. Given the importance of these co-stimulatory molecules in determining the immune reaction that appears in response to antigen our data suggest that B cells might have an important antigen presentation and immunoregulatory role in the pathogenesis of multiple sclerosis.