tokens: These mutations prevent CD40 ligand ( CD40L ) -mediated degradation of inhibitor of NF-kappaB alpha ( IkappaB-alpha ) and account for the following observations : B cells from XHM-ED patients are unable to undergo immunoglobulin class-switch recombination and antigen-presenting cells ( APCs ) are unable to synthesize the NF-kappaB-regulated cytokines interleukin 12 ( IL-12 ) or tumor necrosis factor alpha ( TNF-alpha ) when stimulated with CD40L .