CD40 and cerebral toxoplasmosis: gondii activity induced by CD40 ligation may provide an explanation for susceptibility to ocular and cerebral toxoplasmosis in PKR−/− mice since macrophages/microglia are considered to be key effectors of protection against the parasite in neural tissue and CD40−/− and CD154−/− mice are susceptible to these forms of the disease [26], [47]–[50].