The Third Signal in T Cell-Mediated Autoimmune Disease?

The initial event in the pathogenesis of autoimmune disease is thought to be the priming of naive autoreactive T cells by an infection with a cross-reactive microorganism. Although such cross-reactive priming should be a common event, autoimmune disease does not frequently develop. This situation is reflected after the immunization of C57BL/6 mice with the neuroantigen myelin oligodendrocyte glycoprotein (MOG) with CFA, which primes a type 1 T cell response but does not lead to clinical or histological manifestation of experimental allergic encephalomyelitis unless pertussis toxin is injected in addition. We show in this study that, in MOG:CFA-primed mice, the autoimmune CNS pathology develops after intracerebral deposition of TLR9-activating CpG oligonucleotides, but not following non-CpG oligonucleotide injection or after aseptic cryoinjury of the brain. Thus, access of primed MOG-specific Th1 cells to the uninflamed CNS or to CNS undergoing sterile inflammation did not suffice to elicit autoimmune pathology; only if the APC in the target organ were activated in addition by the TLR9-stimulating microbial product did they exert local effector functions. The data suggest that such licensing of APC in the target organ by microbial stimuli represents a checkpoint for functional self-tolerance. Therefore, microorganisms unrelated to the cross-reactive agent that primes the autoreactive T cells could dictate the onset and exacerbation of autoimmune diseases.