The danger model: a renewed sense of self

Activation of APCs through CD40 or Toll-like receptor 9 overcomes tolerance and precipitates autoimmune disease

Some autoreactive T cells normally escape thymic selection and persist in the periphery. This is true of myelin-reactive CD4(+) T cells, the effectors of experimental autoimmune encephalomyelitis (EAE) in laboratory animals and the presumed mediators of multiple sclerosis in humans. Nonetheless, most individuals do not succumb to autoimmune disease. There is growing evidence that while peripheral APCs stimulate immune responses against foreign Ags in the setting of tissue destruction and "danger," they actually maintain tolerance against self Ags under steady state conditions. We hypothesized that tolerance against candidate autoantigens could be reversed by activation of APCs via CD40 or Toll-like receptor 9 signaling. Adult SJL mice injected i.p. with a peptide fragment of proteolipid protein (a candidate autoantigen in multiple sclerosis) emulsified in IFA fail to mount lymphoproliferative or cytokine responses and are protected from EAE upon subsequent challenge with the Ag combined with adjuvants. Here we report that tolerized proteolipid protein-specific lymph node cells regain the ability to divide, differentiate along a Th1 lineage, and transfer EAE when reactivated in the presence of agonistic Abs against CD40 or CpG oligonucleotides. The effects of both anti-CD40 and CpG oligonucleotides are dependent upon induction of IL-12. Our findings suggest two mechanisms to explain the well-documented association between infectious illnesses and flare-ups of multiple sclerosis. Microbial pathogens could 1) release molecules that bind Toll-like receptors, and/or 2) stimulate microbe-specific T cells to express CD40 ligand, thereby licensing APCs that bear both microbial and autoantigens to break tolerance.

The inflammasome: a molecular platform triggering activation of inflammatory caspases and processing of proIL-beta

Generation of Interleukin (IL)-1beta via cleavage of its proform requires the activity of caspase-1 (and caspase-11 in mice), but the mechanism involved in the activation of the proinflammatory caspases remains elusive. Here we report the identification of a caspase-activating complex that we call the inflammasome. The inflammasome comprises caspase-1, caspase-5, Pycard/Asc, and NALP1, a Pyrin domain-containing protein sharing structural homology with NODs. Using a cell-free system, we show that proinflammatory caspase activation and proIL-1beta processing is lost upon prior immunodepletion of Pycard. Moreover, expression of a dominant-negative form of Pycard in differentiated THP-1 cells blocks proIL-1beta maturation and activation of inflammatory caspases induced by LPS in vivo. Thus, the inflammasome constitutes an important arm of the innate immunity.

Removing the vertebrate-specific TBP N terminus disrupts placental beta2m-dependent interactions with the maternal immune system

Mammalian TBP consists of a 180 amino acid core that is common to all eukaryotes, fused to a vertebrate-specific N-terminal domain. We generated mice having a modified tbp allele, tbp(DeltaN), that produces a version of TBP lacking 111 of the 135 vertebrate-specific amino acids. Most tbp(DeltaN/DeltaN) fetuses (>90%) died in midgestation from an apparent defect in the placenta. tbp(DeltaN/DeltaN) fetuses could be rescued by supplying them with a wild-type tetraploid placenta. Mutants also could be rescued by rearing them in immunocompromised mothers. In immune-competent mothers, survival of tbp(DeltaN/DeltaN) fetuses increased when fetal/placental beta2m expression was genetically disrupted. These results suggest that the TBP N terminus functions in transcriptional regulation of a placental beta2m-dependent process that favors maternal immunotolerance of pregnancy.

Decoding the patterns of self and nonself by the innate immune system

The innate immune system evolved several strategies of self/nonself discrimination that are based on the recognition of molecular patterns demarcating infectious nonself, as well as normal and abnormal self. These patterns are deciphered by receptors that either induce or inhibit an immune response, depending on the meaning of these signals.

If the immune repertoire evolved to be large, random, and somatically generated, then

The evolution of the somatically generated random combining site repertoire of the "adaptive" immune system depended on the concurrent appearance of a somatic process that sorted the repertoire into anti-self and anti-nonself. Unlike the germline-selected sorting process characteristic of "innate" defense mechanisms, somatic sorting of the repertoire requires that antigens be classified based on their behavior, not on their physical or chemical properties. As specific recognitive combining sites (paratopes) define antigenic determinants (epitopes), the sorting of the repertoire operates epitope-by-epitope. By contrast, the coupling of the paratope to effector function must operate antigen-by-antigen because the response to each epitope on the antigen must be in the same effector class (i.e., coherent). This distinction resolves a long standing debate and provides a basis for analyzing the various models.