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Science 3 October 1997:
Vol. 278. no. 5335, p. 9
DOI: 10.1126/science.278.5335.9m
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This Week in Science

Antigen presentation to the right T cells will initiate an immune response, but there are times when the wrong (self) peptides may be bound to the class II molecules, and an autoimmune response could develop. How is peptide loading controlled? (Denzin et al. find that the system that helps load peptides onto class II molecules, coordinated by a class II like molecule called DM, is itself regulated by another class II-like molecule, DO. DO binds to DM and makes it much less efficient at loading peptides onto class II. During an infection, cells that normally do not express class II or DM respond to interferon- gamma by turning on those genes and the cells become as good as "professional" antigen presenting cells (APCs), like macrophages, at presenting antigen and turning on T cells. Professional APCs, however, always express DO, which could conceivably be normally functioning to put a damper on expression of self peptides (and inadvertent activation of autoimmunity). During an infection, activated professional APCs would increase the ratio of DM to DO and thereby overcome the block on presentation.

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Science. ISSN 0036-8075 (print), 1095-9203 (online)