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Science  18 Jun 2004:
Vol. 304, Issue 5678, pp. 1719
DOI: 10.1126/science.304.5678.1719b

During an immune response, CD4+ T cells are activated by peptides derived from endocytosed and processed proteins, which bind class II molecules of the major histocompatibility complex (MHC) and eventually traffic to the cell surface. Although this process has been assumed to operate exclusively on protein antigens, some evidence has suggested that specific types of bacterial repeating carbohydrate motifs might also undergo intracellular processing to activate T cells.

Using confocal microscopy, Cobb et al. followed zwitterion polysaccharides (ZPS) as they colocalized with MHC class II in the endocytic pathway after uptake by antigen-presenting cells (APCs). First ZPS was partially degraded by the action of nitric oxide, generated during the oxidative burst of the cell, leading to the presence of reduced-molecular-weight fragments within the endosome. Fusion with MHC class II-containing endocytic vesicles then allowed for presentation of ZPS fragments to T cells and the formation of immune synapses between the ZPS-presenting APCs and the responding T cells. The processing of specific carbohydrates by APCs, and the fact that this shares much of the pathway used for conventional peptide presentation, could have important implications for T cell immunity and vaccine design. — SJS

Cell 117, 677 (2004).

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