Dendritic cells constitutively sample the tissue microenvironment and phagocytose both microbial and host apoptotic cells1,2,3,4. This leads to the induction of immunity against invading pathogens or tolerance to peripheral self antigens, respectively5,6,7,8,9. The outcome of antigen presentation by dendritic cells depends on their activation status, such that Toll-like receptor (TLR)-induced dendritic cell activation makes them immunogenic, whereas steady-state presentation of self antigens leads to tolerance5,6,8,10. TLR-inducible expression of co-stimulatory signals is one of the mechanisms of self/non-self discrimination5,11. However, it is unclear whether or how the inducible expression of co-stimulatory signals would distinguish between self antigens and microbial antigens when both are encountered by dendritic cells during infection6,8. Here we describe a new mechanism of antigen selection in dendritic cells for presentation by major histocompatibility complex class II molecules (MHC II) that is based on the origin of the antigen. We show that the efficiency of presenting antigens from phagocytosed cargo is dependent on the presence of TLR ligands within the cargo. Furthermore, we show that the generation of peptide–MHC class II complexes is controlled by TLRs in a strictly phagosome-autonomous manner.
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We are grateful to I. Mellman, M. Bevan, J. Monaco and R. Germain for reagents, and E. Kopp for the flagellin–OVA fusion construct. We thank P. Cresswell and A. Rudensky for reagents, suggestions and critical reading of the manuscript. We are indebted to I. Mellman, E.S. Trombetta, L. Delamarre and J. Kagan for discussions; M. Pypaert and C. Marks for electron microscopy; C. Chalouni and R. Montgomery for confocal microscopy; A. Hartley for 3D reconstructions; D. Amsen, A. Giodini, R. Teal, T. Horng, T. Potseleuva, C. Annicelli, S. Holley, R. Couture, D. Matosian, T. Taylor, G. Tokmoulina, J. Shanabrough, M. Blander and S.J. Blander for assistance and support. J.M.B. is supported by an Arthritis Investigator award. R.M. is supported by the Howard Hughes Medical Institute and the NIH.
A confocal three dimensional reconstructed image from Fig. 4e (top panels) is shown rotating at different angles. CD11c+ DC at 5 h post phagocytosis of both 4.5 µm HEL and 10 µm HEL/LPS conjugated microspheres. The C4H3 epitope (red) was detected around the phagosome containing the 10 µm microsphere by intracellular staining. I-Ak is shown in green and surface CTB in blue.
A confocal three dimensional reconstructed image from Fig. 4e (bottom panels) shown rotating at different angles. CD11c+ DC at 5 h post phagocytosis of both 4.5 µm HEL/LPS and 10 µm HEL conjugated microspheres. Intracellular C4H3 epitope (red) was detected staining the phagosome containing the 4.5 µm microsphere. I-Ak is shown in green and surface CTB in blue.