FcRγ-Chain ITAM Signaling Is Critically Required for Cross-Presentation of Soluble Antibody-Antigen Complexes by Dendritic Cells

The uptake of Ag-Ab immune complexes (IC) after the ligation of activating Fc gamma R on dendritic cells (DC) leads to 100 times more efficient Ag presentation than the uptake of free Ags. Fc gamma Rs were reported to facilitate IC uptake and simultaneously induce cellular activation that drives DC maturation and mediates efficient T cell activation. Activating FcgRs elicit intracellular signaling via the ITAM domain of the associated FcR gamma-chain. Studies with FcR gamma-chain knockout (FcR gamma(-/-)) mice reported FcR gamma-chain ITAM signaling to be responsible for enhancing both IC uptake and DC maturation. However, FcR gamma-chain is also required for surface expression of activating Fc gamma Rs, hampering the dissection of ITAM-dependent and independent Fc gamma R functions in FcR gamma(-/-) DCs. In this work, we studied the role of FcR gamma-chain ITAM signaling using DCs from NOTAM mice that express normal surface levels of activating Fc gamma R, but lack functional ITAM signaling. IC uptake by bone marrow-derived NOTAM DCs was reduced compared with wild-type DCs, but was not completely absent as in FcR gamma(-/-) DCs. In NOTAM DCs, despite the uptake of ICs, both MHC class I and MHC class II Ag presentation was completely abrogated similar to FcR gamma(-/-) DCs. Secretion of cytokines, upregulation of costimulatory molecules, and Ag degradation were abrogated in NOTAM DCs in response to Fc gamma R ligation. Cross-presentation using splenic NOTAM DCs and prolonged incubation with OVA-IC was also abrogated. Interestingly, in this setup, proliferation of CD4(+) OT-II cells was induced by NOTAM DCs. We conclude that FcR gamma-chain ITAM signaling facilitates IC uptake and is essentially required for cross-presentation, but not for MHC class II Ag presentation.