Colocalization of FcγRI-targeted antigen with class I MHC:: Implications for antigen processing

The high-affinity receptor for IgG (CD64 or Fc gamma RI) is constitutively expressed exclusively on professional APCs (monocytes, macrophages, and dendritic cells), When Ag is targeted specifically to Fc gamma RI, Ag presentation is markedly enhanced, although the mechanism of this enhancement is unknown. In an effort to elucidate the pathways involved in Fc gamma RI. targeting, we developed a model targeted Ag using enhanced green fluorescent protein (eGFP), This molecule, wH22xeGFP, consists of the entire humanized anti-Fc gamma RI mAb H22 with eGFp genetically fused to the C-terminal end of each CH3 domain. wH22xeGFP binds within the ligand-binding region by its Fc end, as well as outside the ligand-binding region by its Fab ends, thereby cross-linking Fc gamma RI. Confocal microscopy studies revealed that wH22xeGFP was rapidly internalized by the high-Fc gamma RI-expressing cell line U937 10.6, but did not associate with intracellular proteins Rab4, Rab5a, or Lamp-1, suggesting that the targeted fusion protein was not localized in early endosomes, recycling vesicles, or lysosomes, Interestingly, wH22xeGFP was found colocalized with intracellular MHC class I, suggesting that Fc gamma RI-targeted Ags may converge upon a class I processing pathway, These data are in agreement with studies in the mouse showing that Fc gamma RI targeting can lead to Ag-specific activation of cytotoxic T cells. Data obtained from these studies should lead to a better understanding of how Ags targeted to Fc gamma RI are processed and under what conditions they lead to presentation of antigenic peptides in MHC class I, as a foundation for the use of Fc gamma RI-targeted Ags as vaccines. The Journal of Immunology, 2001, 166: 2469-2478.