Colocalization of Fc gamma RI-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 den dritic cells), When Ag is targeted specifically to Fc gamma RI, Ag presenta tion is markedly enhanced, although the mechanism of this enhancement is un known. In an effort to elucidate the pathways involved in Fc gamma RI. targ eting, we developed a model targeted Ag using enhanced green fluorescent pr otein (eGFP), This molecule, wH22xeGFP, consists of the entire humanized an ti-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 F c end, as well as outside the ligand-binding region by its Fab ends, thereb y cross-linking Fc gamma RI. Confocal microscopy studies revealed that wH22 xeGFP 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, wH22xeGF P 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 t argeting can lead to Ag-specific activation of cytotoxic T cells. Data obta ined from these studies should lead to a better understanding of how Ags ta rgeted 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, 200 1, 166: 2469-2478.