Macrophage-targeted killing and vaccines

Evidence for the expression of the EGF receptor on human monocytic cells

Several malignancies over-express the epidermal growth factor receptor, ligation of which results in cellular differentiation and multiplication. Mononuclear phagocytes secrete this cytokine and its receptor has been detected on microglial cells. This communication describes the expression (and its regulation) of epidermal growth factor receptor (EGFR) on U937 cells. We have shown that a few are EGFR-positive, with expression being up regulated by interleukin 6 (IL-6). Also, when cultured in the presence of serum with the monoclonal anti-EGFR, ICR62, U937s showed a reduced growth rate. By contrast, ICR9 caused a significant increase in cellular proliferation. Both antibodies induced cycle arrest in late G(1)/S phase. When the cells were cultured in the absence of serum, low antibody concentration (10 microg/ml) showed an early inhibitory effect on cell proliferation. By contrast, at high antibody concentrations (50 micro/ml), ICR62 significantly increased the proliferation of U937 cells. We suggest that these results provide indirect evidence for an autocrine action of EGF on U937 cells.

Receptor modulation by Fc gamma RI-specific fusion proteins is dependent on receptor number and modified by IgG

The high-affinity IgG receptor, FcgammaRI (CD64), is constitutively expressed exclusively on professional APCs. Human FcgammaRI binds monomeric IgG with high affinity and is, therefore, saturated in vivo. The binding of IgG to FcgammaRI causes receptor recycling, while Abs that cross-link FcgammaRI cause rapid down-modulation of surface FcgammaRI. Because studies performed in the absence of ligand may not be representative of FcgammaRI modulation in vivo, we investigated the ability of FcgammaRI-cross-linking Abs and non-cross-linking derivatives to modulate FcgammaRI in the presence and absence of ligand. In the absence of ligand mAb H22 and wH22xeGFP, an enhanced green fluorescent protein (eGFP)-labeled fusion protein of H22, cross-linked and rapidly down-modulated surface FcgammaRI on the human myeloid cell line, U937, and its high FcgammaRI-expressing subclone, 10.6. This effect was dependent on the concentration of fusion protein and the level of FcgammaRI expression and correlated with internalization of both wH22xeGFP and FcgammaRI, itself, as assessed by confocal microscopy. A single-chain Fv version, sFv22xeGFP, which does not cross-link FcgammaRI, was unable to modulate FcgammaRI in the absence of IgG. However, if ligand was present, treatment with either monovalent or cross-linking fusion protein led to intracellular receptor accumulation. These findings suggest at least two alternate mechanisms of internalization that are influenced by ligand and demonstrate the physiologic potential of FcgammaRI to transport a large antigenic load into APCs for processing. These studies may lead to the development of better FcgammaRI-targeted vaccines, as well as therapies to down-modulate FcR involved in autoimmune diseases.