Function and regulation of Fc epsilon RI expression on monocytes from non-atopic donors.

Background: The high affinity receptor for IgE (Fc epsilon RI) has recently been identified on antigen presenting cells, i.e. Langerhans cells and monocytes from atopic donors and it was hypothesized that Fc epsilon RI expression levels correlated with allergy.

Objective: The aims of the study was to investigate the function and expression of Fc epsilon RI on monocytes from non-atopic donors.

Methods: Purified monocytes or peripheral blood mononuclear cells were used to study Fc epsilon RI expression and signal transduction on CD14 positive cells by flow cytometry and/or confocal laser microscopy.

Results: Freshly isolated monocytes from healthy individuals (n = 58) were shown to express Fc epsilon RI (median 18%, range 2-66%). No IgE was bound to these receptors in vivo, and in vitro no significant binding of complete IgE molecules could be obtained. IgE positive monocytes from atopic donors were also found to have free Fc epsilon RI incapable of binding IgE in vitro. On all CD14 positive cells free Fc epsilon RI expression was rapidly and completely lost during culture in conventional culture media (IMDM, RPMI) but not in phosphate buffered saline (PBS). Moreover, signal transduction through free Fc epsilon RI appeared to be inhibited. However, both IgE binding and calcium mobilization were restored by treatment of fresh non-atopic monocytes with neuraminidase. Importantly, culturing these monocytes overnight in conventional medium containing 2 micrograms/mL IgE induced a cycloheximide insensitive accumulation of IgE bound to Fc epsilon RI and, in addition, led to cell activation.

Conclusion: Monocytes from both atopic donors and healthy individuals express Fc epsilon RI, but the previously reported different expression levels between the two groups seem to be directly related to the absence or presence of IgE in the serum. This may be due to the fact that Fc epsilon RI is subjected to a constant turnover process which is slowed down but not prevented by ligand binding. In addition, free Fc epsilon RI on non-atopic monocytes are under control of a neuramindase sensitive structure(s), which influences signal transduction and IgE binding.