Membrane-spanning Fc gamma receptor III isoform expressed on human placental trophoblasts

Fc receptor for immunoglobulin (Fc gamma R) is an important mediator of immunological functions in the feto-maternal relationship. We have demonstrated by immunohistochemical means that three distinct classes of Fc gamma Rs are expressed in the different cell components of the human placenta.

In this study, Fc gamma RIII isoform expressed on placental trophoblasts (PTs) was investigated by indirect immunofluorescence and cDNA cloning. PTs, isolated from human term placenta by digestion with proteolytic enzyme, were reacted with monoclonal antibodies (MAb) against the Fc gamma Rs and other surface markers of leukocytes and subjected to flow cytometric analysis.

PTs were positively stained with 3G8 and Leu1 1b against Fc gamma RIII, partially stained with MAb against MHC class I, but not with 32.2 (Fc gamma RI), IV3 (Fc gamma RII), or MAbs against CD4, CD19, or CD56, indicating that only low affinity receptor, Fc gamma RIII, is expressed on PTs. The DNA sequence of cloned Fc gamma RIII CDNA from PTs by PCR was identical to that of natural killer (NK) cell isoform, including the position of the stop codon that differs from the granulocyte isoform by several nucleotide substitutions. We further analyzed the susceptibility of PTs against phosphatidylinositol specific phospholipase C (PI-PLC) to determine the structural topology of PT isoform. While the reactivity with 3G8 on PTs was not influenced by treatment with PI-PLC, that on granulocytes was significantly diminished with PI-PLC.

This result confirmed that Fc gamma RIII on PTs is a membrane-spanning molecule, and that it is distinctive from PI anchoring Fc gamma RIII on granulocytes.