PROBLEM: Fc receptor for immunoglobulin (FcγR) is an important mediator of immunological functions in the feto-maternal relationship. We have demonstrated by immunohistochemical means that three distinct classes of FcγRS are expressed in the different cell components of the human placenta.
METHOD: In this study, FcγRIII 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-γRs and other surface markers of leukocytes and subjected to flow cytometric analysis.
RESULTS: PTs were positively stained with 3G8 and Leul lb against FcγRIII, partially stained with MAb against MHC class I, but not with 32.2 (FcγRI), IV3 (FcγRII), or MAbs against CD4, CD19, or CD56, indicating that only low affinity receptor, FC7RIII, is γexpressed on PTs. The DNA sequence of cloned FcγRIII 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.
CONCLUSIONS: This result confirmed that FcγRIII on PTs is a membrane-spanning molecule, and that it is distinctive from PI anchoring FcγRIII on granulocytes.