Receptors for the Fc region of immunoglobulin G (IgG) (Fc gamma Rs) ex
ist in three main forms: membrane bound, soluble and cytoplasmic. The
function of cytoplasmic Fc gamma Rs is poorly understood. We have prev
iously demonstrated cytoplasmic Fc gamma RII (cCD32) within most norma
l human peripheral blood lymphocytes (PBL), including T cells. In this
study we have investigated the hypothesis that following lymphocyte a
ctivation, up-regulation of cCD32 occurs, resulting in increased expre
ssion at the cell surface. Normal PBL were activated in vitro using a
two-way mixed lymphocyte reaction (MLR) and expression of CD32 monitor
ed by flow cytometry and by immunoperoxidase staining using specific m
onoclonal antibodies and aggregated mouse IgG subclasses. Furthermore,
we designed oligonucleotide probes specific for the three main isofor
ms of CD32 and looked for changes in mRNA expression throughout the ML
R using an in situ hybridization technique. Increased surface expressi
on of CD32 was found on both activated human T and B lymphocytes, but
this was found only in the early stages of the MLR, on days 3 and 4, a
nd was virtually absent by day 7. An inverse relationship between cell
surface expression of CD32 and mRNA for the IIb isoforms was noted wi
th strong mRNA expression for IIb isoforms occurring in the later stag
es of the MLR (days 6-7) when interleukin-2R (IL-2R)-positive T cells
were predominant. A soluble IgG binding factor (soluble CD32?) was als
o detected in the MLR culture supernatant. These observations provide
support for the hypothesis that synthesis of IIb isoforms of CD32 occu
rs following alloantigen activation of human T lymphocytes.