DIFFERENTIAL EXPRESSION OF CD32 ISOFORMS FOLLOWING ALLOACTIVATION OF HUMAN T-CELLS

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.