A SOLUBLE FORM OF THE HUMAN TRANSFERRIN RECEPTOR IS RELEASED BY ACTIVATED LYMPHOCYTES INVITRO

Soluble transferrin receptors (sTfR) were detected in culture supernat ants of activated human peripheral blood mononuclear cells (PBMC) usin g a sandwich ELISA technique with two non-cross-reacting TfR MoAbs. Mi togenic stimulation of lymphoid cells induced both up-regulation of Tf R surface density and release of sTfR to the medium. Peak levels of sT fR in culture supernatants occurred at day 4 after activation, 1 day l ater than maximum expression of TfR in the plasma membrane. Production of sTfR was independent of proliferation, as demonstrated by measurin g sTfR release by PBMC, which had been irradiated with a dose of 20 Gy before activation. In addition to these in vitro experiments, we test ed the sera of 85 patients with systemic lupus erythematosus (SLE), an autoimmune disease accompanied by in vivo activation of lymphocytes, for their sTfR levels. No correlation of these data was detectable to serum concentrations of the soluble alpha-chain of the IL-2 receptor, an unequivocal marker of lymphocyte activation. However, they correlat ed negatively to the haemoglobin content of the patients' erythrocytes , indicating that erythroid progenitors are the predominant source of sTfR in SLE patients' sera.