A population of human cord blood mononuclear cells with surface alpha fetoprotein

Suspensions of mononuclear cells from adult peripheral blood (PBL) and mononuclear cells from cord blood (CBL) were examined for the presence of surface alpha-fetoprotein (AFP) using a fluoresceinated F(ab')2 fragment of rabbit IgG anti-human AFP. The mean proportion of CBL with AFP was increased (10%) when compared with PBL (1%) although some CBL specimens did not demonstrate such an increase (range 0--15%). The presence of AFP on CBL could be either due to cytophilic AFP attached to a unique surface receptor or intrinsic AFP synthesis. The following observations could not distinguish between these two possibilities: (1) After treatment with trypsin, only minor reappearance of surface AFP could be observed in AFP-free medium in contrast to the larger numbers observed in medium containing AFP. Such selective reappearance depending on the media could be related to either cytophilic attachment of heterologous or homologous AFP or preferential stimulation of intrinsic AFP synthesis. (2) The reappearance of AFP positive CBL following trypsin treatment and incubation in media with or without AFP containing sera was inhibited by cyclohexamide. Such inhibition could be due to inhibition of synthesis of an AFP surface receptor or intrinsic AFP. (3) The shedding of surface AFP observed at 2--4 degrees C could be due to release of exogenous cytophilic AFP or the continued "turnover" of intrinsic AFP without concomitant AFP synthesis due to the cold temperature. Finally, the removal of AFP positive cells via selective depletion of B cells using bead columns coated with IgG-anti-IgG and the absence of depletion of AFP positive cells after successive gradient centrifugation of E-rosettes and cells with IgG-Fc receptors are consistent with the identity of AFP positive CBL as cells without IgG-Fc receptors or lymphocytes without conventional T-cell markers as defined by E-rosettes.