Unmasking of fetal determinants on adult bone marrow cells

Abstract

The phasing-out of an SV40 cross-reactive fetal antigen in the developing hamster fetus after day 10 occurs concomitantly with an increase of bound sialic acid in membrane, microsomal and soluble cell fractions¹. A masking phenomenon is suggested by the fact that the antigen can be uncovered on older fetal tissue by trypsinisation². Similarly, a murine oncofetal antigen under study in our laboratory is optimally expressed on the haematopoietic stem cells of the fetal liver at the 15th day of gestation³, just before the onset of haematopoiesis and collagen production in the spleen⁴. The expression of this antigen on human leukaemic blast cells has, furthermore, been correlated with low levels of sialic acid on the surface of these tumour cells⁵. As fetal determinants can be expressed on adult bone marrow in disease states not involving malignant transformation⁶, it is of interest to know whether production of this oncofetal antigen is repressed, or whether the determinants are in cryptic form at the plasma membrane due to some masking phenomenon. The investigation described here demonstrates that determinants which cross-react immunologically with fetal liver tissue can be enzymatically unmasked on normal adult murine haematopoietic stem-cell surfaces. This finding suggests that the expression of these oncofetal antigens on malignant cells does not depend on de novo production of the antigens, and that these determinants may play some part in normal differentiation control mechanisms.