739 C3d, A MARKER OF EARLY T-CELLS IN MAN?

Abstract

A small proportion (2-3%) of normal human peripheral blood lymphocytes (PBL) have receptors for both E (SRBC) and the third component of complement (C3). C3 complexes were formed with ox red cells (0), IgM rabbit anti-ox (μ), and different sources of complement. Using OμC3 complexes and fluorescinated SRBC, corosetting cells were enumerated in normal tissues, T-cell lines, E-positive ALL (E+-ALL) blasts, and PBL from a patient with severe combined immunodeficiency disease (SCID). The detection of co-rosetting cells was optimized using mouse sera as the source of complement with prolonged incubation to insure generation of C3d. Cells from four T-cell lines, and six E+ -ALL had a high proportion of co-rosetting cells (20-55%). A high proportion of bone marrow T-cells also demonstrated C3d receptors. C3d+/E+ T-cells in normal PBL were enriched in the low density precursor cell fractions after separation by albumin gradient centrifugation. These cells were induced to become C3d−/E+ following incubation with thymic humoral factors or modulators of cAMP. The patient with SCID demonstrated large numbers of C3d+/E+ cells early in his course following transplantation with cultured thymic epithelium. As this patient's E-rosettes increased from <1% to 30%, co-rosetting cells dramatically declined. These data suggest that a C3d+ T-cell occurs early in ontogeny and becomes C3d− at a post-thymic epithelial stage of development. Furthermore, E+-ALL may represent a clonal expansion of T-lymphocytes at this stage of ontogeny.