A pair of naturally occurring antibodies may dampen complement-dependent phagocytosis of red cells with a positive antiglobulin test in healthy blood donors

BACKGROUND AND OBJECTIVE

It is known that red blood cells (RBC) from healthy blood donors with a positive direct antiglobulin test (DAT) for IgG continue to circulate despite carrying elevated numbers of IgG molecules. To unravel the properties of these RBC-bound IgG, we studied them not only on whole RBC populations, but also on density-fractionated RBCs.

MATERIALS AND METHODS

The properties of acid-eluted RBC-bound IgG and plasma IgG were studied by ELISA for binding to RBC proteins and opsonins, and by blotting. In vitro phagocytosis was studied on density-separated RBCs.

RESULTS

IgG-DAT-positive blood donors carried most IgG molecules on dense RBCs and had more RBCs of high density than DAT-negative controls. Their densest RBCs were older than the oldest RBCs of DAT-negative controls, based on the band 4.1a/b ratio. In vitro phagocytosis of senescent RBCs from IgG-DAT-positive donors was 1.5 to 2 fold higher than that of senescent control cells, but the same or less in the presence of physiological IgG concentrations, implying that RBC-bound IgGs impaired complement-dependent uptake. The IgG molecules on these DAT-positive RBCs comprised anti-band 3 naturally occurring antibodies (NAbs) and were two- to fivefold enriched in anti-C3 and framework-specific anti-idiotypic NAbs as compared to controls. Correspondingly, anti-C3 and framework-specific anti-idiotypic NAbs were proportionally elevated in the plasma of two-thirds of DAT+ donors.

CONCLUSIONS

Extra-binding of anti-C3 together with anti-idiotypic NAbs to senescent RBC-associated C3 fragments may suppress complement-dependent RBC phagocytosis and may prolong the in vivo life span of RBCs.

Altered expression of integrins in RSV-transformed chick epiphyseal chondrocytes

Chondrocytes have been shown to express both in vivo and in vitro a number of integrins of the beta1-, beta3- and beta5-subfamilies (Biorheology 37 (2000) 109). Normal and v-Src-transformed chick epiphyseal chondrocytes (CEC) display different adhesion properties. While normal CEC with time in culture tends to increase their adhesion to the substrate by organizing focal adhesions and actin stress fibers, v-Src-transformed chondrocytes display a refractile morphology and disorganization of actin cytoskeleton. We wondered whether the reduced adhesion and spreading of v-Src-transformed chondrocytes could be ascribed to changes in integrin expression and/or function. Integrin expression by normal CEC is studied and compared to v-Src-transformed chick chondrocytes, using monoclonal and polyclonal antibodies to integrins alpha- and beta-chains. We show the presence of alpha1-, alpha3-, alphav-, alpha6-, beta1- and beta3-chains on CEC, with very low levels of alpha2- and alpha5-chains. Alphav chain associates with multiple beta subunits in normal and transformed chondrocytes. With the exception of alpha1- and alpha2-chains, the levels of the integrin chains analyzed are higher in transformed chondrocytes as compared with normal chondrocytes. In spite of the increased levels of integrin expression, transformed chondrocytes exhibit loss of focal adhesion and actin stress fibers and low adhesion activity on several extracellular matrix constituents. These observations raise the possibility that, in addition to its effects on global pattern of integrin expression, v-Src can influence integrin function in chondrocytes.