Dextran Abrogates Stimulation of IgG Antibody Synthesis by Allogeneic Effect Factor

Abstract

Abstract The influence of dextran upon in vitro immunization of unfractionated splenic cells and of purified splenic B cells against sheep erythrocytes has been studied with particular emphasis upon the relationship of dextran to the augmentation of antibody synthesis by allogeneic effect factor (AEF). Dextran enhanced the number of SRBC-specific IgM PFC when added to unfractionated splenic cells undergoing a primary immune response in vitro. When it was added to purified splenic B cells no such effect was observed. The established capacity of AEF to facilitate antibody synthesis by splenic B cells was found to be markedly altered by addition of dextran to the cultures. Although the capacity of AEF to increase the number of IgM PFC remained virtually unchanged, the enhancement of IgG PFC brought about by AEF was abrogated. These observations suggest that either two separate molecules may be responsible for the activities of AEF or that one molecule possesses two separable functions: one increases IgM PFC and another stimulates IgG PFC. Data are given that indicate the activity of the latter may be interfered with by dextran at the B cell surface membrane. We hypothesize that the receptor for AEF on the IgG-producing B cell may be, in part, structurally related to dextran (i.e., a polysaccharide molecule with α-1,6 glucosidic linkages) and for this reason may interfere with reception of an AEF signal by B cells. If this is the case, then antigens that cause the appearance of IgM antibodies but not IgG antibodies may be incorrectly regarded as “T cell-independent antigens” primarily because they intercept the latter activity of AEF.