Differences in Ultrastructural Appearance and Modulation of Endogenous and Exogenous Surface Immunoglobulin of Rabbit Lymphocytes

Abstract

Abstract Endogenous and exogenous surface Ig (sIg)3 of rabbit peripheral blood lymphocytes (PBLs) differ in ultrastructural appearance and modulation characteristics. Endogenous sIg occurs as small sessile patches on 70% of freshly prepared PBLs after specific mixed anti-globulin immunoferritin labeling. These patches are closely associated with the surface membrane and readily undergo endocytosis when labeled cells are warmed to 37°C. The binding of exogenous Ig may be demonstrated by exposure of PBLs to normal allogenic or xenogenic serum in vitro. Aggregated IgG is responsible for the majority of the serum Ig binding; deaggregated IgG binds poorly and Fab′ fragments of IgG do not bind. Exogenous Ig exists as large aggregates precariously attached to the cell surface on approximately 70% of the PBLs which do not undergo endocytosis when warmed to 37°C but appear to be shed or sloughed from the cell surface. We conclude a) that most rabbit PBLs have both endogenous surface Ig and Fc receptors for aggregates of IgG; b) that endogenous and exogenous sIg can be differentiated by ultrastructural features; c) that exogenous sIg is not detectable on freshly prepared PBLs unless incubated in vitro with aggregated Ig; and d) the exogenous sIg may not have a functional role in lymphocyte activation.