Endocytosis and recycling of the complex between CD23 and HLA‐DR in human B cells

Abstract

SUMMARYThe presentation of extremely low doses of antigen to T cells is enhanced by immunoglobulin E (IgE)-dependent antigen focusing to CD23, the low-af®nity receptor for IgE, expressed on activated B cells. CD23 contains a C-type lectin domain in its extracellular sequence and a targeting signal for coated pits, required for endocytosis, in its cytoplasmic sequence. CD23 is non-covalently associated with the major histocompatibility complex class II antigen, human leucocyte antigen HLA-DR, on the surface of human B cells, but the fate of this complex following endocytosis is unknown. To answer this question we have labelled these proteins on the surface of RPMI 8866 B cells and traced their route through the cytoplasm. Endocytosis mediated by anti-CD23 antibodies (BU38 and MHM6) led to the loss of CD23 from the cells. Endocytosis mediated by an antibody to HLA-DR (CR3/43) or an antigen±IgE complex (NP-BSA±anti-NP IgE), however, led to recycling of the HLA-DR±CD23 complex to the cell surface on a time scale (3±6 hr) consistent with the recycling of HLA-DR in antigen presentation. Along the latter pathway CD23 label was observed in cytoplasmic organelles that resembled the`compartments for peptide loading' or`class II vesicles' described by previous authors. Two features of the recycling process may contribute to the ef®ciency of antigen presentation. Peptide exchange may be facilitated by the proximity of HLA-DR and antigen in peptide loading compartments of the endosomal network. The return of CD23 with HLA-DR to the cell surface may then help to stabilize speci®c B-cell±T-cell interactions, contributing to T-cell activation.

A Novel B Cell-Mediated Transport of IgE-Immune Complexes to the Follicle of the Spleen

“…Especially when using EBV-transformed B cell lines, which should be free of contaminating cell types, contribution of DCs to the T cell stimulation/Ag presentation is excluded (26 -28). This, taken along with the fact that human B cells have been shown to internalize IgE/Ag via CD23 (57,58), strongly suggest that CD23 ϩ B cells can indeed present IgE-Ag complexes to specific T cells in vitro. Whether this also takes place in vivo is more difficult to ascertain.…”

Polymorphism R62W results in resistance of CD23 to enzymatic cleavage in cultured cells

“…[25][26][27] The proposed roles for CD23 in antigen presentation include the binding of antigen-IgE-antibody complexes, internalization of the complexes, transport to compartments of the endosomal network containing proteolytic enzymes, interaction with major histocompatibility complex (MHC) class II antigens and interaction with CD21 at the points of contact on the Band T-cell surfaces. 28,29 An integrated CD23 is believed to be important for the IgE-mediated antigen presentation. A stabilized CD23 on the antigen presenting cells may result in enhanced T-cell proliferation in response to antigen.…”

“…43 As the binding of CD23 to MHC II has been observed to be involved in CD23-mediated antigen presentation, it is quite possible that loss of N-glycosylation may lead to functional changes in the binding affinity of CD23 to MHC II and result in enhanced IgEantigen complex presentation. 28 The genetic polymorphism R62W is expected to affect these various functions via the involvement of N-glycosylation. A further clarification of association of the genetic polymorphism R62W with IgE binding affinity and its interaction with MHCII will offer other potential mechanisms to understand the significance of R62W in specific immunological and allergic reactions.…”

CD23-Bound IgE Augments and Dominates Recall Responses through Human Naive B Cells

“…1A), indicating that BCRm + B cells have multiple Ig surface receptors, likely of many differing specificities. Previous studies have demonstrated that Ag captured via CD23-bound IgE is processed and presented on surface MHC class II within hours (15), suggesting that CD23 is highly efficient in inducing Ag presentation functions. These observations raise the question of how a B cell would "decide" which receptor (CD23 or BCR) or Ag, is the priority in generating a response.…”

“…However, the influence of CD23-bound IgE on human B cellmediated immunity is poorly described. Experimental evidence suggests that cross-linking CD23-bound IgE on B cells increases the recycling of HLA-DR complexes to the surface of the cell, suggesting a role in Ag presentation (15); however, there are conflicting reports as to whether surface CD23 is a negative or positive regulator of IgE in humans (9,16). We therefore sought to better define the effect of CD23-bound IgE on human B cell activation and present a potential role for CD23 + B cells in the development of resistance to schistosomiasis.…”