Joachim Hombach

The B-cell antigen receptor complex

Abstract Here, Michael Reth, John Cambier and colleagues review the genetics, biochemistry and cell biology o f the recently-identified accessory components o f the B-cell antigen receptor.

A novel 34-kd protein co-isolated with the IgM molecule in surface IgM-expressing cells.

Plasmacytoma cells, transfected with a vector encoding a membrane‐bound IgM molecule, do not show cell surface IgM expression, although complete IgM molecules are assembled intracellularly. The isolation of a surface IgM‐positive variant allowed us to analyse molecular requirements of surface IgM expression. Only in surface IgM‐positive cells, a 34‐kd protein (B34) was found to be associated with IgM. B34 is a glycoprotein which forms a disulphide‐linked homodimer. The surface IgM‐positive variant cell line expressing B34 also contains transcripts of the pre‐B and B cell specific mb‐1 gene. The data are discussed in the context of a possible IgM‐antigen receptor complex.

Molecular components of the B cell antigen receptor complex of class IgD differ partly from those of IgM.

Two classes of immunoglobulin, IgM and IgD, are present as antigen receptors on the surface of mature B lymphocytes. We show here that IgD molecules are noncovalently associated in the B cell membrane with a heterodimer consisting of two proteins of 35 kd (IgD‐alpha) and 39 kd (Ig‐beta), respectively. The two novel proteins are not found in the IgD‐expressing myeloma J558L delta m, which fails to bring IgD antigen receptor onto the cell surface. In a surface IgD positive variant line of this myeloma, however, membrane‐bound IgD molecules are associated with the heterodimer, suggesting that the formation of an antigen receptor complex is required for surface IgD expression. We further demonstrate that the IgD‐associated heterodimer differs partly from that of the IgM antigen receptor and that its binding to the heavy chain only requires the presence of the last constant domain and the transmembrane part of the delta m chain.

Antigen persistence and time of T-cell tolerization determine the efficacyof tolerization protocols for prevention of skin graft rejection

We studied antigen-specific T-cell tolerization therapy using skin transplantation across a defined minor histocompatibility antigen difference. Specific tolerization protocols using short-lived peptide or long-lived spleen cells presenting the peptide as antigen prevented graft rejection without immunosuppression when started before or as long as 10 days after transplantation. Peptide-induced T-cell tolerance was transient, and antigen presentation by the graft was not sufficient to maintain tolerance. In contrast, transfer of antigen-expressing lymphoid cells induced long-lasting tolerance correlating with donor cell chimerism. These findings show that antigen-specific tolerization can induce graft acceptance even when begun after transplantation and that long-term graft survival depends on persistence of the tolerizing antigen.

A protective cytotoxic T cell response to a subdominant epitope is influenced by the stability of the MHC class I/peptide complex and the overall spectrum of viral peptides generated within infected cells

This study identifies instability of MHC class I/peptide complexes and intermolecular competition for MHC class I presentation as factors responsible for the subdominance of cyto toxic T lymphocyte (CTL) epitopes. This evidence is based on the characterization of a new CTL epitope derived from the glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV). This epitope, peptide GP117‐125 (GP117) is presented to T cells by the mouse MHC class I molecule, H‐2Db. In short‐term experiments induction of GP117‐specific CTL by vaccination rendered C57BL/6 mice only partially resistant to infection with wild‐type LCMV (LCMV‐WE) but completely resistant to challenge with a previously described LCMV variant. The variant virus, LCMV‐8.7B23, bears point mutations within both known LCMV‐GP, H‐2 Db‐restricted epitopes GP33‐41 (GP33) and GP276‐286 (GP276) resulting in a valine to leucine change at position 35 in peptide GP33 (V35L) and an asparagine to serine change at position 280 in peptide GP276 (N280S). Although variant peptide GP33/V35L stimulates a weak CTL response, GP276/N280S does not. Elution of peptide GP117 from both LCMV‐WE‐ and LCMV‐8.7B23‐infected cells revealed that the difference in the capacity of GP117‐specific CTL to protect against LCMV‐WE and the virus variant LCMV‐8.7B23 was due to differences in the level of GP117 presentation on the surface of both types of cells. Thus, it appears that the protective capacity of CTL specific for the subdominant epitope GP117 is influenced by the extent of presentation of other immunodominant peptide epitopes present within infected cells.

Donor cell persistence and activation-induced unresponsiveness of peripheral CD8 + T cells

We studied the impact of the duration of donor cell persistence on CD8+ T cell responsiveness after adoptive transfer of antigen‐expressing lymphoid cells. Naive or immunized female mice were treated by adoptive transfer of spleen cells from mice ubiquitously expressing a lymphocytic choriomeningitis virus‐derived cytotoxic T lymphocyte (CTL) epitope (gp33 – 41) either alone or in combination with the male H‐Y antigen providing additional antigenic CTL and T helper cell determinants. Low doses of male spleen cells (or sorted B cells) primed CTL, while high doses of the same cells rendered them unresponsive. CTL unresponsiveness induced by high numbers of male spleen cells was dependent upon prolonged persistence of antigen‐expressing donor cells. Unresponsive CTL reverted to a state of activation when the duration of donor cell chimerism was limited. Memory CTL could be rendered unresponsive if antigen‐expressing donor cells were allowed to persist. These results suggest that, irrespective of the type of antigen‐presenting cell and the functional state of the responding T cell, activation and unresponsiveness can represent two different outcomes critically determined by quantitative and kinetic differences of antigen persistence.

Identification of Components of the B Cell Antigen Receptor Complex

Immunoglobulins (Ig) are well-known as antibody molecules which are present in the serum of all vertebrate species. The mouse has five classes of Ig (IgM, IgD, IgG, IgA and IgE) and most of these antibody classes have different effector function during an immune response.1 Heavy chains of each Ig class exist also in a membrane-bound form23 which is expressed on the B cell surface as part of the antigen receptor. During B cell development the different antigen receptors are expressed in an ordered fashion.4 Immature B cells carry only surface Ig (slg) of the IgM class5 while mature B cells co-express slgM and slgD.6,7 Upon activation some B cells are generated which after a class switch express slgG, slgA or slgE. Cells with these antigen receptors are dominant in the pool of memory B cells.

Structure and Signaling Function of B Cell Antigen Receptors of Different Classes

Publisher Summary This chapter discusses the structure and signaling function of B-cell antigen receptors of different classes. Early B cells express antigen receptors of the IgM class on their surface, and mature B cells coexpress IgM and IgD. The other Ig classes are expressed on the B-cell surface only after a primary activation and a class switch event. After cross-linking by antigen or anti-Ig antibodies, the antigen receptor is activated and transduces signals to the cytoplasm. Depending on the developmental stage of the B cell, the signals can result in different biological responses. In early B cells, they can result in cell death or apoptosis, and in mature B cells, they can result in either activation or deactivation of the B cells. The K46 B lymphoma line expresses the genes of all three α components. This is indicated by the finding that IgM, IgD, and IgG2a molecules are expressed right away on the surface of the different K46 transfectants. The genes for the different α components seem to be expressed early during B-cell development.