Jean Salamero

Role of B cell receptor Igα and Igβ subunits in MHC class II-restricted antigen presentation

Abstract The ability of the B cell antigen receptors (BCRs) to enhance MHC class II-restricted antigen presentation was ascribed to mlg-associated Igα/Igβ heterodimers. The relative role of Igα and Igβ subunits in antigen presentation was investigated by fusing their cytoplasmic tails to the extracellular and transmembrane domains of Fc receptors. Igα and Igβ chimera mediate antigen internalization and increase the efficiency of antigen presentation, but they drive antigens to different endosomal compartments. Furthermore, antigens internalized by either chimera are degraded and presented with different kinetics. The cytoplasmic tail of Igα targets antigen towards a major population of newly synthesized MHC class II located in class II-rich compartments. In contrast, Igβ targets antigen towards a minor population of recycling MHC class II molecules, located in transferrin receptor-containing endosomes. Altogether, our data indicate that the composition of BCR could be therefore an important way to modulate the immune response.

Syngeneic sensitization of mouse lymphocytes on monolayers of thyroid epithelial cells. VII. Generation of thyroid-specific cytotoxic effector cells.

We observed T lymphoblast generation after three days of culture of normal spleen lymphocytes on monolayers of syngeneic thyroid epithelial cells. It appears that only these T lymphoblasts sensitized on thyroid monolayers are specifically labelled by fluorescein-conjugated thyroglobulin. In this study, the role of thyroglobulin, and the manner in which it is presented to syngeneic T cells bearing receptors for thyroglobulin were investigated. It appears that thyroglobulin plays a key role in primary syngeneic sensitization of spleen cells in vitro, but its action is not exclusive. Proteins, which are encoded by a gene located in the I-A subregion level of the major histocompatibility complex, are capable of inducing a primary proliferative signal when the presentation of syngeneic class II antigens by syngeneic thyroid epithelial cells is simultaneous. In addition, thyroglobulin-pulsed macrophages are not able to do so. In contrast, once this primary syngeneic T cell proliferation has been accomplished, soluble thyroglobulin is sufficient to induce a secondary response by these syngeneic T cells.

Primary syngeneic sensitization on monolayers of thyroid epithelial cells. X. Inhibition of T-cell proliferative response by thyroglobulin-specific monoclonal antibodies.

We studied the biochemical properties and the reactivity of 13 monoclonal anti-thyroglobulin antibodies, obtained by fusing P3 X 63 (8653) myeloma cells with spleen cells from CBA and C57BL/6 mice previously immunized with syngeneic thyroglobulin. We demonstrated that among the 13 anti-thyroglobulin monoclonal antibodies that we studied, each of them was specific for only syngeneic thyroglobulin, as each of them was able to cross-react with heterologous or xenogeneic thyroglobulin. Two major kinds of monoclonal antibodies were obtained: the first ones are able to block in vitro primary syngeneic sensitization and, moreover, they mutually compete for thyroglobulin binding; the second ones are not involved in this in vitro PSS and do not compete for binding to thyroglobulin. These results suggest that a defined particular epitope, borne by thyroglobulin and expressed on the thyroid epithelial cells, is responsible for in vitro primary syngeneic sensitization.

Endocytosis and Biosynthetic Transport of Murine MHC Class II Molecules in Antigen Presenting Cells

In B lymphocytes, the processing of exogenous proteins and the subsequent binding of antigenic peptides to MHC class II molecules occurs most likely within endocytic compartments. Using various biochemical and morphological techniques, we show that the endosomal compartments are loaded with a pool of MHC class II molecules which are permanently endocytosed and recycled to the cell surface in murine B lymphoma cells. L fibroblasts transfected with I-A αβ K or I-E α K β K/d fail to internalize their surface class II molecules even when expressed in conjunction with the class II associated invariant chain (Ii). We suggest that in murine B lymphoma cells, antigenic peptides can gain access to a pool of recycling class II molecules whereas in L cells they meet newly synthesized class II molecules targeted to the endosomal compartments. In B lymphocytes and macrophages, the invariant chain (Ii) is synthesized in large excess with respect to MHC class II α and β chains. Ii binds to MHC class II molecules in endoplasmic reticulum and is partially degraded presumably at the level of the trans-Golgi network or of prelysosomal compartments. To estimate the role fo Ii molecules in the function and the transport of class II molecules, we have surtransfected murine L fibroblasts expressing IAk molecules using as controls L fibroblasts expressing only IAk or Ii. As pointed out before for the presentation of cytochrome B5 in the context of IEk (Stockinger et al., 1989 Cell 56, 683–689) we found that Ii expression enhance by a hundred fold the capacity of IAk positive L cell to present hen egg lysozyme.

Induction of Ia antigens on thyroid cultures by syngeneic T lymphocytes: a model of autoimmune disease.

We previously reported that the expression of Ia antigens on cultured monolayers of murine thyroid epithelial cells (TEC) occurred with a particular distribution exclusively on the basal part of the cultured thyroid cells, while class I antigens of the major histocompatibility complex (MHC) are only detected on the apical surface. It appears that deposition of syngeneic lymphocytes induces, 24 h later, Ia expression on the apical side of cultured TEC, the surface that is in direct contact with the responder lymphocytes during syngeneic sensitization of T lymphocytes. We hypothesized that this phenomenon could represent, in syngeneic situations, the restriction process in antigen recognition by T cells, as demonstrated by Ia restricted primary syngeneic sensitization (PSS) on murine TEC.