Viral Superantigen-Induced Negative Selection of TCR Transgenic CD4+ CD8+ Thymocytes Depends on Activation, but not Proliferation

T cell-B cell thymic cross-talk: maintenance and function of thymic B cells requires cognate CD40-CD40 ligand interaction

Thymic development requires bidirectional interaction or cross-talk between developing T cells and thymic stromal cells, a relationship that has been best characterized for the interaction between thymocytes and thymic epithelial cells. We have characterized in this article the requirement for similar cross-talk in the maintenance and function of thymic B cells, another population that plays a role in selection of developing thymic T cells. We found that maintenance of thymic B cells is strongly dependent on the presence of mature single-positive thymocytes and on the interactions of these T cells with specific Ag ligand. Maintenance of thymic B cell number is strongly dependent on B cell-autonomous expression of CD40, but not MHC class II, indicating that direct engagement of CD40 on thymic B cells is necessary to support their maintenance and proliferation. Thymic B cells can mediate negative selection of superantigen-specific, self-reactive, single-positive thymocytes, and we show that CD40 expression on B cells is critical for this negative selection. Cross-talk with thymic T cells is thus required to support the thymic B cell population through a pathway that requires cell-autonomous expression of CD40, and that reciprocally functions in negative selection of autoreactive T cells.

AIRE-1 (autoimmune regulator type 1) as a regulator of the thymic induction of negative selection

The monogenic autoimmune syndrome, APS-1 (autoimmune polyglandular syndrome type 1), is characterized by the loss of self-tolerance to multiple organs. Although mutations in the AIRE (autoimmune regulator) gene are responsible for the APS-1, the function of AIRE is not known. AIRE may determine thymic induction of tolerance to self-antigens in multiple organs. To study the function of AIRE in induction of self-tolerance, an in vitro negative selection system was made using 10(6) DO11.10 TCR transgenic thymocytes, 10(5) antigen-presenting cells (APC), and the different constructs of ovalbumin (OVA). In this system, the addition of the immunodominant epitopes of OVA peptide, the antigenic ligand for the DO11.10, made the thymocytes apoptotic and negatively selected. Overexpression of the AIRE gene in APC using retroviral transduction did not cause more thymocytes to become apoptotic. However, the suppression of the expression of AIRE in APC using the dominant-negative gene made the recovery rates of the thymocytes higher than those with the expression of LacZ as a control, and consequently inducing loss of self-tolerance. From these studies, it might be possible to suggest that the AIRE gene might regulate thymic induction of the negative selection process. The target genes for transcriptional regulation by AIRE have been investigated to study the influence of AIRE expression on other proteins in antigen presentation. The expression level of B7.1 was higher in APC expressing the dominant-negative form of AIRE. The target gene regulated by AIRE in transcription will be screened using cDNA microarray.

Functional and phenotypic analysis of thymic B cells: role in the induction of T cell negative selection

The phenotype of mouse thymic B cells and their capacity to induce T cell negative selection in vitro were analyzed. Thymic B cells expressed B cell markers such as IgM, Fc gamma receptor, CD44, heat-stable antigen, LFA-1 and CD40. In addition, they were positive for the activation molecule CD69 and displayed high levels of B7-2. Although thymic B cells expressed CD5 on their surface, no CD5-specific mRNA was detected. Moreover, thymic B cells induced a stronger deletion of TCR-transgenic (TG) thymocytes than splenic B cells, which had low CD69 and B7-2 levels. Interestingly, CD40-activated splenic B cells up-regulated CD69 and B7-2 and acquired a capacity to induce T cell deletion comparable to that of thymic B cells. Moreover, thymic B cells from CD40-deficient mice displayed lower CD69 and B7-2 levels than control thymic B cells, and lower capacity to induce the deletion of TCR TG thymocytes. These results support the hypothesis that CD40-mediated activation of thymic B cells determines a high efficiency of antigen presentation, suggesting that within the thymus B cells may play an important role in the elimination of autoreactive thymocytes.