Factors regulating thymic dendritic cell homeostasis and function

The immune system must balance the generation of a diverse T cell repertoire responsive to various pathogens versus the prevention of tissue destruction by self-antigen-specific T cells. Thymic dendritic cells (DC) are known to eliminate self-antigen-specific thymocytes. Despite this, little is known about the factors regulating thymic DC homeostasis and antigen presentation, which were investigated in this study. In the first part of our study, it was hypothesized that factors within the thymus regulate thymic DC homeostasis. Specifically, the role of CD4+ and CD8+ single-positive thymocytes (SP) was investigated. In mice lacking SP, significantly fewer DC resided in the steady-state thymus, and those DC displayed a less mature phenotype (e.g. reduced expression of major histocompatibility [MHC] and T cell costimulatory molecules) and a markedly reduced T cell stimulatory capacity compared to wild-type thymic DC. When CD4+SP or CD8+SP were individually restored, thymic DC activation status was only recovered under conditions where antigen-specific interactions with SP occurred. During antigen-specific interactions, the restoration of thymic DC activation status depended on CD40 ligand from CD4+SP but not CD8+SP. Thus, thymic DC homeostasis is regulated by antigen-specific interactions with CD4+SP or CD8+SP by distinct molecular mechanisms. To promote thymic tolerance, thymic DC must acquire and present cognate antigens to thymocytes; for example, through "nibbling" of antigens from thymic epithelial cells (TEC). However, the mechanism by which this occurs is largely unknown. Using an in vitro co-culture system, thymic DC nibbling of MHC was investigated. Thymic DC efficiently nibbled MHC from TEC and DC, but not from B cells. It was hypothesized that the cell surface organization of MHC may regulate nibbling by thymic DC. Specifically, the role of lipid rafts was analyzed. Thymic DC acquired lipid rafts from TEC, DC, and B cells. Interestingly, when another B cell surface molecule, immunoglobulin M (IgM), was actively clustered to lipid rafts, nibbling of IgM occurred. These data suggest that cell surface organization of clustered molecules may facilitate acquisition by thymic DC via nibbling. Together, the studies presented herein provide novel insight into the factors regulating thymic DC function and T cell central tolerance.Doctor of Philosoph