International Immunology Advance Access originally published online on March 28, 2006
International Immunology 2006 18(5):729-739; doi:10.1093/intimm/dxl010
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The quantitative assessment of MHC II on thymic epithelium: implications in cortical thymocyte development
1 Laboratory of Cellular and Molecular Immunology and
2 Flow Cytometry Section, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA
3 Department of Microbiology and Immunology, Indiana University School of Medicine, Indianapolis, IN, USA
4 Department of Biological Sciences, Inha University, Incheon 402-751, Korea
5 Present Address: Department of Pathology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea
Correspondence to: M. G. Kim; E-mail: mgkim{at}inha.ac.kr
The dynamics of MHC II expression in various thymic stromal compartments was investigated. By including MHC II in flow cytometry in addition to the cortical CDR1, medullary UEA-1 and pan-epithelial G8.8 markers, thymic stromal compartments were subdivided into at least six different populations. The total level of surface and cytoplasmic MHC II from fresh cortical thymic epithelial cells (cTECs) of normal mouse was as high as MHC II levels in medullary thymic epithelial cells (mTECs). MHC II levels as well as the percentages and cycling status of thymic epithelial cell populations expressing MHC II were not static during post-natal development, suggesting quantitative flexibility in presenting signals to the developing thymocytes. Although there was no evidence found for regulation of surface MHC II levels by TCR or by IFN-
, the absence of class II transactivator reduced both the level of MHC II expression and the number of MHC II+ cells. Surprisingly, MHC II molecules were found to form distinct focal aggregates on the surface of cTEC but not mTEC using high-resolution analysis by confocal microscopy. Moreover, these aggregates were formed independent of TCR or TCR-bearing cells in the thymus. These aggregates could potentially generate a functional unit containing a much higher local MHC II concentration to yield a higher avidity interaction. We discuss possible mechanisms for positive selection by weak interactions in the presence of such preformed MHC II aggregate units in cTEC.
Keywords: cell cycle, MHC II regulation, positive selection, thymic compartments, thymic development
Transmitting editor: A. Singer
* These authors contributed equally to this study.
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