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K21-antigen: a molecule shared by the microenvironments of the human thymus and germinal centers
N Imami1, H M Ladyman, B Vincents
1Department of Immunology, Imperial College School of Medicine, Hammersmith Hospital, London, United Kingdom.
Insights
The K21 monoclonal antibody (mAb) identifies a molecule (K21-Ag) in the thymus and tonsils. This antigen is found on B cells within germinal centers, suggesting a role in B-cell selection and memory development.
Area of Science:
- Immunology
- Cell Biology
Background:
- Hassall's corpuscles in the human thymus contain a 230-kD molecule recognized by the K21 monoclonal antibody (mAb).
- This molecule, K21-antigen (K21-Ag), is also found intracellularly in cultured thymic epithelial cells and other epithelial cell lines.
Purpose of the Study:
- To investigate the expression and potential function of K21-Ag in lymphoid tissues.
- To determine the role of K21-Ag in B-cell selection and memory development within germinal centers.
Main Methods:
- Immunohistochemistry using K21 mAb on human thymus and tonsil sections.
- Double immunostaining with anti-CD22 and anti-CD23 mAbs.
- Flow cytometry on thymocytes, tonsillar lymphocytes, and peripheral blood mononuclear cells.
- Analysis of purified tonsillar T and B lymphocytes.
Main Results:
- K21-Ag is expressed on B cells and some follicular dendritic cells in tonsillar germinal centers, particularly in the apical light zone.
- A subpopulation of lymphoid cells expressing K21-Ag also expresses CD23 (K21++/CD23++), suggesting a role in centrocytic selection.
- T cells do not express K21-Ag, while B cells express low levels that are upregulated with CD23 upon mitogenic stimulation.
- K21-Ag is expressed on the surface of a very low percentage of thymocytes, tonsillar lymphocytes, and peripheral blood mononuclear cells.
Conclusions:
- K21-Ag is a potential marker for selected centrocytes in germinal centers.
- K21-Ag may play a role in B-cell selection and the development of B-cell memory.
- The upregulation of K21-Ag on B cells suggests its involvement in B-lymphocyte activation and selection.
Abstract:
The mouse IgG1 monoclonal antibody (mAb) K21 recognizes a 230-kD molecule (K21-Ag) on Hassall's corpuscles in the human thymus. This mAb also stains cultured thymic epithelial cells as well as other epithelial cell lines, revealing a predominant intracellular localization. Further analysis with mAb K21 on other lymphoid tissues showed that it also stains cells within the germinal centers of human tonsils, both lymphoid (B) cells and some with the appearance of follicular dendritic cells. Double immunostaining of tonsil sections shows that K21-Ag is not expressed by T cells, whereas staining with anti-CD22 and -CD23 mAb revealed some double-positive cells. A subpopulation of the lymphoid cells express the K21-Ag much more strongly. This K21++/CD23++ subpopulation of cells is localized in the apical light zone of germinal centers, suggesting that K21-Ag may be an important marker for the selected centrocytes within germinal centers and may play a role in B-cell selection and/or development of B-cell memory. Flow cytometric analysis showed that K21-Ag is expressed on the surface of a very low percentage of thymocytes, tonsillar lymphocytes, and peripheral blood mononuclear cells. Analysis of purified/separated tonsillar T and B lymphocytes showed that T cells do not express the K21-Ag; in contrast, B cells express low levels of the K21-Ag, and this together with CD23 is upregulated after mitogenic stimulation. Our data therefore raise the possibility that the K21-Ag may play a role in B-lymphocyte activation/selection.