Characterization of mouse CD53: Epitope mapping, cellular distribution and induction by T cell receptor engagement during repertoire selection

The pan‐leukocyte antigen CD53 is a member of the poorly understood transmembrane 4 superfamily (TM4SF) of cell membrane glycoproteins. CD53 is proposed to play a role in thymopoiesis, since rat CD53 is expressed on immature CD4⁻8⁻thymocytes and the functionally mature single‐positive subset, but is largely absent from the intermediate CD4⁺8⁺ cells. We have characterized CD53 in the mouse through the production of two new monoclonal antibodies, MRC OX‐79 and OX‐80, which were raised against the RAW 264 cell line and screened on recombinant CD53 fusion proteins. The epitopes recognized by both antibodies are dependent on disulfide bonding and map to the major extracellular region of CD53, requiring the presence of a single threonine residue at position 154. Mouse CD53 has a molecular mass of 35‐45 kDa and is expressed on virtually all peripheral leukocytes, but not on cells outside the lymphoid or myeloid lineages. CD53 expression distinguishes subpopulations of thymocytes in the mouse and resembles the expression pattern of rat CD53. Amongst the immature CD4⁻8⁻ thymocytes, mouse CD53 is clearly detectable on the earliest CD44^high25⁻ subset, but down‐regulated on the later CD44^high25⁺, CD44^low25⁺ and CD44^low25⁻ stages. Also, the subsequent transient TcR⁻/low CD4⁻8⁺ cells and most CD4⁺8⁺ thymocytes express little or no CD53. This is consistent with the idea that cells which are committed to enter the selectable CD4⁺8⁺ compartment switch off CD53. The effect of T cell receptor (TcR) engagement on the reexpression of CD53 on CD4⁺8⁺ thymocytes was studied both ex vivo and in vitro using F5 mice, transgenic for the H‐2^b/influenza nucleoprotein‐peptide‐specific TcR, back‐crossed onto an H‐2^q or H‐2^b background of RAG‐2‐deficient mice. CD4⁺8⁺ thymocytes from non‐selecting H‐2^q F5 mice are CD53 negative, but in vitro stimulation through the TcR dramatically induces CD53 expression. In contrast, a fraction of CD4⁺8⁺ thymocytes from positively selecting H‐2^b F5 transgenic mice express CD53. Therefore TcR engagement by selecting major histo‐compatibility complex peptide complexes, or surrogate ligands, induces CD53 expression on otherwise CD53‐negative, non‐selected CD4⁺8⁺ thymocytes. Whether CD53 itself participates as a signaling molecule in further stages of thymic selection is still a matter of speculation.