Serological identification of thymocyte differentiation antigens

Engagement of CD99 Induces Up-Regulation of TCR and MHC Class I and II Molecules on the Surface of Human Thymocytes

CD99 is a cell surface molecule involved in the aggregation of lymphocytes and apoptosis of immature cortical thymocytes. Despite its high level expression on immature cortical thymocytes, the functional roles of this molecule during thymic selection are only now being elucidated. Examination of the effects of CD99 engagement on the expression kinetics of the TCR and MHC class I and II molecules, which are involved primarily in thymic positive selection, revealed a marked up-regulation of these proteins on the surface of immature thymocytes. This increase was the result of accelerated mobilization of molecules stored in cytosolic compartments to the plasma membrane, rather than increased RNA and protein synthesis. Confocal microscopic analysis revealed the changes in subcellular distribution of these molecules. When CD99 was engaged, TCR and MHC class I and II molecules were concentrated at the plasma membrane, particularly at cell-cell contact sites. The TCRlow subpopulation of immature double positive thymocytes was much more responsive to CD99-mediated up-regulation than was the TCRhigh population. These findings suggest that CD99-dependent up-regulation may have possible implication in positive selection during thymocyte ontogeny.

Thymocytes positively select thymocytes in human system

We previously demonstrated the expression of MHC class II molecules in a significant percentage of human fetal and postnatal thymocytes. These results, at that time, raised the question as to whether the MHC class II molecules on immature thymocytes could actively be involved in the selection of immature T cells. We have developed a human reaggregate culture system to address this issue. Surprisingly, despite the fact that thymic epithelial cells (TECs) have been shown to be a major selecting cell type of positive selection, we were clearly able to see the involvement of MHC class II+ thymocytes during selection process through T-T interaction. In addition, maturation to single positive (SP) cells occurred only in the presence of MHC class II molecules and immature thymocytes were found to be arrested at the double positive (DP) stage of differentiation by blocking of TCR recognition of MHC class II molecules. All these results strongly suggest that human MHC class II+ thymocytes actively participate in the selection of the TCR repertoire, for which TCR recognition of peptide/MHC class II may be an initial determining step.

Role of a genetic region on chromosome 4 in the regulation of natural killer cell activity in mice

Natural killer cell (NK) activity is regulated by both the H-2 and non-H-2 genes. Using bilineal congenic HW26C and HW13 mice which differ from the background strain C57BL/6By (B6) in a region of chromosome 4, we investigated the role played by a gene/genes in a segment of chromosome 4 of BALB/cBy on NK cell activity. Percoll separated low density spleen cells from young HW26C and HW13 mice showed a 3.5 fold higher NK activity than the B6. We also observed that the increase in NK activity of HW26C was not due to an increase in the number of NK cells. Using five other bilineal congenics containing different regions of chromosome 4 of BALB/cBy, we observed that the putative gene(s) regulating NK activity may be located between b and IFN-alpha/beta genes of chromosome 4. The level of NK activity of (B6xHW26C)F1 ranked between the HW26C and B6 suggesting that the gene product described is inherited in an incompletely dominant fashion.

JL1, a novel differentiation antigen of human cortical thymocyte

Expression of a novel thymocyte differentiation antigen, JL1, defined by a monoclonal antibody (mAb) developed against human thymocytes showed a specificity for stage II double positive (CD4+CD8+) human cortical thymocytes. This antigen was not expressed at detectable levels on medullary thymocytes, mature peripheral leukocytes, bone marrow cells or on other types of tissues elsewhere in the human body. Immunohistologic analysis revealed that JL1 had a clear pattern of distribution on cortical thymocytes. Immunoprecipitation of 125I-labeled cell lysates from human thymocytes and Molt-4 leukemic cell line with anti-JL1 mAb yielded a 120-130-kD single chain glycoprotein. When immunoprecipitation of cell lysate was done after endoglycosidase F treatment, JL1 antigen was still detected by antibody but the band showed a reduction in apparent molecular mass of approximately 5 kD. This suggests that, although JL1 molecule contains carbohydrate group, this does not form a critical part of the antigenic determinant for anti-JL1 antibody. JL1 antigen appears to be the first double positive, stage-specific differentiation antigen of human thymocyte reported so far. This antigen would be a useful marker for lymphoblastic malignancy of stage II thymocyte origin and it may be involved in the thymocyte education process.