OVER-EXPRESSION OF CD3-EPSILON TRANSGENES BLOCKS T-LYMPHOCYTE DEVELOPMENT

We have reported previously that mice carrying >30 copies of the human CD3 epsilon transgene completely lose their T lymphocytes and NK cell s (36), Here we demonstrate by immunohistology that in the most severe ly immunodeficient mouse, tg epsilon 26, the thymus is very small, has sizeable vacuoles and does not contain recognizable T lymphocytes exc ept for a small percentage of Thy-1(+) cells and B cells, Cell surface phenotyping and TCR alpha and -beta rearrangement studies confirm tha t the arrest in T lymphocyte development precedes the arrest in rag-1( null), rag-2(null) and TCR beta(null) mice, Since the T cell progenito rs in which the arrest occurred were absent in the transgenic mice, in direct approaches were taken to examine the causes of the block in T c ell development, Analyses of 12 independently established mutant mouse lines, generated with five different transgenic constructs, revealed that the severity of the abrogation in T cell development was dependen t on the number of copies of transgenes, Since the number of transgene copies generally correlated with the levels of expression of the tran sgenic CD3 epsilon proteins, we concluded that over-expression of the CD3 epsilon protein was the likely cause of the block in T lymphocyte development, The T cell immunodeficiency was caused by either the huma n or the murine CD3 epsilon protein, Since transgene coded mRNAs were found in significantly higher quantities than endogenous CD3 epsilon m RNAs in fetal thymi on days 13 and 14 of gestation, over-expression to ok place very early in development, probably prematurely, Over-express ion of the CD3 epsilon transgene in thymocyte precursors may therefore affect T lymphocyte development in the absence of TCR and possibly in the absence of the other CD3 proteins, More importantly, over-express ion of the CD3 epsilon protein in thymocytes of mice with a low copy n umber of transgenes had a significant effect on late thymic developmen t, Over-expression of the CD3 epsilon protein in immature thymocytes m imicked the effects caused by exposure of CD4(-)CD8(-)thymocytes to an ti-CD3 epsilon treatment: apoptosis and lack of TCR beta expression, W e therefore speculate that in the homozygous tg epsilon 26 animals the arrest in T cell development was caused by excessive signal transduct ion events rather than by a toxic effect of the transgenic protein.