CD3 eta and CD3 zeta are alternatively spliced products of a common genetic locus and are transcriptionally and/or post-transcriptionally regulated during T-cell development.

The CD3 eta subunit of the T-cell receptor is thought to subserve an important role in signal transduction and possibly T-cell development. Herein we characterize the organization of the mouse CD3 eta gene and show that it is part of one gene locus that also encodes CD3 zeta on chromosome 1. The NH2-terminal sequence of CD3 zeta and CD3 eta, which share the same leader peptide and are identical through amino acid 122 of each mature protein, is encoded by exons 1-7. However, exons 8 and 9 are differentially spliced to give rise to CD3 zeta and CD3 eta: exons 1-8 encode CD3 zeta and exons 1-7 plus 9 encode CD3 eta. RNase protection analysis with RNA from a variety of fetal, neonatal, and adult cell types indicates that expression of both gene products is T-lineage-restricted. Importantly, expression of CD3 zeta and CD3 eta mRNA appears before or on day 16 of fetal gestation. Expression is apparently coordinate since no cell types tested express CD3 zeta or CD3 eta alone. The steady-state level of CD3 zeta mRNA is greater than or equal to 40-60 times that of CD3 eta mRNA. In immature CD4+CD8+CD3low double-positive thymocytes and CD4+CD8-CD3high or CD4-CD8+CD3high single-positive thymocytes, the respective steady-state CD3 zeta and CD3 eta mRNA levels are equivalent, whereas the amount of receptor-associated CD3 zeta and CD3 eta proteins in double-positive thymocytes is approximately 10 times less than in single-positive thymocytes. Nevertheless, the CD3 zeta/CD3 eta protein ratio remains constant in all populations (40-60:1). Furthermore, discordance between mRNA and protein levels for CD3 zeta and CD3 eta is also observed in splenic T cells. Thus, posttranscriptional and/or transcriptional regulatory mechanisms control CD3 zeta and CD3 eta expression during T-cell development.