Physical and functional association of CD45 and CD3-TCR complex on CD1(+) human thymocytes. Evidence that the engagement of CD45 molecules can prevent CD1(+) thymocytes from apoptosis

In this study the effects of CD45 engagement on CD3-TCR-driven stimulation of CD1(+) human immature thymocytes have been analyzed, Simultaneous cross-linking of CD45 and CD3 antigens on highly purified CD1(+) thymocytes reduced the number of cells undergoing apoptosis after 16 h of in vitro culture, This cell population might represent immature thymocytes committed in vivo to die by programmed cell death (PCD), CD45 engagement could also increase the number of cycling CD1(+) thymocytes; of note, the large majority (>95%) of dividing cells expressed the CD1 molecule at the cell surface, indicating that proliferating cells were actually represented by immature thymocytes, These data suggest that the CD45 molecule might play a role in the rescue of immature thymocytes from PCD during differentiation, Along this line, we found that activation of CD1(+) thymocytes via the CD3-TCR complex could be enhanced by CD45, both in terms of transcription and surface expression of IL-2R. These effects might be explained by the finding that the CD45 molecule (but not its isoforms CD45RO and RA) was physically associated with the CD3-TCR complex at the cell surface of CD1(+) human thymocytes, as shown by co-precipitation and co-capping experiments, Finally, cross-linking of CD45 and CD3 antigens led to the expansion of CD3(+) thymocytes co-expressing CD4 and One, indicating that simultaneous engagement of CD45 and CD3 molecules can block CD1(+) cells at the double-positive (CD3(+)CD4(+)CD8(+)) differentiation stage, On the other hand, stimulation through CD3 resulted in the expansion of thymocytes showing a mature phenotype (CD3(+)CD4(+) or CD3(+)CD8(+)), Altogether, these findings suggest that the CD45 molecule is involved both in early activation and in the regulation of CD1(+) thymocyte differentiation.