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*
Laboratório de Imunologia Molecular, Instituto de Biologia Molecular e Celular, Universidade do Porto, Porto, Portugal; and
Medical Research Council Cellular Immunology Unit, Sir William Dunn School of Pathology, University of Oxford, Oxford, United Kingdom
In T lymphocytes, the CD2 and CD5 glycoproteins are believed to be
involved in the regulation of signals elicited by the TCR/CD3 complex.
Here we show that CD2 and CD3 independently associate with CD5 in human
PBMC and Jurkat cells. CD5 coprecipitates with CD2 in CD3-deficient
cells and, conversely, coprecipitates with CD3 in cells devoid of CD2.
In unstimulated CD2+ CD3+ Jurkat cells, CD5
associates equivalently with CD2 and CD3 and is as efficiently
phosphorylated in CD2 as in CD3 immune complexes. However, upon
activation the involvement of CD5 is the opposite in the CD2 and CD3
pathways. CD5 becomes rapidly tyrosine phosphorylated after CD3
stimulation, but is dephosphorylated upon CD2 cross-linking. These
opposing effects correlate with the decrease in the activity of the SH2
domain-containing protein phosphatase 1 (SHP-1) following CD3
activation vs an enhanced activity of the phosphatase after CD2
triggering. The failure of CD5 to become phosphorylated on tyrosine
residues in the CD2 pathway has no parallel with the lack of use of
-chains in CD2 signaling; contrasting with comparable levels of
association of CD2 or CD3 with CD5, associates with CD2 only
residually and is nevertheless slightly phosphorylated after CD2
stimulation. The modulation of CD5 phosphorylation may thus represent a
level of regulation controlled by CD2 in signal transduction mechanisms
in human T lymphocytes.
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