Regulation of beta1 integrin functional activity by the CD7 antigen in human T lymphocytes.

Abstract

Modulation of the functional activity of integrin adhesion receptors on T lymphocytes is a dynamic process initiated by activation of cell surface receptors. On human T cells, antibody-mediated crosslinking of the CD7 antigen results in a rapid increase in the functional activity of $\beta$1 and $\beta$2 integrin receptors. The objective of this thesis is to elucidate the intracellular signaling pathway utilized by CD7 to regulate $\beta$1 integrin function on human T lymphocytes. The kinetics of CD7-mediated activation of integrin function was distinct from that observed following phorbol ester stimulation and anti-CD3 crosslinking. CD7 stimulation also did not induce an increase in CD2-mediated adhesion to LFA-3. A role for tyrosine kinases in CD7 regulation of integrin function is suggested, since CD7 stimulation results in increased tyrosine phosphorylation and CD7-mediated activation of integrin function is blocked by tyrosine kinase inhibitors. Stimulation of CD7 was also found to induce the association of CD7 with the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K) via tyrosine phosphorylation of the CD7 cytoplasmic SH2 binding motif, tyr-glu-asp-met (YEDM). Treatment of T cells with two structurally distinct PI 3-K inhibitors blocked CD7-mediated activation of integrin function. In addition, expression of a dominant-negative form of the p85 subunit of PI 3-K specifically inhibited CD7-mediated increases in integrin-mediated T cell adhesion. The role of the CD7 cytoplasmic domain in activating PI 3-K and integrin function was investigated further by expressing chimeric receptors containing the CD7 cytoplasmic domain in the myelomonocytic cell line HL60. Stimulation of chimeric receptors containing the wild-type CD7 cytoplasmic domain resulted in tyrosine phosphorylation, as well as association and activation of PI 3-K. Stimulation of chimeric receptors containing the CD7 cytoplasmic domain with a deletion of the YEDM motif resulted in a similar pattern of tyrosine phosphorylation but failed to associate with and activate PI 3-K. However, neither chimeric receptor was able to increase $\beta$I integrin-mediated adhesion of HL60 cells to fibronectin. In summary, these results suggest that PI 3-K is necessary but not sufficient for CD7-mediated regulation of integrin-mediated adhesion.