To detect the presence of CD4-T cell receptor (TCR) complexes, we previously developed a flow cytometric method for measuring singlet-singlet energy transfer on human T cells labeled with fluorescein isothiocyanate-conjugated anti-CD4 and tetramethylrhodamine isothiocyanate-conjugated anti-TCR. Using the same procedure, we have now studied changes in the expression of CD4, TCR, and CD4-TCR complexes following CD4 engagement. Ligation of the D3 domain with OKT4, or the D1 domain with anti-Leu3a, induced CD4 and TCR down-regulation, while ligation of the D1 domain with gp120 did not. OKT4 caused a transient decrease in CD4-TCR association over 1 hr at 37°C, while anti-Leu3a caused a steady-state decrease. In contrast, gp120 decreased CD4-TCR association mainly at 0°C, rather than at 37°C. Such alteration in CD4-TCR assembly may underly anti-Leu3a- and gp120-mediated inhibition of T cell antigen recognition and account for the negative effect of CD4 ligation on TCR-triggered responses.
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