Inhibition of phosphatidylserine synthesis induced by triggering CD2 or the CD3-TCR complex in a human T cell line. Relationships with G proteins and receptors modulation.

Activation of Jurkat T cells with phytohemagglutinin, CD3 or CD2 mAbs results in a marked inhibition of phosphatidylserine (PS) synthesis. Monitoring PS synthesis in T cells shows that: (i) after modulation of CD3 molecules the cells become refractory to further treatment with CD3 mAbs as well as to a further challenge with CD2 mAbs; and (ii) treatment of T cells with fluoride ions and cholera toxin, two known effectors of guanosine triphosphate-binding proteins, also resulted in a strong inhibition of the synthesis of this phospholipid. The inhibition of PS synthesis thus appears to be regulated similarly to the other activation events, suggesting that transmembrane signalling mechanisms leading to PS inhibition are the same as those previously proposed for increasing phosphatidylinositides turnover and subsequent rise in the intracellular calcium concn in lymphocytes.

Effect of hypertonicity and monensin on CD3/TCR surface expression in human T cells.

Pretreatment of T lymphocytes with anti-CD3 or anti-TCR mAb results in the disappearance of the T cell receptor complex (TCR/CD3) from the cell surface. The mechanisms of this down-regulation have not been fully elucidated. We demonstrate here that the modulation of the CD3/TCR complex can be completely inhibited by hypertonic medium, a condition known to block receptor-mediated endocytosis. Consequently, the sequestration of the CD3/TCR complex associated to relevant mAb in acid vesicles did not occur under hypertonicity condition. Moreover, monensin, which is known to interfere with receptor recycling, was found to amplify the CD3/TCR modulation induced by specific mAb and decrease the uptake of 125I-labeled anti-CD3/TCR mAb by T cells. We therefore propose that mAb-CD3/TCR complexes are internalized via coated pits by a receptor-mediated endocytosis and then transported to acid compartments. MAb are degraded in lysosomes during this process, whereas CD3/TCR molecules recycle back to cell surface.

Monoclonal antibody internalization and degradation during modulation of the CD3/T-cell receptor complex.

Although it is well known that the CD3/T-cell receptor (TCR) complex modulates from the surface of T cells upon exposure to monoclonal antibodies (mAb) directed against it, the fate of bound mAb has not been yet elucidated. We therefore perform direct binding experiments of 125I-labeled mAb against CD3 or TCR to investigate their fate in Jurkat T cells. We demonstrated that all mAb were progressively internalized and degraded in Jurkat T cells and that this degradation was inhibited by chloroquine, an inhibitor of lysosomal degradation enzymes. The sequestration of anti-CD3 mAb in acid compartments was furthermore shown using cytofluorometry. All together our results show that antibodies against CD3 or against TCR follow the same endocytic pathway.