CD4 proteins on helper and regulatory T cells serve as co-receptors that bind to

Chapter 0, Problem 24-11

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CD4 proteins on helper and regulatory T cells serve as co-receptors that bind to invariant parts of class II MHC proteins. CD4 is thought to increase the adhesion between T cells and antigen-presenting cells (APCs) that are initially connected only weakly by the T cell receptor bound to its specific peptideMHC complex. To test this possibility, you label cell-surface MHC molecules with a fluorescently labeled peptide so that you can detect individual peptideMHC complexes at the interface between the APCs and the T cells in a culture dish. To detect T cell responsesthe sign of a productive contactyou load them with a Ca2+ indicator dye, as cytosolic Ca2+ increases when lymphocytes are active. You now count the peptide MHC complexes at a large number of interfaces (immunological synapses) and measure the resulting uptake of Ca2+ in the adherent T cells (Figure Q244, red circles). When you repeat the experiment in the presence of blocking antibodies against CD4, you get a different result (blue circles). Do these results support or refute the notion that CD4 augments T cell receptor binding? Explain your answer.