Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 5

Which statements are true? Explain why or why not.
-1 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 peptide-MHC complex. To test this possibility, you label cell-surface MHC molecules with a fluorescently labeled peptide so that you can detect indi- vidual peptide-MHC complexes at the interface between the APCs and the T cells in a culture dish. To detect T cell responses-the sign of a productive contact-you 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 (immuno- logical synapses) and measure the resulting uptake of Ca²+ in the adherent T cells (Figure Q24-4, red circles). When you repeat the experiment in the presence of blocking anti- bodies against CD4, you get a different result (blue circles). Do these results support or refute the notion that CD4 aug- ments T cell receptor binding? Explain your answer. CHAPTER:Glossary CHAPTER:Index