The immunological synapse and CD28-CD80 interactions
Bromley SK, Iaboni A, Davis SJ, Whitty A, Green JM, Shaw AS, Weiss A, Dustin ML. (2001), Nat Immunol. 2, 1159-66
According to the two-signal model of T cell activation, costimulatory molecules augment T cell receptor (TCR) signaling, whereas adhesion molecules enhance TCR-MHC-peptide recognition. The structure and binding properties of CD28 imply that it may perform both functions, blurring the distinction between adhesion and costimulatory molecules. Our results show that CD28 on naïve T cells does not support adhesion and has little or no capacity for directly enhancing TCR-MHC-peptide interactions. Instead of being dependent on costimulatory signaling, we propose that a key function of the immunological synapse is to generate a cellular microenvironment that favors the interactions of potent secondary signaling molecules, such as CD28.
Key figure: CD28-CD80 binding parameters
Jurkat cells were injected into a flow cell and incubated for 30 min at 37 °C on planar phospholipid bilayers that contained Cy3-CD80 and Cy5-CD48. (a) The percentage of adherent cells was determined by comparing the interference reflection and transmitted light images. (b) Average contact area. (c) Average number of CD80 molecules bound per contact. (d) 2D Kd plot. B/F denotes [bound CD80]/[free CD80]; B*p denotes [bound CD80]*(contact area/cell area). At least 50 cells were analyzed for each datapoint. 2D Kd values from these experiments were 0.6−0.8 molecules/μm2. Data are representative of three experiments. (e−g) 3D binding affinity of murine sCD80 to immobilized hCD28-Fc. (e) Injections of sCD80 at 37 °C started at 16 μM and were followed by seven twofold dilutions, flowing over hCD28-Fc immobilized at a concentration of 3000 response units (RU). The curves represent total specific binding after subtraction of the background responses observed in a control flow cell with immobilized SLAM protein. A nonlinear fit of the Langmuir binding isotherm53 (f) and the linear analysis of a Scatchard plot (g) yielded a Kd of 2 μM. Comparison of the membrane and solution affinities yielded a confinement region of 3 nm.