TCR triggering by pMHC ligands tethered on surfaces via poly(ethylene glycol) depends on polymer length - PubMed (original) (raw)

TCR triggering by pMHC ligands tethered on surfaces via poly(ethylene glycol) depends on polymer length

Zhengyu Ma et al. PLoS One. 2014.

Abstract

Antigen recognition by T cells relies on the interaction between T cell receptor (TCR) and peptide-major histocompatibility complex (pMHC) at the interface between the T cell and the antigen presenting cell (APC). The pMHC-TCR interaction is two-dimensional (2D), in that both the ligand and receptor are membrane-anchored and their movement is limited to 2D diffusion. The 2D nature of the interaction is critical for the ability of pMHC ligands to trigger TCR. The exact properties of the 2D pMHC-TCR interaction that enable TCR triggering, however, are not fully understood. Here, we altered the 2D pMHC-TCR interaction by tethering pMHC ligands to a rigid plastic surface with flexible poly(ethylene glycol) (PEG) polymers of different lengths, thereby gradually increasing the ligands' range of motion in the third dimension. We found that pMHC ligands tethered by PEG linkers with long contour length were capable of activating T cells. Shorter PEG linkers, however, triggered TCR more efficiently. Molecular dynamics simulation suggested that shorter PEGs exhibit faster TCR binding on-rates and off-rates. Our findings indicate that TCR signaling can be triggered by surface-tethered pMHC ligands within a defined 3D range of motion, and that fast binding rates lead to higher TCR triggering efficiency. These observations are consistent with a model of TCR triggering that incorporates the dynamic interaction between T cell and antigen-presenting cell.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Schematic illustration of IEkMCC ligands tethered onto a plastic surface with PEG polymer linkers.

IEkMCC proteins with free c-terminal cysteines were first conjugated with heterobifunctional PEG linkers Mal-PEG-Bio through interactions between the sulfhydryl group and the maleimide group. Conjugates with biotin at the free ends of the polymer were then tethered to a plastic surface coated with streptavidin.

Figure 2

Figure 2. Characterization and separation of PEG polymer linkers and IEkMCC-PEG conjugates.

(A) Compiled elution curves of nine PEG polymer linkers from a Superdex 200 10/300 GL gel filtration column. The polymers were detected through the weak UV absorption of the biotin group using a 245 nm UV detector. (B) Separation of the IEkMCC and PEG polymer reaction products. The reaction products were loaded on a Superdex 200 10/300 GL gel filtration column to separate IEkMCC-PEG conjugates, unreacted IEkMCC, and unreacted PEG polymers. The reaction products of PEG 15000, PEG 30000 and PEG 60000 were first purified with an IEk-binding affinity column to eliminate unreacted PEG polymers. The dotted vertical line indicates the elution volume of IEkMCC protein. The late elution peaks of unreacted polymers can be seen for PEGs ranging from PEG 88 to PEG 5000. In reaction with PEG 7500, unreacted IEkMCC and unreacted polymer formed a single peak that was eluted at a position between unconjugated IEkMCC and pure PEG 7500.

Figure 3

Figure 3. FRET between streptavidin on plastic plates and IEkMCC tethered with PEG polymers.

(A) Measured FRET efficiencies of IEkMCC tethered with six different PEG polymers. The intensity of DyLight 549 was captured before and after DyLight 649 was photobleached. The measured FRET efficiency (formula image) was calculated using the intensity of DyLight 549 before (formula image) and after (formula image) DyLight 649 photobleaching (formula image ). The averaged values of two measurements were plotted with standard deviations. (B) After normalization, the measured FRET efficiencies match those calculated based on the Flory radius (formula image) of the PEG polymers. The formula image of the PEG polymer of formula imagesubunits and unit length formula image was calculated using formula image, where formula image is 0.28 nm . Theoretical FRET efficiency (formula image) was calculated using the equation formula image, where the Förster distance (formula image) of the DyLight 549-DyLight 649 donor-acceptor pair is 5 nm and the distance between the pMHC ligand and streptavidin formula image is formula image of the PEG polymer plus the pMHC radius of 2 nm. The FRET efficiencies were normalized by dividing the FRET efficiencies by the FRET efficiency of PEG 88.

Figure 4

Figure 4. T cell activation by IEkMCC tethered with PEG polymers of different lengths.

(A) T cell IL2 production in response to IEkMCC-PEG ligands of varying coating densities after 6 hours of stimulation. Data are representative of three independent experiments. The percent of T cells producing IL2 was determined by intracellular staining and flow cytometry. Three experiments using T cells from three different mice were performed (see Fig. S8 for flow cytometry plots). The percent of T cells producing IL2 was normalized to the highest value in each experiment. The data points are averages of the normalized values with standard errors of the means. (B) The rate of T cell response to IEkMCC ligands tethered with PEG polymers of different lengths. T cell IL2 production in response to stimulation on 96 well plates coated with 110 pM IEkMC-PEG ligands. T cells were harvested every hour for 6 hours and levels of IL2 expression were assayed by flow cytometry. Three experiments using T cells from three different mice were performed (see Fig. S9 for flow cytometry plots). The percent of T cells producing IL2 was normalized to the highest value in each experiment. The data points are averages of the normalized values with standard errors of the means. (C) The rates of T cell IL2 responses to IEkMCC ligands tethered with PEG polymers were extracted from the slope of linear fitting curves in Fig. 4B and plotted against the Flory radius of the polymers. The linear regressions and equations for deriving the rates are shown in Fig. S7.

Figure 5

Figure 5. The impact of polymer length on pMHC-TCR binding kinetics based on CG-MD simulation.

The on-rates and off-rates derived from the simulation for the PEG 4000, PEG 10000 and PEG 20000 were scaled against the data for the PEG 4000 and plotted as a function of the polymer Flory radius. To display the relationship between binding rates and TCR triggering efficiency, the experimentally determined IL2 expression rates for PEG 3500, PEG 5000, PEG 7500, PEG 150000 and PEG 30000 shown in Fig. 4A were scaled against PEG 3500 and plotted as a function of the polymer Flory radius.

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