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Direct observation of ligand recognition by T cells


The activation of T cells through interaction of their T-cell receptors with antigenic peptide bound to major histocompatibility complex (MHC) on the surface of antigen presenting cells (APCs) is a crucial step in adaptive immunity. Here we use three-dimensional fluorescence microscopy to visualize individual peptide–I-Ek class II MHC complexes labelled with the phycobiliprotein phycoerythrin in an effort to characterize T-cell sensitivity and the requirements for forming an immunological synapse1,2,3 in single cells. We show that T cells expressing the CD4 antigen respond with transient calcium signalling to even a single agonist peptide–MHC ligand, and that the organization of molecules in the contact zone of the T cell and APC takes on the characteristics of an immunological synapse when only about ten agonists are present. This sensitivity is highly dependant on CD4, because blocking this molecule with antibodies renders T cells unable to detect less than about 30 ligands.

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Figure 1: Detection of specific peptide–MHC complexes formed on the surface of APCs using streptavidin-PE versus a complex-specific antibody.
Figure 2: Calcium increases in T cells in response to defined numbers of peptide–MHC ligands.
Figure 3: Dose response of T-cell calcium signals to specific peptide–MHC complexes in the T-cell–APC interface.
Figure 4: Effect of peptide number on endogenous accumulation of I-Ek.
Figure 5: Role of CD4 in the T-cell response to low numbers of agonist peptides.


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We thank B. Newsome, W. E. Moerner, J. Huppa, L. Wu and P. Ebert for discussions. This work was supported by grants (to M.M.D.) from the NIH and the Howard Hughes Medical Institute. D.J.I. was supported by the Cancer Research Fund of the Damon Runyon-Walter Winchell Foundation Fellowship; M.A.P. was supported by the Howard Hughes Medical Institute; and M.K. was supported by a fellowship from the Alfred Benzon Foundation.

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Correspondence to Mark M. Davis.

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Irvine, D., Purbhoo, M., Krogsgaard, M. et al. Direct observation of ligand recognition by T cells. Nature 419, 845–849 (2002).

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