Quantification of helper T lymphocyte/antigen-presenting cell interactions: A single-cell study.

Abstract

Helper T lymphocytes (Th cells), a key component of the immune system's response against antigen, cannot recognize antigen in its native form but instead recognize a processed form of the antigen on the surfaces of antigen-presenting cells (APCs). The focus of this research was to develop a quantitative understanding of the Th-APC interaction and the resulting Th cell response. Responses of individual Th cells to their physiological stimuli, MHC-peptide complexes on APC surfaces, were measured. A dynamic, single-cell assay involving alternating differential interference contrast and fluorescence microscopy, together with digital imaging, was developed for viewing the physical Th-APC interaction and monitoring the increases in intracellular free calcium concentration of the Th cell, an early event in Th cell activation. Intracellular calcium responses of individual Th cells were found to be heterogeneous and an all-or-none phenomenon, independent of antigen concentration; however, the fraction of Th-APC conjugates involving responding Th cells was an increasing function of antigen concentration. Mathematical modeling was used in conjunction with experimental results to estimate that 1 to 20 MHC-peptide complexes are required in the initial Th-APC contact area to elicit a Th cell calcium response. Mathematical modeling was also used to investigate two classes of methods, altering properties of the antigen and altering the method of antigen uptake, employed to modify antigen processing and presentation and to hypothesize their effects on Th cell response. Two mathematical models were used, one for relating the antigen concentration to the number of MHC-peptide complexes presented on the APC surface and the second for relating the number of MHC-peptide complexes to the number of bound TCRs on the Th cell surface. Key parameters for each of the methods were identified and their effects on antigen processing and presentation and the resulting Th cell response were determined. For example, high values of MHC/peptide affinity were predicted to compensate for low values of TCR/MHC-peptide affinity in particular parameter ranges. Results were in good qualitative agreement with a variety of experimental data. This work is useful for not only interpreting experimental data but also guiding future experiments aimed at manipulating the Th cell response.