Mathematical modeling of helper T lymphocyte/antigen-presenting cell interactions: analysis of methods for modifying antigen processing and presentation.

Helper T lymphocytes (Th cells) are activated by contact with antigen-presenting cells (APCs) that have processed and presented the appropriate MHC-peptide complexes. Two experimental methods for modifying antigen processing and presentation include altering the properties of the antigen and altering the method of antigen uptake. Mathematical modeling was used to investigate the effects of these two methods on the Th cell response. Two mathematical models were used, one for relating the external antigen concentration to the number of MHC-peptide complexes to the number of bound T cell receptors (TCRs) on the Th cell. Large values of MHC/peptide affinity were predicted to compensate for small values of TCR/MHC-peptide affinity in particular parameter ranges. Similarly, large values of antigen receptor number were predicted to compensate for small values of antigen receptor affinity in particular parameter ranges. Results were shown to agree with a variety of experimental data. In addition, model predictions suggest that knowledge of MHC/peptide, TCR/MHC-peptide, and receptor/antigen affinities is not sufficient to accurately describe an experimental system; the kinetic rate constants can dramatically affect antigen processing and presentation, the Th-APC interaction, and the Th cell response. This theoretical approach is useful not only for interpreting experimental data but also for guiding future experiments aimed at manipulating the Th cell response.