Peptide binding affinity and pH variation establish functional thresholds for activation of HLA-DQ-restricted T cell recognition

Peptides derived from the HSV-2 VP16 protein were utilized for studies of p eptide binding to DQ0302 molecules and T cell activation at both neutral an d acidic pH. The native peptide VP16 430-444 contains an Asp at position 44 2, binds to DQ0302 strongly, with a Kd value of 50nM at acidic pH and very weakly, with a Kd value of greater than 10 mu M at neutral pH. A truncated version of 430-444, i.e., VP16 433-442, binds with an affinity 10-fold lowe r compared to 430-444 at acidic pH, and binding at neutral pH was barely de tectable. The homologous peptide 430-444,442A has an Asp to Ala substitutio n at position 442 and binds-to DQ0302 with a Kd similar to 433-442. The sho rt truncated analog 433-442A binds very poorly at both acidic and neutral p H. Both the wild type 430-444 and 433-442 peptides stimulated a HSV-specific T cell clone after a brief incubation with antigen presenting cells (APC) ex pressing DQ0302 at acidic pH. Much higher concentrations of wild type pepti des were needed to activate T cells at neutral pH. In contrast, APC pulsed with Ala-substituted peptides 430-444,442A or 433-442A at neutral pH failed to stimulate the T fell clone, while APC pulsed at acidic pH and subsequen tly washed led to successful T cell activation. The Ala-substituted peptide was recognized by the T cell clone at neutral pH only when it was present in the APC culture throughout the stimulation process. While the MHC-peptid e complexes formed with the native peptide are stable, complexes formed wit h the Ala-substituted peptide had a functional t(1/2) Of less than 4 hr at neutral pH. Human Immunology 60: 619-626 (1999). (C) American Society for H istocompatibility and Immunogenetics, 1999. Published by Elsevier Science I nc.