How well can a T-cell epitope replace its parent carrier protein? A dose-response study.

Purpose: This work examines the effectiveness of synthetic peptide immunogens derived from immunodominant T-cell epitopes as replacements for their intact parent protein in vaccines.

Methods: Fluorescein was conjugated to hen egg lysozyme (FL-HEL, positive control) and three synthetic peptide immunogens: (a) murine B10.A (H-2a) immunodominant T-cell epitope of HEL [FL-(T-cell epitope)]; (b) multiple antigenic peptide (MAP) multimer of this epitope ([FL-(T epitope)]n-MAP, n = 2-4); and (c) negative control MAP with T-cell epitope residues replaced with glycine [(FL-Gly18)4-MAP]. The dose response of each immunogen was examined over a 300-fold range in B10.A mice. The immune response was monitored using antifluorescein ELISA assays.

Results: FL-(T epitope)'s immune response correlated positively with dose, with maximum response comparable to that of [FL-(T epitope)]n-MAP, or FL-HEL. This trend was consistent across 1 degrees, 2 degrees, and 3 degrees responses, although interanimal variability was higher in the latter two because of an all-or-none response in mice immunized with this peptide. [FL-(T epitope)]n-MAP's immune response was consistently high and nearly dose independent, a trend observed across 1 degrees, 2 degrees, and 3 degrees responses. FL-HEL's immune response correlated negatively to dose in the 1 degrees response but was nearly dose independent in the 2 degrees and 3 degrees responses. The magnitude of these latter responses was comparable to that observed for [FL-(T epitope)]n-MAP. (FL-Gly18)4-MAP did not elicit an immune response except at the highest dose. This trend was consistent across 1 degrees, 2 degrees, and 3 degrees responses.

Conclusions: The monomeric epitope was 300-fold less potent than its parent carrier protein, but increasing immunogen valency using MAP technology compensated totally for reduced potency. (FL-Gly18)4-MAP's lack of response at all but the highest dose strongly suggests that a specific immunodominant T-cell epitope sequence for HEL is necessary for successful peptide mimicry of HEL. This work also demonstrates the importance of quality assessment of commercial MAP core resins.