High-affinity T helper epitope induces complementary helper and APC polarization, increased CTL, and protection against viral infection

Abstract

IntroductionViral proteins have not evolved to be optimal vaccines. We have demonstrated previously proof of principle for the approach of "epitope enhancement," whereby modifying the amino acid sequence of a Th epitope in the HIV-1 envelope to enhance binding to a class II MHC molecule can increase immunogenicity (1, 2). Attaching this modified Th epitope to a cytotoxic T lymphocyte (CTL) epitope enhanced CTL induction in vivo (2). Genetic mapping showed that the enhanced CD8 + CTL response was due to an improved CD4 + cell response. We (3) and others (4-6) have shown similar epitope enhancement for peptides binding to class I MHC molecules. However, the molecular mechanisms by which increased affinity for class II MHC translates into increased efficacy remain unclear. Is the enhanced peptide simply active at lower concentrations? Is the difference only quantitative, or are there qualitative differences in the response? One might have expected a peptide with higher affinity for MHC to be more immunogenic, but not necessarily to induce a qualitatively different response.Here we focus on the mechanism of epitope enhancement for class II MHC-restricted helper T cells in augmenting a CTL response. We and others have found an essential role for CD4 + T helper cells in induction of CTL responses to peptide vaccines (7-9). The mechanism of CD4 + cell help for CTL induction appears to involve an intermediate step requiring conditioning of the APC via CD40 ligation by CD40L (10-13), but the downstream effects of this ligation have not been determined. Although an upregulation of essential costimulatory molecules on the antigen-presenting cells (APCs) has been suggested as a possible mechanism of help for CTL induction, the identity of the specific costimulatory molecules involved and the mechanism enhancing CD8 + CTL activation remain unclear. Also, it was not known how the modified peptide increases help; we show here it acts by increasing CD40L expression. Furthermore, it was not known whether an increase in APC conditioning by augmentation of help would simply result in more effective antigen presentation or would qualitatively alter the APC to induce a more polarized response.All of these questions are addressed in the current study. We find that modification of a Th cell epitope to increase binding to MHC causes quantitatively more T cells to undergo upregulation of CD40L and to express higher levels of CD40L per cell, which interacts with CD40 to condition the APC for CTL induction. The primary measurable effect of the improved APC conditioning by the higher-affinity peptide is upregulation of IL-12 production, along with some difference in B7-1 and B7-2 expression. Thus, unexpectedly, the APC becomes more polarizing toward a Th1 response. Furthermore, the conditioned APCs, removed from the Th Natural viral proteins do not always make optimal vaccines. We have found that sequence modification to increase epitope affinity for class II MHC molecules (epitope enhancement) can improve immunogenicity. Here we show firs...

Strategies for designing and optimizing new generation vaccines

“…Epitope enhancement for MHC class II molecules can also result in increased CD4 + T-cell help for a CD8 + cytotoxic T-lymphocyte (CTL) response, even when the CTL epitope itself is not altered 10 . In this case, we have recently found that epitope enhancement can also induce a qualitatively different response, more skewed toward T HELPER 1 (T H 1) cytokine production by a mechanism involving increased CD40L (CD40 ligand; CD154) on the helper cell, which, in turn, induces more interleukin (IL)-12 production by the APC 11 .…”

Progress on new vaccine strategies against chronic viral infections

“…The mechanism could be explained by a reciprocal interaction between helper T cells and dendritic cells in which the higher affinity peptide induced more CD40L expression on the surface of the helper T cells. These in turn induced more IL-12 production by the dendritic cells as well as more costimulatory molecule expression, which skewed the helper T cell phenotype to the Th1 type, made the dendritic cells more effective at activating CTL precursors, and improved protective efficacy (116). Several studies have described epitope enhancement of HIV peptides with low affinity for the most common human class I HLA molecule HLA-A*0201 (117-119), but when modified peptides are used, care must be taken to induce T cells that still respond well to the natural viral sequence (119).…”

Integrated assessment of predicted MHC binding and cross-conservation with self reveals patterns of viral camouflage

“…Many algorithms rely on the linear nature of T cell epitopes as it simplifies modeling their interactions with MHC in defined binding pockets, which contrasts with the complexities of B cell epitope prediction that involves the interaction of discontinuous antigen sequences with highly variable complementarity determining regions of antibodies. Such in silico predictions of T-helper epitopes have been successfully applied to the design of vaccines [5,6] and to the selection of epitopes in studies of autoimmunity [7]. We use here the EpiMatrix system, a suite of epitope mapping tools that has been validated over the course of more than a decade, both in vitro and in vivo (for example, see references [8-14]).…”