Characterization of a helper T cell epitope recognized by mice of a low responder major histocompatibility type

Polygene DNA vaccine induces a high level of protective effect against HIV-vaccinia virus challenge in mice

Single HIV-1 subtype DNA vaccine is unlikely to provide reactive protection across a wide range of HIV strains since the HIV virus changes the antigenic sites, particularly, in env gene. To overcome these issues, we constructed a multivalent poly-epitope DNA vaccine. A polygenic DNA vaccine encoding 20 antigenic epitopes from the HIV-1 Env, Gag, and Pol proteins of several clades was constructed using humanized and optimized codons and it was named here hDNA vaccine. In mice, this hDNA vaccine stimulated the following strong (1) antigen-specific serum antibody (Ab) responses, (2) delayed-type hypersensitivity, (3) the activation of IFN-gamma secretion cells targeting gp120 and synthetic antigenic peptides, in addition (4) a significant level of several peptide specific cytotoxic T lymphocytes (CTL) responses. Challenged with modified vaccinia viruses vPE16 and vP1206 expressing HIV-1 env and gag.pol genes, respectively, demonstrated the viral titers in the ovary of the mice vaccinated with hDNA significantly less compared to the unvaccinated mice. Thus, the use of polygene DNA vaccine appears to induce a high level of HIV-specific immune responses and is very effective against challenge with recombinant HIV-vaccinia viruses.

Fine specificity and cross-clade reactivity of HIV type 1 Gag-specific CD4+ T cells

Despite growing evidence that HIV-1-specific CD4(+) T helper (Th) cells may play a role in the control of viremia, discrete Th cell epitopes remain poorly defined. Furthermore, it is not known whether Th cell responses generated using vaccines based on clade B virus sequences will elicit immune responses that are effective in regions of the world where non-clade B viruses predominate. To address these issues we isolated CD4(+) T cell clones from individuals with vigorous HIV-1-specific Th cell responses and identified the minimum epitopes recognized. The minimum peptide length required for induction of CD4(+) T cell proliferation, IFN-gamma secretion, and cytolytic activity ranged from 9 to 16 amino acids in the five epitopes studied. Cross-clade recognition of the defined epitopes was examined for variant peptides from clades A, B, C, D, and AE. Over half the variant epitopes (17 of 32) exhibited impaired recognition, defined as less than 50% of the IFN-gamma secretion elicited by B clade consensus sequence. There was no evidence for antagonistic activity mediated by the variant peptides, and despite strong responses there was no escape of autologous virus from Th responses in the epitopes we studied. Abrogated recognition of variant CD4(+) T cell epitopes presents a potential obstacle to vaccine development.

Sequence features that correlate with MHC restriction

Identification of common sequence motifs in antigenic peptides restricted to a specific class II molecule has not been easy due to the large variation in length and sequence that is observed in these peptides. The goal of this study is to develop an automated computerized method for the identification of sequence features and structural determinants that play a role in the MHC restriction of helper T-cell antigenic peptides. For this, we compiled an extended database of helper T-cell sites, including the information on MHC restriction, when available. Two groups of peptides are assigned to each MHC type: (1) peptides that bind to that MHC molecule to elicit a T-cell response, and (2) peptides that were shown experimentally either not to bind to or not to elicit a T-cell proliferative response in association with that MHC molecule. We search for common motifs in the group of binding peptides, and identify significant motifs that are frequent among these peptides but almost absent in the group of non-binding peptides. A motif consists of physical-chemical and structural properties that may be responsible for binding specificity and can be extracted from sequence data, such as, hydrophobicity, charge, hydrogen bonding capability, etc. The first search is performed on the non-aligned binding peptides. Next, the sequences are aligned according to an identified motif and a search for additional, conserved, properties is performed. The statistical significance of the motifs is evaluated as well as their compatibility with published experimental results on substitution effects. Here we demonstrate the general scheme of the analysis and results for I-Ek and I-Ak associated peptides.

Antigen processing by endosomal proteases determines which sites of sperm-whale myoglobin are eventually recognized by T cells

This study reports an identification of the major processing products of an exogenous protein antigen, viz, sperm-whale myoglobin, as obtained after cell-free processing with partially purified macrophage endosomes. It is demonstrated that such a system yields fragments that are indistinguishable by high performance liquid chromatography analysis from those generated after uptake of myoglobin inside live macrophages. The concerted action of the endosomal proteases cathepsin D and cathepsin B can account for nearly all cleavages observed. Cathepsin D appears to be mainly responsible for the initial cleavage of myoglobin, while cathepsin B catalyzes the C-terminal trimming of initially released fragments. The fragments released by cathepsin D contain most, if not all, major epitopes for murine myoglobin-specific helper T cells. Interestingly, each known T cell epitope of myoglobin is located at the very N terminus of a different myoglobin fragment released upon processing. In order to explain this correspondence, noted also in several other protein antigens, a structural relationship is proposed between antigen processing by cathepsin D and antigen recognition by major histocompatibility complex (MHC) class II products. As is demonstrated here, this relationship may be used as a predictive tool for the identification of MHC-binding sequences as well as of T cell epitopes in their naturally occurring form.