Despite twenty-five years of global effort, an effective vaccine against the human immunodeficiency virus type 1 (HIV-1) remains elusive. Induction of broadly neutralizing antibodies against HIV-1 is very difficult, yet it is the key to all other protective anti-viral vaccines
. Therefore consideration of HIV-1 vaccine candidates that stimulate cellular immunity has been the focus of many recent vaccines
. Although recent advances in vector design have generated optimism in this field
, these technologies still need to address the extreme variability of HIV-1, whereby co-circulating viruses may differ in over 20% of their proteome
. Thus, while novel vectors and heterologous prime-boost combinations are getting better at inducing higher frequencies of HIV-1-specific T cells, less attention has been paid to how these vaccines can elicit T cells capable of recognizing multiple HIV-1 variants.
There are several approaches for dealing with the HIV-1 diversity. One optimistic view is that a single clade may induce sufficiently cross-reactive T-cell responses to protect against other variants of both the same and heterologous clades. The choice of a natural isolate can be based on having the closest sequence to all others, or picking a strain derived from acute infection and arguing that there is a convergence of viral sequences during transmission
. However, even if a single variant elicits responses that confer some cross-reactive protection, such protection is likely to be only partial and thus it is well worth attempting to design vaccine immunogens with enhanced cross-reactive potential. Although there are numerous reports of cross-clade reactive HIV-1-specific CD8+
T cell responses
, use of unphysiologically high concentrations of variant peptides make the biological relevance of many of these results uncertain. In contrast, there are ample examples of highly specific T cell receptors sensitive to single amino acid (aa) changes
, as well as compelling evidence of HIV-1 variants escaping existing T cell responses in infected individuals by single mutations in epitopes
. In vitro, systematic studies employing all possible single aa substitutions in each position of an MHC class I epitope indicated that as few as one third of such epitope variants were recognized by a given T cell receptor
. These results are in agreement with theoretical predictions proposed for cross-recognition of MHC class I-presented peptides by T cell receptors
. Thus the use of a single natural isolate for a vaccine has a high risk of not protecting against a different clade, nor against many variants of the same clade.
A second approach to HIV-1 diversity derives vaccine immunogens from ‘centralized’ sequences, which employ consensus/average, or centre-of-the-tree
sequences or extrapolated aa to a common clade or group ancestor
. Centralized sequences are designed to minimize the sequence differences between a vaccine immunogen and circulating viruses
. So far they have proven immunogenic and able to elicit T cell responses in small animal studies
and clinical trials
, providing experimental support for their further development. Early results for centralized immunogens for the entire group M are promising in that initial immunogenicity studies in mice yielded T-cell responses that were comparable to within-clade responses for many clades
, however, this strategy may be stretched too far for optimal coverage of CD8+
T cell epitope variants of the whole group M
In a third approach, vaccines deliver a cocktail of immunogens derived from different clades
. While initial results have been encouraging and responses to each antigen in the cocktail were observed
, attention still needs be paid to possible immune interference, such as epitope antagonism, between different, but closely related peptide sequences in the vaccine, which may be limiting responses to some epitopes. Antagonism of T cell responses by altered epitope peptide ligands has been demonstrated both in vitro
and in vivo
. It can occur when a host capable of mounting a response to an agonist epitope is simultaneously exposed to an antagonist epitope variant, which interferes with the induction of the T cell response to the agonist epitope and leads to a defective response. Thus, the breadth of responses induced by cocktail approaches should be carefully monitored when such vaccines are used
A fourth approach uses computational methods for assembling a polyvalent vaccine candidate that optimize the coverage of T cell epitopes. ‘Mosaic’ immunogens
are based on intact proteins and retain the probability for natural processing and presentation of T cell epitopes. Their potential problems are similar to those of other cocktails of natural proteins, i.e. immune interference and inclusion of both variable and conserved regions, whereby responses to variable regions may draw attention away from potentially more useful conserved targets. The impact of these processes will be only resolved in vaccine studies. An alternative means of designing immunogens contending with the HIV-1 variation is the COT+ method
, which combines a central sequence with a set protein fragments designed to help cover diversity.
Here, we describe a further alternative that may have considerable advantages. We describe the construction and experimental testing in mice and humans of a novel multi-clade immunogen derived only from highly conserved regions of the HIV-1 consensus proteome, which was designed to provide extensive coverage of the principle HIV-1 clades A, B, C and D, while minimizing the possible occurrence of the epitope interference. It has the potential considerable advantage of focusing the T cell responses on the most conserved parts of the virus and thus overcoming the usual patterns of immunodominance, while making it difficult for the virus to escape without a likely significant cost to its fitness. This approach has also merit in the simplicity of design and delivery, requiring only a single immunogen to provide extensive coverage of global HIV-1 population diversity.