These epitopes were identified click here mostly in chronically infected individuals, who had mounted T-cell responses against them. Moreover, preliminary immunogenicity results from the first trials of the conserved vaccines show encouraging
immunogenicity. Nevertheless, as with any approach, vaccines based on the conserved regions have their theoretical caveats. First, conserved immunogens are chimeric proteins assembled from protein sub-regions and, as such, have sequence junctions where the sub-regions meet. These junctions may create novel irrelevant epitopes (not present in HIV-1), which could, for certain HLAs, be immunodominant and suppress induction of protective responses. However, based on the likelihood of creating such immunodominant interfering junctional epitopes, these will almost certainly be the exception rather than the rule. Second, CD4+ T cells, the main natural target cells for HIV-1 replication, do not have co-stimulatory molecules check details on their surface and, therefore, are not potent primers of T-cell responses. Thus, in natural HIV-1 infection, many or most T-cell responses are primed either by direct infection of ‘professional’ antigen-presenting cells or through cross-priming, for instance via the uptake of HIV-1-infected apoptotic cell debris by ‘professional’ antigen-presenting cells. While
it is known that most immunodominant epitopes are expressed on HIV-1-infected cells, this has not been explored in great detail for subdominant epitopes such as those derived from the HIV-1 conserved regions. Thus, it is not guaranteed that HIV-1-infected cells express conserved epitopes on their surface in sufficient amounts for effective and timely killing by cytotoxic T cells, acetylcholine i.e. before the infected cells produce HIV-1 progeny, which is key for the success of conserved T-cell
vaccines (Fig. 2). Both of these caveats are being investigated in the on-going clinical trials of the conserved vaccines by e.g. in vitro virus suppression assays utilising vaccine-induced T-cell effectors 21. The strategy for controlling HIV-1 by the use of conserved T-cell epitopes has been proposed on several occasions 22–24. However, an actual T-cell vaccine employing conserved regions (rather than epitopes) of HIV-1, thus preserving the natural epitope adjacent sequences and also the possibility of inducing responses to as yet unidentified epitopes, was first reported by Letourneau et al., who employed the 14 most conserved regions of the proteome as 27- to 128-amino acid-long consensus sequences alternating the four major main global clades A, B, C, and D 25. At about the same time, such an approach was theoretically proposed by Rolland et al., who suggested the use of 45 conserved elements (CEs) at least 8 amino acids long that fulfilled stringent conservation criteria 26.