Background. Virus-specific CTL responses are critical in the control of HIV-1 infection and their induction will play an important part in therapeutic and prophylactic HIV vaccines. The high levels of HIV-specific CTLs induced by the natural infection are clearly inadequate to eradicate the virus, with the majority of patients progressing to AIDS. Thus, the quality as well as magnitude of the response is important for protection. Since the native proteins do not produce an optimal antiviral immunity, we propose to identify cross-reactive peptide mimetopes that may be more effective. CTL responses to SL9 are uniquely prominent among chronically infected patients (pts) (75% pts). We showed from ex vivo priming of SL9-CTLs from naive CD8⁺ T-cells of seronegative healthy donors that this epitope is inappropriately immunogenic, producing unstable, highly activated CTLs sensitive to apoptosis. Although the native SL9 sequence is a poor immunogen, it remains a good vaccine target, since it is abundant in infected cells and recognized by 75% of A2⁺ people (40% of general population is A2⁺).

Methods. To identify peptide mimetopes of SL9 that are better immunogens, we have used in vitro primed SL9-CTL lines to scan a large combinatorial nonapeptide library consisting of 3 trillion distinct peptides.

Results. We have identified 6 peptides to be immunogenic and more importantly, to be “public” mimic epitopes, cross-recognized by CTLs from 2 to 5 of our panel of 5 donors. MEP-41, the best characterized to date, also provokes more consistent and stable cross-reactive CTL responses (“better” CTLs) than the native epitope in vitro (immunizations of human naïve CD8⁺ T-cells) and in vivo (vaccinations of HHD mice). It is also capable of expanding SL9-specific CTLs from 2 HIV-infected patients studied to date.

Conclusions. These peptides are prototypes of a new generation of alternative peptide ligands identified by the unbiased scanning of synthetic peptide libraries. They are unique with respect to the position and the number of the substitution(s) on the nonameric peptide sequence. MEP-41 has a substitution in position 8 while MEP-3, -4, -11, and –12 are substituted in 5 of the 9 positions, some of which are nonconservative substitutions. Our data suggest that peptide mimetopes can be selected for this prototype CTL epitope that generate a qualitatively superior anti-HIV CTL response and, thus, are better vaccine candidates.