Influence of strong CD4 epitope on long-term virus-specific cytotoxic T cell responses induced in vivo with peptides.

Free unmodified peptides are poor immunogens for cytotoxic T lymphocytes (CTL) in mice, unless they are injected with adjuvant. Although it is generally accepted that CD4+ Th cells are essential for CTL priming with peptides, it is not clear how long sequences devoid of any CD4 epitope, or strict CD8 epitopic sequences too short to be presented in a MHC class II-restricted fashion, can generate such responses. We thus have examined the extent to which the immunization protocol affects the need for a CD4 epitope. Since peptides are potentially important for vaccination, we also examined the duration of the CTL responses using a set of peptides that contained a CD4 epitope, or a CD8 epitope, or both epitopes in the same sequence, and compared immunization protocols previously found to induce CTL responses with peptides. The in vivo injection protocol had a marked effect, since the same CD8 sequence could generate a CTL response in a Th-dependent or Th-independent fashion, depending on the protocol used. The T cell help provided by natural CD4-CD8 sequences was inefficient in Th-dependent CTL priming and CTL generation required help from a stronger exogenous CD4 peptide. The peptides could be injected either as a single tandem CD8-CD4 peptide or as a mixture of two separate peptides. Th-independent CTL responses primed in other conditions proved to be as strong as Th-dependent responses, at least when the animals were killed shortly after the last injection. However, only CTL responses generated together with specific Th cells persisted for several months. Moreover, the efficacy of CTL persistence seemed to be correlated with the strength of the CD4 epitope for priming. This has important implications for the design of peptide vaccines.