Prime–boost immunization generates a high frequency, high-avidity CD8+ cytotoxic T lymphocyte population

Development and expansion of high-avidity T cell populations may be important for the success of immunization strategies against HIV and other pathogens that have presented major problems for vaccine development. We have used tetrameric–MHC complexes ex vivo and lytic assays to show that `prime-boost' immunization with DNA vaccines and recombinant poxvirus vectors generates high frequencies of cytotoxic T lymphocytes (CTL) that recognize target cells expressing very low levels of specific antigen. These cells persist for at least 6 months at levels representing ~10% of the CD8+ T cell population. Using a novel in vivo assay, we also found that prime-boost immunized animals were capable of eliminating target cells expressing 10- to 100-fold less immunogenic peptide than mice given either vector alone. In addition, viral challenge led to rapid expansion of CTL effectors in prime-boost groups, to levels representing >30% of total CD8+ T cell numbers. Strategies that generate specific T cells of high avidity, optimizing early detection of infected cells, offer new hope for effective prophylaxis and immunotherapy.