IFN-γ-secreting Th cells regulate both the frequency and avidity of epitope-specific CD8+ T lymphocytes induced by peptide immunization:: an ex vivo analysis

CD8+ T lymphocytes are involved in protective immune responses to infected or tumor cells. In this report, we examined the regulation of antigen-specific CD8+ T cell frequency and avidity by distinct T-h cell subsets. Peptide-specific CD8+ T cells were induced by immunization of mice with a MHC class I-restricted epitope, co-injected with a MHC class II-restricted epitope to recruit T-h cells. CD8+ T cell responses were assessed directly ex vivo for lytic activity and IFN-gamma secretion using the enzyme-linked immunospot (ELISPOT) assay. Go-immunization in incomplete Freund's adjuvant (IFA) with three different helper peptides induced IFN-gamma- and IL-2-secreting T-h cells, in the absence of IL-4 secretion, suggesting preferential Th1 profiles. Such immunization resulted in the increase of antigen-specific CD8+ T cell frequency, which was detected in blood as efficiently as in lymph nodes and spleen, and elicited high-avidity CD8+ T cells, We investigated whether these effects were dependent upon a particular T-h profile. When alum was used instead of IFA, the production of IL-2 by T-h cells was still significant, while the production of IFN-gamma was undetectable, Such T-h cell activation failed to support an increase of antigen-specific CD8+ T cell frequency. Altogether, these results document in vivo the regulatory role played by T-h cells in CD8+ T cell activation and may be relevant for the design of efficient vaccination schedules.