Two overlapping subdominant epitopes identified by DNA immunization induce protective CD8⁺ T-cell populations with differing cytolytic activities

Subdominant CD8⁺ T-cell responses contribute to control of several viral infections and to vaccine-induced immunity. Here, using the lymphocytic choriomeningitis virus model, we demonstrate that subdominant epitopes can be more reliably identified by DNA immunization than by other methods, permitting the identification, in the virus nucleoprotein, of two overlapping subdominant epitopes: one presented by L^d and the other presented by K^d. This subdominant sequence confers immunity as effective as that induced by the dominant epitope, against which >90% of the antiviral CD8⁺ T cells are normally directed. We compare the kinetics of the dominant and subdominant responses after vaccination with those following subsequent viral infection. The dominant CD8⁺ response expands more rapidly than the subdominant responses, but after virus infection is cleared, mice which had been immunized with the "dominant" vaccine have a pool of memory T cells focused almost entirely upon the dominant epitope. In contrast, after virus infection, mice which had been immunized with the "subdominant" vaccine retain both dominant and subdominant memory cells. During the acute phase of the immune response, the acquisition of cytokine responsiveness by subdominant CD8⁺ T cells precedes their development of lytic activity. Furthermore, in both dominant and subdominant populations, lytic activity declines more rapidly than cytokine responsiveness. Thus, the lysis^low-cytokine^competent phenotype associated with most memory CD8⁺ T cells appears to develop soon after antigen clearance. Finally, lytic activity differs among CD8⁺ T-cell populations with different epitope specificities, suggesting that vaccines can be designed to selectively induce CD8⁺ T cells with distinct functional attributes.