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Dendritic cell maturation enhances CD8+ T‐cell responses to exogenous antigen via a proteasome‐independent mechanism of major histocompatibility complex class I loading

Immune stimulating complexes (ISCOMS) containing the saponin adjuvant Quil A are vaccine adjuvants that induce a wide range of immune responses in vivo, including strong class I major histocompatibility complex (MHC) ‐restricted cytotoxic T‐lymphocyte activity. However, the antigen‐presenting cell responsible for the induction of these responses has not been characterized. Here we have investigated the role of dendritic cells (DC) in the priming of antigen‐specific CD8+ T cells in vitro by ISCOMS containing ovalbumin. Resting bone marrow DC pulsed with ovalbumin ISCOMS efficiently prime resting CD8+ T cells through a mechanism that is transporter associated with antigen processing (TAP) dependent, but independent of CD40 ligation and CD4+ T‐cell help. Lipopolysaccharide‐induced maturation of DC markedly enhances their ability to prime CD8+ T cells through a mechanism which is also independent of CD4+ T‐cell help, but is dependent on CD40 ligation. Furthermore, DC maturation revealed a TAP‐independent mechanism of CD8+ T‐cell priming. Our results also show that class I MHC‐restricted presentation of ovalbumin in ISCOMS by DC is sensitive to chloroquine and brefeldin A but insensitive to lactacystin. We suggest that DC may be the principal antigen‐presenting cells responsible for the priming of CD8+ T cells by ISCOMS in vivo and that targeting these vectors to activated DC may enhance their presentation via a novel pathway of class I antigen processing.