IL-12 Acts Selectively on CD8α− Dendritic Cells to Enhance Presentation of a Tumor Peptide In Vivo

Abstract Previous work has shown that a significant proportion of murine splenic dendritic cells (DC) express a high affinity receptor for IL-12, thus accounting for the adjuvanticity of the cytokine when DBA/2 mice are transferred with syngeneic DC exposed in vitro to rIL-12 and an otherwise poorly immunogenic tumor peptide. In DBA/2 mice, splenic DC consist of 90–95% CD8− and 5–10% CD8+ cells. To detect any difference in IL-12 responsiveness among phenotypically distinct DC subtypes, enriched CD8− (99% pure) and CD8+ (∼80% pure) populations of DC from DBA/2 spleens were assayed for APC function in vivo following exposure to rIL-12 and tumor peptide in vitro. Unlike unfractionated DC, the CD8− fraction was capable of effective presentation of the peptide even when the cells had not been pretreated with IL-12 before peptide pulsing. The addition of as few as 3% CD8+ cells during pulsing blocked in vivo priming by the CD8− fraction. However, pretreatment of CD8− DC with IL-12 before cell mixing and peptide pulsing ablated the inhibitory effect of the CD8+ fraction. CD8−, but not CD8+, DC showed significant message expression for the β1 and β2 subunits of the IL-12 receptor. These data suggest that a minority population of CD8+ DC, which appeared to secrete IL-10 in vitro, negatively regulates the induction of T cell reactivity by peptide-loaded CD8− DC in DBA/2 mice. However, the CD8− fraction can be primed by IL-12 to overcome the inhibitory effect of the CD8+ subtype.