IL-12 acts selectively on CD8 alpha(-) dendritic cells to enhance presentation of a tumor peptide in vivo

Previous work has shown that a significant proportion of murine splenic den dritic cells (DC) express a high affinity receptor for IL-12, thus accounti ng for the adjuvanticity of the cytokine when DBA/2 mice are transferred wi th syngeneic DC exposed in vitro to rIL-12 and an otherwise poorly immunoge nic 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 p henotypically distinct DC subtypes, enriched CD8- (>99% pure) and CD8(+) (s imilar to 80% pure) populations of DC from DBA/2 spleens mere assayed for A PC function in vivo following exposure to rIL-12 and tumor peptide in vitro . Unlike unfractionated DC, the CD8- fraction was capable of effective pres entation 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 durin g pulsing blocked in vivo priming by the CD8(-) fraction, However, pretreat ment 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 s howed significant message expression for the beta 1 and beta 2 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 ind uction of T cell reactivity by peptide-loaded CD8(-) DC in DBA/2 mice, Howe ver, the CD8(-) fraction can be primed by IL-12 to overcome the inhibitory effect of the CD8(+) subtype.