CTL GENERATION IN THE PRESENCE OF IL-4 IS INHIBITED BY FREE P40 - EVIDENCE FOR EARLY AND LATE IL-12 FUNCTION

The cytokine requirements for the generation of a CTL response were ex amined using thymocyte responders stimulated with Con A, In agreement with previous reports, IL-4 alone can generate a strong CTL response, To determine whether IL-12 was required for this response, we added ei ther a neutralizing Ab against IL-12 or an antagonist of IL-12 functio n, p40, Both anti-IL-12 and p40 were found to inhibit the generation o f CTL in the presence of IL-4, Consistent with these data, exogenous I L-12 was found to synergize with IL-4, particularly, when IL-4 was add ed at levels unable to generate a CTL response alone. This synergistic response was observed whether IL-12 was added at the beginning of cul ture along with IL-4 or after the first day to cultures initiated with IL-4, Likewise, the combination of IL-2 plus IL-12 evoked a strong sy nergistic response when cultures were initiated with IL-2 and IL-12 wa s added initially or after the first day of culture. Addition of eithe r IL-2 or IL-12 alone did not generate a response. In contrast to thes e data showing that IL-12 was able to signal late in culture, p40 inhi bited CTL generation in response to IL-4 only when added at the beginn ing of culture, These results imply that CTL induction involves a cruc ial early IL-12 signal, which once delivered permits cells to respond to a later IL-12 signal, These data are consistent with the idea that free circulating p40 could inhibit the generation of CTL by antagonizi ng the Early IL-12 ''differentiatian'' signal.