ALLOANTIGEN-REACTIVE TH1 DEVELOPMENT IN IL-12-DEFICIENT MICE

IL-12p70, a 70- to 75-kDa heterodimer consisting of disulfide-bonded 3 5-kDa (p35) and 40-kDa (p40) subunits, enhances Th1 development primar ily by its ability to induce IFN-gamma production by NK and Th1 cells, Although homodimers of the p40 subunit of IL-12 are potent IL-12 rece ptor antagonists in some systems, we have reported that p40 homodimer may accentuate alloreactive CD8(+) Th1 function, To test the role of e ndogenously produced p40 in alloimmunity Th1 development was assessed in either IL-12 p35 knockout (p35(-/-)) mice, the cells of which are c apable of secreting p40 or p40 knockout (p40(-/-)) mice, Compared with IL-12 wild-type controls, splenocytes obtained from both p35(-/-) and p40(-/-) mice produced markedly less IFN-gamma after in vitro stimula tion with Con A or alloantigens. Interestingly, in vivo-sensitized Th1 were detected in both p35(-/-) and p40(-/-) cardiac allograft recipie nts, However in vivo Th1 development was enhanced in p35(-/-) recipien ts compared with p40(-/-) animals, suggesting that endogenous p40 prod uced in p35(-/-) mice mag stimulate alloreactive Th1, Indeed, neutrali zing endogenous p40 with anti-IL-12 p40 mAb reduced Th1 development in p35(-/-) allograft recipients to that seen in p40(-/-) mice, To deter mine whether Th1 development that occurred in the absence of IL-12p70 and p40 required IFN-gamma, p40(-/-) allograft recipients were treated with anti-IFN-gamma mAb. Neutralizing IFN-gamma did not inhibit in vi vo Th1 development in p40(-/-) recipients and resulted in a unique pat hology of rejection characterized by vascular thromboses, Collectively , these data suggest that 1) endogenous p40 may substitute for IL-12p7 0 in alloantigen-specific Th1 sensitization in vivo and 2) in vivo all oreactive Th1 development may occur independent of IL-12 and IFN-gamma , suggesting an alternate Th1-sensitizing pathway.