IL-12 administered in vivo to young and aged mice. Discrepancy between the effects on tumor growth in vivo and cytotoxic T lymphocyte generation ex vivo: dependence on IFN-{gamma}

Because IL-12 restores allogeneic cytotoxic T lymphocyte (CTL)activity by T cells of aged mice in vitro, we initially assessedwhether IL-12 could overcome age-related deficits when givento aged mice in vivo. Growth of P815 (H-2^d) was enhanced inaged compared with young BALB/c (H-2^d) mice and tumor growthwas curtailed by IL-12 in both age groups. Unexpectedly, secondaryCTL stimulated ex vivo with P815 were reduced in IL-12-treatedmice compared with controls. Primary CTL generated ex vivo acrossMHC differences in IL-12-treated BALB/c and C57BL/6 young micewere reduced by 90–99%, were dose- and time-dependent,and were associated with reduced allo-stimulated NK-like activityand ³H]thymidine incorporation. IFN- was elevated in sera andin supernatants from allo-stimulated cultures from IL-12-treatedmice, while IL-4 was reduced in such supernatants, suggestingthat, despite reduced CTL, IL-12 was associated with increasedT_h1- and reduced T_h2-type cytokine production. IL-12 also inducedsplenomegaly, primarily due to increased numbers of cells lackingmarkers of mature T, B and NK cells, or macrophages, or polymorphonuclearleukocyte morphology. IFN- mutant mice exhibited reduced splenicenlargement in response to IL-12, suggesting that the splenomegalywas due, in part, to IFN- production. However, reduced CTL generationwas not due entirely to dilution of CTL precursor cells becausespleen cellularity and size increased 3-fold while CTL activitydecreased 10- to 100-fold, and CTL generation normalized toCD8⁺ T effector cells was still significantly reduced in IL-12-treatedmice. Interestingly, purified CD4⁺ and CD8⁺ T cells from IL-12-treatednormal mice exhibited greater proliferative and cytolytic activitiesrespectively compared with controls. Thus, effector T cellsin IL-12-treated mice were not impaired, but exhibited augmentedresponsiveness, suggesting that IL-12 induced complex interactionsamong spleen cell populations and that these effects, in part,are mediated by IFN-.