Prostaglandin E2 (PGE2) favors T helper type 2 (Th2)-like cytokine secretion profiles in murine and human CD4+ T cells by inhibiting the production of the Th1-associated cytokines interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) and up-regulating the production of the Th2-associated cytokines IL-4 and IL-5 in a dose-dependent way. However, the potent inhibition of IL-2 production by PGE2 seems to be in contrast with the simultaneous up-regulation of IL-4 and IL-5 production, because the induction of these cytokines requires IL-2. We, therefore, investigated to which extent the net modulatory effect of PGE2 is determined by the availability of IL-2. To this aim, we examined the effects of PGE2 on the cytokine secretion profiles of a panel of human Th0 clones upon stimulation via different activation pathways, resulting either in high or low IL-2 production. The differential modulation of Th1 and Th2 cytokines by PGE2 was observed only upon modes of stimulation resulting in high IL-2 production. When IL-2 production was low, PGE2 inhibited the secretion of all four cytokines. These different modulation patterns were directly related to the IL-2 availability, because (i) neutralizing antibody to IL-2 abrogated the up-regulatory effect of PGE2 on IL-4 and IL-5 secretion in experiments with high endogenous IL-2 levels, (ii) lack of differential cytokine modulation by PGE2 in conditions with low levels of endogenous IL-2 could be restored with exogenous IL-2, and (iii) cell viability was comparable in all conditions. These results demonstrate that the net modulatory effect of PGE2 on the cytokine secretion profile of T cells critically depends on the availability of IL-2. Since this parameter varies with the experimental conditions and the T cell population studied, this finding may explain why certain immune responses may be either up- or down-regulated by PGE2 under different conditions.
Dendritic cells (DC) are important initiators of specific primary immune responses because they are the only APC that can efficiently activate naive Th cells. DC have the capacity to produce interleukin-12 (IL-12), a cytokine that plays a pivotal role in the development of Th1-mediated cellular immune responses. The present study focuses on the conditions under which human DC produce bioactive IL-12 p70 and, consequently, direct the development of naive T helper (Th) cells toward the Th1 phenotype. Bacteria or bacterial compounds such as Staphylococcus aureus Cowan strain I (SAC) or lipopolysaccharide (LPS) induced substantial IL-12 levels in DC, which could be further upregulated by interferon-γ (IFN-γ), whereas induction of IL-12 production via CD40 ligation required IFN-γ as an obligatory, complementary signal. Also, activated naive Th cells were poor inducers of IL-12 production, unless exogenous IFN-γ was present, whereas activated memory Th cells were effective inducers of IL-12 production and did not require exogenous IFN-γ. Next, the cytokine profiles of matured Th cells that were primed by DC under different conditions were examined. DC promoted the development of naive Th cells into memory Th0 cells that produced both the type 1 cytokine IFN-γ and the type 2 cytokine IL-4. In contrast, after activation with SAC, DC efficiently directed the development of Th1 cells through the release of IL-12. An APC-independent Th cell maturation model, using either recombinant IL-12 or supernatants of SAC-activated DC and neutralizing anti-IL-12 antibodies, confirmed that DC-derived IL-12 was the major Th1 skewing factor. Together, these data indicate that the contact between DC and naive Th cells during the initiation of specific immune responses does not result in the efficient induction of IL-12 production and that, consequently, exogenous IL-12–inducing factors are required to promote primary Th1-mediated cellular immune responses.
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