The fact that TNF and the mfVSG and Mitat1.5 sVSG regulate only few genes, whereas LPS regulates the same but almost 5000 genes in addition, argues for predominantly quantitative differences between the two types of DC maturation. However, since these quantitative changes led to qualitatively different Th1 or Th2-cell polarization, this may reflect another DC-based aspect of the “strength of signal” theory where peptide titrations and affinities heavily influenced the Th-cell skewing potential 59, 60. The peptide dose dependency has been shown to be independent of the DC subtype but strong LPS or CpG stimulation clearly shifted toward PI3K inhibitor Th1-cell
61. As a mechanism how this could be regulated, others proposed that weak T-cell stimulation prevents CD40L upregulation, which in turn was required to trigger CD40 on DCs for their IL-12 production and Th1-cell immunity 62. Thus, weak DC stimulation would then result in a Th2-cell response, whereas strong DC stimulation, i.e. by DC maturation with LPS or weak maturation but
presenting high doses of peptide, would result in a Th1-cell polarization. The three signal models as initially proposed by Kapsenberg 7 explain how DCs mediate Th-cell differentiation: peptide-MHC ligation (signal 1), costimulatory signaling (signal 2), and a selective cytokine set initiate the differential Th-cell commitment (signal 3). For Th1-cell polarization, IL-12p70 production by DCs is, besides the recently Oxymatrine described CD70-dependent
pathway 63, a clear signal toward Th1-cell polarization but signal 3 for Th2 cells remains less clear. Previous reports have shown that the Th2-cell promoting BVD-523 molecular weight mediator PGE2 induces the secretion of IL-12p40 in DCs thereby inhibiting the production of the Th1-cell driving cytokine IL-12p70 16–18. It has been proposed that blocking or washing out IL-12p70 production is sufficient to drive the differentiation of Th2-cell responses by the so-called default or exhaustion pathway 64, 65. The elimination of IL-12p70 from the context of antigen presentation by mature DCs would result in a similar phenotype of inflammatory semi-mature DCs as we have generated them here. The differences in the production of low levels of IL-6 or IL-12p40 by DCs matured with TNF, mfVSG, or MiTat1.5 sVSG do not seem to shift the qualitative Th2-cell profile but only result in minor quantitatively different amounts of Th2 cells. In addition, these differences did not have functional consequences after injection on asthma or EAE. Due to the fact that VSG-mediated semi-maturation of DCs is dependent on MyD88 signaling, we may have to consider these Th2-cell inducing antigens as weak TLR agonists. Others have shown that especially TLR2 triggering of DCs can lead to a Th2-cell priming with or without coinduction of Th17 cells 66, 67 although there are also other results for Schistosoma antigens that induce Th2-cell responses without the involvement of TLR2, TLR4, or MyD88 68.