Die Rolle der lektin-artigen TNF-Domäne bei der Resolution von Lungenödemen und in der experimentellen Trypanosomiasis

Apart from the interaction with its two receptors, TNF also exhibits a lectin binding capacity for specific oligosaccharides. Although the lectin-like region has been reported to be functionally and spacially separate from the interaction sites with the receptors a role of this TNF domain in the signal transduction of soluble TNF via TNFR2 was suggested. Therefore, using mouse fibroblasts stably expressing receptor chimeras derived from the extracellular domain of the two human TNF receptors and the intracellular death domain of human Fas (MF-R1-Fas/MF-R2-Fas), the involvement of the lectin-like domain in TNFR1/TNFR2 activation of soluble and transmembrane TNF was further investigated. Hence, the lectin-like domain of TNF was shown to be involved in the activation of TNFR2 by both soluble TNF and transmembrane TNF. However, a role of this TNF region in TNFR1 mediated signal transduction could be ruled out, since the induction of TNFR1 by the triple-mutant of TNF, lacking the lectin-like activity of the cytokine, was similar to that of wt TNF.The capacity of TNF to promote lung edema reabsorption and the ability to directly kill the bloodstream forms of African trypanosomes in vitro has been linked to the lectin-like domain of the cytokine. Thus, a peptide mimicking this TNF region, the so-called tip-peptide, exerts both activities as well. To further investigate the function of the lectin-like domain in vivo under physiological and pathological conditions, triple-mutated TNF knock-in-mice (B6-TNFTM1.1Blt), expressing TNF that lacks the trypanolytic activity of the cytokine, have been generated. These mice are viable and fertile. Moreover, the processing of the cytokine is not drastically changed.An in vivo flooded mouse lung model was established to observe the role of the lectin-like domain in lung liquid clearance. Hence, there was no indication for a role of this TNF domain in the regulation of intrapulmonary liquid reabsorption. Moreover, although for human tip-peptide an induction of lung liquid clearance has been reported in rats, no significant effect on edema reabsorption could be detected in the mouse model. This observation was independent of the mouse strain and of the dose and the species specific sequence of the peptide (human, murine).However, the lectin-like domain participates in the host-parasite interrelationship during experimental Trypanosoma brucei infections in vivo. Infection of triple-mutant k.i. mice resulted in increased parasitemia levels and infection associated pathology, when compared to wt mice. In TNFR2-deficient mice, lacking a soluble TNFR2 mediated neutralization of TNF, these effects were even more pronounced. Accordingly, it is concluded that the trypanolytic activity of TNF that was found in vitro does not seem to play an ascertainable role in this context. Rather the involvement of TNF in the pathway of immunosuppression caused by T. brucei or indirect host responses could be a reason for the relatively high parasitemia recorded in TNFR2 deficient mice. Keeping in mind that the triple-mutant of TNF displayed a reduced TNFR2-mediated bioactivity as compared to wt TNF, the increased cachexia observed in triple-mutated k.i. mice could be attributed to reduced interactions between TNF and the neutralizing soluble TNFR2