Regulatory role of a tripeptide (TKP) from the second constant domain of immunoglobulin G--I. Inhibition of rat and human macrophage activities.

We have previously shown that peptides released after the cleavage of IgG by parasite proteinases were strong inhibitors of the macrophage effector functions against schistosome larvae. The results presented here demonstrate that a single tripeptide set, Thr-Lys-Pro (TKP), inhibits various macrophage functions and can be considered as an immunologically active peptide. Indeed, not only IgE-dependent cytotoxicity but also beta-glucuronidase release, chemiluminescence and ILI production were reduced when rat macrophages were previously incubated with TKP or some analogues. Moreover, chemotaxis and IgE-specific receptor expression were inhibited in both rat and human macrophages after treatment with TKP, without affecting the cell viability. The substitution or acetylation of Thr diminished or suppressed the inhibitory effect of TKP.