An unusual host-parasite relationship: the growth hormone-like factor from plerocercoids of spirometrid tapeworms.

Accelerated body growth is associated with experimental infections with the plerocercoid stage of the pseudophyllidean tapeworm, Spirometra mansonoides. The growth response is due to a protein that is synthesized and released by plerocercoids in the host. Plerocercoid growth factor (PGF) is transported by the blood, interacts with growth hormone (GH) receptors and mimics many of the biological actions of GH. Purification and characterization of PGF reveals that the GH-like factor is a 27.5 kDa cysteine proteinase. Sequencing of a cDNA for PGF revealed 40-50% homology to mammalian cysteine proteinases, but no homology to any GH. The GH-like factor/proteinase is found in abundance in the tegument of plerocercoids where the proteinase activity would be most effective in aiding tissue invasion, and possibly evasion, of the host's immune response to the parasite. The advantage of the GH-like activity is less clear, but it may affect the species survival by suppressing specific elements of the host's immune system and by improving the probability that the infected host will be eaten by a definitive host by increasing morbidity and/or mortality, conditions which are associated with long-term exposure to elevated levels of GH and PGF.