Invertebrate immune responses to eukaryotic parasites:: reaction of coelomocytes of Lumbricus terrestris to native and non-native parasites

The oligochaete annelid Lumbricus terrestris is almost universally infected with a gregarine parasite belonging to the genus Monocytsis. The infection occurs in very large numbers in the earthworm's seminal vesicles; however, there is no consensus as to the degree of pathogenicity of this parasite. Moreover, the nature of the worm's immune response and the host-parasite relationship have never been thoroughly investigated. These preliminary experiments were designed to determine the nature and degree of immune responses to Monocystis and the steps necessary for further characterization of host-parasite relationships. We first determined if there is evidence for an immune reaction occurring within the seminal vesicles. Although coelomocytes were found in both preparations, there was no observed direct contact with parasites. We also attempted to determine if number and type of coelomocytes varied between heavily infected and less heavily infected worms. Though coelomocyte populations varied greatly, all of the worms were so heavily parasitized that it was difficult to establish any correlation. In a third study, we exposed earthworm immune cells to parasites by performing in vitro experiments culturing coelomocytes from L. terrestris together with its naturally occurring monocystid parasites, and then comparing this to reactions to a related parasite, Cryptosporidium, that does not normally infect earthworms. These in vitro studies showed rapid and significant reactions both to Monocystis and Cryptosporidium, but the nature of the reaction of cells, size of "granulomas" that formed, and rapidity of reaction and degranulation of cells differed between the two parasites. These differences are the most significant finding thus far, and are currently the focus of further investigations.