Cellular immune competence of a Drosophila mutant with neoplastic hematopoietic organs

In the Tum^l mutant of Drosophila melanogaster, the larval hematopoietic organs undergo neoplastic changes and release into circulation large numbers of blood cells. The lamellocytes, and to a lesser extent the plasmatocytes from which they are derived, are the cells that encapsulate various endogenous tissues and form melanotic tumors. The mutation is temperature sensitive, with maximum gene expression manifested at 29°C. The ability of Tum^l larvae to encapsulate eggs of the wasp parasite Leptopilina heterotoma is dependent not only on temperature, with host larvae much more immune reactive at 29°C than at lower temperatures (15° or 21°C), but also on the interval of time following infection when temperature shift experiments are performed. When the shift of parasitized larvae from 21° to 29°C is delayed by 18 hr the hosts are not as immune reactive as those shifted immediately after infection. Since Tum^l larvae are potentially highly immune reactive at the time of infection (with sufficient numbers of lamellocytes in circulation to encapsulate parasites), the low degree of immune competence in hosts shifted to 29°C after 18 hr or maintained at lower temperatures suggests that the increased capacity of blood cells to react against foreign surfaces is dependent on the cells acquiring new or altered recognition and adherence properties at 29°C. The 18-hr delay may provide the parasite with an opportunity to interfere with the acquisition of these specific cellular alterations. Differential hemocyte counts from parasitized larvae show abnormally low lamellocyte counts in susceptible hosts, indicating that successfully developing parasites interfere with the differentiation of hemocytes.