Labeling of Sporozoites of Plasmodium berghei with Tritiated Purines

Abstract

Sporozoites were labeled in vivo. Mosquitoes (Anopheles stephensi) infected with the NK-65 strain of P. berghei were allowed to feed ad lib. for 14 days subsequent to an infective blood meal on 5% Karo syrup solution containing different levels of tritium-labled purines and pyrimidines. Sporozoites were harvested on day 16 by dissection of mosquito salivary glands. Isotope incorporation in sporozoites was determined by autoradiography. Each of the purines tested, including adenine-8-3H, adenosine-3H (G), deoxyadenosine-3H (G), and deoxyguanosine-3H (G), was incorporated into both nuclear and cytoplasmic regions of sporozoites when fed to mosquitoes at 5 to 100 ,uCi/ml. Similar levels of the pyrimidines tested, including thymine-methyl-3H, thymidine-methyl-3H, or orotic-5-3H acid, failed to label sporozoites. The incorporation of preformed purines, but not pyrimidines, by developing sporozoites suggests that the parasite may require exogenous sources of purine during its development in the mosquito but relies on de novo synthesis of pyrimidines. The adenine8-3H-labeled sporozoites were shown to be infective by intravenous inoculation into A/J mice. These results demonstrate the feasibility of labeling developing sporozoites and should provide a means for further studies on the developmental cycle of malaria in the vertebrate host. After the discovery of exoerythrocytic bodies of malaria in the liver of monkeys and man (Shortt and Garnham, 1948) it was generally assumed that relapses were caused by a continual cycling of histiotrophic merozoites in the liver. More recently, Garnham (1967) has proposed that relapses may be caused by sporozoites which remain latent in parenchymal cells of the liver for various periods of time before undergoing development. To date, these latent forms have not been demonstrated, possibly because of their small size and similarity to the cytoplasm of liver cells in stained preparations. Radionuclide labeling of sporozoites would provide a means for recognizing these latent forms in tissue if, indeed, they exist. In the present study a method for labeling sporozoites in vivo with tritiated purines was developed. In addition, the labeled sporozoites were shown to retain their infectivity when inoculated into susceptible hosts. MATERIALS AND METHODS Anopheles stephensi mosquitoes were allowed to feed on hamsters infected with the NK-65 strain of Plasmodium berghei and maintained at Received for publication 18 September 1973. * This work was presented in part at the 57th Annual Meeting of the Federation of American Societies for Experimental Biology, Atlantic City, New Jersey (1973, Fed. Proc. 32: 704). 21 C as described by Yoeli et al. (1965). Subsequent to the infective blood meal, mosquitoes were allowed to feed from the mouth of a plastic dropping pipette that contained tritium-labeled purines or pyrimidines in 5% Karo syrup solution. The mouth of the pipette was mounted in contact with the mesh on top of the cage. Fresh isotope solution was supplied every other day for a period of 16 days. Sporozoites were harvested on day 18 by dissection of mosquito salivary glands into a 1:1 mixture of saline and mouse plasma. The glands were gently homogenized in a 3-ml tissue grinder fitted with a Teflon pestle in an ice bath. Sporozoite yield was estimated from counts in a Neubauer hemocytometer. Viability of adenine-8-3H-labeled sporozoites was evaluated by the intravenous inoculation of approximately 10,000 sporozoites into 6to 8-weekold A/J mice (Jackson Laboratories, Bar Harbor, Maine) within 1 hr after dissection of salivary glands. Infectivity and prepatent period were determined by daily Giemsa-stained blood films. Isotope incorporation by sporozoites was determined by autoradiography using Kodak NTB-2 nuclear track emulsion (Eastman Kodak Co.). Infected salivary glands were crushed with a glass cover slip on slides previously coated by dipping in 1% bovine serum albumin. Sporozoites were then fixed with methanol and allowed to air-dry. Slides were coated by dipping into undiluted photographic emulsion at 40 C. After exposure to the emulsion for various intervals at 4 C, slides were processed with Kodak D-19 developer and fixer. The preparation was then stained through the emulsion with 7.5% Giemsa for 1 hr at pH 7.2. The degree of labeling was determined microscopically under oil immersion by counting silver grains associated with individual sporozoites.