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Electron Microscopic Study of the Guinea Pig Leukemia Virus¹

Cancer Research Unit, Veterans Administration Hospital, Bronx, New York 10468

Leukemia was induced in either strain 2 guinea pigs or F₁ hybrids with L2C leukemic cell suspensions. Examination of organs from 22 leukemic guinea pigs revealed the presence of leukemia virus particles in all 22 animals. Particles were found in the thymus, spleen, lymph nodes, bone marrow, s.c. tumors, and also in leukemic cells infiltrating the lungs, kidneys, liver, ovary, and uterus. Immature virus particles with electron-lucent centers and 2 concentric shells were observed budding from the membranes or free within the cisternae of the endoplasmic reticulum. Mature virus particles with electron-dense nucleoids and single, thick outer shells were found mainly in the intercellular spaces. On the basis of the electron micrographs studied, it is possible to speculate that particles can move from the interior to the periphery of the cell through a system of vesicles. A fusion of vesicular membranes with the cell membrane followed by an opening in the combined membranes would result in the release of particles from the cell without interrupting the continuity of the cell surface. This process can be described as reverse pinocytosis. No virus particles were found in organs of 16 normal, noninoculated guinea pigs examined.

¹ Aided in part by grants from the Damon Runyon Memorial Fund and the American Cancer Society.

Received 6/ 9/70. Accepted 7/21/70.