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Viropexis of type B particles in reticulum cell sarcoma of R3-Dm strain mice.

Summary Results of ultrastructural studies of generalized reticulum cell sarcomas in virgin RIII/Dm strain female mice with transplants of isologous embryo tissues are presented. The spleen tumors contained numerous type B and type C virus particles and, in smaller numbers, intracytoplasmic type A virus particles. Budding of virus particles was frequently seen. In the spleen tumors, type B virus particles were seen adsorbed on the plasma membrane of the reticulum tumor cells and penetrating into the cytoplasm of the cells by a mechanism described as viropexis. Nucleoids of the virions were apparently released in the cytoplasm of the tumor cells. Both type B and type C particles were phagocytized by macrophages in the spleen tumors. Adsorption of type B particles, budding of type C particles, and intracytoplasmic type A particles were observed in the same tumor cells. Type C particles and budding of type C particles, but not type B particles, were also seen in the lymph nodes and the intrathoracic tumors of these mice. Type B particles and their behavior in reticulum cell sarcoma of mice have not been previously recorded.