Enumeration of Activated Thymus-Derived Lymphocytes by the Virus Plaque Assay

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Abstract
Lymphocytes activated by antigens or mitogens acquire the capacity to replicate viruses, and the number of activated lymphocytes can be estimated by the virus plaque assay. Concanavalin A and pokeweed mitogen produced 33-fold and 17-fold increases in virus plaqueforming cells (V-PFC), respectively, above background, while lipopolysaccharide produced only a 2- to 3-fold increase. T (thymus-derived lymphocyte)-depleted lymphocyte populations, derived from anti-θ-treated or nude (arthymic) mouse spleens, failed to produce V-PFC after culture with concanavalin A or pokeweed mitogen. The present studies thus demonstrate that the virus plaque assay measures activated T-lymphocytes. A dissociation between the V-PFC response and cell proliferation was previously observed in antigen-stimulated cells cultured in the presence of mitotic inhibitors. In the present studies, while stimulation of CBA (H2 k ) lymphocytes by DBA/2 (H2 d ) cells produced high levels of thymidine incorporation, lymphocyte target-cell cytotoxicity, and V-PFC, stimulation of BALB/c (H2 d ) lymphocytes against DBA/2 (H2 d ) cells resulted in even higher levels of thymidine incorporation with a virtual absence of cytotoxic lymphocytes or V-PFC. These results indicate that proliferation is not a sufficient condition for permitting lymphocytes either to exert cytotoxicity on target cells or to replicate viruses, and suggest that there may be a correlation between the development of V-PFC and cytotoxic lymphocytes. They are consistent with the view that there are at least two functional subpopulations of T-lymphocytes.