In the present study, we have examined the identity of the mouse lymphoid cells subject to nonspecific killing by normal guinea pig serum (GPS). In parallel cytotoxicity assays in which the number of cells was varied over a 12-fold range, with all other conditions kept the same, the proportions of cells killed were virtually constant, indicating that a specific cell subpopulation acted as target. Spleen cells surviving treatment with GPS were found to be incapable of responding to the T cell mitogens phytohemagglutinin-P and concanavalin A but responded well to stimulation with endotoxic lipopolysaccharide (LPS), a B cell mitogen, suggesting either a direct or indirect selective effect of GPS on T cells. Similar results were obtained with C4-deficient GPS. LPS-induced DNA synthesis was identical in spleen cells treated with either GPS or anti-Thy-1.2 antibody plus a noncytotoxic source of complement. Spleen cells surviving treatment with anti-Thy-1.2 antibody plus complement were found to be insensitive to GPS cytotoxicity. Conversely, prior incubation of the spleen cells with GPS resulted in the killing of all cells subject to lysis by anti-Thy-1.2 and complement. Spleen cells from nu/nu mice and bone marrow cells from normal mice showed minimal sensitivity to GPS cytotoxicity. A close similarity was observed between the percentages of cells killed by GPS in various lymphoid cell populations and the known proportions of T cells in these populations. In cytotoxicity assays in which normal and T cell-enriched (by passage through nylon wool columns) spleen cell populations were tested with both GPS and anti-Thy-1.2 plus complement, lysis by the two techniques was always comparable. The present results suggest strongly that mouse T cells are selectively killed by GPS and that this cytotoxicity can be used to monitor T cell proportions in mouse lymphoid cell populations with the same efficiency as with the use of anti-Thy-1.2 antibody plus complement.

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This work was supported by United States Public Health Service Grants AI-06112 and AI-06455 and contract 1-CB-43926.

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