AbstractOld (15–20 month) male (NZB × NZW)F1(B/W) mice have severely impaired spleen cell reactivity to phytohemagglutinin (PHA), a mitogen which stimulates mainly T lymphocytes. Spleen cells from old mice markedly suppressed the PHA response of splenocytes from young (3–4 month) B/W males. Similar suppressor activity was not present in the spleens of old mice of four nonautoimmune strains.The suppressor activity of old B/W spleen cells was mediated by a nonphagocytic, radioresistant, mononuclear leukocyte. Although this cell was eluted in the “T lymphocyte” fraction of nylon wool columns, it was not sensitive to treatment with anti‐Thy‐1 antiserum and complement. Suppressor activity was lost after 18 h incubation at 37 °C in tissue culture medium. Supernatants of these overnight cultures had no suppressive effect on fresh young B/W spleen cells.Old B/W spleen cells suppressed PHA reactivity more than concanavalin A or lipopolysaccharide reactivity. Kinetic studies demonstrated an increasing suppression with time over 72 h of culture. This study demonstrates that the severely impaired PHA reactivity of old B/W mice is mediated, at least in part, by activ
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