I. Evidence that lymphotoxin activity involves both cytotoxic and stimulating factors.

This paper describes experiments undertaken to determine more uniform conditions for the generation of cytotoxic lymphokine--'lymphotoxin' (LT) in differently stimulated rat lymphocyte cultures, and to compare the sensitivity of different test systems for detecting rat LT activity in vitro. Wistar rat lymphoid cells were activated by culture with phytohaemagglutinin (PHA) and mixed lymphocyte cultures (MLC) with semi-allogeneic (Wistar x August) F1 lymphoid cells. Lymphocyte supernatants harvested between 1 and 8 days were tested for their effect on metabolism and viability of cultured mouse fibroblasts (L-929 cells) by four methods: (i) inhibition of [14C]leucine incorporation; (ii) total and viable cell counts in test tube cultures ('macro test'); (iii) viable cell counts in microtitre plates ('microtest'); and (iv) chromium (51Cr) release from chromated L-cell monolayers. Cytotoxic effects on target cells of lymphocyte supernatants were evident after 3 days of PHA stimulation and 6 days of mixed lymphocyte culture, and the most sensitive indication of cytotoxic activity provided by inhibition of amino-acid incorporation and by loss of viable L cells from monolayers in tube cultures. In dilutions greater than 1:16-1:32 both cytotoxic supernatants exhibited a stimulating effect on target-cell proliferation. Stimulation of L cells growth was also observed when monolayers were exposed for 24 h to 'early' (24 h) PHA undiluted supernatants. At a later time of exposure to these supernatants a considerable loss of total and viable cells in the monolayers was evident. The results indicated that both cytotoxic and growth-stimulating lymphokines could be generated during activation of rat lymphocytes. A hypothesis is suggested whereby 'lymphotoxin' activity in vitro arises from the sequential effects of stimulating and cytotoxic lymphokine, and whereby the balance of these effects in vivo might determine the response of fibroblasts involved in reactions of chronic allergic inflammation.