Studies of membrane-associated and soluble (secreted) lymphotoxin in human lymphokine-activated T-killer cells in vitro.

The expression, release, and cytolytic activity of membrane-associated lymphotoxin was examined in cultures of phytohemagglutinin-P (PHA-P) and interleukin 2 (IL-2)-stimulated human T-lymphokine-activated killer (T-LAK) cells in vitro. Lymphotoxin (LT/TNF-beta) was identified on the membrane of T-LAK cells using flow cytometry. The membrane form of LT (mLT) is detected not only on T-LAK cells but also on LT-secreting human B-cell lymphoid cell lines, RPMI 1788 and Raji, but not on U937 or K562 cells. Maximum expression of mLT on T-LAK cells and the secretion of LT into the supernatant depended on the concentration of IL-2. Expression of mLT on T-LAK cells was reduced by stimulation with PHA-P; however, supernatant LT levels greatly increased. Both expression of mLT and release of soluble LT was reduced after incubation of T-LAK cells with actinomycin D (ActD) or cycloheximide (CHx). Paraformaldehyde-fixed T-LAK cells caused cytolysis of WEHI 164 cells in vitro, which was blocked by anti-LT but not anti-TNF antibody. These data support the concept that mLT may be an intermediate form to secreted LT, and that the mLT form is cytolytically active.