ALKYLPHOSPHOCHOLINE-INDUCED PRODUCTION OF NITRIC-OXIDE AND TUMOR-NECROSIS-FACTOR-ALPHA BY U-937 CELLS

The human histiocytic cell line U 937, which expresses a number of mon ocyte markers and properties, was investigated with regard to its abil ity to be activated for NO and tumor necrosis factor (TNF) release aft er treatment with alkylphosphocholines (APC) and APC liposomes. Using APC multilamellar vesicles (MLV) a clear dose-dependent increase of NO production could be demonstrated for U 937 cells, whereas the corresp onding soluble substances had no effect. The time course of NO release was characterised by a peak between 2 h and 12 h and a strong decreas e after 24 h. LPS caused no NO release nor the production of TNF in U 937 cells. The simultaneous incubation of the cells with lipopolysacch aride and APC or APC-MLV, led to a strong increase in TNF production. Closer investigation of the time sequence of this synergistic effect d emonstrated that cells, that had first been treated with hexadecylphos phocholine (HPC)MLV and 4 h later with lipopolysaccharride secreted si gnificantly more TNF into the supernatants than in the experiment wher e both substances were added simultaneously. From these results it was concluded that APC-MLV are possibly able to act as a primer in the pr ocess of lipopolysaccharide mediated TNF induction. Furthermore, a pos itive influence of phorbol 12-myristate 13-acetate (PMA) on the abilit y of U 937 cells to produce TNF following a treatment with HPC or HPC- MLV could be observed. PMA-pretreated cells were shown to release much more TNF compared to control cells, which led to the supposition that the immunomodifying activity of APC becomes effective only in more hi ghly differentiated cell types.