Abstract
The specific cell surface receptors for lymphotoxin (LT) which are expressed on murine fibroblast L.P3 cells, a subline of L929 cells, were found to consist of a single class of specific high-affinity receptors with a dissociation constant (Kd) of 3.8×10-10 M and a density of 5.8×103 sites/cell. Similarly, murine fibroblast L929 cells, human melanoma A375 cells and human cervical carcinoma HeLa-S3 cells had about 7.2×103, 3.5×103, and 6.6×103 sites/cell with Kd, values of 1.4×10-10, 0.5×10-10, and 1.1 ×10-10 M, respectively. Among the LT receptor-positive cell lines, there was no direct correlation between the level of specific LT binding and the sensitivity to the cytotoxic or cytostatic effect of LT. Cross-linking of 125I-LT to the cell surface receptors with disuccinimidyl suberate, followed by two-dimensional gel electrophoresis of the cell lysate, revealed two kinds of LT-LT receptor complexes with molecular weights of 70 and 97 kDa, and having the same pI value of 6.8. Cell-bound 125I-LT was internalized within 1 h and degraded intracellularly, and finally secreted into the medium within a few hours. Appropriate concentrations of LT and interferon γ (IFNγ) showed synergistic cytotoxicity toward murine fibroblast L.P3 cells and human' monocytoma U937 cells, but these cytokines were only slightly cytotoxic individually. Preincubation of these cells with IFN γincreased the total number of LT receptors without any significant change in the dissociation constant or in the molecular weight. The binding of 125I-LT to L.P3 cells was inhibited by both unlabeled LT and unlabeled tumor necrosis factor.
