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Cytolysis of actinomycin D-treated target cells by cell-free supernatants from human monocytes
Abstract:
The lytic mechanism of human peripheral blood monocytes was studied by using as targets actinomycin D-treated WEHI-164, an NK-insensitive murine fibrosarcoma cell line. Monocytes, but not lymphocytes, lysed WEHI-164 target cells pre-treated with actinomycin D within 6 h in 51Cr-release assays. Because cytolysis could not be inhibited competitively by unlabeled WEHI/D target cells, contact-independent mechanisms of cytolysis were investigated. Cell-free supernatants collected from monocytes cultured for 4-6 h at 37 degrees C lysed target cells as effectively as effector cell preparations of monocytes. Supernatants from lymphocytes cultured in parallel were not cytolytic. Cytolytic activity was not detected in supernatants from preparations of monocytes that were held on ice. However, monocytes produced cytolytic activity whether they were isolated by adherence or remained unseparated in suspensions of mononuclear cells. The cytolytic activity in cell-free supernatants (CFS) from monocytes was unaffected by incubation with protease inhibitors. CFS activity was destroyed by heat. Storage of CFS at 37 degrees, 22 degrees, 4 degrees, or -20 degrees C for 24 h decreased cytolytic activity; however, loss of cytotoxicity was minimized by storage at 4 degrees C. The cytolytic substance detected in 4-h CFS from monocytes appeared to be a protein(s) based on the sensitivity of the cytolytic activity to proteases. Cytolytic activity of CFS eluted from Sephacryl 200 in a single peak with an apparent molecular weight between 25,000 and 45,000 daltons.
Insights
Human monocytes release a cell-free cytotoxic factor that lyses tumor cells independently of cell contact. This soluble mediator, a protein between 25-45 kDa, is heat-labile and optimally stored at 4°C.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Human peripheral blood monocytes are key immune cells involved in host defense.
- Understanding monocyte-mediated cytotoxicity is crucial for developing immunotherapies.
- Previous studies have focused on contact-dependent mechanisms of monocyte lysis.
Purpose of the Study:
- To investigate the lytic mechanism of human peripheral blood monocytes.
- To determine if monocytes release soluble factors capable of inducing target cell lysis.
- To characterize the nature and stability of this cytolytic activity.
Main Methods:
- Using actinomycin D-treated WEHI-164 cells as NK-insensitive targets.
- Employing 51Cr-release assays to quantify target cell lysis.
- Analyzing cell-free supernatants (CFS) for cytolytic activity.
- Investigating the effect of protease inhibitors, heat, and storage conditions on CFS activity.
- Size exclusion chromatography (Sephacryl 200) to estimate molecular weight.
Main Results:
- Monocytes, but not lymphocytes, lysed target cells via a contact-independent mechanism.
- Cell-free supernatants from cultured monocytes exhibited potent cytolytic activity.
- The cytolytic activity was heat-labile and unaffected by protease inhibitors.
- Optimal storage for preserving activity was at 4°C.
- The active substance was characterized as a protein(s) with an estimated molecular weight of 25,000–45,000 daltons.
Conclusions:
- Human monocytes secrete a soluble cytotoxic factor capable of lysing tumor cells.
- This monocyte-derived factor represents a novel mechanism of cell-mediated cytotoxicity.
- The proteinaceous nature and stability profile of the factor offer potential for therapeutic applications.