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Paraformaldehyde fixation of LPS-stimulated human monocytes: technical parameters permitting the study of membrane
Insights
Investigating cell surface Interleukin-1 (IL-1) activity revealed that standard fixation methods may cause IL-1 leakage. Optimized conditions, however, confirm genuine membrane-associated IL-1 activity on monocytes.
Area of Science:
- Immunology
- Cell Biology
Background:
- The presence of cell surface Interleukin-1 (IL-1) on mononuclear phagocytes is debated.
- Standard paraformaldehyde (PFA) fixation methods are used to detect membrane-associated IL-1, but concerns exist regarding passive IL-1 alpha leakage.
Purpose of the Study:
- To clarify the existence and detection of membrane-associated IL-1 activity.
- To investigate the impact of fixation conditions on IL-1 detection in stimulated monocytes.
Main Methods:
- Utilized specific IL-1 alpha and IL-1 beta assays.
- Employed varying paraformaldehyde (PFA) fixation times and assay timings (up to 144 hours).
- Tested the effect of anti-IL-1 alpha and anti-IL-1 beta antibodies on detected activity.
Main Results:
- Standard PFA fixation led to the release of both IL-1 alpha and IL-1 beta into supernatants, confounding membrane-associated activity measurements.
- Under optimized conditions (144h assay, longer fixation), IL-1 activity was detected on fixed monocytes.
- This detected activity was IL-1 alpha-dependent and independent of fixation duration.
Conclusions:
- Technical conditions critically influence the detection of membrane-associated IL-1 activity.
- Passive leakage, not membrane association, explains IL-1 detection under standard fixation protocols.
- Optimized assay conditions are necessary to accurately measure genuine cell surface IL-1 activity.
Abstract:
The existence of IL-1 activity on the cell surface of stimulated mononuclear phagocytes is a matter of controversy. In particular, fixation of IL-1-expressing cells for 15 min in 1% paraformaldehyde (PFA) is commonly used to evidence such "membrane-associated" IL-1 activity but other authors have attributed this to passive leakage of IL-1 alpha from the cells and report no activity with longer fixation times. Using specific IL-1 alpha and IL-1 beta assays, we found that after the mild standard PFA fixation procedure, not only IL-1 alpha but also IL-1 beta were released into the supernatants for up to 96 h following fixation; membrane IL-1 activity cannot thus be measured in these conditions. However, using conditions in which neither immunoreactive IL-1 molecules nor IL-1 activity are found in the supernatants (i.e. assay at 144 h, increased fixation time), we were still able to detect IL-1 activity on LPS-stimulated, PFA-fixed monocytes. This activity was independent of the duration of PFA fixation and was inhibited by anti-IL-1 alpha but not anti-IL-1 beta antibodies. Our data thus underline the importance of technical conditions in the study of membrane-associated IL-1 activity.