[toggle_content title=”Abstract”] Rats received a single intravenous injection with liposome-encapsulated dichloromethylene diphosphonate (C12MDP). This treatment resulted in the elimination of macrophages in spleen and liver within 2 days. Macrophages ingest the liposomes and are destroyed by the drug, which is released from the liposomes after disruption of the phospholipid bilayers under the influence of lysosomal phospholipases. Repopulation of macrophages in spleen and liver was studied at different time intervals after treatment. Macrophages in the liver (Kupffer cells) and red pulp macrophages in the spleen were the first cells to reappear, followed by marginal metallophilic macrophages and marginal-zone macrophages in the spleen. Different markers of the same cell did not reappear simultaneously. On the other hand, the same marker (recognized by the monoclonal antibody ED2) reappeared much more rapidly in the liver than in the spleen. The present results in the rat were different from those earlier obtained in the mouse. Red pulp macrophages were the first cells and marginal zone macrophages were the last cells to repopulate the spleen in both rodents after treatment with C12MDP liposomes. However, there was much more overlap in the repopulation kinetics of splenic macrophage subpopulations in the rat, when compared with the mouse.[/toggle_content]
[toggle_content title=”Clodronate Liposome Parameters”] [custom_table]
Total Lipid Concentration
Lipid Mole %
10 mg/ml 1
1 Clodronate and lipid concentrations assumed based on information in paper (dosed 2 mg in 0.2 ml).
[toggle_content title=”Animals and Dosing”] [custom_table]
The repopulation kinetics of spleen and liver macrophage subpopulations differ considerably between rats (data from this paper) and mice (historical data).
In rat spleens, only cellular remnants of ED1+/ED2+/ED3+ macrophages remain on days 2 and 4 post-injection. ED1+ cells began to reappear in the red pulp on day 8. ED2+ and ED3+ cells were detected on day 16 in selective regions. ED1+ cells returned to baseline values by day 32. ED2+/ED3+ cells were back to normal levels by day 65.
Interdigitating dendritic cells were not depleted.
Other macrophages in the white pulp appeared rounded and swollen on day 8, but did not die.
Rat liver ED1+/ED2+ macrophages were depleted by day 2 and ED2+ cells did not reappear until day 8. A few small, round ED1+ cells were found on day 4, but had regained their normal morphology by day 8. Both populations had returned to baseline levels on day 16.
Bone marrow, peritoneal and other lymphoid tissues were examined, however only some showed a slight decrease in ED1+ and ED3+ cells. Alveolar macrophages were not collected.
After day 16, ED3+ cells appeared in lymphoid tissues where they are not normally found; the authors speculated that autoimmune mechanisms were at play.
The presence of swollen high endothelial venules were observed in bronchial and tracheal lymph nodes which the authors also proposed to be due to autoimmune responses.
The fact that various subpopulations return to tissues at various rates should be considered when interpreting results from depletion experiments. Cell-surface receptor variability and functional differences between the subpopulations may result in abnormal macrophage responses when experimental data is collected during macrophage repopulation after depletion.
Repopulation rates must be carefully considered when experimental protocols are dependent on the absence of phagocytic activity in all or select tissues. For example, monocytes may begin to repopulate before maximal depletion is reached in the spleen and liver.
To reiterate, depletion and repopulation rates will also differ between species including within rodent species. Therefore, historical data from mice may not be applicable when running depletion experiments in rats or guinea pigs, for example.
We also wonder if the various immunocompromised mutants could demonstrate atypical depletion and repopulation kinetics within a species.
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