Effect of Aliskiren, a Direct Renin Inhibitor, on Macrophage Accumulation in a Rat Model of Pulmonary Fibrosis Induced by Fat Embolism
Authors: Siddiqi A. , Alam S. , Ahmad Y. , Arif D. , Ponnapureddy R. , Tappeta K. , Fletcher A. , Patel C. , Hamidpour S. , Poisner A. , Molteni A. 
Published: November 2023

Introduction

In a rat model of fat embolism using triolein (T), we have shown that the histopathological pulmonary effects of inflammation and fibrosis were reduced by administration of captopril and losartan treatment (1) and recently by the renin inhibitor aliskiren (2), underscoring the role of the Renin‐Angiotensin System in the pathogenesis of this condition. Aliskiren treatment also reduced the number of mast cells (MC) (3). In view of the known interactions of MC with macrophages in the inflammatory process leading to fibrosis, we extended our study to the lung macrophages.

Methods

22 Sprague Dawley rats received T (0.2 ml IV, ‐n=18) or saline (n=4). The T‐treated rats were divided into three groups of 6 rats each and injected IP one hour later with 0.2 ml saline, aliskiren 50mg/kg or aliskiren 100mg/kg. Rats were killed at 48 hours, lungs collected for histological studies and stained by H & E, SMA‐1 and Masson trichrome for evaluation of inflammation and fibrosis. Macrophages were stained by CD68. Two pathologists unaware of the slides identity evaluated the lung damage and took 10 photographs of each slide at 400× for the macrophage count.

Results

Rats injected with T + saline showed the expected severe pulmonary inflammation and fibrosis markedly reduced by both aliskiren doses. Macrophages showed differences in size and histological appearance. The most frequently observed type showed a compact cytoplasm and dimensions similar to those of the MC. A second type was twice as large and showed many droplet‐like cytoplasmic vacuoles. In contrast to what we saw for MC, the compact size macrophages showed no variation in number in any of the 4 groups and were distributed throughout the entire lung parenchyma. The T + saline group showed a small decline vs the controls. However, the T + 50 mg/kg aliskiren group showed a decline in macrophage numbers vs the other 3 groups (p<0.016, <0.025, and <0.007) respectively. The larger macrophages were distributed in all groups, like the MC around the small caliber pulmonary arteries.

Conclusion

Morphological differences were observed in the macrophage appearance in this model, one with a compact cellular structure and a second with intracellular vacuoles. In contrast to what was observed with MC after T treatment, there was no significant increase for those macrophages with the compact cellular structure and the two doses of aliskiren did not reduce their numbers. The large vacuolated macrophages showed no difference between controls and T‐treated rats but the aliskiren (50 mg/kg) dose produced a significant decrease in their number. There was a wide variation in the cellular response to T. The decrease in the larger vacuolated macrophages after the low dose of aliskiren is consistent with the decrease in fat accumulation that we reported (2).

Support or Funding Information

Supported by the Catherine G. Gelmacher Foundation, St. Louis, MO.