Presence of pulmonary intravascular macrophages in the equine lung: some structuro-functional properties.

This study provides the first evidence that horse lungs have pulmonary intravascular macrophages (PIMs) which are cells that ingest harmful particles. The research also discusses the unique properties of these cells and provides a foundation for further exploration of their implications in horse health.

Pulmonary Intravascular Macrophages (PIMs) in Horses

• The presence of PIMs in horses was confirmed through this research. These cells are known to be present in several other animal species including calf, goat, and sheep. PIMs serve as resident phagocytes, cells that engulf and ingest harmful microorganisms or particles.
• The structure of these PIMs, like their globular surface coat and the structures of the endocytic pathway, are similar to those found in the aforementioned species. This suggests a shared functional pathway across different species.

Functional Properties of Equine PIMs

• The study further analyzed the functional properties of these equine PIMs using Monastral Blue (MB) as a tracer. MB is used to assess the phagocytic properties of the cells, which helps in understanding how these cells perform in ingesting and neutralizing harmful particles in the body.
• PIMs react to MB by modifying their coat globules into lipid droplets, which suggests the presence and possible conversion of neutral lipids. The lipid droplets are then transported into specific cell compartments known as endosomes and lysosomes, responsible for breaking down waste materials within the cell.
• Conversely, an alternate pathway is followed by the unaltered globules of the coat, suggesting the possibility of different endocytic process for different substances.

Implication of Equine PIMs

• Another important finding from this study is that these equine PIMs seem capable of large-scale platelet uptake. This was observed in MB-treated ponies that exhibited reduced platelet count, a condition known as thrombocytopenia. This could suggest a significant role of PIMs in platelet regulation.
• Finally, the researchers propose a hypothesis that the coat of the PIM could be a form of low-density lipoprotein (LDL) similar to other species and that their endocytic organelles could act as producers of vasoactive lipids. However, these remain hypothetical and require further studies for confirmation.

The study provides a detailed characterization of previously unknown PIMs resident in the equine lungs, their structural characteristics, their functional implications, and potential physiological relevance. By expanding the knowledge about PIMs across species, this research opens up new avenues for understanding and potentially addressing health conditions in horses.