# Spatial Phenotype of the Mast Cell Population in Endometritis of Various Severities

## TL;DR

This study maps how mast cells and their interactions with other immune cells change in the endometrium as endometritis worsens, offering potential for better diagnosis and treatment.

## Contribution

The first to map mast cell accumulation and their spatial interactions with immune cells in chronic endometritis.

## Key findings

- Mast cell content in the endometrium increases with the severity of chronic endometritis.
- Mast cell colocalization with CD8+ lymphocytes, monocytes, and macrophages rises with disease severity.
- Spatial patterns of mast cell interactions may help diagnose chronic endometritis and prevent fibrosis.

## Abstract

Endometritis features an inflammatory milieu in the endometrium, accompanied by the recruitment of immunocompetent cells, including mast cells (MCs). The mechanisms underlying MC involvement in chronic endometritis (CE) and fibrous niche formation remain poorly understood, particularly regarding spatial intercellular interactions in situ. In this study, we used multiplex immunohistochemistry and quantitative immunofluorescence analysis to map the spatial phenotype of MC distribution. Standard histochemical techniques, monoplex and multiplex immunohistochemical staining technologies, light-field microscopy, epifluorescence, and confocal microscopy with multispectral imaging, combined with quantitative immunofluorescence analysis with AI application, were used to identify the spatial phenotyping of quantitative and qualitative features of the endometrial MC population in CE. The increased intensity of endometrial inflammation was accompanied by a rise in the profile of MC content in the endometrium; this accounted for a 0.014% increase in the control and 0.067%, 0.113%, and 0.206% increases in mild, moderate, and severe CE, respectively. We are the first to map the number of MCs that demonstrated loci of accumulations in the endometrium coinciding with foci of fibrous changes. The number of these foci correlated with the severity of chronic endometritis and the development of clinical signs. The frequency of juxtacrine and paracrine MC colocalization with other immunocompetent cells increased with increased CE activity and fibrotic changes: For CD8+ lymphocytes, colocalization increased from 4.6% in the control to 11.6%, 18.5%, and 28.0% in mild, moderate, and severe CE, respectively. For monocytes, colocalization increased from 5.6% in the control to 18.7%, 26.8%, and 28.8% in mild, moderate, and severe CE, respectively. For type 1 macrophages, colocalization increased from 5.6% in the control to 13.5%, 17.4%, and 24.6% in mild, moderate, and severe CE, respectively. For type 2 macrophages, colocalization increased from 3.4% in the control to 9.6%, 9.1%, and 21.5% in mild, moderate, and severe CE, respectively. Spatial patterns of juxtacrine and paracrine MC interactions with other immune cells may provide diagnostic algorithms for chronic endometritis, enabling targeted therapy and preventing fibrotic changes.