# Phenotypic Profiling and Activation-Associated Expression of CD99 Ligands on Human Leukocytes

## TL;DR

This study explores how CD99 ligands are expressed on immune cells and how their levels change during immune activation, particularly in NK cells and monocytes.

## Contribution

The study identifies CD99 ligand expression patterns on human leukocytes and their regulation during immune activation, especially with IL-2 stimulation.

## Key findings

- CD99 ligands are predominantly expressed on NK cells and monocytes.
- Ligand expression increases significantly after IL-2 stimulation in non-classical monocytes and CD56 Dim NK cells.
- T lymphocytes show low CD99 ligand expression that increases modestly upon activation.

## Abstract

Protein–ligand interactions are ubiquitous in biological cells. Transmembrane protein CD99 is expressed on all leukocytes, but information on the distribution and behavior of its ligands is still lacking. Here, we report the distribution of CD99 ligands across various immune cell types and examine how their levels vary upon immune cell activation. Our findings indicate that CD99 ligands are predominantly expressed on natural killer (NK) cells and monocytes, with their expression further increasing after interleukin (IL)-2 stimulation. These data suggest that CD99 expression is associated with immune activation and may serve as a marker for activated immune cells.

The immune system comprises a complex network of cells that continuously change during activation, infection, and the maintenance of balance. Immunophenotyping offers valuable insights into the regulation of immune responses. We systematically characterized the expression profile of CD99 ligands across distinct immune cell subsets using both conventional and high-dimensional flow cytometry. CD99 ligands were detected on NK cells and monocytes under both resting and IL-2-activated conditions, with non-classical monocytes and CD56 Dim NK cells exhibiting the highest expression levels. Notably, ligand expression in these subsets was further enhanced following IL-2 activation. In contrast, T lymphocytes (CD3+) displayed low basal levels of CD99 ligand expression, which increased modestly upon activation. Cellular activation was accompanied by an expansion of specific immune phenotypes characterized by elevated CD99 ligand expression alongside the upregulation of activation markers such as CD69 and CD137. Collectively, these findings suggest that the expression of the CD99 ligands may serve as an indicator of immune activation and demonstrate subset-specific regulation, particularly in response to IL-2 stimulation. These findings have revealed the distinct expression patterns of CD99 ligands, emphasizing their crucial role in modulating immune responses.