# CaSSiDI: novel single-cell “Cluster Similarity Scoring and Distinction Index” reveals critical functions for PirB and context-dependent Cebpb repression

## TL;DR

This study introduces CaSSiDI, a new method for analyzing single-cell RNA data, and reveals new roles for PirB in regulating macrophage functions and Cebpb expression.

## Contribution

The paper introduces CaSSiDI, a novel computational framework for optimizing clustering in single-cell RNA-sequencing data.

## Key findings

- PirB regulates Cebpb expression in macrophages, affecting Ly6Clo monocyte generation and peritoneal macrophage expansion.
- PirB's effect on Cebpb is tissue-specific and not observed in splenic red pulp macrophages.
- CaSSiDI identified distinct subsets of red pulp macrophages with unique gene expression profiles.

## Abstract

PirB is an inhibitory cell surface receptor particularly prominent on myeloid cells. PirB curtails the phenotypes of activated macrophages during inflammation or tumorigenesis, but its functions in macrophage homeostasis are obscure. To elucidate PirB-related functions in macrophages at steady-state, we generated and compared single-cell RNA-sequencing (scRNAseq) datasets obtained from myeloid cell subsets of wild type (WT) and PirB-deficient knockout (PirB KO) mice. To facilitate this analysis, we developed a novel approach to clustering parameter optimization called “Cluster Similarity Scoring and Distinction Index” (CaSSiDI). We demonstrate that CaSSiDI is an adaptable computational framework that facilitates tandem analysis of two scRNAseq datasets by optimizing clustering parameters. We further show that CaSSiDI offers more advantages than a standard Seurat analysis because it allows direct comparison of two or more independently clustered datasets, thereby alleviating the need for batch-correction while identifying the most similar and different clusters. Using CaSSiDI, we found that PirB is a novel regulator of Cebpb expression that controls the generation of Ly6Clo patrolling monocytes and the expansion properties of peritoneal macrophages. PirB’s effect on Cebpb is tissue-specific since it was not observed in splenic red pulp macrophages (RPMs). However, CaSSiDI revealed a segregation of the WT RPM population into a CD68loIrf8+ “neuronal-primed” subset and an CD68hiFtl1+ “iron-loaded” subset. Our results establish the utility of CaSSiDI for single-cell assay analyses and the determination of optimal clustering parameters. Our application of CaSSiDI in this study has revealed previously unknown roles for PirB in myeloid cell populations. In particular, we have discovered homeostatic functions for PirB that are related to Cebpb expression in distinct macrophage subsets.