# InterCellar User Guide

Marta Interlandi1*

1Institute of Medical Informatics, University of Muenster, Muenster (DE)

*marta.interlandi@uni-muenster.de

#### 26 April 2023

#### Package

InterCellar 2.7.0

# 1 Introduction

`InterCellar` is a Bioconductor package that provides an interactive Shiny application to enable the analysis of cell-cell communication from single-cell RNA sequencing (scRNA-seq) data. Every step of the analysis can be performed interactively, thus not requiring any programming skills. Moreover, `InterCellar` runs on your local machine, avoiding issues related to data privacy.

## 1.1 Installation

`InterCellar` is distributed as a Bioconductor package and requires R (version 4.1) and Bioconductor (version 3.14).

To install `InterCellar` package enter:

    if (!requireNamespace("BiocManager", quietly = TRUE))
        install.packages("BiocManager")

    BiocManager::install("InterCellar")

## 1.2 Launching the app

Once InterCellar is successfully installed, it can be loaded as follow:

    library(InterCellar)

In order to start the app, please run the following command:

    InterCellar::run_app( reproducible = TRUE )

`InterCellar` should be opening in a browser. If this does not happen automatically, please open a browser and navigate to the address shown (for example, `Listening on http://127.0.0.1:6134`). The flag `reproducible = TRUE` ensures that your results will be reproducible across R sessions.

# 2 Data upload

The first step of the workflow requires the upload of pre-computed results generated by an external tool capable of predicting cell-cell communication mediated by ligand-receptor interactions. `InterCellar` supports both published tools such as CellPhoneDBv2 (Efremova et al. 2020), CellChat(Jin et al. 2021), ICELLNET(Noël et al. 2021), and SingleCellSignalR (Cabello-Aguilar et al. 2020), and custom results output of ad hoc methods, which must contain necessary information as described in the panel From custom analysis. For this user guide, we will use CellPhoneDB (CPDB) results computed on a scRNA-seq dataset from Chua et al.(Chua et al. 2020). This dataset comprises data of COVID-19 patients, divided in critical and moderate cases, as well as healthy controls. Cell-cell interaction (CCI) data output of CPDB on each condition can be found at InterCellar-reproducibility.

By navigating to 1. Data and Upload, we can import our 3 CCI data from the Supported tools panel. We specify an existing local folder where `InterCellar` will create output folders to save figures and tables results of the analysis. To upload a CCI data, we must specify an ID and an output folder tag. Next, we can select the folder containing CellPhoneDB results from our local drive. `InterCellar` will read and pre-process the data and show the resulting table in Table view. The pre-processing step consists of:

- Mapping interaction pairs to the corresponding genes (when necessary)
- Annotating genes to their molecular function: L (ligand) or R (receptor)
- Re-ordering the interaction pairs listed as R-L to L-R

Finally, we can switch active CCI data on the left menu, to easily analyze multiple datasets in parallel.

# 3 Data exploration: Universes

Once the input data has been uploaded, `InterCellar` takes us to the exploration of three Universes. Each universe has its focus on a different biological domain: cell clusters, genes and functions. Specific filtering options can be applied and multiple visualization choices are available to enable a deep exploration of the cellular communication.

## 3.1 Cluster-verse

Focus of this universe are clusters of cells participating in the communication. The filtering options allow the user to subset the dataset by:

- excluding entire clusters from the data. All interactions related to the excluded clusters will be excluded as well from further steps of the analysis;
- setting a minimum interaction score;
- (when available) changing the p-value threshold for significant interactions (default to 0.05).

The analyst will be able to see the effect of these filtering steps by looking at the box showing the number of total interactions.

Warning: these filters have global influence on the analysis, since they subset the input data!

Three tabs are part of the Cluster-verse: Network, Barplot and Table.

The Network of clusters shows the overall cellular communication. Nodes represent different clusters while edges show the (total or weighted by interaction score) number of paracrine interactions occurring between two clusters. Edges that fall back on the same cluster represent autocrine interactions.

Barplot offers two different barplots representing: (1) the total number of interactions per cluster, divided in paracrine and autocrine interactions; and (2) the relative number of interactions for a certain cell type.

In the Table panel, the analyst can restrict the data exploration to a specific focus, by subsetting the data to one cluster of interest, called viewpoint, and one flow of communication among:

- Directed, outgoing interactions (L-R): for which the viewpoint cluster sends (expresses) the ligand to the other clusters, that are in turn expressing the corresponding receptor;
- Directed, incoming interactions (R-L): for which the viewpoint expresses the receptor that binds to the corresponding ligand sent by other clusters;
- Undirected interactions (L-L and R-R): for which both elements of an interaction pair are either ligands or receptors.

## 3.2 Gene-verse

`InterCellar` second universe focuses on the genes. Filtering options to exclude interaction pairs (int-pairs) are available and are specific to the input tool chosen by the user. The Table shows all distinct int-pairs enriched in our data, regardless of the clusters in which these are found. Included in this Table are Ensembl and UniProt IDs of each gene, with hyperlinks to the respective web pages to facilitate investigation of unfamiliar genes.

Upon selection of one or multiple int-pairs from the previous Table, a dot plot is generated and visible in the Dot Plot panel. The analyst can decide to select a subset of clusters for the visualization as well as choose different colors for high and low int-pair score.

Network panel visualizes the selected int-pairs in a cluster network.

## 3.3 Function-verse

In the Function-verse, the analyst is required to perform a functional annotation, before proceeding to the next steps of the analysis. To this scope, `InterCellar` offers multiple sources of functional annotations in terms of Gene Ontology (queried from Ensembl, via the package biomaRt) and pre-downloaded pathway databases (from the package graphite). After selection of suitable sources, the annotation can be performed and a Table showing all functional terms annotated to each int-pair is displayed. Worth to note is the fact that a functional term is annotated to an int-pair only when the functional term is enriched in both genes (or gene complexes), partners of the interaction.

The Barplot panel summarizes the number of functional terms annotated for each source.

In the following panel, Ranking, functional terms are listed individually, along with information on occurrence (i.e. how many int-pairs have been annotated to this term).

By selecting one row of the Ranking table, we can explore the term of interest in the Sunburst plot. This visualization allows to connect functions to int-pairs and clusters. On the left side of the panel, a table lists all int-pairs annotated to the term. The user can choose to visualize the number of interactions or the weighted number (by score). On the right side, the sunburst plot is composed as follows:

- the selected functional term is shown in the inner circle of the plot;
- the inner ring shows all “first partner” clusters, enriched by the relevant int-pairs. Specifically, clusters on the inner ring express the first gene of each int-pair;
- the outer ring displays all “second partner” clusters, expressing the second gene of each int-pair;
- the width of each section represents the fraction of int-pairs found in that section (also shown when hovering on the inner ring sections);
- hovering on outer ring sections will show the individual int-pairs enriched in the cluster pairs.

# 4 Data-driven analysis

## 4.1 Int-Pair Modules

This step of `InterCellar`’s workflow allows the analyst to define and analyze Int-Pair Modules, i.e. groups of int-pairs that share a similar functional profile. To this aim, the choice of a viewpoint cluster and communication flow is required. `InterCellar` will subset the input data accordingly. This analysis can be repeated for each viewpoint and flow of interest.

To define the number of int-pair modules in the data subset, four visualizations are provided. On the left hand side, the optimal number of modules is calculated by `InterCellar` using (1) the elbow method on the total within-clusters sum of squares (which should be minimized) and (2) the average silhouette width (which should be maximized). Both methods are standard practice in cluster analysis and are supposed to help the choice of the optimal number of modules. However, the user is free to choose the best number of modules depending on each case. In general, high (low) number of groups is reflected in high (low) specificity of a module. For this purpose, two visualization offer yet another way to investigate the optimal number of modules. A dendrogram of int-pairs shows the results of a hierarchical clustering obtained on the first two components of the UMAP underneath. Each point of the UMAP represents one int-pair (shown by hovering) and color-coding is consistent for both UMAP and dendrogram, showing the number of modules chosen. Moreover, dendrogram and UMAP are initialized with the optimal number of modules chosen by the elbow method (giving usually higher resolution compared to the average silhouette).

Once the int-pair modules have been defined, `InterCellar` offers the possibility to visualize the int-pairs belonging to each module and the respective clusters in a Circle plot. Directed interactions are represented here by arrows originating from ligands (double segment) towards receptors (single segment). The Table panel summarizes the same info in a tabular format.

Last step of the int-pair modules analysis concerns functional terms. `InterCellar` performs a permutation test to calculate empirical p-values assessing the significance of functional terms annotated to int-pairs of each module. A Table displays functional terms that are found significant (p-value <= 0.05 by default) for the chosen int-pair module. The significant functional terms listed in these tables can help the user to “manually” select terms that are of biological interest and can be used to annotate the UMAP, as we did in our manuscript.

## 4.2 Multiple conditions

The final step of `InterCellar`’s analysis allows the comparison of cell-cell communication from different conditions (up to 3). The user can choose which conditions to compare (we recommend having the same -or very similar- composition in terms of cell clusters). The analysis is then structured as follow:

- Cluster-based: the comparison focuses on the number of interactions per cluster. Panel Back-to-Back Barplot considers the first two conditions and plots a barplot comparing the total number of interactions per cluster. Panel Radar Plot compares the relative numbers of interaction from a certain viewpoint cell cluster, for two or three conditions. Here we compare COVID-19 critical cases VS moderate ones. For the radar plots, we consider also control cases.

- Gene-based: `InterCellar` computes int-pair/cluster-pair couplets that are unique to each condition and displays them in the Table. Upon selection of one or multiple unique couplets, a Dot Plot is generated, showing the occurrence of each couplet in the respective condition. A Pie Chart summarizes the contribution of the selected couplets to each condition.

- Function-based: here only int-pairs that are uniquely found in each condition are considered. These are shown in Table-UniqueIntPairs along with the condition and cluster-pairs. Based on the functional annotation performed in the function-verse `InterCellar` implements a permutation test to calculate an empirical p-value of significance for the functional terms that were annotated to these unique int-pairs. Table-FuncTerms shows all functional terms that are significantly enriched by int-pairs unique to each condition. Finally, by selecting a term of interest, the user can visualize it in a Sunburst Plot.

# References

Cabello-Aguilar, Simon, Mélissa Alame, Fabien Kon-Sun-Tack, Caroline Fau, Matthieu Lacroix, and Jacques Colinge. 2020. “SingleCellSignalR: Inference of Intercellular Networks from Single-Cell Transcriptomics.” Nucleic Acids Research 48 (10): e55–e55.

Chua, Robert Lorenz, Soeren Lukassen, Saskia Trump, Bianca P Hennig, Daniel Wendisch, Fabian Pott, Olivia Debnath, et al. 2020. “COVID-19 Severity Correlates with Airway Epithelium–Immune Cell Interactions Identified by Single-Cell Analysis.” Nature Biotechnology 38 (8): 970–79.

Efremova, Mirjana, Miquel Vento-Tormo, Sarah A Teichmann, and Roser Vento-Tormo. 2020. “CellPhoneDB: Inferring Cell–Cell Communication from Combined Expression of Multi-Subunit Ligand–Receptor Complexes.” Nature Protocols 15 (4): 1484–1506.

Jin, Suoqin, Christian F Guerrero-Juarez, Lihua Zhang, Ivan Chang, Raul Ramos, Chen-Hsiang Kuan, Peggy Myung, Maksim V Plikus, and Qing Nie. 2021. “Inference and Analysis of Cell-Cell Communication Using Cellchat.” Nature Communications 12 (1): 1–20.

Noël, Floriane, Lucile Massenet-Regad, Irit Carmi-Levy, Antonio Cappuccio, Maximilien Grandclaudon, Coline Trichot, Yann Kieffer, Fatima Mechta-Grigoriou, and Vassili Soumelis. 2021. “Dissection of Intercellular Communication Using the Transcriptome-Based Framework Icellnet.” Nature Communications 12 (1): 1–16.