nf-core/pixelator

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Introduction

This document describes the output produced by the pipeline. The directories listed below will be created in the results directory after the pipeline has finished. All paths are relative to the top-level results directory.

Pipeline overview

The pipeline is built using Nextflow and processes data using multiple subcommands of pixelator.

The pipeline consists of the following steps:

• Preprocessing
• Quality control
• Demultiplexing
• Duplicate removal and error correction
• Compute connected components
• Filtering, annotation, cell-calling
• Downstream analysis
• Generate layouts for visualization
• Generate reports
• Pipeline information - Report metrics generated during the workflow execution

Preprocessing

The preprocessing step uses pixelator single-cell amplicon to create full-length amplicon sequences from both single-end and paired-end data. It returns a single FASTQ file per sample containing fixed length amplicons. This step will also calculate Q30 quality scores for different regions of the library.

These amplicon FASTQ files are intermediate and by default not placed in the output folder with the final files delivered to users. Set --save_amplicon_reads or --save_all to enable publishing of these files to:

Quality control

Quality control is performed using pixelator single-cell preqc and pixelator single-cell adapterqc.

The preqc step performs QC and quality filtering of the raw sequencing data using Fastp internally. It generates a QC report in HTML and JSON formats. It saves processed reads as well as reads that were discarded (i.e. were too short, had too many Ns, or too low quality, etc.). Internally preqc

The adapterqc stage checks for the presence and correctness of the pixel binding sequences, using Cutadapt internally. It also generates a QC report in JSON format. It saves processed reads as well as discarded reads (i.e. reads that did not have a match for both pixel binding sequences).

These processed and discarded FASTQ reads are intermediate and by default not placed in the output folder with the final files delivered to users. Set --save_qc_passed_reads and/or --save_qc_passed_reads to enable publishing of these files. Alternatively, set --save_all to keep all intermediary outputs of all steps.

Demultiplexing

The pixelator single-cell demux command assigns each read to a marker (with a certain barcode) file. It also generates QC report in JSON format. It saves processed reads (one file per antibody) as well as discarded reads (in a different file) with no match to the given barcodes/antibodies.

These processed and discarded FASTQ reads are intermediate and by default not placed in the output folder with the final files delivered to users. Set --save_demux_failed_reads and/or --save_demux_processed_reads to enable publishing of these files. Alternatively, set --save_all to keep all intermediary outputs of all steps.

Duplicate removal and error correction

This step uses the pixelator single-cell collapse command.

The collapse command quantifies molecules by performing error correction and detecting PCR duplicates. This is achieved using the unique pixel identifier and unique molecular identifier sequences to check for uniqueness, collapse and compute a read count. The command generates a QC report in JSON format. Errors are allowed when collapsing reads if --algorithm is set to adjacency (this is the default option).

The output format of this command is a parquet file containing deduplicated and error-corrected molecules.

The collapsed reads are intermediate and by default not placed in the output folder with the final files delivered to users. Set --save_collapsed_reads to enable publishing of these files. Alternatively, set --save_all to keep all intermediary outputs of all steps.

Compute connected components

This step uses the pixelator single-cell graph command. The input is the edge list parquet file generated in the collapse step. The molecules from edge list are filtered by count (--graph_min_count) to form the edges of the connected components of the graph. When graphs are computed and identified, their ID names are added back to the edge list in a column called “component”.

The graph command has the option to recover components (technical multiplets) into smaller components using community detection to find and remove problematic edges (see --multiplet_recovery).

The edge list is intermediate and by default not placed in the output folder with the final files delivered to users. Set --save_edgelist to enable publishing of these file.

Alternatively, set --save_all to keep all intermediary outputs of all steps.

Cell-calling, filtering, and annotation

This step uses the pixelator single-cell annotate command.

The annotate command takes as input the molecule list file generated in the graph command. It parses, and filters the molecules grouped by “component” ID to find putative cells, and it will generate a PXL file containing the edges of the graphs in an edge list, and an (AnnData object)[https://anndata.readthedocs.io/en/latest/] as well as some useful metadata.

Some summary statistics before filtering are stored in raw_components_metrics.csv.gz. This file is not included in the output folder by default, but can be included by passing --save_raw_component_metrics.

By default, the PXL file after annotate will not be saved to the results directory unless --skip_analysis and --skip_layout is passed. Set --save_annotate_dataset to include these files.

Downstream analysis

This step uses the pixelator single-cell analysis command. Downstream analyses are performed on the PXL file generated by the previous stage. The results of the analysis are added to the PXL file produced in this stage.

Currently, the following analyses are performed:

• polarization scores (enable with --compute_polarization)
• co-localization scores (enable with --compute_colocalization)

Each analysis can be disabled by using respectively --compute_polarization false or --compute_colocalization false. This entire step can also be skipped using the --skip_analysis option.

By default, the PXL file after analysis will not be saved to the results directory unless --skip_layout is passed. Set --save_analysis_dataset to include these files.

Alternatively, set --save_all to keep all intermediary outputs of all steps.

Compute layouts for visualization

This step uses the pixelator single-cell layout command. It will generate precomputed layouts that can be used to visualize cells as part of the downstream analysis. This data will be appended to a PXL file.

This entire step can also be skipped using the --skip_layout option.

Set --save_all to keep all intermediary outputs of all steps.

Generate reports

This step uses the pixelator single-cell report command. This step will collect metrics and outputs generated by previous stages and generate a report in HTML format for each sample.

This step can be skipped using the --skip_report option.

More information on the report can be found in the pixelator documentation

Pipeline information

Nextflow provides excellent functionality for generating various reports relevant to the running and execution of the pipeline. This will allow you to troubleshoot errors with the running of the pipeline, and also provide you with other information such as launch commands, run times and resource usage.

Output directory structure

With default parameters, the pixelator pipeline output directory will only include the latest PXL file generated by the pipeline (with the most “complete” information) and an interactive HTML report per sample. The PXL dataset files can be from either the annotate, analysis or layout step.

With default parameters, the <sample-id>.layout.datasets.pxl will be copied to the output directory. If the layout stage is skipped (using --skip_layout) the <sample-id>.analysis.datasets.pxl files will be included and if the analysis stage is skipped (using --skip_analysis) the <sample-id>.annotate.datasets.pxl will be copied.

Various flags are available to store intermediate files and are described in the input parameter documentation. Alternatively, you can keep all intermediate files using --save_all.

Below is an example output structure for a pipeline run using the default settings.

• pipeline_info/

• pixelator/

  • logs/

    • <sample-id>/:
      • *.log

  • pbmcs_unstimulated.layout.dataset.pxl

  • pbmcs_unstimulated.qc-report.html

  • uropod_control.layout.dataset.pxl

  • uropod_control.qc-report.html