TCRemP: T-Cell Receptor sequence embedding via Prototypes

TCRemP is a package developed to perform T-cell receptor (TCR) sequence embedding. TCR sequences encode antigen specificity of T-cells and their repertoire obtained using AIRR-Seq family of technologies serves as a blueprint the individual's adaptive immune system. In general, it is very challenging to define and measure similarity between TCR sequences that will properly reflect closeness in antigen recongition profiles. Defining a proper language model for TCRs is also a hard task due to their immense diversity both in terms of primary sequence organization and in terms of their protein structure. Our pipeline follows an agnostic approach and vectorizes each TCR based on its similarity to a set of ad hoc chosen TCR "probes". Thus we follow a prototype-based approach and utilize commonly encountered TCRs either sampled from a probabilistic V(D)J rearrangement model (see Murugan et al. 2012) or a pool of real-world TCR repertoires to construct a coordinate system for TCR embedding.

The workflow is the following:

• TCRemP pipeline starts with a selection of k prototype TCR alpha and beta sequences, then it computes the distances from every of n input TCR alpha-beta pairs to 2 * k prototypes for V, J and CDR3 regions, resulting in 6 * k parameters (or 3 * k for cases when only one of the chains is present).

Distances are computed using local alignment with BLOSUM matrix, as implemented in our mirpy package; we plan to move all computationally-intensive code there.

• Resulting distances are treated as embedding co-ordinates and and are subject to principal component analysis (PCA). One can monitor the information conveyed by each PC, whether they are related to features such as Variable or Joining genes, CDR3 region length or a certain epitope.

N.B. TCRemP is currently in active development, please see below for the list of features, current documentation, a proof-of-concept example. All encountered bugs can be submitted to the issues section of the @antigenomics repository.

Using TCRemP one can:

• perform an embedding for a set of T-cell clonotypes, defined by TCR’s Variable (V) and Joining (J) gene IDs and complementarity determining region 3 (CDR3, amino acid sequence placed at the V-J junction). The embedding is performed by mapping those features to real vectors using similarities to a set of prototype TCR sequences
• embed a set of clones, pairs of TCR alpha and beta chain clonotypes
• analyze the mapping by performing dimensionality reduction and evaluating principal components (PCs)
• cluster the embeddings using DBSCAN method with parameter selection using knee/elbow method
• visualize T-cell clone and clonotype embeddings using tSNE, coloring the visualization by user-specified clonotype labels, such as antigen specificities
• infer cluster that are significantly enriched in certain labels, e.g. TCR motifs belonging to CD8+ T-cell subset or specific to an antigen of interest

Planned features:

• [in progress] co-embed samples with VDJdb database to predict TCRs associated with certain antigens, i.e. “annotate” TCR repertoires
• [in progress] perform imputation to correctly handle mixed single-/paired-chain data
• [in progress] implement B-cell receptor (BCR/antibody) prototypes to apply the method to antibody sequencing data

Getting started

Installation procedure and first run

One can simply install the software out-of-the-box using pip with py3.11:

conda create -n tcremp ipython python=3.11
conda activate tcremp
pip install git+https://github.com/antigenomics/tcremp

Or, in case of package version problems or other issues, clone the repository manually via git, create corresponding conda environment and install directly from sources:

git clone https://github.com/antigenomics/tcremp.git
cd tcremp
conda create -n tcremp ipython python=3.11
conda activate tcremp
pip install .

If the installation doesn't work for Apple M1-M3 processors install the required libraries yourself.

Check the installation by running:

tcremp-run -h # note that first run may be slow
cd $tcremp_repo # where $tcremp_repo is the path to cloned repository
tcremp-run -i data/example/v_tcrpmhc.txt -c TRA_TRB -o data/example/ -n 10 -x clone_id

check that there were no errors and observe the results stored in data/example folder. You can then go through the example.ipynb notebook to run the analysis and visualize the results. You can proceed with your own datasets by substituting example data with your own properly formatted clonotype tables.

Preparing the input data

The input data typically consists of a table containing clonotypes as defined above, either TCR alpha, or beta, or both. One can additionally tag clonotypes/clones with user-defined ids, e.g. cell barcodes, and labels, e.g. antigen specificity or phenotype. One can also use a custom clonotype table instead of a pre-built set of prototypes ( see data/example/VDJdb_data_paired_example.csv).

Input format

Common requirements

1. V and J gene names should be provided based on IMGT naming, e.g. TRAV35*03 or TRBV11-2. TCRemP will always use the major allele, so the alleles above will be transformed into TRBV11-2*01
2. The data should not contain any missing data for any of the columns: V, J and CDR3.
3. There should be no symbols except for 20 amino acids in CDR3s

Input columns

Column name	Description	Required
clone_id	clonotype id which will be transferred to the output file and which will be used for paired chain data mapping	optional (required for TRA_TRB mode)
v_call	TCR V gene ID	required
j_call	TCR J gene ID	required
junction_aa	TCR CDR3 amino acid sequence	required
locus	either alpha or beta	required

Single chain table example

Either wide with missing values

clone_id	junction_aa	v_call	j_call	locus
1	CASSIRSSYEQYF	TRBV19	TRBJ2-7	beta
2	CASSWGGGSHYGYTF	TRBV11-2	TRBJ1-2	beta

Paired chain example

A simple flat format

clone_id	junction_aa	v_call	j_call	locus
GACTGCGCATCGTCGG-28	CAGHTGNQFYF	TRAV35	TRAJ49	alpha
GACTGCGCATCGTCGG-28	CASSWGGGSHYGYTF	TRBV11-2	TRBJ1-2	beta

Running TCRemP

Basic usage

Run the tool as

tcremp-run --input my_input_data.txt --output my_folder --chain TRA_TRB

The command above will:

• checks input data format and proofreads the dataset
• extracts TCR alpha and beta clonotypes from my_input_data.txt
• calculates distance scores from clonotypes for the built-in set of 3000 prototypes for each chain

Command line parameters

The parameters for running tcremp-run main script are the following:

parameter	short usage	description	available values	required	default value
--input	-i	input clonotype table	path to file	yes	-
--output	-o	pipeline output folder	path to directory	no	tcremp_{inputfilename}/
--prefix	-e	prefix name for distance file	str	no	tcremp_{inputfilename}/
--index-col	-x	index column where the clonotype IDs are stored	str	no	tcremp_{inputfilename}/
--chain	-c	single or paired clonotype chains	TRA, TRB, TRA_TRB	yes	-
--prototypes_path	-p	path to the custom input prototype table	path to file	no	data/example/v_tcrpmhc.txt
--n-prototypes	-n	number of prototypes to be selected for embedding supplemented prototype table	integer	no	None
--sample-random-prototypes	-sample-random-p	whether to sample the prototypes randomly or not	bool	no	False
--n-clonotypes	-nc	number of clonotypes to be selected from input file	integer	no	None
--sample-random-clonotypes	-sample-random-c	whether to sample the clonotypes randomly or not	bool	no	False
--species	-s	species of built-in prototypes to be used	HomoSapiens, MusMusculus, MacacaMulatta	no	HomoSapiens
--random-seed	-r	random seed for random prototype selection	integer	no	None
--nproc	-np	number of processes to perform calculcation with	integer	no	1
--lower-len-cdr3	-llen	filter out cdr3 with len <llen	integer	no	30
--higher-len-cdr3	-hlen	filter out cdr3 with len >hlen	integer	no	30
--metrics	-m	which type of matrics to use: similarity or dissimilarity one	similarity, dissimilarity	no	dissimilarity

Output

The output file will contain the following columns:

• clone_id - assigned identifier to each row of the input table (either tranferred from initial data or generated)
• {i}_a_v, {i}_a_j, {i}_a_cdr3 - columns with distances to each alpha prototype
• {i}_b_v, {i}_b_j, {i}_b_cdr3 - columns with distances to each beta prototype

Each line of the output file corresponds to one input clonotype.