arda — Antigen Receptor Domain Annotation

Versatile, fast, exact FR/CDR annotation of TCR and BCR sequences — mRNA and protein in FASTA, and reads in FASTQ from both amplicon and bulk RNA-seq — for nucleotide and amino-acid input, across all loci at once.

arda does the expensive IgBLAST work once, offline — building a pre-aligned reference database of every in-frame V·J germline scaffold with FR1–4 / CDR1–3 markup — then at runtime maps your sequences to that database with MMseqs2 and transfers the markup through the alignment in a small C++ hot path. The result is a spec-valid AIRR Rearrangement annotation that matches IgBLAST (98–99.7% region concordance on real GenBank mRNA), from a plain CLI + Python library — no Docker, no workflow engine.

It also annotates records that have no read behind them — a CDR3 amino acid, a V call and a J call, as in a VDJdb row — marking up which residues each germline templates, repairing the junction, and inferring the D gene from the junction's length.

Why

IgBLAST is the gold standard but is slow to invoke per-batch and awkward to embed. arda keeps IgBLAST-quality region calls while being:

• Fast & scalable — MMseqs2 search + a C++ projection step; multiprocessing and SLURM-friendly from small FASTA to large FASTQ.
• Embeddable — import arda; arda.annotate_sequences(...).
• Honest — a D call is gated on an E-value that ships with it (d_support), a germline allele with no derivable anchor is flagged rather than guessed, and a repair that would not produce a canonical junction is refused.
• Easy to install — conda for the mmseqs binary, pip install -e . for the package + C++ extension; IgBLAST is fetched into a gitignored bin/ and is only needed to (re)build the reference DB, not at runtime.

Install

pip install arda-mapper             # from PyPI (imports as `arda`); binary wheels ship the C++ extension
pip install 'arda-mapper[rnaseq]'   # + seqtree, required by `arda rnaseq correct`

mmseqs2 (the search backend) is fetched/managed by arda at runtime. For development — and to get the committed germline references on disk — use setup.sh:

bash setup.sh            # creates conda env `arda`, fetches IgBLAST, pip install -e .
conda activate arda

Flags: --no-conda (use the active env), --build-db (rebuild references after install), --tests (run the fast suites). The committed database/vdj/<organism>/ references mean most users never need to build anything. A pip install arda-mapper with no source checkout auto-fetches the curated references into ~/.cache/arda on first use (the arda-reference-vdj.tar.gz release asset) and builds the MMseqs2 index there — no $ARDA_HOME and no build step required (set ARDA_NO_AUTO_FETCH for air-gapped runs with a pre-populated cache).

Supported organisms: human, mouse (full IG + TR), rat, rabbit, rhesus_monkey (IG only — IgBLAST ships no TR internal annotation for these).

CLI

arda info                                   # resolved paths + tool availability
arda annotate -i reads.fastq -o out.airr.tsv --organism human --seqtype nt
arda annotate -i prot.fasta  -o out.airr.tsv --organism human --seqtype aa
arda annotate -i reads.fastq -o out.airr.tsv --strand forward   # plus-strand only
arda markup -i vdjdb.txt -o marked.tsv --vdjdb --report -       # mark up + repair bare (CDR3aa, V, J) records
arda rnaseq map --r1 R1.fq.gz --r2 R2.fq.gz -o mapped.airr.tsv  # filter receptor reads from bulk RNA-seq
arda rnaseq assemble -i mapped.airr.tsv -o assembled.airr.tsv   # rescue CDR3s no single read spans
arda rnaseq correct -i mapped.airr.tsv -o clones.tsv            # collapse CDR3 errors into clonotypes
arda rnaseq run --r1 R1.fq.gz --r2 R2.fq.gz -p SAMPLE -d out/   # one-shot map+assemble+correct for pipelines
arda igblast -i reads.fastq -o truth.airr.tsv                   # gold-standard IgBLAST (all loci)
arda build-db   --organism all              # rebuild references (needs IgBLAST)
arda build-index --organism all             # (re)build the precompiled mmseqs DBs
arda slurm -i big.fastq -o big.airr.tsv --shards 50 --partition cpu   # cluster scale

examples/ is a runnable tour, every artifact derived from real data committed to this repo and regenerated by python examples/regenerate.py: one real mRNA per locus; the two human reads (of 7,341, across five organisms) that carry a tandem D-D; seven VDJdb records covering every junction-repair outcome, including one arda reports and refuses to rewrite and one it refuses outright; and a 1,035-read FASTQ that runs the whole bulk RNA-seq pipeline in ~6 s. Two tests re-run that script and fail if a committed artifact stops reproducing.

See CHANGELOG.md for what changed per release and benchmarks/RESULTS.md for measured speed and accuracy.

The reference database ships with precompiled MMseqs2 indexes (database/vdj/<organism>/mmseqs/), so annotation runs out of the box with no build step. They are used automatically when the local MMseqs2 version matches the shipped one; otherwise arda transparently rebuilds a private cache on first run (arda build-index regenerates the shipped DBs for your version).

Input may be FASTA or FASTQ, plain or gzipped. Nucleotide input is searched on both strands by default (reverse-complement reads are re-oriented and flagged rev_comp=T); a single search annotates a mixed bulk RNA-seq file across all loci.

Pipeline integration

arda rnaseq run is a one-shot map+assemble+correct for bulk RNA-seq: given paired (or single) gzipped FASTQ it writes <prefix>.clones.tsv (AIRR clonotypes), <prefix>.airr.tsv (mapped reads), <prefix>.assembled.airr.tsv (assembled long-CDR3 reads) and <prefix>.arda.json (run report). Because it is a plain CLI over named files, it drops into any workflow engine with no glue code.

A ready-to-use Nextflow module lives in integrations/nextflow/arda/: copy it to modules/local/arda/ in an nf-core/rnaseq (or similar) checkout, feed it the trimmed per-sample FASTQ channel the aligners already use, and it publishes per-sample clonotype tables to ${params.outdir}/arda/. It ships a conda environment.yml (works with -profile conda out of the box) and a Dockerfile, and emits a versions.yml. See its README and the pipeline-integration guide for the five-line drop-in.

arda is CPU-bound and low-memory: ~40k reads/s on 32 cores (~2.4 M reads/min, < 400 MB RAM), so a full-depth bulk RNA-seq sample of ~50 M read pairs finishes in ~45 min. Throughput scales with cores.

Library

import arda

records = arda.annotate_sequences(
    ["GACGTGCAG...", ("clone7", "CAGGTG...")],  # strings or (id, seq) pairs
    seqtype="nt", organism="human",
)
# -> list of AIRR record dicts: v_call, d_call/d2_call, j_call, c_call/c_class,
#    fwr1..fwr4, cdr1..cdr3, *_start/*_end (1-based closed), *_aa, junction(_aa),
#    np1/np2/np3, d_support/d2_support, {v,j,c,d}_cigar, sequence_alignment,
#    germline_alignment, productive, ...
# The TSV is a spec-valid AIRR Rearrangement file (passes airr.schema validation).

Records with no read behind them

A VDJdb row is a CDR3 amino acid, a V call and a J call. There is nothing to align, but the germlines still template a known run of residues into each end of the junction:

from arda.cdr3fix import markup_cdr3
from arda.annotate.dmap import map_d_junction
from arda.dpost import posterior_d

mk = markup_cdr3("CAIRDDKII", "TRAV12-3*01", "TRAJ30*01", "HomoSapiens")
mk.cdr3_repaired            # 'CAIRDDKIIF'  -- the Phe118 anchor restored
[str(e) for e in mk.errors] # ["J del@8 missing 'F' d=0"]
mk.good                     # True: both sides repaired, both anchors present

map_d_junction(junction_nt, "TRDV1*01", "TRDJ1*01", "human").d2_call   # 'TRDD3*01'
posterior_d("CASSPLGQAYEQYF", "TRBV5-1*01", "TRBJ2-7*01", "human").d_call  # 'TRBD1'

Coordinates here are junction space — Cys104 through Phe/Trp118, both anchors included. That is what VDJdb's cdr3 column holds, and it is not arda's cdr3 field, which excludes both. Conflating them silently corrupts every coordinate.

Repair is conservative: only edits adjacent to a conserved anchor are applied, everything deeper is reported and left alone, and a repair is refused outright unless the result opens with Cys104 and closes with Phe/Trp118.

Annotating bare germline segments

There is no coverage filter, so a V-only or J-only query maps to its scaffold and only the regions inside the query's coverage are returned. This lets you annotate isolated germline V or J alleles without synthesising a rearrangement — a bare V yields fwr1..fwr3, a bare J yields fwr4:

from arda.annotate.mapper import annotate_records

recs = annotate_records(
    [("TRBV9*01", v_germline_nt), ("TRBJ2-7*01", j_germline_nt)],
    organism="human", seqtype="nt", strand="forward", map_d=False,
)
# V record -> fwr1/cdr1/fwr2/cdr2/fwr3 (+ v_sequence_end = CDR3 start)
# J record -> fwr4 (+ j_sequence_start = CDR3 end / FR4 start)

(mirpy uses exactly this to bake per-allele FR/CDR subsequences into its gene library; see tests/synthetic/test_germline_segments.py.)

Bulk RNA-seq mode

arda rnaseq is a recall-first pipeline for extracting the receptor repertoire from bulk RNA-seq, where 1–5% of reads are receptor-derived:

• map streams paired FASTQ, keeps only reads that map to a receptor scaffold, and writes them as AIRR. The reference includes J + C constant-region scaffolds, so a read spanning the J→C splice — which ends in the constant region and has no V to anchor — still maps, and carries a c_call (the CH1 exon) plus a c_class isotype (IGHG/IGHM/IGHA … — the class, never the noise-prone subclass). In paired mode the isotype of a CDR3-bearing read is recovered from its constant-region mate. --reconstruct merges each overlapping mate pair into one fragment, resolving overlap mismatches by the higher-Phred base.
• assemble (Stage 3) reconstructs clonotypes whose CDR3 is too long for any single 100–150 bp read to span (V(DD)J ultralong, ~20–40 aa) by greedy overlap-extension anchored on Stage-1's per-read cdr3_start, and folds the recovered reads back into correct.
• correct aggregates reads into clonotypes and collapses sequencing-error CDR3 variants. Abundance is the AIRR duplicate_count — every read that encompasses the junction (spanning or partial), the true expression estimate — with consensus_count for distinct fragments. The error model is per-base with a length-scaled threshold (a mismatch over a longer junction is likelier an error) and is SHM-indel-tolerant, keeping only complete junctions.

arda rnaseq run --r1 R1.fq.gz --r2 R2.fq.gz -p SAMPLE -d out/   # one-shot map + assemble + correct

arda igblast -i reads.fastq -o truth.airr.tsv runs IgBLAST across all loci as a gold-standard reference for benchmarking (see the arda-benchmark project).

How it works

1. Reference build (arda.refbuild, offline): download IMGT/V-QUEST germlines → enumerate deduplicated in-frame V×J scaffolds (D only affects CDR3 interior, so it isn't enumerated) plus J + C constant-region scaffolds (the CH1 exon spliced onto each J, so J→C reads have somewhere to land) → annotate with igblastn -outfmt 19 → translate → write database/vdj/<organism>/{alleles.fasta, alleles.aa.fasta, markup.tsv, markup.aa.tsv, combinations.tsv, d_germlines.fasta, cdr3_anchors.tsv, d_prior.tsv, build.log}.

2. Runtime (arda.annotate): MMseqs2 search query→scaffolds → best hit → C++ transfer_regions projects scaffold region coordinates onto the query (handling indels, truncation, mid-codon alignment starts, reverse strand) → for VDJ loci a gapless C++ local alignment of the CDR3 interior against the D germlines adds d_call/d2_call + np*; a hit on a J + C scaffold adds c_call/c_class → AIRR TSV. Ambiguous D and C calls are comma-joined allele lists, as V/J already are. Out-of-frame junctions are reported with an N-bridge (_) so FR4 still reads.

   The V..J interior is bounded by the per-allele junction anchors in cdr3_anchors.tsv, not by the scaffold projection — a scaffold has a 9 nt N-pad where a read has a 20–40 nt N-D-N region, so the projection collapses the very window the D lives in. The D call is then accepted on a Karlin–Altschul E-value (d_support) rather than a per-locus score floor, and is constrained by germline geometry: TRBD2 lies 3′ of the entire TRBJ1 cluster, so a TRBJ1 rearrangement can never be assigned TRBD2. D mapping also runs on --seqtype aa, against each D germline's three translated frames.

3. Bare records (arda.cdr3fix, arda.dpost): a VDJdb-style row — CDR3 amino acid, V, J, species, and no read — is marked up against the same anchors (arda markup), its errors located and conservatively repaired, and optionally given a D gene inferred from the junction length (--d-posterior).

See memory/ for design rationale and gotchas. Fast sequence primitives (translate, detect_coding_frame, reverse_complement, back_translate) live in the C++ extension and are re-exported from arda.refbuild.translate — mirpy-API-compatible, so mirpy can import arda and reuse them.

Performance

Exact annotation that matches IgBLAST while being several times faster, scaling to large FASTQ. Synthetic human IGH, 16 threads (scripts/bench_vs_igblast.py):

sequences	arda	arda rate	speedup vs IgBLAST	region concordance
10,000	5.5s	~1.8k/s	4.4×	98.9%
50,000	16s	~3.0k/s	7.3×
100,000	30s	~3.3k/s	7.9×

On ~7.3k real GenBank mRNA records spanning all five organisms and their loci (committed, gzipped test fixtures), region concordance with IgBLAST on productive records is 98–99.7% per organism; junction_aa/cdr3_aa match IgBLAST ~99% and satisfy the AIRR invariants exactly. V-gene assignment agrees ~100%. (GenBank also contains genomic/partial/non-productive entries that confuse both tools; those are excluded from the comparison.)

Bulk RNA-seq is much faster than amplicon, because mmseqs prefilters by k-mer matching — reads with no receptor k-mer are rejected before alignment. At 150 nt reads, 16 threads (scripts/bench_prefilter.py):

receptor content	throughput
100% (amplicon)	~5.7k reads/s
10%	~19k reads/s
1% (blood RNA-seq)	~25k reads/s

Extrapolated to a 32-core node, a 30M-read bulk RNA-seq library (~1% receptor) annotates in roughly 10–20 min — the same order of magnitude as a STAR genome alignment pass on the same data (STAR is faster per read, but arda maps only to a tiny germline DB and the non-receptor majority costs just prefilter rejection). Large FASTQ is streamed in bounded chunks (a background reader prefetches the next chunk while the current one is annotated), so memory stays flat regardless of input size — --chunk-size tunes it.

Roadmap / TODO

See ROADMAP.md. Done: V·J reference build (5 organisms), MMseqs2 mapping, C++ markup transfer, reverse-complement, all-loci querying, streaming I/O, out-of-frame junctions, D-segment mapping incl. D-D fusions (IGH/TRB/TRD, on nucleotide and protein input, E-value gated, genomic-order constrained), constant-region J + C scaffolds (c_call/c_class isotype), bulk RNA-seq mode (rnaseq map/assemble/correct/run), long-CDR3 contig assembly, coverage-based expression (duplicate_count/consensus_count), **junction markup

• repair on bare records** (arda markup, arda.cdr3fix), D on a bare junction (arda.annotate.dmap) and the aa D posterior (arda.dpost), precompiled indexes, multi-node (SLURM) sharding. Next: full-depth clonotype benchmarking, and an arda.hmm semi-Markov model of V→N→D→N→J that would subsume the E-value gate, the genomic-order constraint and the D posterior into one forward–backward pass.

Development

pip install -e .                                      # rebuilds the C++ ext on import
python -m pytest tests/unit tests/synthetic -q        # fast suite (needs mmseqs)
python -m pytest tests/realworld -q                   # vs IgBLAST, on committed fixtures — offline
env RUN_BENCHMARK=1 python -m pytest tests/benchmark -s   # timing/memory/scaling

Optional extras gate optional suites: .[rnaseq] (seqtree) for arda rnaseq correct, .[groundtruth] (olga) for the generative ground-truth tests that keep arda.cdr3fix honest, .[test] for airr schema validation. Without them those tests skip, so pip install -e '.[test,rnaseq]' before reading a green suite as full coverage.

Layout: src/arda/{refbuild,annotate}, C++ in src/_markup/markup.cpp, references in database/, downloads in gitignored bin/ + data/. Design rationale and the gotchas that cost us the most live in memory/ — read memory/d-mapping.md and memory/junction-markup.md before touching either.