Characterization of the lambda light chain repertoire and non-coding regions of equine immunoglobulins using the EquCab3 genome.

Overview

• This study investigates the lambda light chain immunoglobulin (Igλ) repertoire in horses and characterizes related non-coding regulatory regions using the updated equine genome assembly EquCab3.
• The research advances knowledge of horse antibody genetics by annotating Igλ gene segments, analyzing promoter regions, and identifying key motifs conserved between horses and humans.

Background and Importance

• Horse immunoglobulins have been used for over a century in serotherapy, such as treatments for venomous bites and other conditions.
• More than 90% of circulating equine antibodies consist of lambda light chains (Igλ), but detailed molecular information about these chains is limited.
• Despite full genome sequencing of horses, the prominent immunogenetics database (IMGT) has not yet fully annotated the horse Igλ repertoire, limiting deeper molecular analyses.

Goals of the Study

• To characterize the equine Igλ repertoire at the molecular level using the newest genome assembly (EquCab3).
• To identify and analyze the non-coding regions surrounding these immunoglobulin genes, especially promoter regions and co-stimulatory elements.
• To improve annotation of Igλ gene segments (V and J) and discover potential new alleles.

Methods

• Used the EquCab3 genome assembly to map variable (V) and joining (J) gene segments from a previous assembly (EquCab2) to new genomic coordinates.
• Compared conserved regulatory motifs from human immunoglobulin promoter regions to equine sequences to infer promoter characteristics.
• Analyzed a dataset of immunoglobulin clones, doubling the number used in prior studies, to get a more accurate representation of equine Igλ gene usage and diversity.

Key Findings – Igλ Gene Repertoire

• Created a refined library of Igλ gene segment alleles, including newly recognized ones such as the J4S1 gene segment.
• Discovered that J4S1 has the same sequence as the J5 and J6S1 gene segments and constitutes over 50% of annotated J genes, indicating a dominant role in the repertoire.
• Observed a restricted usage of V gene segments:
• Approximately 70% of the antibody repertoire comes from just four V gene segments, all belonging to subgroup 8.
• This suggests selective preference or evolutionary expansion of these segments.

Key Findings – Non-Coding and Regulatory Elements

• Identified conserved promoter regions, including the well-known octamer motif, which showed strong sequence similarity between human and horse promoters.
• Promoters contain a TATA-Box, a common feature that helps initiate transcription.
• Found other co-stimulatory elements near promoter regions, such as pyrimidine-rich sequences and E-box motifs, which may enhance or regulate Igλ gene expression.

Complementarity Determining Region (CDR-L3) Characteristics

• Analyzed the length and amino acid composition of the CDR-L3 region, a critical part of the antibody that binds antigen.
• Most CDR-L3 regions were 10-11 amino acids long, indicating length conservation.
• Serine (Ser) was the most common amino acid found in this region, suggesting a potential role in antigen interaction or structural stability.

Significance and Implications

• This study improves annotation and understanding of the horse Igλ repertoire, filling a gap in equine immunogenetics resources.
• The identification of conserved promoter motifs and co-stimulatory elements provides insight into how Igλ expression is regulated in horses, potentially reflecting evolutionary conservation with humans.
• The restricted gene usage and CDR-L3 characteristics suggest specific mechanisms shaping antibody diversity in horses.
• These findings lay the groundwork for future research and better use of horse antibodies in therapeutic and diagnostic applications.