The Complexity and Diversity of Gallus gallus Antibody Repertoire

The immune system of birds, particularly chickens (Gallus gallus), has been found to possess a unique antibody repertoire that exhibits distinct characteristics when compared to other species such as humans and mice. A recent study isolated a chicken Fab with four noncanonical cysteines in the V H region, which demonstrated a binding affinity of 64 nM for its target antigen. This discovery confirmed that these constituents are part of the humoral response, rather than artifacts.

Further analysis revealed that all features of the naive repertoire were fully replicated in the target-selected, phage-displayed repertoire. The study also observed probable formation of disulfide bonds between CDR3 and CDR1, FW2, or CDR2, similar to those found in camelids. This suggests that noncanonical disulfides are strongly favored in chickens, potentially increasing CDR stability and complexity in the topology of the combining site.

The study also examined the correlation between CDR3 length and cysteine content. A strong positive correlation (R² = 0.97) was observed, indicating that longer CDR3 sequences tend to have higher cysteine content. In comparison with humans and mice, the chicken CDR3 repertoire was found to be skewed towards longer sequences, dominated by small amino acids such as G/S/A/C/T, and had a higher cysteine content (9.4%) but lower tyrosine content (9.2%).

The study also analyzed the frequency of mutations in the V H regions and found that CDRs 1 and 2 exhibited frequent mutation in solvent-exposed positions, while common structural residues were conserved at the same positions as in human CDRs. Many framework (FW) mutations could be clearly related to the modulation of CDR structure or the V H–V L interface.

Examination of 1269 unique naive chicken V H sequences revealed that the majority of positions in the FW regions were maintained as germline, with high mutation rates observed in the CDRs. This study provides new insights into the complexity and diversity of the Gallus gallus antibody repertoire and highlights its unique features compared to other species.