Evolution of Alternative Adaptive Immune Systems in Vertebrates

Area of Science:

• Immunology
• Evolutionary Biology
• Comparative Genomics

Background:

• Jawless fishes possess a unique adaptive immune system utilizing variable lymphocyte receptors (VLRs).
• VLRs are structurally distinct from the immunoglobulin-based receptors found in jawed vertebrates.
• Two T-like lymphocyte lineages and one B-like lymphocyte lineage express different VLRs.

Purpose of the Study:

• To elucidate the mechanisms of VLR gene assembly and expression in jawless fishes.
• To investigate the developmental origins and tissue-specific differentiation of lymphocyte lineages.
• To compare the evolutionary trajectory of adaptive immunity in jawless versus jawed vertebrates.

Main Methods:

• Analysis of VLR gene organization and germline sequences.
• Investigation of gene conversion mechanisms involving cytidine deaminases.
• Characterization of lymphocyte development in specific hematopoietic tissues (thymoid, typhlosole, kidneys).

Main Results:

• VLRs are assembled from incomplete germline genes via a gene conversion-like process using flanking sequences as templates.
• T-like cells express VLRAs or VLRCs, while B-like cells secrete VLRB antibodies recognizing diverse antigens.
• Lymphocyte development exhibits lineage and tissue specificity, with T-like cells in the thymoid and B-like cells in the typhlosole and kidneys.

Conclusions:

• The dichotomy of T-like and B-like cells and dedicated lymphopoietic tissues are ancestral vertebrate features.
• Somatic diversification of antigen receptor genes evolved independently in jawless and jawed vertebrates.
• Jawless fish VLR system represents an independent evolutionary path for adaptive immunity.