Re-evaluation of the immunological Big Bang

Area of Science:

• Immunology
• Evolutionary Biology
• Genomics

Background:

• The traditional view of adaptive immunity's origin ('Big Bang') involved immunoglobulin gene rearrangement by RAG recombinase in jawed vertebrate ancestors.
• The discovery of variable lymphocyte receptors (VLRs) in jawless fish revealed a second, convergent adaptive immune system, challenging previous models.
• Recent findings indicate a third lymphocyte lineage in lampreys, defined by VLR C (VLRC), further complicating the understanding of early adaptive immunity.

Purpose of the Study:

• To investigate the evolutionary origins of adaptive immunity.
• To understand the diversification of lymphocyte lineages and antigen receptor systems.
• To explore the early development and transcriptional control of immune systems in vertebrates.

Main Methods:

• Comparative genomics of jawless and jawed vertebrates.
• Analysis of lymphocyte lineages and antigen receptor genes (immunoglobulins, VLRs, VLRC).
• Inferred transcriptional control mechanisms based on genomic data and existing knowledge.

Main Results:

• Evidence suggests three distinct lymphocyte lineages existed in the common ancestor of jawless and jawed vertebrates.
• These lineages utilized different antigen receptor supertypes, indicating an early 'division of labor'.
• Most effector cytokines and chemokines were present in cartilaginous fish, suggesting their ancient origins.

Conclusions:

• The emergence of adaptive immunity was more complex than previously thought, involving multiple lymphocyte lineages and receptor systems.
• An early functional specialization among lymphocytes likely drove the evolution of adaptive immunity.
• Further genomic studies of lampreys and hagfish are crucial to fully understand the 'Big Bang' of adaptive immunity.