Adaptive immunity in jawless fish
Jawless vertebrates, which today consist entirely of lampreys and hagfish, have an adaptive immune system similar to that found in jawed vertebrates. The cells of the agnathan AIS have roles roughly equivalent to those of B-cells and T-cells, with three lymphocyte lineages identified so far:
- VLRA (most similar to α/β T cells, in its role and pathway of differentiation)[1]
- VLRB (most similar to B cells)[1]
- VLRC (most similar to γ/δ T cells)[2]
VLRA and VLRB were identified in 2009,[1] while VLRC was discovered in 2013.[3] Instead of immunoglobulins, they use variable lymphocyte receptors.
Antigen receptors
Jawless vertebrates do not have immunoglobulins (Igs), the key proteins to B-cells and T-cells. However, they do possess a system of leucine-rich repeat (LRR) proteins that make up variable lymphocyte receptors (VLRs). This system can produce roughly the same number of potential receptors that the Ig-based system found in jawed vertebrates can.[4] Instead of recombination-activating genes (RAGs), genes coding for VLRs can be altered by a family of cytidine deaminases known as APOBEC, possibly through gene conversion.[5] Cytidine deaminase 1 is associated with the assembly of VLRA and VLRC and cytidine deaminase 2 appears to assemble VLRB.[1][3]
Evolution
The gene expression profiles of lymphocyte-like cells (LLCs) in jawless vertebrates indicate that VLRB+ LLCs and B cells share a common ancestor, and VLRA+ and VLRC+ LLCs and T cells share a common ancestor.
References
Further reading
- Flajnik, Martin F. (19 March 2018). "A cold-blooded view of adaptive immunity". Nature Reviews Immunology. 18 (7): 438–453. PMID 29556016.