Adaptive immunity in jawless fish

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.^[6] Instead of using recombination-activating genes (RAGs) to randomly incorporate segments like bony fish Igs, genes coding for VLRs are assembled randomly via a process resembling gene conversion, mediated by a family of cytidine deaminases known as APOBEC.^[7][8][a] Cytidine deaminase 1 (CDA1) is associated with the assembly of VLRA and VLRC and cytidine deaminase 2 (CDA2) appears to assemble VLRB.^[1][3]

The product of VLR gene conversion is a functional gene expressing a VLR protein. The VLR typically consists of a signal peptide (SP), an N-terminal capping domain (LRRNT), a largely invariant 24-residue first LRR segment (LRR1), up to 9 24-residue variable LRRs (LRRV), a final truncated LRR called the connecting peptide (CP), a C-terminal capping domain including a stalk (LRRCT).^[10] This gene is assembled starting from a "skeleton" gene in the germline genome, which contains the promoters, the SP+LRRNT, a large gap, followed by LRRCT. Each VLR type uses its own skeleton gene. (The skeletons vary in their domain composition: VLRB and VLRD skeletons only have part of LRRNT; VLRE skeleton includes part of LRR1; VLRB skeleton has a split LRRCT; VLRF only has the C-terminal part of LRRCT.) Serial gene conversion replaces the gap with LRR1/LRRV/CP segments from donor cassettes located elsewhere in the genome. Some cassettes are shared among multiple VLR types, others are specific for one type only.^[11][4]

It is unclear whether there is a system in place to weed out lymphocytes expressing self-reactive VLRs in the cyclostomian AIS. Selection for length (number of LRRVs) and diversity of the N-terminal domain has been identified, however.^[12]

The gene expression profiles of lymphocytes (also "lymphocyte-like cells", LLCs) in jawless vertebrates indicate that VLRB⁺ LLCs and B cells, VLRA⁺ LLCs and α/β T cells, and VLRC⁺ LLCs and γ/δ T cells each share a common ancestor.^[2] Like B cells and T cells, the development of VLRB⁺ LLCs is spatially separated from the development of VLRA⁺ and VLRC⁺ LLCs. VLRB⁺ LLCs and B cells develop in hematopoietic tissues: VLRB⁺ LLCs develop in the typhlosole and kidneys and B cells develop in bone marrow. VLRA⁺ and VLRC⁺ LLCs develop in a thymus-like organ called the thymoid, similar to T cells developing in the thymus.^[13] The expression of VLRA and VLRC are mutually exclusive, much like the α/β and γ/δ TCRs.^[14]

There is also a population of triple-negative (VLR{A,B,C}^-) lymphocytes that express none of these types. The additional lymprey types (VLRD/E/F) show the highest expression in these cells, but VLRD/E/F also show a little bit of expression in VLRA⁺ and VLRC⁺ cells. These additional types are therefore also considered "T-like".^[11][4]

VLRB molecules and LLBs can directly bind to antigens and VLRB-transfected cells secrete VLRB protein products, similar to B cells in jawed vertebrates. VLRA⁺ LLCs were unable to bind Bacillus anthracis, Escherichia coli, Salmonella typhimurium, or Streptococcus pneumoniae before or after immunization, suggesting that VLRAs require antigen processing like TCRs.^[1] However, MHCs or MHC-like molecules that could present processed antigens have not been found in lampreys,^[15] and some VLRAs expressed in yeast were able to directly bind to antigens.^[16] The antigen binding of VLRCs has not been studied.^[17] However, the VLRC gene is close in proximity and sequence to the VLRA gene and the two are often co-expressed in LLCs, suggesting that both are TCR-like receptors.^[3]

VLR-like proteins have been identified in the amphixous Branchiostoma floridae. One identified protein specifically recognizes Gram-positive bacteria and is likely highly expressed in the gill, a potential immune organ. There is no evidence of it being somatically variable.^[18]

A VLR-like gene in the Chinese mitten crab Eriocheir sinensis has ten different isoforms from alternative splicing, with different antigen specificities. This offers a limited degree of antigen receptor diversity. Many organisms without an adaptive immune system have expansions in their pattern recognition receptors to provide diversity and this is one example.^[19]