Immunoglobulin-bearing blood leucocytes in the Pacific hagfish

Functional aspects of fish lymphocytes

After almost 40 years of studies in comparative immunology, some light has been shed on the evolutive immunobiology of vertebrates, and experimental evidences have shown that acquired immunity, defined by somatic recombination of antigen-binding molecules and memory, is an achievement as ancient as jawless vertebrates. However, the molecular processes generating antigen receptors evolved independently between jawless and jawed fishes, and produced lymphocytic cells with similar functions but employing different sets of genes. In recent years, data have been provided describing some in vitro and in vivo functional responses of fish lymphocytes. After a long gap, the number of specific markers for fish lymphocytes is increasing, thus allowing a first characterisation of lymphocyte subsets. Overall, in the near future it will be possible to open a new chapter in fish immunology and investigate functional immunity of lymphocyte responses by combining the extensive knowledge on immune gene products with markers for molecules and cells. The present review summarizes current knowledge on functional features of fish lymphocytes.

Variable lymphocyte receptors in hagfish

A previously uncharacterized type of variable lymphocyte receptors (VLR) was identified recently in the Sea lamprey. This jawless vertebrate generates an extensive VLR repertoire through differential insertion of neighboring diverse leucine-rich repeat (LRR) cassettes into an incomplete germ-line VLR gene. We report here VLR homologs from two additional lamprey species and the presence of two types of VLR genes in hagfish, the only other order of contemporary jawless vertebrates. As in the Sea lamprey, the incomplete hagfish germ-line VLR-A and -B genes are modified in lymphocyte-like cells to generate highly diverse repertoires of VLR-A and -B proteins via a presently undetermined mechanism. This jawless-fish mode of VLR diversification starkly contrasts with the rearrangement of Ig V(D)J gene segments used by all jawed vertebrates to produce diverse repertoires of T and B lymphocyte antigen receptors. The development of two very different strategies for receptor diversification at the dawn of vertebrate evolution approximately 500 million years ago attests to the fitness value of a lymphocyte-based system of anticipatory immunity.

Chemotactic responses of hagfish (Vertebrata, Agnatha) leucocytes

The chemotactic responses of hagfish leucocytes were tested using a variety of chemoattractants. Leucocyte migration was significantly enhanced by purified mammalian complement anaphylotoxin (C5a) and LPS-activated hagfish plasma. Checkerboard analyses confirmed that the responses of leucocytes to both of these chemoattractants were directed along concentration gradients (chemotaxis) and did not result from accelerated random movement (chemokinesis). Chemotaxis was undertaken by leucocyte fractions that were enriched in granulocytes, the predominant phagocytic cells of hagfish. The data suggest that chemotactic mechanisms may have been conserved during evolution to such a degree that mammalian chemoattractants can bind and activate chemotactic receptors on hagfish leucocytes. Moreover, hagfish appear to express plasma proteins that are structurally and functionally homologous to mammalian complement anaphylotoxins.

Hagfish humoral defense protein exhibits structural and functional homology with mammalian complement components

A genomic clone and cDNA fragment encoding a portion of a humoral recognition molecule from the hagfish were isolated and sequenced. The serum protein has previously been described as having structural features that are immunoglobulin-like. Amino acid sequence obtained from the 77-kDa H1 heavy chain facilitated the isolation of a genomic clone containing at least two coding regions. Through use of primers derived from the genomic sequences, a 231-base-pair cDNA fragment was obtained by PCR from liver RNA. Comparison of the deduced 120-amino acid sequence from the N terminus of H1 with known protein sequences revealed substantial sequence similarity with the beta chain of the murine fourth complement component C4 and with the related third and fifth complement molecules C5 and C3 and the major histocompatibility complex-encoded sex-limited protein. Observation of structural and functional similarities associated with the sequence similarity indicate that these molecules share an evolutionary relationship: the polypeptide chain structure of hagfish complement-like protein (CLP) resembles that of C4; CLP contains a hidden thioester group on the 70-kDa chain; CLP binds to streptococcal cells and enhances the phagocytosis of yeast by hagfish leukocytes. These data suggest that CLP forms part of a non-clonally-derived complement-related humoral defense system in the hagfish.

Cellular requirements for mixed leucocyte reactivity in the cyclostome, Eptatretus stoutii

Unfractionated peripheral blood leucocytes from allogeneic hagfish proliferated vigorously in one-way mixed leucocyte reaction (MLR) at 20-22 degrees. Maximum responsiveness was observed after 5 days of co-culture of responder and irradiated stimulator cells. Separation of leucocytes on the basis of laser scatter parameters yielded a small leucocyte population which responded but did not stimulate in MLR and a large leucocyte population capable of stimulating but not responding. Adherent cell depletion of the total leucocyte population did not affect the ability to respond in MLR but did ablate the stimulatory capacity of hagfish leucocytes. Together with previous phenotypic studies, these results confirm the presence of lymphocyte and macrophage/accessory cell populations in hagfish peripheral leucocytes.