Cartilaginous and bony fish are the most primitive vertebrates with a thymus, and possess T cells equivalent to those in mammals. There are a number of studies in fish demonstrating that the thymus is the essential organ for development of T lymphocytes from early thymocyte progenitors to functionally competent T cells. A high number of T cells in the intestine and gills has been reported in several fish species. Involvement of CD4+ and CD8α+ T cells in allograft rejection and graft-versus-host reaction (GVHR) has been demonstrated using monoclonal antibodies. Conservation of CD4+ helper T cell functions among teleost fishes has been suggested in a number studies employing mixed leukocyte culture (MLC) and hapten/carrier effect. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ginbuna and rainbow trout. Furthermore, the important role of cell-mediated immunity rather than humoral immunity has been reported in the protection against intracellular bacterial infection. Recently, the direct antibacterial activity of CD8α+, CD4+ T-cells and sIgM+ cells in fish has been reported. In this review, we summarize the recent progress in T cell research focusing on the tissue distribution and function of fish T cells.
A novel Asfarvirus-like virus is proposed as the etiological agent responsible for mass mortality in abalone. The disease, called abalone amyotrophia, originally was recognized in the 1980s, but efforts to identify a causative agent were unsuccessful. We prepared a semi-purified fraction by nuclease treatment and ultracentrifugation of diseased abalone homogenate, and the existence of the etiological agent in the fraction was confirmed by a challenge test. Using next-generation sequencing and PCR-based epidemiological surveys, we obtained a partial sequence with similarity to a member of the family Asfarviridae. BLASTP analysis of the predicted proteins against a virus database resulted in 48 proteins encoded by the novel virus with top hits against proteins encoded by African swine fever virus (ASFV). Phylogenetic analyses of predicted proteins of the novel virus confirmed that ASFV represents the closest relative. Comparative genomic analysis revealed gene-order conservation between the novel virus and ASfV. In situ hybridization targeting the gene encoding the major capsid protein of the novel virus detected positive signals only in tissue from diseased abalone. The results of this study suggest that the putative causative agent should be considered a tentative new member of the family Asfarviridae, which we provisionally designate abalone asfa-like virus (AbALV). African swine fever virus (ASFV) is the causative agent of African swine fever (ASF). The virus causes a hemorrhagic fever with high mortality, with rates approaching 100% in domestic pigs 1. The virus infects domestic pigs and their relatives and ticks 2. ASF outbreaks had been recorded in Africa and Europe, but in recent years the disease has spread to China, Vietnam, Cambodia, Mongolia, Hong Kong, and Korea, becoming a threat to the swine industry worldwide 3. ASFV is a member of nucleocytoplasmic large DNA viruses (NCLDVs) with an average diameter of 200 nm. Although some related viruses, such as faustovirus 4 , kaumoebavirus 5 , and pacmanvirus 6 , have been reported, ASFV is the only member of the Asfarviridae family 7. In the present paper, we describe a virus likely to be the closest ASFV relative found to date; this novel virus was isolated as the presumptive causative agent of abalone amyotrophia. Mass mortalities of abalone have been reported since the early 1980s, during seed production in Japan. The disease was designated abalone amyotrophia because diseased abalone develop muscle atrophy in the mantle and foot 8. Diseased abalone show reduced ability to adhere to the substrate, and some diseased abalone exhibit incisions on the front margin of the shell and brown pigmentation inside of the shell 9. Histopathological evaluation has revealed the presence of abnormal cell masses that are produced extensively, primarily in the ganglion and peripheral nerve of the foot muscle 9. Cumulative mortality can reach 50% and higher 10. Abalone herpesvirus (AbHV) 11,12 and abalone shriveling syndrome-associated virus (AbSV) 13 also cause mortality ac...
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