The immune system of Atlantic halibut is relatively undeveloped at the time of hatching, and thus larvae are vulnerable to bacterial and viral diseases that can result in high mortalities. To enable establishment of effective prophylactic measures, it is important to know when the adaptive immune system is developed. This depends on both B- and T-cell functions. In the present study the expression of RAG1, TCRα, TCRβ, CD3γδ, CD3ɛ, CD3ζ, CD4, CD4-2, CD8α, CD8β, Lck, and ZAP-70 was analyzed in larval and juvenile stages during halibut development. Using real time RT-PCR, low basal mRNA levels of all 12 genes could be detected at early stages. An increase in mRNA transcripts for the genes was seen at different time points, from 38 days post hatching (dph) about the time when the first anlage of thymus is found, and onwards. The transcription patterns of the 12 mRNAs were found to be similar throughout the developmental stages tested. In situ hybridization on larval cross-sections showed that RAG1 and Lck could be detected in lymphocyte like cells within the thymus at 42 dph. CD4 expression could not be detected within the thymus before 66 dph, however, positive cells were restricted to the cortical region. At 87 dph, the zonation of the thymus in a cortical, cortico-medullary, and a medullary region seemed to be more evident with CD8α expressing cells found in all regions, indicating the presence of mature T-cells. This correlates with previous results describing thymus development and the appearance of IgM(+) cells during halibut ontogenesis.
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