In vivo allograft rejection in a bony fish Dicentrarchus labrax (L.): characterisation of effector lymphocytes.

Lymphoid cell subpopulations involved in allograft rejection in the teleost Dicentrarchus labrax were characterised at the ultrastructural level and quantified by using monoclonal antibodies against T- and B-lymphocytes. T-cells positive for T-cell receptor β-chain (TcRβ) were detected by reverse transcription/polymerase chain reaction (RT-PCR) and in situ hybridisation by using RNA probes for TcRβ. Flow cytometry detected a similar percentage of T- and B-lymphocytes (around 17%) in the leucocyte-enriched fraction from allografts. Two different types of T-lymphocytes (DLT15-immunoreactive) infiltrating the allografts were identified by cytomorphology: small cells with high nuclear/cytoplasmic ratio and cells with a higher cytoplasmic content. RT-PCR revealed a single band (513 bp) corresponding to the TcRβ. In situ hybridisation showed that TcRβ-positive cells in the grafted muscle fibres were less numerous compared with DLT15-positive cells, as evidenced in parallel sections, suggesting that cytotoxic cells might express different TcR phenotypes. DLIg3- immunoreactive Ig-producing lymphocytes had: 1) a high nuclear/cytoplasmic ratio or 2) a larger size similar to that of pre-plasma cells (plasma cells lacked any membrane labelling)