Abundantly expressed transcripts in the lymphoid organ of the black tiger shrimp, Penaeus monodon, and their implication in immune function.

The lymphoid organ of penaeid shrimps is proposed to play an important role in the innate immune system. To investigate the potential immune function of the lymphoid organ, we analyzed the expressed genes from the lymphoid organ of normal and Vibrio harveyi-infected Penaeus monodon using an expressed sequence tag (EST) approach. Sequence analysis of the EST clones derived from the two lymphoid organ cDNA libraries (408 clones from the normal and 625 clones from the infected libraries), revealed a high redundancy of specific transcripts. Transcripts of the lysosomal cysteine proteinases, cathepsins B and L, were abundantly expressed in the lymphoid organ of both libraries, whilst the transcripts of the related genes peritrophin and thrombospondin predominated and were found only in the V. harveyi-infected library, making them interesting candidate functional genes. Moreover, immune-related genes were found at a significant proportion (approximately 15%) in both normal and infected libraries, but different expressed genes were observed between the two libraries. The expression levels of P. monodon cathepsins B and L in the lymphoid organ following injection with either V. harveyi or white spot syndrome virus (WSSV) showed only a slight change in the transcript abundance compared to that seen in the mock-infection (control). Immunohistochemistry confirmed that cathepsin L protein was localized in the lymphoid organ with intense cathepsin L staining observed in the lymphoid organ spheroids of WSSV-infected shrimps. The results suggest that cathepsins L and B likely play a major role in the lymphoid organ function and are probably implicated in degradation of foreign material that is sequestrated in the lymphoid organ spheroids, although any additional role in control of viral or cellular mediated apoptosis remains to be evaluated.