Insights into B cell heterogeneity in two Antarctic teleost species

Interest in fish immunity has risen recently due to the discovery of some peculiar features. In particular, many studies have focused on teleost immunoglobulins (Ig) and their adaptation to the environmental conditions. Ongoing research is studying the B lineage of two species living at sub- zero temperatures in Antarctic seas, the red-blooded Trematomus bernacchii (TB) and the hemoglobinless icefish Chionodraco hamatus (CH). Transcripts for the secreted and membrane form (1089 and 864 bp, respectively) of TB Igμ were amplified by RT-PCR using specific primers. B cells were identified by immunohistochemistry (IHC) using mutually no cross-reactive polyclonal antisera against TB purified serum IgM heavy (μ, 76 kDa) and light (L, 25 kDa) chains, and a polyclonal antiserum against CH purified serum IgM. Proteins extracted from TB tissues and separated by reducing SDS- PAGE were immunoblotted using the antiserum to homologous Igμ. Both secreted and membrane Igμ transcripts as well as 76- and 66-kDa proteins were detected in TB head-kidney (HK), thymus, spleen, blood cells, gills and intestine, at apparently varied expression levels. Quantitative IHC confirmed that Ig+ cells were more numerous in lymphoid than mucosal organs, in all instances with a prevailing perivascular distribution, but also outlined some differences in size and relative density among cells expressing μ and L chains in the different tissues. Quantitative IHC on available CH tissues also reported more Ig+ cells in the HK and spleen than in the intestine, predominantly perivascular. Furthermore, cross-reacting sera anti-TB μ and L chains suggested some degree of B cell heterogeneity. The occurrence along the intestine of TB and CH of reverse gradients (decreasing or increasing towards the anus, respectively) in the number of Ig+ cells needs to be carefully interpreted in light of immune regulation and control of a huge parasitic load, qualitatively and quantitatively different at the gut levels. Indeed, evidence is accumulating in both species that liver and exocrine pancreas would provide to Ig transport toward the intestinal lumen as well as to local defence against parasite invasion (peritoneal side) and/or ascending infections. Reported findings raise important questions about existence and function of other Ig isotypes (and subtypes) in Antarctic fish, that need to be addressed on a wider set of tissues using proper analytical tools.