Histological and molecular studies on IgM expressing cells of the B-lineage of two Antarctic teleost species

Life at sub-zero temperatures, such in Antarctic seas, has selected especial adaptations of cell molecules, among them the immunoglobulins (Ig). Differently from species living in temperate water, many Antarctic teleost bony fish showed peculiar protein structure and molecular dynamics of IgM, acquired during radiation of the Suborder Notothenoidei (~24 Mya). To gather further information about IgM producing cells, we sampled two common Antarctic species, the red-blooded Trematomus bernacchii (Tb) and the haemoglobinless icefish Chionodraco hamatus (Ch). Our studies were focused on two main lymphoid organs: the head-kidney (HK), thought a primary lymphoid organ for cells of the B-lineage, and the spleen (SPL), a secondary lymphoid tissue involved in effector B cells differentiation. The occurrence of secreted and membrane Igμ transcripts, IgM proteins and numerous cells containing IgM has been demonstrated in both tissues of Tb and Ch. The combined use of rabbit polyclonal antisera against homologous/heterologous IgM (in some instances specific for heavy or light chains) and B-specific transcription factors (with DNA binding domains highly conserved throughout evolution) proved that, at least in Ch, the HK houses both B-cell progenitors, B lymphocytes (especially abundant in the peripheral blood) and plasma cells, mainly accumulated around the arteries. These findings strongly suggest that the HK also behaves as secondary lymphoid tissue, in keeping with previous data from rainbow trout and other few teleost species. The peculiar characteristics of the IgM molecule of many Antarctic teleosts (e.g., three constant Igμ domains in Tb, only two constant Igμ domains in Ch, regarding the membrane antigen receptor) prompted us to start investigating in detail the organization of the B Cell Receptor (BCR) complex. Preliminary data will be presented about the two accessory molecules CD79A (Ig-α) and CD79B (Ig-β).