Differentiation and proliferation of B lymphocytes in a culture of carp hematopoietic cells

Cultivation of cells from carp (Cyprinus carpio) kidney tissues including hematopoietic tissues resulted in the formation of a Feeder-like cell layer. The carp hematopoietic cells were found to grow in contact with the cell layer. Numerous non-attaching cells were observed in the supernatant of the culture medium. These non-attaching cells were constantly produced in the culture medium, half of which was renewed at 1-4 days intervals for about 1 month. To characterize the proliferating cells, we observed the non-attaching cells with a scanning electron microscope. At a higher magnification, the cells had many short microvilli on their cell surface which is a common feature of carp lymphocytes. Immunofluorescent staining by monoclonal antibodies against carp IgM demonstrated that 1.6-5.6% of the cells were surface immunoglobulin (sIg) positive and 0.1-0.5% of the cells were cytoplasmic immunoglobulin (cIg) positive. Moreover, the percentage of sIg- or cIg-positive cells was maintained during the culture, even after the total number of non-attaching cells rapidly increased (more than 20 population doublings) between 10-30 days. These results suggest that immature B-lymphocytes (sIg-negative cells) differentiated into a more matured type of B lymphocytes (sIg- or cIg- positive cells) during the culture.