B cell and immunoglobulin heterogeneity in carp (Cyprinus carpio L.); an immuno(cyto)chemical study

B cell and immunoglobulin (Ig) heterogeneity was demonstrated in carp, Cyprinus carpio L., using two monoclonal antibodies (MAbs; WC14, WCI12) produced against carp serum Ig. Immunochemical results showed that both WCI4 and WCI12 react with a protein determinant on the heavy chain of Ig (relative molecular mass approximately 70,000). Immunofluorescence microscopic and flow cytometric analyses of lymphoid cells suggest three distinct subpopulations of B cells and plasma cells: WCI4+12- cells, WCI4-12+ cells, and WCI4+12+ cells. WCI4-12+ and WCI4+12+ anti-DNP antibody-secreting cells were also demonstrated with the ELISPOT assay in pronephros and spleen cell suspensions from primary immunised carp. Affinity chromatography of carp serum and sequential immunoprecipitation of 125I-labelled peripheral blood leucocyte (PBL) membrane proteins only indicated the presence of two antigenically different Ig molecules, i.e., WCI4-12+ and WCI4+12+ molecules. WCI4+12- molecules could not be detected by affinity chromatography or immunoprecipitation. During ontogeny, a shift in percentages of WCI4+12- and WCI4-12+ cells was found in the spleen and the pronephros. In these organs, WCI4+12- cells formed the majority of B cells at 2 weeks of age, but the percentages of this cell type decreased during ontogeny. On the other hand, the percentages of WCI4-12+ cells increased during development, and these cells became the major population of B cells from 13 weeks onward. The proportion of WCI4+12+ cells remained almost constant during ontogeny. The distribution of B cell subpopulations in blood was more or less stable at all ages. The functional significance of Ig heterogeneity in fish and in particular carp is discussed.