Co-ligation of surface IgM and CD40 on naive B lymphocytes generates a blast population with an ambiguous extrafollicular/germinal centre cell phenotype.
We have previously shown that dual occupancy of sigM and CD40 - essential receptors in T-dependent B cell responses - by antibodies held on CD32-L cells results in the rapid proliferation of resting human B lymphocytes in a cytokine-independent manner. Here we report the detailed phenotype of the blast population emerging in such cultures. By 3 days the levels of CD19 and CD20 have increased 4- and 2-fold respectively: such high level expression of these two pan-B markers is characteristic of cells of germinal centre (GC) origin. B cells co-stimulated via sIgM and CD40 express low level CD23 and almost half become CD5(+); they also acquire CD38 and - importantly - CD77, both of these being selective markers of GC B cells. Expression of sigM and IgD is down-regulated on these cells and a minor, but significant, population of IgG+ cells appears. In marked contrast to GC B cells, the population proliferating in response to dual occupancy of sigM and CD40 has up-regulated and strongly expresses CD44. Morphologically, the cells are heterogeneous but there is a dominant blastic cell type with relatively scanty cytoplasm and having multiple nucleoli, both of which are characteristic of centroblasts; nevertheless, these cells remain morphologically distinct from freshly isolated GC B cells and do not show hallmark features of centrocytes. Although there is substantial cell death occurring by days 6-7 in these cultures, there is no morphological evidence for apoptosis. Thus, the proliferating population that emerges from the dual engagement of antigen receptor and CD40 on resting B cells appears to be bestowed with some features of GC B cells but has others which are incompatible with that particular stage of differentiation. The possibility that it might represent (i) a blast stage that is transitional between activation in T zones and entry into the follicle or (ii) a precursor population that colonizes the primary follicle prior to GC formation is discussed.
Wheeler
K
K
Department of Immunology, The Medical School, Vincent Drive, Birmingham B15 2TT, UK.
Gordon
J
J
D036541:ADP-ribosyl Cyclase; D051997:ADP-ribosyl Cyclase 1; D000911:Antibodies, Monoclonal / Q000378:metabolism; D015703:Antigens, CD; D018941:Antigens, CD19 / Q000096:biosynthesis; D018951:Antigens, CD20 / Q000096:biosynthesis; D000943:Antigens, Differentiation / Q000096:biosynthesis; D001402:B-Lymphocytes / Q000276:immunology* / Q000378:metabolism*; D019013:CD40 Antigens / Q000378:metabolism*; D002478:Cells, Cultured; D005434:Flow Cytometry; D018858:Germinal Center / Q000276:immunology*; D006801:Humans; D007075:Immunoglobulin M / Q000378:metabolism*; D016130:Immunophenotyping; D015847:Interleukin-4 / Q000378:metabolism; D008213:Lymphocyte Activation; D008562:Membrane Glycoproteins; D008565:Membrane Proteins / Q000378:metabolism*; D009699:N-Glycosyl Hydrolases / Q000096:biosynthesis; D011485:Protein Binding / Q000276:immunology; D015398:Signal Transduction / Q000276:immunology; D014281:Trihexosylceramides / Q000096:biosynthesis
D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D018941:Antigens, CD19; D018951:Antigens, CD20; D000943:Antigens, Differentiation; D019013:CD40 Antigens; D007075:Immunoglobulin M; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D014281:Trihexosylceramides; D015847:Interleukin-4; C018549:globotriaosylceramide; D009699:N-Glycosyl Hydrolases; D036541:ADP-ribosyl Cyclase; C498154:CD38 protein, human; D051997:ADP-ribosyl Cyclase 1