IDENTIFICATION OF A TONSIL IGD(-CELL SUBSET WITH PHENOTYPICAL AND FUNCTIONAL-CHARACTERISTICS OF GERMINAL CENTER B-CELLS() B)

We have identified and isolated a subpopulation of IgD(+) B cells (IgD (+) CD38(+) B cells) from human tonsils which expresses the germinal c enter (GC)-associated surface markers CD10, CD38, CD75, CD77 and CD95/ Fas. The heterogeneity of expression of several markers on IgD(+) CD38 (+) B cells suggests that this population can be further subdivided in to two discrete subtypes. On a functional basis, IgD(+) CD38(+) B cell s behave as GC B cells as they rapidly and spontaneously undergo apopt osis in vitro and cannot be stimulated to synthesize DNA upon cross-li nking of the antigen receptor. However, in contrast with most GC B cel ls, IgD(+) CD38(+) B cells have not completed Ig class switching since they predominantly secrete IgM following stimulation in vitro and lac k surface expression of secondary isotypes. Immunoenzymatic staining p erformed on tonsil tissue sections revealed that IgD(+) CD38(+) B cell s are located in two distinct histological structures: within the GC o f a few classical secondary follicles, in which they appear as scatter ed cells, and within rare atypical GC, homogeneously constituted of Ig D(+) B cells. Taken together, our findings indicate that IgD(+) CD38() B cells constitute a novel subset of GC B cells. The possibility tha t these cells could represent an early stage of the follicular reactio n or be generated in response to certain bacterial carbohydrate antige ns is discussed.