The present study demonstrates that an agonistic anti-CD38 monoclonal antibody (mAb) (IB4) is capable of preventing apoptosis of human tonsillar germinal center (GC) B cells as measured by either morphological methods on Giemsa-stained cytospin preparations or flow cytometry on propidium iodide-stained cells. Two other anti-CD38 mAb (Leu-17 and OKT10) consistently failed to prevent apoptosis in the same cells, even when tested over a wide range of concentrations. Furthermore, exposure of GC B cells to IB4 mAb up-regulates the bcl-2 proto-oncogene product in a manner similar to that observed with CD40 ligand (CD40L). The ability of IB4 mAb to prevent apoptosis of GC B cells was inferior to that of both anti-CD40 mAb and CD40L. No synergistic or additive effects were observed when IB4 mAb was used together with CD40L. Unlike anti-CD40 mAb or CD40L, IB4 mAb neither induced a proliferation of GC B cells nor increased their proliferative response to anti-CD40, CD40L or recombinant interleukin-4, used alone or in combination. The present results are consistent with the recent findings on either the feature of the CD38 molecules to deliver activation signals and on the mechanisms of selection of B cells that operates in the GC.
This study describes the purification of a subset of tonsillar B cells which share phenotypic, morphologic and cytochemical features with subepithelial (SE) B cells. These cells, which represented the 5-10% of the total tonsillar B cells, were found in the Percoll gradient fraction of highest density, together with resting follicular mantle (FM) B cells. The latter B cells, however, expressed surface CD5 and could be removed by an immune rosetting procedure. The remaining small CD5- B cells had a surface phenotype (IgM+, IgD+, CD23-, CD38+/-, CD10-, CD44+) that was different from that of FM (IgM+, IgD+, CD23+, CD39+, CD38-, CD10-, CD44+2) and of germinal center (GC) (CD23-, CD39-, CD38+, CD10+, CD44+/-, IgG+) B cells isolated from the same cell suspensions. Furthermore, the absence of surface activation markers (CD71 and CD69) and of surface IgG allowed us to distinguish small CD5- B cells from activated and memory cells migrating within Percoll fractions of lower density. In situ immunohistochemical studies revealed that B cells with an identical phenotype as that of small CD5- B cells could be detected predominantly in the SE region (lamina propria) of the tonsil, and also within the epithelium lining the cryptae. This area was also comprised of a relatively minor proportion of activated B cells, not found in the small CD5- B cell fraction owing to the separation procedure used. Consistent with the notion that the SE area could be a site of B cell activation was also the presence of activated macrophages and of plasma cells. Thirty to forty percent of small CD5- B cells isolated in suspension were positive for the endogeneous alkaline phosphatase (ALP) activity. In contrast, only a few FM B cells were ALP+, while GC cells were consistently ALP-. In situ studies also demonstrated a prevalent expression of ALP activity by the B cells in the SE area. At the ultrastructural level, small CD5- B cells were clearly different from both FM and GC B cells. They displayed a cytoplasm more extended than that of FM B cells with abundant endosomes and plasma membrane projections, and a speckled pattern of nuclear heterochromatin distribution. When fixed tissue sections were examined, cells with identical ultrastructural features could be demonstrated in the tonsillar lamina propria. Collectively, the above data demonstrate an identity of features between the small CD5- B cells isolated in suspension and SE B cells analyzed in situ. Since tonsillar SE B cells are generally thought to represent the homolog of the extrafollicular B cells (including those of the splenic marginal zone), these studies may provide new opportunities for functional studies on this so far incompletely characterized B cell subset.
This study describes the features of the marginal zone (MZ) B cells of human tonsils and spleens and compares them with those of the follicular mantle (FM) B cells from the same tissues. The two B cell subpopulations displayed marked differences in phenotype, in response capacity to T cell-independent antigens and polyclonal B cell activators, and in presentation of antigens to T cells. FM B cells expressed surface CD5, and hence should be considered as B1 cells by current nomenclature. Fractionation of MZ B cells according to the presence or absence of surface IgD revealed the presence of two subsets. These subsets were characterized by different properties, including the presence of Ig V(H) gene mutations and the response capacity to TI-2 antigens, this latter property being associated with IgD-positive cells. Comparison of the data with those reported for mice revealed that human MZ B cells had strong analogies with both the murine MZ and B1 cells. In contrast, human B1 cells (that is, CD5-positive FM cells) were considerably different, an observation that should prompt further studies. Indeed, B cells with characteristics analogous to those of murine B1 cells were detected in small but definite proportions in the peripheral blood and tonsils. If the current distinction into B1 and B2 cells has to be maintained also for humans, it is likely that only these CD5-positive cells rather than the FM B cells should be called B1 cells.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.