Our data reveal an unproven in vivo role for murine B cells in the regulation of serum IgE levels in vivo. B cells act as a sink to eliminate excess serum IgE by binding to IgE in a CD23-dependent manner. Though B cells eliminated excess serum IgE, the overall rate of serum IgE catabolism remained largely unchanged.
The lack of contribution by FcεRI+
cells to the clearance of IgE has previously been reported (26
). These observations are particularly striking given the affinity of the FcεRI for IgE (sub-nanomolar range) (9
). One possible explanation is that only a limited number of unoccupied receptors exist on the surface of basophils and mast cells. Although quantifying the numbers of unoccupied receptors is difficult (39
), it was clear that both basophils and mast cells had sufficient unoccupied receptors to bind exogenous IgE (). Indeed, the regulation of surface FcεRI is thought to occur mainly through stabilization of unoccupied receptors after IgE binding with a dynamic balance between de novo
IgE production and endocytosis of unoccupied receptors (40
). The number of target cells available may also have been a factor. Basophils are the least represented hematopoietic cell in the peripheral blood, found at levels approximately 10% of the numbers of B cells in normal mice and humans. Further, B cells are heavily represented in secondary lymphoid organs such as the spleen and lymph nodes, while basophils are more rarely observed in these tissues (41
). Basophils undergo rapid turnover and could thereby regulate serum IgE levels (43
); however, these effects would need to be in an FcεRI-independent manner. By contrast, mast cells are widely distributed in peripheral tissues, including the skin, intestines, and respiratory tract (44
). Traditionally, IgE loading of tissue mast cells has been thought to occur through a passive diffusion model. If so, we would have expected mast cells to play some role in IgE clearance. However, mast cell-deficient mice showed overlapping peak IgE levels and rates of clearance as compared to wild-type mice (data not shown
). Therefore, while similar logic regarding a limited number of unoccupied receptors is certainly possible, we speculate that tissue mast cells may have limited access to serum IgE by virtue of the endothelial barrier. If such limitations in access existed, mast cells might then be expected to contribute little to serum IgE clearance, unless local changes in delivery or vasopermeability occurred. These possibilities require further study.
The absence of B cells (or blockade of CD23) led to an approximately 2-fold increase in the peak serum IgE concentration ( and ). Despite this increase in peak serum IgE concentration, we observed no clear change in the overall rate of IgE clearance from the serum (). This lack of change in IgE clearance is in agreement with published findings in CD23−/−
). However, in these CD23-deficient mice, no difference in peak IgE was observed after intravenous infusion. This likely reflects the 30–40 fold larger amount of infused IgE (~75–100 μg). The peak IgE level after such infusions is in the 30–40 μg range, which may exceed physiologic limits that might have been detected using the amounts we used (13
). Additionally, the use of the epitope-tagged IgE molecule allowed us to isolate the kinetics of an individual group of IgE molecules independent of the native IgE pool.
The similar rate of IgE clearance in B cell-deficient mice suggests that IgE homeostasis is regulated on several levels. While B cells limit the overall size of the IgE pool by rapidly binding free IgE molecules, other mechanisms exist to determine the catabolism and clearance of IgE. IgE-binding factors have long been postulated to contribute to the clearance of IgE (8
), but a detailed understanding is lacking. The regulation of IgE levels by CD23 has focused primarily on alterations in IgE production. In particular, the first published CD23-deficient mouse had a moderate 2-fold increase in total serum IgE, along with an up to 10-fold increase in antigen-specific IgE production following immunization (37
). Subsequent mouse strains had a mixture of results including normal baseline serum IgE levels, no alteration in antigen-specific IgE production, and defects in antigen focusing following hapten treatment in mice pre-sensitized with antigen-specific IgE (45
). These data supplemented previous work suggesting defective IgE production in anti-CD23 treated rodents (47
). Many of these differences likely relate to the inter-strain variation, genetic backgrounds, and different experimental model systems, as well as to the inherent complexity of CD23 biology, including the binding characteristics of the IgE-CD23 interaction and subsequent effects on B cell survival (8
). In humans, the biology is further complicated by interactions between CD23 and CD21, which enhance IgE production (21
Anti-CD23 has been considered as a biotherapeutic in human allergic disease. The impact of anti-CD23 treatment on allergic disease remains unclear, but early data pointed to a rapid drop in IgE levels following anti-CD23 treatment (50
). Though these data would appear to counter ours, the antibody targets as well as differences in human and mouse CD23 biology, including an approximately 10-fold lower affinity of human CD23 for IgE, may account for these outcomes (13
CD23 exists in both a membrane-bound and soluble form. Our data indicate that B cells can directly bind to serum IgE, although the low-affinity of the interaction required us to perform fixation to detect this binding. While our data do not exclude a role for soluble CD23 in the regulation of IgE levels, previous work with isolated overexpression of each form of CD23 has suggested that the membrane-bound form has a much greater impact on IgE biology than does the soluble form (53
). The lower affinity of soluble CD23 for IgE, and an inability of mouse CD23 to interact with CD21, further diminishes the likelihood of a significant role for soluble CD23 in regulating serum IgE levels or IgE production (49
The amount of CD23-associated IgE on B cells was directly proportional to the serum IgE level and showed rapid turnover (). Though it is possible that ADAM10-mediated CD23 shedding increases with ligand binding (54
), no data yet exist to demonstrate such a correlation, and ligand binding may in fact decrease CD23 shedding (13
). As our data indicate, IgE molecules are internalized and likely degraded. This receptor-mediated endocytosis has also been demonstrated by in vitro
experiments using murine B cell hybridomas expressing CD23, Epstein-Barr virus (EBV)-transformed human B cells, and additional cell lines (38
The size of the free serum IgE pool is a function of production, clearance of IgE molecules, and capture of IgE by peripheral FcεRI+
cells. Our data indicate that the latter appears to make little contribution to the regulation of the free IgE pool and that B cells, through a low-affinity receptor, are a primary regulator of the IgE pool. Binding of monomeric IgE by B cells likely plays at least two important roles in the expression of allergic inflammation. First, CD23 binding of monomeric IgE serves to enhance antigen presentation and antigen focusing as has been previously described (46
). Serum IgE levels can be detected within the first 5–8 days of an immune response (58
), and these antibodies could facilitate antigen capture and presentation by B cells. Second, B cell capture of serum IgE levels decreases the availability of IgE molecules for loading onto mast cells in peripheral tissue. As loss of this binding capacity enhances hypersensitivity responses (), we speculate that B cells act as a checkpoint to guard against the potentially fatal consequences of inappropriate mast cell activation by IgE and antigen. This system biases mast cells toward loading with IgE specificities that are highly represented in the serum. Presumably, these specificities would be the most relevant for host defense, though in the modern age, allergic manifestations are the more common result. Further work in humans to examine if human CD23 plays a similar regulatory role as murine CD23 and to clarify the precise role of specific domains of CD23 as well as the cell types involved will give us a better perspective on the role of CD23 in allergic inflammation and novel insights to target CD23 function in disease.