Lymphocyte Fc receptors: the immunobiology and pathology of CD23

IgE secretion is attenuated by an inhibitor of proteolytic processing of CD23 (Fc epsilonRII)

CD23, the low-affinity IgE receptor, is up-regulated on interleukin (IL)-4-stimulated B cells and monocytes, with a concomitant increase in the release of soluble fragments of CD23 (sCD23) into the medium by proteolytic processing of the surface-bound intact CD23. The effect of inhibition of the processing of CD23 on IgE production in human and mouse cells and in a mouse model in vivo was evaluated. CD23 processing to sCD23 from RPMI 8866 (a human Epstein-Barr virus-transformed B cell line) cell membranes was inhibited by a broad-spectrum matrix-metalloprotease inhibitor, batimastat, with an IC50 of 0.15 microM. Batimastat also inhibited CD23 processing in whole RPMI 8866 cells as well as in IL-4-stimulated purified human monocytes with similar IC50. Batimastat inhibited IgE production from IL-4/anti-CD40-stimulated human tonsil B cells as well as mouse splenic B cells in a manner consistent with inhibition of CD23 processing. Release of soluble fragments of CD23 in the cell supernatants of tonsil B cells was inhibited over the concentration range of 1-10 microM batimastat and intact cell surface CD23 was increased on mouse splenic B cells in the presence of these concentrations of batimastat. IgE production of IL-4-stimulated human peripheral blood mononuclear cells was also blocked by 1-10 microM batimastat, again with comparable inhibition of sCD23 release over the same concentration range. Finally, in a mouse model of IgE production, batimastat inhibited IgE production in response to ovalbumin challenge as determined by serum IgE levels. Taken together, the data support a role of CD23 in IgE production and point to CD23 processing to sCD23 as a therapeutically relevant control point in the regulation of IgE synthesis.

Functional significance of CD23- on CD23-transfected Th2 clone

CD23, a low-affinity IgE Fc receptor, is not displayed on most resting T cells but its expression has been shown to be transiently induced in vivo and in vitro on some CD4+ T cells [1-4] and in vivo on CD8+ T cells by IgE-secreting hybridoma tumors [5]. To investigate the functional role of CD23 on T cells, we inserted a CD23 construct into an expression vector driven by a CD2 promoter and transfected it into a murine Th2 clone D10.G4.1 (D10). We stimulated the transfected D10 cells (D10.3M.24) with anti-TCR antibody in the presence or absence of IgE, and measured IL-4, IL-5 and IL-6 production in the culture supernatants. Activation of D10.3M.24 cells by anti-TCR antibody induced greater levels of IL-4, IL-5 and IL-6 production, when the TCR and CD23 were co-crosslinked by TNP anti-TCR and IgE anti-TNP antibodies. IgG anti-TNP antibody did not enhance lymphokine production by D10.3M.24 cells. The enhanced lymphokine production by IgE was blocked by monoclonal anti-CD23 antibody. IgE anti-TNP antibody did not enhance lymphokine production by the wild-type D10 cells induced by TNP anti-TCR antibody. These studies show that when co-crosslinked with the TCR, CD23 can modulate the lymphokine production in activated Th2 cells. Since CD23 binds to IgE and also binds to CD21 [6], a complement receptor commonly expressed on B cells, T-cell CD23 could play an immunoregulatory role during cognate T-B cell interaction and during IgE antibody responses.

Allergen-induced CD23 on CD4+ T lymphocytes and CD21 on B lymphocytes in patients with allergic asthma: evidence and regulation

Interaction of CD4+ T cells and B cells is necessary for IgE production. It has been recently demonstrated that cell surface antigen CD21 is a ligand for CD23 (Fc epsilon RII) and that the pairing of these molecules may participate in the control of IgE production. In this study we investigated the effect of the Dermatophagoides pteronyssinus (Dpt) allergen and recombinant interleukin(rIL)-4 on the expression of CD21 and CD23 on T and B cells of asthmatic patients allergic to Dpt and of healthy controls. Peripheral blood mononuclear cells (PBMC) were incubated alone or with Dpt allergen (100 biological units/ml) and/or rIL-4 (100 U/ml) for up to 7 days. The flow-cytometric analysis of double-fluorescence staining revealed that Dpt allergen and/or rIL-4 induced CD23 on CD4+ T lymphocytes only in allergic patients. The allergen-induced CD23 on T cells is de novo synthesized antigen since no induction of CD23 on T cells was observed in cultures with 0.4 microgram/ml actinomycin D. Moreover, 100 U/ml of interferon-gamma inhibited the induction of CD23 on CD4+ T cells. T cells obtained from healthy donors did not express CD23 or CD21 antigen upon incubation with allergen and/or rIL-4. Although rIL-4 also induced CD23 in controls, the expression was only observed on CD20+ cells. The allergen alone induced a significant elevation of the mean fluorescence intensity of both CD21 and CD23 only in allergic individuals. When the cell proliferation was analyzed, a slightly increased stimulation index upon cultivation of PBMC was obtained from non-allergic donors as well, but less than in allergic patients. The co-expression of major histocompatibility complex class II molecules and CD23 on CD4+ T lymphocytes in allergic patients, as assessed by the three-color immunofluorescence analysis, indicates that these cells were activated. We conclude that CD4+ T lymphocytes possess a unique capability to express CD23 upon exposure to allergen. Moreover, the allergen-mediated induction of CD23 on T cells observed only in allergic patients may be the reason for the increase of IgE production. This would not occur in non-allergic individuals as there is no CD23 expression on T cells.

Alternative transcripts of the human CD23/Fc epsilon RII. A possible novel mechanism of generating a soluble isoform in the type-II cell surface receptor

Human CD23/Fc epsilon RII is a 45 kDa type-II membrane glycoprotein having two isoforms (a and b) that only differ in the structures of their intracytoplasmic tails. CD23/Fc epsilon RII has been demonstrated to have multiple roles in the immune system such as regulation of lymphocyte growth and differentiation and IgE-mediated immune responses. Here, we found that the human B-cell line RPMI8866, in addition to a and b transcripts, contained shorter transcripts (a' and b') that lack the entire third exon. These alternative transcripts were also detected in peripheral blood lymphocytes as well as other hematopoietic cell lines with CD23/Fc epsilon RII. Because exon 3 encodes all of the transmembrane segment and the anchoring region of the cytoplasmic tail, it is suggested that a' and b' transcripts encode secretory forms of CD23/Fc epsilon RII or they may function as regulatory transcripts involved in the control of CD23/Fc epsilon RII expression.

CD21-CD23 ligand pair expression in children with allergic asthma

The CD23 antigen, a low affinity receptor for IgE, was recently shown to interact with another ligand, CD21, and the pairing of these molecules is important in T cell-B cell interaction and control of IgE production. Here, we analysed the expression of CD21 and CD23 on CD4+ and CD20+ lymphocytes in 25 allergic children and 12 age-matched non-allergic controls. Both the percentage (P < 0.01) and the absolute number (P < 0.001) of CD23+ cells were increased in allergic children. There was no difference of CD21+ cells. Double positive CD4+ CD23+ cells (2.5%) were only detected in one patient, in others all CD23 being expressed on B cells. The CD21 antigen was expressed only on B cells. Furthermore, allergic children had an increased mean fluorescence intensity of both the CD21 (P < 0.001) and the CD23 (P < 0.001) receptor. To analyse the possible difference in B cell subsets expressing CD21 and CD23 antigens, three-colour fluorescence analysis was performed. In allergic children the subset of CD20+ CD21- cells expressed more CD23 than in controls (P < 0.001). These results may mean an impaired expression and possibly regulation of CD21-CD23 interaction in allergic conditions.

Lymphocyte Fc receptors: the special case of T cells

The different cell types of the lymphoid-myeloid lineage constitutively express various Fc receptors. The exception is the T-cell lineage where most subsets express Fc receptors only during a narrow window following cellular activation. M. Sandor and R.G. Lynch summarize information which identifies a multi-level relationship between Fc receptors and clonotypic T-cell receptors and conclude that this relationship might account for the restricted expression of Fc receptors on T cells.