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Low‐affinity receptors (Fc epsilon R) and secreted factors (IgE‐BF) which bind to immunoglobulins of the IgE isotype play a key role in the regulation of human IgE synthesis. We report here the cloning of a cDNA coding for the Fc epsilon R of the human B‐lymphoblast cell line RPMI 8866. The nucleotide sequence of this cDNA predicts a polypeptide with 321 amino acids and a mol. wt of 36,281 daltons. A functional Fc epsilon R capable of binding IgE was expressed in Chinese hamster ovary cells after stable transformation with the cDNA which had been cloned into a mammalian expression vector. Amino acid sequence analysis of IgE‐BF purified from RPMI 8866 cells revealed an amino‐terminal sequence of 19 residues which coincides with the predicted amino acid sequence of the Fc epsilon R, starting at residues 148 and 150. A computer search with the translated amino acid sequence of the Fc epsilon R revealed a domain of 120 amino acids having striking homology to the human asialoglycoprotein receptors.

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Prognosis of B-cell chronic lymphocytic leukemia (CLL) is based on clinical staging whose limitation is the failure to assess whether the disease will progress or remain stable in early stage (Binet A, or Rai 0, I, II) patients. We previously reported that soluble CD23 (sCD23), a protein derived from the B-cell membrane CD23 Ag, is selectively elevated in the serum of CLL patients. This prospective study assessed the predictive value of serum sCD23 level measured at study entry on the overall survival of all CLL patients and on disease progression of stage Binet A patients. Prognostic value of repeated measurements of sCD23 over time in stage A patients was also analyzed. One hundred fifty-three CLL patients were prospectively followed with a median follow-up of 78 months. Eight clinical or biological parameters were collected from the date of the first sCD23 measurement. At study entry, by Cox model, Binet staging (P = .0001) and serum sCD23 level (P = .03) appeared as prognostic factors for survival. Patients with sCD23 level above median value (> 574 U/mL) had a significantly worse prognosis than those with lower values (median survival of 53 v 100+ months, P = .0001). During follow-up, sCD23 doubling time increased by 3.2 the risk of death (P = .001). Among stage A patients (n = 100), sCD23 determination at study entry was the sole variable predictive of disease progression, patients with sCD23 level above 574 U/mL had a median time progression of 42 months versus 88 months for those with lower levels (P = .0001). Stage A patients who doubled their sCD23 level exhibited a 15-fold increased risk of progression (P = .0001) and, in addition, the sCD23 increase preceded by 48 months disease progression. We conclude that in CLL patients, serum sCD23 level provides significant additional prognostic information in terms of overall survival. Most interestingly, among early stage patients, sCD23 determination at diagnosis and during the course of the disease may help to the early identification of patients who will rapidly progress to upper stages.

B lymphocytes from patients with chronic lymphocytic leukemia (B-CLLs), strongly express the CD23 antigen, a surface marker with significant prognostic importance in this disease. Because we previously reported that IL4 shows a poor capacity for CD23 expression on B-CLLs, we first examined the possible mechanisms underlying CD23 overexpression on BCLLs and found that mitogen-activated CLL T cells release soluble factors that are capable, in synergy with IL4, of strongly inducing CD23. Using neutralizing Abs, we noticed that the T-cell-derived enhancing activity is entirely ascribed to the combined effects of IFN'y (potent inhibitor of CD23 on normal B cells), TNFa (which has no effect on normal B cells), and IL-2 (which has a slight enhancing effect on both CLL and normal B cells). Furthermore, recombinant IFN'y as well as IFNa, TNFa, and IL-2 (but not IL-3, IL-5, IL-6, IL-7, and lymphotoxin) significantly enhance CD23 protein and mRNA expression on B-CLLs, in the presence or absence ofIL4. Inasmuch as optimal CD23 expression absolutely requires the combination of IFN'y, IL-2, TNFa (the production of which is increased in CLL disease), and IL4, it was relevant to show that IL4 mRNA is indeed expressed in fresh T-CLL cells. We next examined the possible role of CD23 in the regulation of B-CLL proliferation. Signaling through CD23 via ligation of the antigen by F(ab)2 anti-CD23 MAb but not Fab fragments inhibits the cytokine-induced B-CLL DNA synthesis. It is concluded that the CD23 gene is abnormally regulated in B-CLL disease and that cross-linking of CD23 molecule delivers a negative growth signal to the leukemic B cells. (J.

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SumnlarySoluble CD23 (sCD23) has multiple IgE-independent biological activities. In the present study, we examined the regulatory effect of sCD23 on cytokine production by human peripheral blood mononuclear cells (PBMC). We show that sCD23 enhances by about 80-fold the interleukin 2 (IL-2)-induced interferon 7 (IFN-3') production and by about 10-fold the response to IL-12. This potentiating activity is time and dose dependent and is not associated with a significant effect on DNA synthesis. The sCD23 costimulatory activity for IFN-3' synthesis is drastically reduced in monocyte-depleted PBMC, suggesting that monocytes may be the target for sCD23, This hypothesis was supported by the following observations. First, sCD23 alone is a potent inducer of tumor necrosis factor a (TNF-ot) production by PBMC and this effect disappears after monocyte depletion. The triggering of TNF-ot release is specifically inhibited by neutralizing anti-CD23 monoclonal antibody (mAb). In addition, IL-2 and IL-12 synergize with sCD23 to induce TNF-ot production. Second, sCD23 triggers the release of other inflammatory mediators such as IL-lot, IL-1/3, and IL-6. Finally, TNF-a production in response to IL-2 and sCD23 precedes IFN-y and IFN-3' secretion is significantly inhibited by anti-TNF-c~ mAb, indicating that the sCD23 costimulatory signal for IFN-y production may be partially mediated by TNF-ot release. It is proposed that sCD23 is a proinflammatory cytokine that, in addition, may play an important role in the control of the immune response via the enhancement of IFN-N production.

Human CD23 and its soluble forms (sCD23) display various biological activities, in addition to their IgE binding function (IgE/BF). The IgE binding domain was recently mapped to residues between Cys163 and Cys282 but its involvement in IgE‐independent, CD23 functions remains unknown. In order to clarify this point, a series of N‐terminal, C‐terminal and internal deletion mutants of CD23 or sCD23 were expressed in CHO cells and tested for their ability (i) to bind to IgE, (ii) to induce colony formation by human myeloid precursor cells, (iii) to promote mature T cell marker expression by early prothymocytes, and (iv) to regulate IgE synthesis. The present study indicates that cytokine activities require the presence of Cys288, while this amino acid is not necessary for IgE/BF. Blocking experiments using various conformation‐sensitive monoclonal antibodies further suggest that active epitope(s) of CD23 in cytokine assays is(are) distinct from those involved in IgE/BF.