Targeting of CD22‐positive B‐cell lymphoma cells by synthetic divalent sialic acid analogues

Abstract

CD22 is an inhibitory co-receptor of the B-cell receptor (BCRThe ITIMs are phosphorylated by the Src-family kinase Lyn upon BCR activation, leading to the activation and recruitment of SH2-domain containing protein tyrosin phosphatase 1 (SHP-1) [3,4]. SHP-1 is a tyrosine phosphatase that dephosphorylates signaling molecules, which are critical for initiation and maintenance of the BCR-induced Ca 2+ pathway in B cells [1]. The function of CD22 is regulated by interaction of the first extracellular immunoglobulin (Ig) domain of CD22 to glycan ligands. It recognizes specifically glycans that carry sialic acids attached in α2,6-linkage to an underlying β1, 4-linked galactose residue. The affinity for its native ligand with the sequence Siaα2,6Gal is low * These authors contributed equally to this work.C 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2012. 42: 2792-2802 Clinical immunology 2793(KD = 0.2 mM) [5] and due to the high concentration of sialic acids on the B-cell surface (25-50 mM), the ligand binding sites of CD22 are masked in cis by self ligands [6,7]. In a structure-aided design, which indicated a hydrophobic pocket within the CD22 ligand-binding site, synthetic glycan ligands were designed [8,9]. To test the affinity of these synthetic glycans to CD22 on the cellular surface, the masking by cis-ligands had to be destroyed by sialidase-treatment. An optimal synthetic ligand targeting CD22 should have a rather high affinity and should be able to overcome the receptor masking without sialidase pretreatment. We had previously determined that the C-9 position of neuraminic acid can be modified with a biphenyl carbonyl-amino (BPC) substituent that increases the affinity for human CD22 about 200 times [8]. Treatment with this synthetic glycan ligands blocks CD22-mediated signal inhibition in human B-cell lines, thereby augmenting BCR signaling [8].CD22 shows a B-cell specific expression from the pre-B to mature B-cell stage but is downregulated on plasma cells. Therefore, it is an attractive target for B-cell targeted immunotherapy and is already used in clinical studies to treat B-cell lymphomas and systemic autoimmune diseases. Several therapeutics based on anti-CD22 antibodies have been developed. Epratuzumab is in phase I/II clinical trials for non-Hodgkin lymphoma (NHL) treatment and systemic lupus erythematosus (SLE) [10,11]. Furthermore, conjugates of single chain anti-CD22 antibodies with RNAses [12] or toxins like Pseudomonas exotoxin A [13] have been tested. CD22 is able to shuttle bound effector molecules inside the cell, since it is endocytosed quickly upon binding of soluble ligands [14,15].The approach we and others use is to replace the anti-CD22 antibodies with high-affinity synthetic glycan ligands as targeting moiety. Until now, this has only been done by attaching synthetic CD22 ligands to liposomes filled with doxorubicin [16] or by attaching high-molecular-weight CD22 ligands to saporin [17]. Here, we show that we can target CD22 by very small, dival...

CD22 Ligands on a Natural N-Glycan Scaffold Efficiently Deliver Toxins to B-Lymphoma Cells

“…41 Of particular note was that these ligands, as single molecules and without further multimerization, are capable of being bound and endocytosed by hCD22 on Daudi B lymphoma cells, which are known to have abundant natural ligands of CD22 that bind in cis and prevent binding of weak exogenous ligands in trans . 22–23, 30 These ligands provides an alternative to nanoparticle- and Ab-mediated approaches for delivery of toxins and other cargo to CD22 bearing B lymphoma cells. The fact that avidity gains are also seen with N-glycan based ligands of mCD22 and mSn suggests that avidity gains could be achieved using the N-linked glycan scaffold for other members of the Siglec family.…”

“…24, 30–31 Several groups successfully produced paucivalent ligands using BPC Neu5Ac as the high affinity sialic acid analog. While they bound to CD22 with significantly higher avidity, the ligands did not bind to B lymphoma cells without first treating cells with sialidase to destroy endogenous cis -ligands, or alternatively by further multimerization.…”

Therapeutic Targeting of Siglecs using Antibody- and Glycan-Based Approaches

“…Accordingly, high affinity CD22 ligands appended to liposomes encapsulated with doxorubicin has been shown to be a potent method for killing human B cell lymphoma cells both in vitro and in mice [15]. Furthermore, a soluble dimeric high affinity CD22 ligand conjugated to a toxin was shown to be efficient in killing B-ALL cells in vitro (Figure 5) [87]. Given the recent development of more potent and selective ligands for other members of the Siglecs, such as CD33 [84], it will be interesting to see if these delivery principles can also be used to target other types of lymphomas and leukemias.…”

Synthesis and biological evaluation of 9- N -oxamyl sialosides as Siglec-7 ligands

“…[20][21][22] Selective ligands were used as well for cell specific delivery of toxic agents to kill malignant cells, for delivery of antigens to improve vaccinations and for tolerance induction. [23][24][25][26][27][28][29][30][31] The group of CD33 (Siglec-3) related Siglecs is undergoing fast evolution and has several members in humans, including Siglec-7 (named p75/AIRM or CD328). 2,32-36 Siglec-7 is expressed on a variety of immune cells, including NK-cells, T-cells, B-cells, eosinophiles, mast cells, basophils, monocytes, macrophages, dendritic cells, granulocytes, platelets and as well as on leukemic cells.…”