ITIM-dependent endocytosis of CD33-related Siglecs: role of intracellular domain, tyrosine phosphorylation, and the tyrosine phosphatases, Shp1 and Shp2.

The leukocyte CD33-related sialic acid-binding Ig-like lectins (Siglecs) are implicated in glycan recognition and host defense against and pathogenicity of sialylated pathogens. Recent studies have shown endocytosis by CD33-related Siglecs, which is implicated in clearance of sialylated antigens and antigen presentation and makes targeted immunotherapy possible. Using CD33 as a paradigm, we have now investigated the reasons underlying the comparatively slow rate of endocytosis of these receptors. We show that endocytosis is largely limited and determined by the intracellular domain while the extracellular and transmembrane domains play a minor role. Tyrosine phosphorylation, most likely through Src family kinases, increases uptake of CD33 depending on the integrity of the two cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Simultaneous depletion of the protein tyrosine phosphatases, Src homology-2-containing tyrosine phosphatase 1 (Shp1) and Shp2, which bind to phosphorylated CD33, increases internalization of CD33 slightly in some cell lines, whereas depletion of spleen tyrosine kinase (Syk) has no effect, implying that Shp1 and Shp2 can dephosphorylate the ITIMs or mask binding of the phosphorylated ITIMs to an endocytic adaptor. Our studies show that restraint of CD33 internalization through the intracellular domain is relieved partly when the ITIMs are phosphorylated and show that Shp1 and Shp2 can modulate this process.

Phosphorylated ITIMs enable ubiquitylation of an inhibitory cell surface receptor.

Immune responses are modulated by activating and inhibitory receptors that traffic to and from the cell surface. Ligands that bind to inhibitory receptors induce phosphorylation of immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic tails, followed by recruitment of inhibitory signaling molecules. Mechanisms that control the surface levels of inhibitory receptors are largely unexplored. Here, we show, using CD33/sialic acid-binding immunoglobulin-like lectin (Siglec)-3 as a paradigm, that ITIMs can bind to the ubiquitin ligase Cbl and that ITIMs are ubiquitylated following Src family kinase-mediated tyrosine phosphorylation. Ubiquitylation is a known signal for endocytosis. Accordingly, cells expressing CD33 mutants that cannot become ubiquitylated show significantly increased cell surface expression of CD33 and have impaired CD33 internalization, whereas in-frame fusion of ubiquitin to CD33 reverses this phenotype. Our results identify a novel function of ITIMs and demonstrate that phosphorylation-dependent ubiquitylation regulates cell surface expression and internalization, and thus possibly function, of CD33/Siglec-3, suggesting an important role of ubiquitin in endocytosis of ITIM-bearing inhibitory immunoreceptors.

Influence of CD33 expression levels and ITIM-dependent internalization on gemtuzumab ozogamicin-induced cytotoxicity.

Gemtuzumab ozogamicin (GO; Mylotarg), a novel immunoconjugate used for treatment of acute myeloid leukemia (AML), contains the humanized anti-CD33 antibody (hP67.6) as a carrier to facilitate cellular uptake of the toxic calicheamicin-gamma(1) derivative. By use of lentivirus-mediated gene transfer to manipulate CD33 expression in myeloid cell lines that normally lack CD33 (murine 32D cells) or have very low levels of CD33 (human OCI-AML3 and KG-1a cells), we here show a quantitative relationship between CD33 expression and GO-induced cytotoxicity. The CD33 cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs) control internalization of antibody bound to CD33. Disruption of the ITIMs by introduction of point mutations not only prevented effective internalization of antibody-bound CD33 but also significantly reduced GO-induced cytotoxicity. Together, our data imply a pivotal role of both the number of CD33 molecules expressed on the cell surface and the amount of internalization of CD33 following antibody binding for GO-induced cytotoxicity and suggest novel therapeutic approaches for improvement of clinical outcome of patients treated with GO.

The peripheral benzodiazepine receptor ligand PK11195 overcomes different resistance mechanisms to sensitize AML cells to gemtuzumab ozogamicin.

The antibody-targeted therapeutic, gemtuzumab ozogamicin (GO, Mylotarg), is approved for treatment of relapsed acute myeloid leukemia (AML). We previously showed that AML blasts from GO refractory patients frequently express the drug transporters P-glycoprotein (Pgp) and/or multidrug resistance protein (MRP). We also previously reported that inhibition of drug transport by the Pgp modulator, cyclosporine A (CSA), can increase GO sensitivity in Pgp(+) AML cells and that the peripheral benzodiazepine receptor ligand, PK11195, sensitizes AML cells to standard chemotherapeutics both by inhibiting Pgp-mediated efflux and by promoting mitochondrial apoptosis. We now show that PK11195 also can overcome multiple resistance mechanisms to increase GO sensitivity in AML cells, including resistance associated with expression of drug transporters and/or antiapoptotic proteins. PK11195 substantially increases GO cytotoxicity in AML cells from many different cell lines and primary patient samples, often more effectively than CSA. We also show that PK11195 is nontoxic in NOD/SCID mice and can sensitize xenografted human AML cells to GO. Since PK11195 is well tolerated in humans as a single agent, its further study as a multifunctional chemosensitizer for anti-AML therapies, including GO-based therapies, is warranted.

Multidrug resistance protein attenuates gemtuzumab ozogamicin-induced cytotoxicity in acute myeloid leukemia cells.

Gemtuzumab ozogamicin (GO) is a novel immunoconjugate therapy for acute myeloid leukemia (AML). P-glycoprotein (Pgp) confers resistance to GO and is associated with a worse clinical response. To address whether multidrug resistance protein (MRP) affects GO susceptibility, we characterized Pgp, MRP1, and MRP2 expression in CD33+ cell lines and CD33+ AML samples and analyzed the effect of the Pgp inhibitor cyclosporine (CSA) and the MRP inhibitor MK-571 on GO-induced cytotoxicity. MRP1, but not MRP2, expression correlated with MRP activity. MK-571 enhanced GO-induced cytotoxicity in Pgp-negative/MRP-positive NB4 and HL-60 cells. CSA, but not MK-571 alone, restored GO susceptibility in Pgp-positive/MRP-positive TF1 cells; however, MK-571 enhanced cytotoxicity in the presence of CSA. All patient samples exhibited MRP activity, and 17 of 23 exhibited Pgp activity. CSA increased GO-induced cytotoxicity in 12 Pgp-positive samples, whereas MK-571 alone was effective in only one sample with minimal Pgp activity. In 3 Pgp-positive/MRP-positive samples, MK-571 enhanced GO-induced cytotoxicity in the presence of CSA. Thus, MRP1 may attenuate susceptibility to GO. This effect was comparatively less than that for Pgp and required the inhibition of Pgp for detection in cells that coexpressed both transporters. Because MK-571 and CSA failed to affect cytotoxicity in a portion of Pgp-positive/MRP-positive AML samples, additional resistance mechanisms are likely important.

Analysis of 4.3 kilobases of divergent locus B of macaque retroperitoneal fibromatosis-associated herpesvirus reveals a close similarity in gene sequence and genome organization to Kaposi's sarcoma-associated herpesvirus.

We previously identified retroperitoneal fibromatosis-associated herpesvirus (RFHV) as a simian homolog of Kaposi's sarcoma-associated herpesvirus (KSHV) in a fibroproliferative malignancy of macaques that has similarities to Kaposi's sarcoma. In this report, we cloned 4.3 kb of divergent locus B (DL-B) flanking the DNA polymerase gene from two variants of RFHV from different species of macaque with a consensus degenerate hybrid oligonucleotide primer approach. Within the DL-B region of RFHV, viral homologs of the cellular interleukin-6, dihydrofolate reductase, and thymidylate synthase genes were identified, along with a homolog of the gammaherpesvirus open reading frame (ORF) 10. In addition, a homolog of the KSHV ORF K3, the modulator of immune recognition-1, was identified. Our data show a close similarity in sequence conservation, gene content, and genomic structure between RFHV and KSHV which strongly supports the grouping of these viral species within the same RV-1 rhadinovirus lineage and the hypothesis that RFHV is the macaque homolog of KSHV.