Internalization of human macrophage surface antigens induced by monoclonal antibodies

Quantification of Antibody Persistence for Cell Surface Protein Labeling

Introduction

Antibodies are an essential research tool for labeling surface proteins but can potentially influence the behavior of proteins and cells to which they bind. Because of this, researchers and clinicians are interested in the persistence of these antibodies, particularly for live-cell applications. We developed an easily adoptable method for researchers to characterize antibody removal timelines for any cell-antibody combination, with the benefit of studying broad, hypothesized mechanisms of antibody removal.

Methods

We developed a method using four experimental conditions to elucidate the contributions of possible factors influencing antibody removal: cell proliferation, internalization, permanent dissociation, and environmental perturbation. This method was tested on adipose-derived stem cells and a human lung fibroblast cell line with anti-CD44, CD90, and CD105 antibodies. The persistence of the primary antibody was probed using a fluorescent secondary antibody daily over 10 days. Relative contributions by the antibody removal mechanisms were quantified based on differences between the four culture conditions.

Results

Greater than 90% of each antibody tested was no longer present on the surface of the two cell types after 5 days, with removal observed in as little as 1 day post-labeling. Anti-CD90 antibody was primarily removed by environmental perturbation, anti-CD105 antibody by internalization, and anti-CD44 antibody by a combination of all four factors.

Conclusions

Antibody removal mechanism depended on the specific antibody tested, while removal timelines for the same antibody depended more on cell type. This method should be broadly relevant to researchers interested in quantifying an initial timeframe for uninhibited use of antibody-labeled cells.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00670-3.

Antibody-Induced Internalization of the Human Respiratory Syncytial Virus Fusion Protein

Respiratory syncytial virus (RSV) infections remain a major cause of respiratory disease and hospitalizations among infants. Infection recurs frequently and establishes a weak and short-lived immunity. To date, RSV immunoprophylaxis and vaccine research is mainly focused on the RSV fusion (F) protein, but a vaccine remains elusive. The RSV F protein is a highly conserved surface glycoprotein and is the main target of neutralizing antibodies induced by natural infection. Here, we analyzed an internalization process of antigen-antibody complexes after binding of RSV-specific antibodies to RSV antigens expressed on the surface of infected cells. The RSV F protein and attachment (G) protein were found to be internalized in both infected and transfected cells after the addition of either RSV-specific polyclonal antibodies (PAbs) or RSV glycoprotein-specific monoclonal antibodies (MAbs), as determined by indirect immunofluorescence staining and flow-cytometric analysis. Internalization experiments with different cell lines, well-differentiated primary bronchial epithelial cells (WD-PBECs), and RSV isolates suggest that antibody internalization can be considered a general feature of RSV. More specifically for RSV F, the mechanism of internalization was shown to be clathrin dependent. All RSV F-targeted MAbs tested, regardless of their epitopes, induced internalization of RSV F. No differences could be observed between the different MAbs, indicating that RSV F internalization was epitope independent. Since this process can be either antiviral, by affecting virus assembly and production, or beneficial for the virus, by limiting the efficacy of antibodies and effector mechanism, further research is required to determine the extent to which this occurs in vivo and how this might impact RSV replication.IMPORTANCE Current research into the development of new immunoprophylaxis and vaccines is mainly focused on the RSV F protein since, among others, RSV F-specific antibodies are able to protect infants from severe disease, if administered prophylactically. However, antibody responses established after natural RSV infections are poorly protective against reinfection, and high levels of antibodies do not always correlate with protection. Therefore, RSV might be capable of interfering, at least partially, with antibody-induced neutralization. In this study, a process through which surface-expressed RSV F proteins are internalized after interaction with RSV-specific antibodies is described. One the one hand, this antigen-antibody complex internalization could result in an antiviral effect, since it may interfere with virus particle formation and virus production. On the other hand, this mechanism may also reduce the efficacy of antibody-mediated effector mechanisms toward infected cells.

Targeting CD13 (aminopeptidase-N) in turn downregulates ADAM17 by internalization in acute myeloid leukaemia cells

Secreted matrix metalloproteinases (MMP)-2 and MMP-9 and membrane-anchored aminopeptidase-N/CD13 are abnormally expressed in human acute myeloid leukaemia (AML). We previously showed that CD13 ligation by anti-CD13 monoclonal antibodies can induce apoptosis in AML cells. Here, we assessed ADAM17 expression in primary blood blasts CD13⁺CD33⁺ from patients with AML. Primary AML cells expressed ADAM17 transcript and its surface expression was higher in subtype M4 (myelomonocytic) and M5 (monocytic) AML specimens than in M0 and M1/M2 (early and granulocytic) specimens. In AML cell lines defining distinct AML subfamilies (HL-60/M2, NB4/M3, THP-1/M5, U937/M5) and primary AML cells cultured ex vivo, anti-CD13 antibodies downregulated surface CD13 and ADAM17 without affecting MMP-2/-9 release. Knockdown of CD13 by siRNA prevented anti-CD13-mediated ADAM17 downregulation, indicating that CD13 is required for ADAM17 downregulation. Soluble ADAM17 was not detected in the medium of anti-CD13 treated cells, suggesting that ADAM17 was not shed. After ligation by anti-CD13, CD13 and ADAM17 were internalized. Subsequently, we found that ADAM17 interacts with CD13. We postulate that the interaction of ADAM17 with CD13 and its downregulation following CD13 engagement has important implications in AML for the known roles of ADAM17 in tumour-associated cell growth, migration and invasion.

Antibody-based therapy of acute myeloid leukemia with gemtuzumab ozogamicin

Antibodies have created high expectations for effective yet tolerated therapeutics in acute myeloid leukemia (AML). Hitherto the most exploited target is CD33, a myeloid differentiation antigen found on AML blasts in most patients and, perhaps, leukemic stem cells in some. Treatment efforts have focused on conjugated antibodies, particularly gemtuzumab ozogamicin (GO), an anti-CD33 antibody carrying a toxic calicheamicin-g 1 derivative that, after intracellular hydrolytic release, induces DNA strand breaks, apoptosis, and cell death. Serving as paradigm for this strategy, GO was the first anti-cancer immunoconjugate to obtain regulatory approval in the U.S. While efficacious as monotherapy in acute promyelocytic leukemia (APL), GO alone induces remissions in less than 25-35% of non-APL AML patients. However, emerging data from well controlled trials now indicate that GO improves survival for many non-APL AML patients, supporting the conclusion that CD33 is a clinically relevant target for some disease subsets. It is thus unfortunate that GO has become unavailable in many parts of the world, and the drug's usefulness should be reconsidered and selected patients granted access to this immunoconjugate.

Targeting tetraspanins in cancer

INTRODUCTION

Tetraspanins are a family of small proteins that cross the membrane four times and form complexes by interacting between themselves and with a variety of transmembrane and cytosolic proteins, building a network of interactions referred to as tetraspanin web or tetraspanin enriched microdomains (TEMs). These domains provide a signaling platform involved in many important cellular functions and malignant processes.

AREAS COVERED

The authors describe the methods and the rationale for targeting tetraspanins in the therapy of cancer in this review.

EXPERT OPINION

Targeting tetraspanins in cancer may be a promising therapy due to the importance of tetraspanins in several steps of tumor formation, communication with the environment, dissemination, and metastasis.

Porcine sialoadhesin (CD169/Siglec-1) is an endocytic receptor that allows targeted delivery of toxins and antigens to macrophages

Sialoadhesin is exclusively expressed on specific subpopulations of macrophages. Since sialoadhesin-positive macrophages are involved in inflammatory autoimmune diseases, such as multiple sclerosis, and potentially in the generation of immune responses, targeted delivery of drugs, toxins or antigens via sialoadhesin-specific immunoconjugates may prove a useful therapeutic strategy. Originally, sialoadhesin was characterized as a lymphocyte adhesion molecule, though recently its involvement in internalization of sialic acid carrying pathogens was shown, suggesting that sialoadhesin is an endocytic receptor. In this report, we show that porcine sialoadhesin-specific antibodies and F(ab')₂ fragments trigger sialoadhesin internalization, both in primary porcine macrophages and in cells expressing recombinant porcine sialoadhesin. Using chemical inhibitors, double immunofluorescence stainings and dominant-negative constructs, porcine sialoadhesin internalization was shown to be clathrin- and Eps15-dependent and to result in targeting to early endosomes but not lysosomes. Besides characterizing the sialoadhesin endocytosis mechanism, two sialoadhesin-specific immunoconjugates were evaluated. We observed that porcine sialoadhesin-specific immunotoxins efficiently kill sialoadhesin-expressing macrophages. Furthermore, porcine sialoadhesin-specific albumin immunoconjugates were shown to be internalized in macrophages and immunization with these immunoconjugates resulted in a rapid and robust induction of albumin-specific antibodies, this compared to immunization with albumin alone. Together, these data expand sialoadhesin functionality and show that it can function as an endocytic receptor, a feature that cannot only be misused by sialic acid carrying pathogens, but that may also be used for specific targeting of toxins or antigens to sialoadhesin-expressing macrophages.

Distinct roles for tetraspanins CD9, CD63 and CD81 in the formation of multinucleated giant cells

Members of the tetraspanin superfamily of proteins are implicated in a variety of complex cell processes including cell fusion. However, the contribution of individual tetraspanins to these processes has proved difficult to define. Here we report the use of recombinant extracellular regions of tetraspanins to investigate the role of specific members of this family in the fusion of monocytes to form multinucleated giant cells (MGC). In contrast to their positive requirement in sperm-egg fusion, previous studies using antibodies and knockout mice have indicated a negative regulatory role for tetraspanins CD9 and CD81 in this process. In an in vitro model of fusion using human monocytes, we have confirmed observations that antibodies to CD9 and CD81 enhance MGC formation; however, in contrast to previous investigations, we found that all members of a panel of antibodies to CD63 inhibited fusion. Moreover, recombinant proteins corresponding to the large extracellular domains (EC2s) of CD63 and CD9 inhibited MGC formation, whereas the EC2s of CD81 and CD151 had no effect. The potent inhibition of fusion and binding of labelled CD63 EC2 to monocytes under fusogenic conditions suggest a direct interaction with a membrane component required for fusion. Our findings indicate that the tetraspanins CD9, CD63 and CD81 are all involved in MGC formation, but play distinct roles.

Siglecs as targets for therapy in immune-cell-mediated disease

The sialic-acid-binding immunoglobulin-like lectins (siglecs) comprise a family of receptors that are differentially expressed on leukocytes and other immune cells. The restricted expression of several siglecs to one or a few cell types makes them attractive targets for cell-directed therapies. The anti-CD33 (also known as Siglec-3) antibody gemtuzumab (Mylotarg) is approved for the treatment of acute myeloid leukemia, and antibodies targeting CD22 (Siglec-2) are currently in clinical trials for treatment of B cell non-Hodgkins lymphomas and autoimmune diseases. Because siglecs are endocytic receptors, they are well suited for a 'Trojan horse' strategy, whereby therapeutic agents conjugated to an antibody, or multimeric glycan ligand, bind to the siglec and are efficiently carried into the cell. Although the rapid internalization of unmodified siglec antibodies reduces their utility for induction of antibody-dependent cellular cytotoxicity or complement-mediated cytotoxicity, antibody binding of Siglec-8, Siglec-9 and CD22 has been demonstrated to induce apoptosis of eosinophils, neutrophils and depletion of B cells, respectively. Here, we review the properties of siglecs that make them attractive for cell-targeted therapies.

Modification of membrane markers on THP-1 cells after ozone exposure in the presence or absence of fMLP

The impacts of ozone and N-formyl-methionyl-leucyl-phenylalanine (fMLP) on the detection of membrane markers on non-differentiated THP-1 cells were evaluated for in vitro exposures. Several markers, specific for monocytes and macrophages, were identified on the THP-1 cells, allowing their use as a model for alveolar macrophages. Ozone exposure modified not only the detection of membrane markers, especially CD13 and CD14, monocyte and macrophage markers, but also the detection of the specific receptor for fMLP, formyl peptide receptor (FPR). Activation by fMLP also reduced the detection of the CD antigens, and a combined exposure to ozone and fMLP amplified this decrease, probably due to an additive effect of these chemicals. Overall, these results suggest important membrane rearrangements for short-term treatments to ozone and/or fMLP.

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-γ1 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.

Human monocyte-derived macrophages and dendritic cells are comparably effective in vitro in presenting HLA class I-restricted exogenous peptides

Recent experimental data have shown that mice could be immunized efficiently, in particular against cancer, by the injection of antigen-loaded dendritic cells (DC) or macrophages (MPH). In the present work, these two antigen-presenting cells (APC) were prepared in humans from circulating mononuclear cells (MNC). MPH were obtained from MNC that were cultured in hydrophobic plastic bags and purified by elutriation. DC were from the culture of adherent elutriation-purified monocytes in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). The two APC were prepared in parallel from the same donors and their phenotype and antigen-presenting capacity were compared. DC differed from MPH by a higher expression of HLA-DR and CD23 and a lower expression of CD14, CD64 and of adhesion molecules. DC and MPH were comparably effective in (a) enhancing the mitotic response of autologous lymphocytes to immobilized anti-CD3 (accessory function); (b) presenting melanoma peptides to specific cytotoxic T lymphocyte (CTL) clones; and (c) stimulating the generation of CTL directed against a myxovirus influenza peptide. However, DC were more effective than MPH in inducing the mitotic response of allogeneic peripheral blood leucocytes (PBL), possibly because of their higher expression of HLA class II molecules. In conclusion, DC and MPH prepared from blood MNC did not differ substantially in their ability to activate HLA class I-restricted T-cell responses by exogenous peptide presentation.