Flow cytometry and receptor occupancy in immune-oncology

Introduction: Immunotherapies are focused on strategies that alter immune responses, using antibodies that binds to receptors on different immune cell subsets and either activate or suppress their functions depending on the immune response being targeted. Hence, the necessity of developing assays that assess the functional and biological effect of a therapeutic on its target. When incorporated into high-parameter flow cytometry panels, receptor occupancy assay can simultaneously evaluate receptor expression and drug occupancy on defined cell subsets, which can provide information related to functional effects, and safety.Areas covered: This review focuses on the importance of developing, optimizing, and validating a robust Receptor Occupancy Assay (ROA) to improve dose selection, pharmacology monitoring and safety mainly in clinical settings.Expert opinion: The designing of an ROA can be challenging and can lead to exaggerated pharmacology if not accurately developed, optimized, and validated. However, improvements in our understanding of epitopes, binding, affinities, and pharmacological effects may lead to improved antibody drug targeting and receptor evaluation.

Cytokines: The Good, the Bad, and the Deadly

Over the past 30 years, the world of pharmaceutical toxicology has seen an explosion in the area of cytokines. An overview of the many aspects of cytokine safety evaluation currently in progress and evolving strategies for evaluating these important entities was presented at this symposium. Cytokines play a broad role to help the immune system respond to diseases, and drugs which modulate their effect have led to some amazing therapies. Cytokines may be "good" when stimulating the immune system to fight a foreign pathogen or attack tumors. Other "good" cytokine effects include reduction of an immune response, for example interferon β reduction of neuron inflammation in patients with multiple sclerosis. They may be "bad" when their expression causes inflammatory diseases, such as the role of tumor necrosis factor α in rheumatoid arthritis or asthma and Crohn's disease. Therapeutic modulation of cytokine expression can help the "good" cytokines to generate or quench the immune system and block the "bad" cytokines to prevent damaging inflammatory events. However, care must be exercised, as some antibody therapeutics can cause "ugly" cytokine release which can be deadly. Well-designed toxicology studies should incorporate careful assessment of cytokine modulation that will allow effective therapies to treat unmet needs. This symposium discussed lessons learned in cytokine toxicology using case studies and suggested future directions.

Engineered protease-resistant antibodies with selectable cell-killing functions

Molecularly engineered antibodies with fit-for-purpose properties will differentiate next generation antibody therapeutics from traditional IgG1 scaffolds. One requirement for engineering the most appropriate properties for a particular therapeutic area is an understanding of the intricacies of the target microenvironment in which the antibody is expected to function. Our group and others have demonstrated that proteases secreted by invasive tumors and pathological microorganisms are capable of cleaving human IgG1, the most commonly adopted isotype among monoclonal antibody therapeutics. Specific cleavage in the lower hinge of IgG1 results in a loss of Fc-mediated cell-killing functions without a concomitant loss of antigen binding capability or circulating antibody half-life. Proteolytic cleavage in the hinge region by tumor-associated or microbial proteases is postulated as a means of evading host immune responses, and antibodies engineered with potent cell-killing functions that are also resistant to hinge proteolysis are of interest. Mutation of the lower hinge region of an IgG1 resulted in protease resistance but also resulted in a profound loss of Fc-mediated cell-killing functions. In the present study, we demonstrate that specific mutations of the CH2 domain in conjunction with lower hinge mutations can restore and sometimes enhance cell-killing functions while still retaining protease resistance. By identifying mutations that can restore either complement- or Fcγ receptor-mediated functions on a protease-resistant scaffold, we were able to generate a novel protease-resistant platform with selective cell-killing functionality.

The use of an anti-CD40 agonist monoclonal antibody during immunizations enhances hybridoma generation

ABSTRACT Herein we describe the use of an agonistic anti-murine CD40 MAb as a B cell proliferative agent to enhance the generation of monoclonal antibodies (MAbs) in Balb/c mice. While hybridoma technology has been validated repeatedly over the decades, little work has been described to improve upon the overall numbers of in vivo B cells and specific antibodies obtained from a fusion. To begin to address this situation, strategies to boost B lymphocyte yields for hybridoma production were employed. Anti-CD40 agonist antibodies have been reported to activate and amplify human resting B lymphocytes in vitro, resulting in increased cell numbers available for the generation of human hybridomas or B cell clones. An agonistic anti-murine CD40 MAb was administered to immunized mice 3 days prior to splenic harvest, and B lymphocyte yields were found to be approximately 2-fold higher in treated animals when compared to untreated animals. Moreover, the resulting hybridoma fusions using lymphocytes from treated animals yielded 5- to 10-fold more antigen reactive hybrids when compared to untreated animals. This novel addition to conventional approaches utilizes the proliferative effects of agonistic anti-CD40 MAbs to markedly enhance monoclonal antibody generation.

Tonic B-cell and viral ITAM signaling: context is everything

The presence of an immunoreceptor tyrosine-based activation motif (ITAM) makes immunoreceptors different from other signaling receptors, like integrins, G-coupled protein receptors, chemokine receptors, and growth factor receptors. This unique motif has the canonical sequence D/Ex(0-2)YxxL/Ix(6-8)YxxL/I, where x represents any amino acid and is present at least once in all immunoreceptor complexes. Immunoreceptors can promote survival, activation, and differentiation by transducing signals through these highly conserved motifs. Traditionally, ITAM signaling is thought to occur in response to ligand-induced aggregation, although evidence indicates that ligand-independent tonic signaling also provides functionally relevant signals. The majority of proteins containing ITAMs are transmembrane proteins that exist as part of immunoreceptor complexes. However, oncogenic viruses also have ITAM-containing proteins. In this review, we discuss what is known about tonic signaling by both cellular and viral ITAM-containing proteins and speculate what we might learn from each context.

Analysis of the Individual Contributions of Igα (CD79a)- and Igβ (CD79b)-Mediated Tonic Signaling for Bone Marrow B Cell Development and Peripheral B Cell Maturation

The individual contribution of Igalpha and Igbeta for BCR-triggered fates is unclear. Prior evidence supports conflicting ideas concerning unique as well as redundant functions for these proteins in the context of BCR/pre-BCR signaling. Part of this ambiguity may reflect the recent appreciation that Igalpha and Igbeta participate in both Ag-independent (tonic) and Ag-dependent signaling. The present study undertook defining the individual requirement for Igalpha and Igbeta under conditions where only ligand-independent tonic signaling was operative. In this regard, we have constructed chimeric proteins containing one or two copies of the cytoplasmic domains of either Igalpha or Igbeta and Igalpha/Igbeta heterodimers with targeted Tyr-->Phe modifications. The ability of these proteins to act as surrogate receptors and trigger early bone marrow and peripheral B cell maturation was tested in RAG2(-/-) primary pro-B cell lines and in gene transfer experiments in the muMT mouse model. We considered that the threshold for a functional activity mediated by the pre-BCR/BCR might only be reached when two functional copies of the Igalpha/Igbeta ITAM domain are expressed together, and therefore the specificity conferred by these proteins can only be observed in these conditions. We found that the ligand-independent tonic signal is sufficient to drive development into mature follicular B cells and both Igalpha and Igbeta chains supported formation of this population. In contrast, neither marginal zone nor B1 mature B cell subsets develop from bone marrow precursors under conditions where only tonic signals are generated.

A rapid and efficient method for generating anti-variable region monoclonal antibodies using type-1 interferons as immune modulators

The generation of anti-variable region monoclonal antibodies (mAbs) against therapeutic antibodies is essential in the pharmacokinetic/pharmacodynamic (PK/PD) assessments of the drugs in clinical study samples. Sandwich EIA and other methods are typically employed to achieve sensitivity and selectivity for the PK/PD analyses. These assays usually require generation of mAb reagents that bind specifically to the drug in non-competing pair combinations. Thus, large panels of anti-variable region mAbs must be generated in an expeditious manner to increase the probability of success. Herein we describe a novel immunization method that utilizes type 1 interferons (IFNs) as immunomodulators coupled with an agonistic anti-CD40 mAb as a B cell proliferative agent to drive immune responses. This novel protocol allows for rapid and robust mAb reagent generation without the use of conventional protein-denaturing adjuvants. The use of IFNs allowed for the generation of comparable and in some cases, increased numbers of anti-variable region mAbs in a dramatically shorter timeframe. This IFN based, immunostimulatory approach utilizing a non-denaturing adjuvant, likely presents conformational epitopes and may optimize the humoral response for the rapid generation of anti-variable region reagents to therapeutic mAb candidates.

Igα/Igβ Complexes Generate Signals for B Cell Development Independent of Selective Plasma Membrane Compartmentalization

Ligand-induced BCR association with detergent-resistant plasma membrane compartments (lipid rafts) has been argued to be essential for initiating and/or sustaining Igalpha/Igbeta-dependent BCR signaling. Because a fraction of the BCR and an even larger fraction of the preBCR associates with lipid rafts in the apparent absence of ligand stimulation, it has been proposed that raft-associated receptor complexes mediate the ligand-independent basal signaling events observed in resting B lineage cells. However, there is no direct evidence that localization of Igalpha/Igbeta-containing complexes to detergent-resistant membrane compartments is absolutely required for the signaling events that drive B cell development. To address these issues we have designed surrogate preBCR/Igalpha/Igbeta complexes that are incapable of ligand-induced aggregation and that are preferentially targeted to either raft or nonraft compartments. An analysis of their ability to promote the preBCR-dependent proB-->preB cell transition of murine B cell progenitors revealed that expression of these surrogate receptor complexes at levels that approximate that of the conventional preBCR can drive B cell development in a manner independent of both aggregation and lipid raft localization.

Basal Igalpha/Igbeta signals trigger the coordinated initiation of pre-B cell antigen receptor-dependent processes

The pro-B to pre-B transition during B cell development is dependent upon surface expression of a signaling competent pre-B cell Ag receptor (pre-BCR). Although the mature form of the BCR requires ligand-induced aggregation to trigger responses, the requirement for ligand-induced pre-BCR aggregation in promoting B cell development remains a matter of significant debate. In this study, we used transmission electron microscopy on murine primary pro-B cells and pre-B cells to analyze the aggregation state of the pre-BCR. Although aggregation can be induced and visualized following cross-linking by Abs to the pre-BCR complex, our analyses indicate that the pre-BCR is expressed on the surface of resting cells primarily in a nonaggregated state. To evaluate the degree to which basal signals mediated through nonaggregated pre-BCR complexes can promote pre-BCR-dependent processes, we used a surrogate pre-BCR consisting of the cytoplasmic regions of Igalpha/Igbeta that is targeted to the inner leaflet of the plasma membrane of primary pro-B cells. We observed enhanced proliferation in the presence of low IL-7, suppression of V(H)(D)J(H) recombination, and induced kappa light (L) chain recombination and cytoplasmic kappa L chain protein expression. Interestingly, Igalpha/Igbeta-mediated allelic exclusion was restricted to the B cell lineage as we observed normal TCRalphabeta expression on CD8-expressing splenocytes. This study directly demonstrates that basal signaling initiated through Igalpha/Igbeta-containing complexes facilitates the coordinated control of differentiation events that are associated with the pre-BCR-dependent transition through the pro-B to pre-B checkpoint. Furthermore, these results argue that pre-BCR aggregation is not a requirement for pre-BCR function.

Positive and negative selection during B lymphocyte development

Our laboratory is interested in a variety of issues related to lymphocyte development. More specifically, we have focused on the processes that regulate the decision to commit to the B lymphocyte (B cell) lineage, then the subsequent signals that are involved in maintaining this commitment to the B cell lineage. These signals result in the positive selection of those B cells that properly execute the complex genetic changes associated with B cell development, then trigger the elimination of B cells that are responsive to self-antigens and, therefore, possess the potential to mediate autoimmune disease. Our general experimental approach has been to address these issues from the perspective of signal transduction. Our goal is to define the biochemical and genetic processes that are integrated in order to accomplish these selection processes. To do so, we employ in vivo animal models as well as more defined in vitro studies, using both primary and transformed cell lines. For the past several years, we have been primarily interested in the precise mechanisms by which the B cell antigen receptor (BCR), and intermediate forms of this receptor, regulate these complex developmental processes. We have used the ongoing studies described below as two representative examples of how we are approaching these issues and some of the insights that we have made. To place both of these studies in context, we will begin with a brief introduction into B cell development.

Basal B-cell receptor signaling in B lymphocytes: mechanisms of regulation and role in positive selection, differentiation, and peripheral survival

B-cell development is a highly ordered multistep process dependent upon signals generated by the pre-B and B-cell antigen receptor (BCR). BCR signals drive maturation of the B cell by integrating a number of parallel and sequential biological processes that result in generation of fully immunocompetent B cells. Among these biological processes are positive selection through several developmental checkpoints, negative selection of potentially self-reactive B cells, and activation of the mature B cell. In addition, recent studies have shown that developing and mature B cells rely on the constant activity of the BCR for their continued survival. Ligand (antigen)-dependent and -independent mechanisms of BCR signaling have been proposed, but their specific contributions to B-cell maturation and differentiation in the bone marrow and periphery are not completely clear. We discuss here a model, whereby ligand-independent basal BCR activity would be sufficient to trigger B-cell development through to the mature stage. However, long-term survival and formation of specific mature B-cell populations may be dependent on ligand-receptor interactions.

Ligand-independent signaling functions for the B lymphocyte antigen receptor and their role in positive selection during B lymphopoiesis

Signal transduction through the B cell antigen receptor (BCR) is determined by a balance of positive and negative regulators. This balance is shifted by aggregation that results from binding to extracellular ligand. Aggregation of the BCR is necessary for eliciting negative selection or activation by BCR-expressing B cells. However, ligand-independent signaling through intermediate and mature forms of the BCR has been postulated to regulate B cell development and peripheral homeostasis. To address the importance of ligand-independent BCR signaling functions and their regulation during B cell development, we have designed a model that allows us to isolate the basal signaling functions of immunoglobulin (Ig)alpha/Igbeta-containing BCR complexes from those that are dependent upon ligand-mediated aggregation. In vivo, we find that basal signaling is sufficient to facilitate pro-B --> pre-B cell transition and to generate immature/mature peripheral B cells. The ability to generate basal signals and to drive developmental progression were both dependent on plasma membrane association of Igalpha/Igbeta complexes and intact immunoregulatory tyrosine activation motifs (ITAM), thereby establishing a correlation between these processes. We believe that these studies are the first to directly demonstrate biologically relevant basal signaling through the BCR where the ability to interact with both conventional as well as nonconventional extracellular ligands is eliminated.

Coordinate regulation of HLA class II genes: a novel DNA binding complex

We have characterized a nuclear protein complex from B lymphoblastoid cell lines that binds to HLA class II promoters as detected by electrophoretic gel mobility shift assays (EMSA). This complex (C1) binds to three independent sites in the proximal DRA promoter which have not been identified previously as cis-acting elements. C1 is very abundant in Burkitt's lymphoma cell lines, but less abundant in "normal" B lymphoblastoid cell lines. The binding specificity of the C1 complex was analysed using competition experiments and chemical footprinting methods. Complexes with specificity similar to C1 also bind the DPA and DQA promoters. Though mutation of the sequences in the DRA promoter that severely reduced binding of the C1 complex had no effect on the ability of the DRA fragment to drive transcription of the reporter gene in transient expression or in vitro transcription assays, this conservation of binding sites among all class II promoters tested suggests functional relevance in transcription. In addition, complexes similar to C1 were observed in nuclear extracts from all cell lines examined, but minor differences in mobility appeared to correlate with class II expression. Thus, the C1 complex may act as a trans-acting factor in MHC class II expression.

The role of the cytoskeleton and intercellular junctions in the transcellular membrane protein polarity of bovine aortic endothelial cells in vitro

This project examines the transcellular membrane protein polarity of bovine aortic endothelial cell (BAEC) monolayers in vitro with respect to the roles that intercellular junctions (as defined by comparing confluent and subconfluent monolayers) and the submembranous cytoskeleton play in controlling this phenomenon. Plasma membrane (PM) proteins obtained from apical (AP) and basolateral (BL) PM domains of confluent BAEC monolayers were isolated using the cationic colloidal silica technique and resolved by two-dimensional gel electrophoresis (2-D PAGE). To facilitate the identification of domain-specific PM proteins, an isoelectric point/molecular weight database of the proteins from AP and BL PM domains was constructed. Domain-specific PM proteins were assessed for their interaction with the cytoskeleton by determining whether they co-isolated with a Triton X-100 detergent-resistant cytoskeletal/extracellular matrix fraction. The maintenance of polarized PM protein segregation by intercellular junctional complexes was determined by comparing AP and BL protein patterns of confluent monolayers with patterns generated by subconfluent monolayers, which lack such junctional structures. Proteins isolated from AP and BL PM domains from both confluent states were immunoblotted with antibodies to angiotensin-converting enzyme (ACE) and collagen receptors (CR). ACE was restricted exclusively to the AP PM domain in the subconfluent condition, even though no apparent cytoskeletal interaction was observed. CRs, found to interact with the cytoskeleton in either confluence state, were predominantly segregated to the BL PM domain regardless of the presence or absence of cell-cell contact. Membrane proteins found by 2-D PAGE to be asymmetrically distributed in the absence of intercellular junctions were assessed for cytoskeletal interaction by their inability to be extracted by Triton X-100 from monolayers in the subconfluent state. Computer cross-referencing of 2-D PAGE peak lists and immunodetection generated from the above fractionation protocols identifies a set of four proteins associated with the cytoskeleton that remain segregated in the proper domain, and five proteins associated with the cytoskeleton that become equally distributed between AP and BL PM domains in the absence of intercellular junctions. Additionally, six proteins not associated with the cytoskeleton remain asymmetrically distributed to the AP domain in the subconfluent state. The data suggest that BAEC monolayers have unknown mechanisms, apart from intercellular junctions expressed at confluency or cytoskeletal binding, for maintaining transcellular PM protein polarity.