Coordinate analysis of murine immune cell surface markers and intracellular phosphoproteins by flow cytometry

Receptor cross-linking on human plasmacytoid dendritic cells leads to the regulation of IFN-alpha production

Plasmacytoid dendritic cells (PDC) are the natural type I IFN-producing cells that produce large amounts of IFN-alpha in response to viral stimulation. During attempts to isolate PDC from human PBMC, we observed that cross-linking a variety of cell surface receptors, including blood DC Ag (BDCA)-2, BDCA-4, CD4, or CD123 with Abs and immunobeads on PDC leads to inhibition of IFN-alpha production in response to HSV. To understand the mechanisms involved, a number of parameters were investigated. Cross-linking did not inhibit endocytosis of soluble Ag by PDC. Flow cytometry for annexin V and activated caspase-3 indicated that PDC are not undergoing apoptosis after receptor cross-linking. Cross-linking of CD123, but not the other receptors, caused the up-regulation of costimulatory molecules CD80 and CD86, as well as the down-regulation of CD62L, indicating PDC maturation. Thus, anti-CD123 Ab may be acting similar to the natural ligand, IL-3. Anti-phosphotyrosine Ab, as well as Ab to the IFN regulatory factor, IRF-7, was used in intracellular flow cytometry to elucidate the signaling pathways involved. Tyrosine phosphorylation occurred after cross-linking BDCA-2 and BDCA-4, but not CD4. Cross-linking did not affect IRF-7 levels in PDC, however, cross-linking BDCA-2, BDCA-4, and CD4, but not CD123, inhibited the ability of IRF-7 to translocate to the nucleus. Taken together, these results suggest that cross-linking BDCA-2, BDCA-4, and CD4 on PDC regulates IFN-alpha production at the level of IRF-7, while the decrease in IFN-alpha production after CD123 cross-linking is due to stimulation of the IL-3R and induction of PDC maturation.

Cutting edge: Paracrine, but not autocrine, IL-2 signaling is sustained during early antiviral CD4 T cell response

IL-2 is expressed predominantly by activated T cells, and regulates T cell function by activating, via its receptor, the latent transcription factor STAT5. This signaling can occur in either a paracrine (between cells) or an autocrine (same cell) manner, although the kinetics by which these two signaling modes operate during in vivo T cell responses are unknown. In the current study, IL-2 expression and signaling in a clonotypic population of antiviral CD4+ T cells was analyzed by flow cytometry during the initial 24 h of priming. IL-2 expression and STAT5 activation peaked in parallel, but surprisingly, were almost completely mutually exclusive. Thus, only paracrine IL-2 signaling could be observed. As an additional indication of the efficiency of paracrine IL-2 signaling, polyclonal CD4+CD25+Foxp3+ regulatory T cells displayed detectable STAT5 activation under steady-state conditions, which was strongly enhanced by neighboring IL-2-expressing antiviral CD4 cells.

ShcA mediates the dominant pathway to extracellular signal-regulated kinase activation during early thymic development

During thymic development, the beta selection checkpoint is regulated by pre-T-cell receptor-initiated signals. Progression through this checkpoint is influenced by phosphorylation and activation of the serine/threonine kinases extracellular signal-regulated kinase 1 (ERK1) and ERK2, but the in vivo relevance of specific upstream players leading to ERK activation is not known. Here, using mice with a conditional loss of the shc1 gene or expressing mutants of ShcA, we demonstrate that the adapter protein ShcA is responsible for up to 70% of ERK activation in double-negative (DN) thymocytes in vivo and ex vivo. We also identify two specific tyrosines on ShcA that promote ERK phosphorylation in vivo, and mice expressing ShcA with mutations of these tyrosines show impaired DN thymocyte development. This work provides the first in vivo demonstration of the relative requirement of upstream adapters in controlling ERK activation during beta selection and suggests a dominant role for ShcA.

Fluorescent cell barcoding in flow cytometry allows high-throughput drug screening and signaling profiling

Flow cytometry allows high-content, multiparameter analysis of single cells, making it a promising tool for drug discovery and profiling of intracellular signaling. To add high-throughput capacity to flow cytometry, we developed a cell-based multiplexing technique called fluorescent cell barcoding (FCB). In FCB, each sample is labeled with a different signature, or barcode, of fluorescence intensity and emission wavelengths, and mixed with other samples before antibody staining and analysis by flow cytometry. Using three FCB fluorophores, we were able to barcode and combine entire 96-well plates, reducing antibody consumption 100-fold and acquisition time to 5-15 min per plate. Using FCB and phospho-specific flow cytometry, we screened a small-molecule library for inhibitors of T cell-receptor and cytokine signaling, simultaneously determining compound efficacy and selectivity. We also analyzed IFN-gamma signaling in multiple cell types from primary mouse splenocytes, revealing differences in sensitivity and kinetics between B cells, CD4+ and CD4- T cells and CD11b-hi cells.

Phospho-proteomic immune analysis by flow cytometry: from mechanism to translational medicine at the single-cell level

Understanding a molecular basis for cellular function is a common goal of biomedicine. The complex and dynamic cellular processes underlying physiological processes become subtly or grossly perturbed in human disease. A primary objective is to demystify this complexity by creating and establishing relevant model systems to study important aspects of human disease. Although significant technological advancements over the last decade in both genomic and proteomic arenas have enabled progress, accessing the complexity of cellular interactions that occur in vivo has been a difficult arena in which to make progress. Moreover, there are extensive challenges in translating research tools to clinical applications. Flow cytometry, over the course of the last 40 years, has revolutionized the field of immunology, in both the basic science and clinical settings, as well as having been instrumental to new and exciting areas of discovery such as stem cell biology. Multiparameter machinery and systems exist now to access the heterogeneity of cellular subsets and enable phenotypic characterization and functional assays to be performed on material from both animal models and humans. This review focuses primarily on the development and application of using activation-state readouts of intracellular activity for phospho-epitopes. We present recent work on how a flow cytometric platform is used to obtain mechanistic insight into cellular processes as well as highlight the clinical applications that our laboratory has explored. Furthermore, this review discusses the challenges faced with processing high-content multidimensional and multivariate data sets. Flow cytometry, as a platform that is well situated in both the research and clinical settings, can contribute to drug discovery as well as having utility for both biomarker and patient-stratification.

Deeper insights into hematological oncology disorders via single-cell phospho-signaling analysis

An understanding of hematological cancer cell signaling processes poses one of the most complex and intractable problems in modern biomedical inquiry. While we understand some of the fundamental players that contribute to oncogenic processes, significant effort is focused upon determining how these individual players relay information to each other to create the composite functions of a cancer cell. Efforts designed to understand these processes at the single cell level will undoubtedly allow for understanding of the heterogeneity of hematological tumors as well as, simultaneously, the function of the 'responding' immune system. I will relate some of the insights our laboratory has developed over the last several years applying single-cell phospho-flow cytometry to the study of signaling in primary patient material and murine models. While it is clear that this analysis now allows us to accomplish phospho-signaling biochemistry at the single cell level with primary cell material, we are only beginning to develop some of the bioinformatics tools to appropriately display the vast amount of information collected by such approaches. These approaches, however, have already allowed us to develop approaches that prognosticate patient outcomes based on signaling status, prior to any treatment, as well as subgroup patient subtypes according to signaling states. The modest efforts to date presage a time where it should be possible to provide far more tailored therapies specific to the varied diseases represented by the hematological malignancies.

Characterization of the murine immunological signaling network with phosphospecific flow cytometry

The immune system is a multitiered network that at the first level uses changes to intracellular signaling proteins to commit cells to determined fates. At the second tier, cells interact with one another via specifically expressed surface receptors and their cognate signaling molecules. At the third level, the local environments of immune cells change the outcomes of intracellular signaling pathways and thereby the role of cells during immune challenge. The interplay among these three tiers allows the distinct cell types of the immune system to respond cohesively to eliminate foreign Ags. In this study, using phosphospecific flow cytometry, we analyze elements of these network tiers by generating profiles of single-cell phosphoprotein responses in B cells, T cells, and myeloid cells to a number of mechanistically and clinically relevant cytokines (IFN-gamma, GM-CSF, IL-2, and IL-10) as well as LPS at key regulatory interfaces (Jak-Stat and MAPK pathways). The stimuli typically induced phosphorylation of specific signaling pathways and exerted their effects on distinct subsets of immune cells. However, upon comparison of stimulation in vitro and in vivo, we noted that signaling pathway specificity and cell type specificity were influenced strongly by the external environment. When taken from the in vivo environment, certain cell subsets became hypo- or hyper-responsive, showed profound differences in sensitivity to cytokine levels, or displayed altered phosphorylation kinetics. Thus, simultaneous analysis of the three tiers of the immune system network illustrates the principles by which immune regulation is context dependent and how in vitro culture systems compare with the in vivo environment.