Applications of Flow Cytometry in Drug Discovery and Translational Research

Flow cytometry is a mainstay technique in cell biology research, where it is used for phenotypic analysis of mixed cell populations. Quantitative approaches have unlocked a deeper value of flow cytometry in drug discovery research. As the number of drug modalities and druggable mechanisms increases, there is an increasing drive to identify meaningful biomarkers, evaluate the relationship between pharmacokinetics and pharmacodynamics (PK/PD), and translate these insights into the evaluation of patients enrolled in early clinical trials. In this review, we discuss emerging roles for flow cytometry in the translational setting that supports the transition and evaluation of novel compounds in the clinic.

DOCK8 enforces immunological tolerance by promoting IL-2 signaling and immune synapse formation in Tregs

Patients deficient in the guanine nucleotide exchange factor DOCK8 have decreased numbers and impaired in vitro function of Tregs and make autoantibodies, but they seldom develop autoimmunity. We show that, similarly, Dock8-/- mice have decreased numbers and impaired in vitro function of Tregs but do not develop autoimmunity. In contrast, mice with selective DOCK8 deficiency in Tregs develop lymphoproliferation, autoantibodies, and gastrointestinal inflammation, despite a normal percentage and in vitro function of Tregs, suggesting that deficient T effector cell function might protect DOCK8-deficient patients from autoimmunity. We demonstrate that DOCK8 associates with STAT5 and is important for IL-2-driven STAT5 phosphorylation in Tregs. DOCK8 localizes within the lamellar actin ring of the Treg immune synapse (IS). Dock8-/- Tregs have abnormal TCR-driven actin dynamics, decreased adhesiveness, an altered gene expression profile, an unstable IS with decreased recruitment of signaling molecules, and impaired transendocytosis of the costimulatory molecule CD86. These data suggest that DOCK8 enforces immunological tolerance by promoting IL-2 signaling, TCR-driven actin dynamics, and the IS in Tregs.

A DOCK8-WIP-WASp complex links T cell receptors to the actin cytoskeleton

Wiskott-Aldrich syndrome (WAS) is associated with mutations in the WAS protein (WASp), which plays a critical role in the initiation of T cell receptor-driven (TCR-driven) actin polymerization. The clinical phenotype of WAS includes susceptibility to infection, allergy, autoimmunity, and malignancy and overlaps with the symptoms of dedicator of cytokinesis 8 (DOCK8) deficiency, suggesting that the 2 syndromes share common pathogenic mechanisms. Here, we demonstrated that the WASp-interacting protein (WIP) bridges DOCK8 to WASp and actin in T cells. We determined that the guanine nucleotide exchange factor activity of DOCK8 is essential for the integrity of the subcortical actin cytoskeleton as well as for TCR-driven WASp activation, F-actin assembly, immune synapse formation, actin foci formation, mechanotransduction, T cell transendothelial migration, and homing to lymph nodes, all of which also depend on WASp. These results indicate that DOCK8 and WASp are in the same signaling pathway that links TCRs to the actin cytoskeleton in TCR-driven actin assembly. Further, they provide an explanation for similarities in the clinical phenotypes of WAS and DOCK8 deficiency.

Heterozygosity for transmembrane activator and calcium modulator ligand interactor A144E causes haploinsufficiency and pneumococcal susceptibility in mice

BACKGROUND

The B-cell receptor transmembrane activator and calcium modulator ligand interactor (TACI) is important for T-independent antibody responses. One in 200 blood donors are heterozygous for the TACI A181E mutation.

OBJECTIVE

We sought to investigate the effect on B-cell function of TACI A181E heterozygosity in reportedly healthy subjects and of the corresponding TACI A144E mutation in mice.

METHODS

Nuclear factor κB (NF-κB) activation was measured by using the luciferase assay in 293T cells cotransfected with wild-type and mutant TACI. TACI-driven proliferation, isotype switching, and antibody responses were measured in B cells from heterozygous TACI A144E knock-in mice. Mouse mortality was monitored after intranasal pneumococcal challenge.

RESULTS

Levels of natural antibodies to the pneumococcal polysaccharide component phosphocholine were significantly lower in A181E-heterozygous than TACI-sufficient Swedish blood donors never immunized with pneumococcal antigens. Although overexpressed hTACI A181E and mTACI A144E acted as dominant-negative mutations in transfectants, homozygosity for A144E in mice resulted in absent TACI expression in B cells, indicating that the mutant protein is unstable when naturally expressed. A144E heterozygous mice, such as TACI^+/- mice, expressed half the normal level of TACI on their B cells and exhibited similar defects in a proliferation-inducing ligand-driven B-cell activation, antibody responses to TNP-Ficoll, production of natural antibodies to phosphocholine, and survival after intranasal pneumococcal challenge.

CONCLUSION

These results suggest that TACI A181E heterozygosity results in TACI haploinsufficiency with increased susceptibility to pneumococcal infection. This has important implications for asymptomatic TACI A181E carriers.

The Thoc1 encoded ribonucleoprotein is required for myeloid progenitor cell homeostasis in the adult mouse

Co-transcriptionally assembled ribonucleoprotein (RNP) complexes are critical for RNA processing and nuclear export. RNPs have been hypothesized to contribute to the regulation of coordinated gene expression, and defects in RNP biogenesis contribute to genome instability and disease. Despite the large number of RNPs and the importance of the molecular processes they mediate, the requirements for individual RNP complexes in mammalian development and tissue homeostasis are not well characterized. THO is an evolutionarily conserved, nuclear RNP complex that physically links nascent transcripts with the nuclear export apparatus. THO is essential for early mouse embryonic development, limiting characterization of the requirements for THO in adult tissues. To address this shortcoming, a mouse strain has been generated allowing inducible deletion of the Thoc1 gene which encodes an essential protein subunit of THO. Bone marrow reconstitution was used to generate mice in which Thoc1 deletion could be induced specifically in the hematopoietic system. We find that granulocyte macrophage progenitors have a cell autonomous requirement for Thoc1 to maintain cell growth and viability. Lymphoid lineages are not detectably affected by Thoc1 loss under the homeostatic conditions tested. Myeloid lineages may be more sensitive to Thoc1 loss due to their relatively high rate of proliferation and turnover.

TRIF signaling is essential for TLR4-driven IgE class switching (IRM5P.707)

The TLR4 ligand LPS causes mouse B cells to undergo IgE and IgG1 isotype switching in the presence of IL-4. TLR4 activates two signaling pathways mediated by the adaptor molecules MyD88 and TRAM, which recruits TRIF. Following stimulation with LPS+IL-4, Tram-/- and Trif-/- B cells completely failed to express Cε germ line transcripts (GLT) and secrete IgE. In contrast, Myd88-/- B cells had normal expression of Cγ1 GLT, but reduced IgE secretion in response to LPS+IL-4. Following LPS+IL-4 stimulation, Cγ1 GLT expression was modestly reduced in Tram-/- and Trif-/- B cells, whereas Aicda expression and IgG1 secretion were reduced in Tram-/-,Trif-/-, and Myd88-/- B cells. B cells from all strains secreted normal amounts of IgE and IgG1 in response to anti-CD40+IL-4. Following stimulation with LPS+IL-4, Trif-/- B cells failed to sustain NFκB p65 nuclear translocation beyond 3 hours and had reduced binding of p65 to the Iε promoter. Addition of the NFκB inhibitor, JSH-23, to wild-type B cells 15 hours after LPS+IL-4 stimulation selectively blocked Cε GLT expression and IgE secretion, but had little effect on Cγ1 GLT expression and IgG secretion. These results indicate that sustained activation of NFκB driven by TRIF is essential for LPS+IL-4 driven activation of the Cε locus and class switching to IgE.

TRIF signaling is essential for TLR4-driven IgE class switching

The TLR4 ligand LPS causes mouse B cells to undergo IgE and IgG1 isotype switching in the presence of IL-4. TLR4 activates two signaling pathways mediated by the adaptor molecules MyD88 and Toll/IL-IR domain-containing adapter-inducing IFN-β (TRIF)-related adaptor molecule (TRAM), which recruits TRIF. Following stimulation with LPS plus IL-4, Tram(-/-) and Trif(-/-) B cells completely failed to express Cε germline transcripts (GLT) and secrete IgE. In contrast, Myd88(-/-) B cells had normal expression of Cε GLT but reduced IgE secretion in response to LPS plus IL-4. Following LPS plus IL-4 stimulation, Cγ1 GLT expression was modestly reduced in Tram(-/-) and Trif(-/-) B cells, whereas Aicda expression and IgG1 secretion were reduced in Tram(-/-), Trif(-/-), and Myd88(-/-) B cells. B cells from all strains secreted normal amounts of IgE and IgG1 in response to anti-CD40 plus IL-4. Following stimulation with LPS plus IL-4, Trif(-/-) B cells failed to sustain NF-κB p65 nuclear translocation beyond 3 h and had reduced binding of p65 to the Iε promoter. Addition of the NF-κB inhibitor, JSH-23, to wild-type B cells 15 h after LPS plus IL-4 stimulation selectively blocked Cε GLT expression and IgE secretion but had little effect on Cγ1 GLT expression and IgG secretion. These results indicate that sustained activation of NF-κB driven by TRIF is essential for LPS plus IL-4-driven activation of the Cε locus and class switching to IgE.

Abstract 5193: The RNP biogenesis factor Thoc1 is required for proliferative intestinal stem cell viability

Thoc1 encodes an RNA binding protein (Thoc1) which interacts with the tumor suppressor retinoblastoma (Rb1). Thoc1 levels have been found to be elevated in multiple cancers relative to normal tissue. Additionally cancer cells but not most normal cells undergo apoptosis following Thoc1 deletion, suggesting Thoc1 may be a good therapeutic target. In normal tissue, Thoc1 is needed for embryonic inner cell mass viability and Thoc1 hypomorphic mice are small and sterile. Based on these data we hypothesize highly proliferative cells, possibly those with an extended replicative potential, such as stem cells, require Thoc1. To test this, we examined the effect of Thoc1 deletion in the intestine since it contains well-defined stem cells Thoc1 was deleted in the adult mouse using the Tamoxifen/CreER system with Rosa26 driving CreER expression. Mice were treated with 2 mg/day Tamoxifen for 1, 2, 3, 4, 5, or 6 days. Thoc1 deletion completely disrupts small intestine (SI) structure. Specifically, Thoc1 deletion impairs SI crypt cell function and viability, which leads to a complete loss of SI epithelial cell turnover. The SI crypts contain rapidly proliferating intestinal stem (ISC) and progenitor cells that continuously feed and replace the differentiated SI villi epithelium every 3-5 days. Within the proliferative crypts, multiple ISC populations exist. These include the active, homeostatic Lgr5+ ISCs and the reserve, more quiescent mTert+ and Bmi1+ ISCs. Lgr5+ ISCs are the most actively proliferating ISC population, whereas mTert+ and Bmi1+ ISCs are believed to become proliferative upon tissue insult. QRT-PCR data of ISC (Lgr5, mTert, Bmi1) marker transcripts from crypt purified epithelial cells suggests Thoc1 loss initially induces ISC populations to proliferate and expand (transcripts increase), likely a direct response to the initial Thoc1 deletion. However, once active it appears proliferating ISCs become susceptible to Thoc1 deletion, as ISC marker transcripts ultimately approach zero on day 6 in test mice. In addition, ISC fate mapping studies show recombined ISCs in test mice are less likely to give rise to long lived stable progeny than recombined ISCs in control mice. Despite a similar structure to the SI, Thoc1 deletion does not appear to affect the colon. The colon too contains ISCs that give rise to all cells in the tissue; however, colon ISCs are more quiescent versus the SI ISC, which suggests actively proliferating stem/progenitor cell types may specifically require Thoc1. The results of this study suggest actively proliferating stem cell types require Thoc1, most likely to support the coordinated gene expression necessary for rapid proliferation and differentiation. This finding is significant as it sheds light on Thoc1 function and may help explain why Thoc1 levels are increased in various cancers, as proliferating stem cells and cancer cells share similar traits.

Citation Format: Laura B. Pitzonka, Sumana Ullas, David Goodrich. The RNP biogenesis factor Thoc1 is required for proliferative intestinal stem cell viability. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5193. doi:10.1158/1538-7445.AM2013-5193

Abstract 1850: The role of Thoc1 in tissue homeostasis in the adult mouse

Thoc1 encodes a protein (Thoc1) that interacts with the retinoblastoma protein (Rb). Thoc1 functions in an evolutionary conserved protein complex called THO that is involved in nuclear ribonucleoprotein particle (RNP) formation. Loss of THO function compromises the packaging of nascent RNA, which causes defects in transcription, RNA processing and mRNA export. In mice, Thoc1 is required for embryonic development. It is currently unknown whether Thoc1 is required in adult mice for normal tissue homeostasis. We hypothesize that different tissues will vary in their dependence on Thoc1, possibly based on their proliferative potential. To test this hypothesis, we conditionally deleted Thoc1 in adult mice in a wide variety of tissues using the Tamoxifen/CreER recombination system, and assessed the consequences on tissue homeostasis. Thoc1F/F:Rosa26CreER+/− (test) and Thoc1+/+:Rosa26CreER+/− (control) adult mice were treated with 2 mg/day Tamoxifen for 6 days. Slowly proliferating (liver, kidney, colon) and rapidly proliferating (small intestinal (S.I.)) tissue were collected 24 hours after the last treatment. Histological analysis shows Thoc1 loss disrupts the architecture of the small intestine (S.I.) but does not affect the other tissues examined. The functional unit of the S.I. is the crypt-villus axis. S.I. crypts contain rapidly proliferating stem and progenitor cells that continuously feed the differentiated, non-proliferative villus epithelium. The crypts proliferate so rapidly that the entire S.I. epithelium is replaced every three to five days. Interestingly, Thoc1 loss only causes apoptosis in the S.I. crypts, suggesting disruption of villi architecture following Thoc1 loss is a result of altered crypt function, not a direct effect of Thoc1 loss. Supporting this, the crypts of mice in which Thoc1 has been deleted also have a loss of cell proliferation. While the colon and S.I. have similar tissue architecture, the colon's rate of turnover is much slower. Therefore, it is not surprising the colon is not affected by Thoc1 loss. These findings support the hypothesis that tissues with a high cell proliferative potential require Thoc1. To explore this further, we examined how Thoc1 deletion would affect the highly proliferative hematopoietic system using the same test and control mice as previously listed. Further supporting the hypothesis, Thoc1 loss decreases the number of lymphocytes in the blood, but does not affect spleen and thymus (slowly proliferating tissues) histology. The results of this study suggest highly proliferative tissues that turnover rapidly require Thoc1, most likely to support the gene expression and cell growth necessary for rapid cell division. This hypothesis is significant as it predicts cancer cells will be more sensitive to Thoc1 loss than normal cells, a prediction verified by preliminary results in our lab.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1850. doi:1538-7445.AM2012-1850