T cells Use Focal Adhesions to Pull Themselves Through Confined Environments

Immune cells are highly dynamic and able to migrate through environments with diverse biochemical and mechanical composition. Their migration has classically been defined as amoeboid under the assumption that it is integrin-independent. Here we show that activated primary Th1 T cells require both confinement and extracellular matrix protein to migrate efficiently. This migration is mediated through small and dynamic focal adhesions that are composed of the same proteins associated with canonical mesenchymal focal adhesions, such as integrins, talin, and vinculin. These focal adhesions, furthermore, localize to sites of contractile traction stresses, enabling T cells to pull themselves through confined spaces. Finally, we show that Th1 T cell preferentially follows tracks of other T cells, suggesting that these adhesions are modifying the extracellular matrix to provide additional environmental guidance cues. These results demonstrate not only that the boundaries between amoeboid and mesenchymal migration modes are ambiguous, but that integrin-mediated adhesions play a key role in T cell motility.

Ccr2+ Monocyte-Derived Macrophages Influence Trajectories of Acquired Therapy Resistance in Braf-Mutant Melanoma

Therapies targeting oncogene addiction have had a tremendous impact on tumor growth and patient outcome, but drug resistance continues to be problematic. One approach to deal with the challenge of resistance entails extending anticancer treatments beyond targeting cancer cells by additionally altering the tumor microenvironment. Understanding how the tumor microenvironment contributes to the evolution of diverse resistance pathways could aid in the design of sequential treatments that can elicit and take advantage of a predictable resistance trajectory. Tumor-associated macrophages often support neoplastic growth and are frequently the most abundant immune cell found in tumors. Here, we used clinically relevant in vivo Braf-mutant melanoma models with fluorescent markers to track the stage-specific changes in macrophages under targeted therapy with Braf/Mek inhibitors and assessed the dynamic evolution of the macrophage population generated by therapy pressure-induced stress. During the onset of a drug-tolerant persister state, Ccr2+ monocyte-derived macrophage infiltration rose, suggesting that macrophage influx at this point could facilitate the onset of stable drug resistance that melanoma cells show after several weeks of treatment. Comparison of melanomas that develop in a Ccr2-proficient or -deficient microenvironment demonstrated that lack of melanoma infiltrating Ccr2+ macrophages delayed onset of resistance and shifted melanoma cell evolution towards unstable resistance. Unstable resistance was characterized by sensitivity to targeted therapy when factors from the microenvironment were lost. Importantly, this phenotype was reversed by coculturing melanoma cells with Ccr2+ macrophages. Overall, this study demonstrates that the development of resistance may be directed by altering the tumor microenvironment to improve treatment timing and the probability of relapse.

SIGNIFICANCE

Ccr2+ melanoma macrophages that are active in tumors during the drug-tolerant persister state following targeted therapy-induced regression are key contributors directing melanoma cell reprogramming toward specific therapeutic resistance trajectories.

Spatially Restricted T-cell Activation in Inflamed Tissues

Pathogen control requires T-cells to locate and make direct contact with antigen-presenting cells (APCs) for peripheral reactivation and delivery of effector molecules. The signals that orchestrate T-cell-APC interactions are poorly understood in inflamed peripheral tissues. Using intravital imaging (IVI), we have found that Th1 cell entry into the inflamed dermis is nucleated by perivascular CXCL10+ cellular clusters enriched for APCs. Th1 cells form more stable interactions with APCs within these clusters compared to outer regions. CXCL10+ clusters are amplified in a IFNγ-dependent manner, where CD4+ T-cell IFNγ release increases APC recruitment and CXCL10 expression. We hypothesize that these chemokine clusters serve as CXCL10 Peripheral Activation (PAC-10) niches, where Th1 cells achieve early peripheral activation away from the pathogen modulated milieu. We seek to determine the relationship between these spatially restricted PAC-10 niches, infection foci, and pathogen clearance. Using a combination of IVI, flow cytometry, and transcriptomics we characterized niche specific Th1 cells and niche cellular composition to determine if PAC-10 niches modulate Th1 cells for pathogen control. Initial studies reveal that niche specific Th1 cells have increased CD69 and IFNγ expression compared to non-niche counterparts. PAC-10 niches appear to have a spatially distinct innate cell population, likely with enhanced ability to support Th1 activation. Future studies will characterize the functional status of niche specific Th1 cells and evaluate the necessity of PAC-10 niches for pathogen control. These studies will inform therapeutic strategies to boost T-cell activation in infection or hinder activation in autoimmunity.

Supported by NIH NIAID P01 AI02851 NIH NIAID R01 AI070826

Understanding the temporal requirement for T follicular helper cells for germinal center output of long-lived plasma cells and B memory cells

T follicular helper cells (Tfh) play a critical role in supplying help to B cells for the selection of high-affinity B cells in germinal centers (GC), that give rise to memory B cells (Bmem) and long-lived plasma cells (LLPCs). The products of the GC reaction appear to be temporally distinct, with Bmem cells produced early and LLPCs late.

The temporal requirements for Tfh help are not well understood and we still do not know when and for how long this help is critical. In the total absence of help supplied by Tfh cells, both GCs and LLPCs are greatly diminished. Our previous work (Schrock, Leddon et al 2019, PNAS) suggested that sustained GC Tfh were critical for LLPC generation, but interestingly not for Bmem generation. We hypothesize that a temporal requirement for GC Tfh regulates LLPC versus Bmem GC output. To test this, we have established a number of models that allow for deletion of Tfh at different times post OVA/CFA immunization. To specifically target Tfh cells, CD4 iCRE Bcl6fl/fl mice were used for time-dependent deletion of Bcl6 upon tamoxifen administration at different stages of the GC reaction. Tfh and GC output are determined using multicolor Flow Cytometry panels, BM B cell Elispot for LLPCs and a Bmem Elispot. With these systems in place, we aim to pinpoint the temporal requirement for Tfh for both Bmem and LLPCs, speculating that timing of Tfh help will differ for these two arms of the GC response.

Understanding this temporal requirement of Tfh help within the GC can not only help us to better design future vaccines that promote LLPCs, but also guide the development of drugs to modulate Tfh longevity in autoimmune disorders, where aberrant Tfh development or maintenance contribute to pathology.

IL-17-Dependent Dysregulated Cutaneous Immune Homeostasis in the Absence of the Wiskott-Aldrich Syndrome Protein

Wiskott-Aldrich Syndrome (WAS) is characterized by recurrent infections, thrombocytopenia, and eczema. Here, we show that WASp-deficient mice on a BALB/c background have dysregulated cutaneous immune homeostasis with increased leukocyte accumulation in the skin, 1 week after birth. Increased cutaneous inflammation was associated with epithelial abnormalities, namely, altered keratinization, abnormal epidermal tight junctional morphology and increased trans-epidermal water loss; consistent with epidermal barrier dysfunction. Immune and physical barrier disruption was accompanied by progressive skin dysbiosis, highlighting the functional significance of the disrupted cutaneous homeostasis. Interestingly, the dysregulated immunity in the skin preceded the systemic elevation in IgE and lymphocytic infiltration of the colonic lamina propria associated with WASp deficiency. Mechanistically, the enhanced immune cell accumulation in the skin was lymphocyte dependent. Elevated levels of both Type 2 (IL-4, IL-5) and Type 17 (IL-17, IL-22, IL-23) cytokines were present in the skin, as well as the 'itch' factor IL-31. Unexpectedly, the canonical WAS-associated cytokine IL-4 did not play a role in the immune dysfunction. Instead, IL-17 was critical for skin immune infiltration and elevation of both Type 2 and Type 17 cytokines. Our findings reveal a previously unrecognized IL-17-dependent breakdown in immune homeostasis and cutaneous barrier integrity in the absence of WASp, targeting of which may provide new therapeutic possibilities for the treatment of skin pathologies in WAS patients.

T cell activation niches-Optimizing T cell effector function in inflamed and infected tissues

Successful immunity to infection, malignancy, and tissue damage requires the coordinated recruitment of numerous immune cell subsets to target tissues. Once within the target tissue, effector T cells rely on local chemotactic cues and structural cues from the tissue matrix to navigate the tissue, interact with antigen-presenting cells, and release effector cytokines. This highly dynamic process has been "caught on camera" in situ by intravital multiphoton imaging. Initial studies revealed a surprising randomness to the pattern of T cell migration through inflamed tissues, behavior thought to facilitate chance encounters with rare antigen-bearing cells. Subsequent tissue-wide visualization has uncovered a high degree of spatial preference when it comes to T cell activation. Here, we discuss the basic tenants of a successful effector T cell activation niche, taking cues from the dynamics of Tfh positioning in the lymph node germinal center. In peripheral tissues, steady-state microanatomical organization may direct the location of "pop-up" de novo activation niches, often observed as perivascular clusters, that support early effector T cell activation. These perivascular activation niches appear to be regulated by site-specific chemokines that coordinate the recruitment of dendritic cells and other innate cells for local T cell activation, survival, and optimized effector function.

The spatio-temporal control of effector T cell migration

Effector T cells leave the lymph nodes armed with specialized functional attributes. Their antigenic targets may be located anywhere in the body, posing the ultimate challenge: how to efficiently identify the target tissue, navigate through a complex tissue matrix and, ultimately, locate the immunological insult. Recent advances in real-time in situ imaging of effector T cell migratory behaviour have revealed a great degree of mechanistic plasticity that enables effector T cells to push and squeeze their way through inflamed tissues. This process is shaped by an array of 'stop' and 'go' guidance signals including target antigens, chemokines, integrin ligands and the mechanical cues of the inflamed microenvironment. Effector T cells must sense and interpret these competing signals to correctly position themselves to mediate their effector functions for complete and durable responses in infectious disease and malignancy. Tuning T cell migration therapeutically will require a new understanding of this complex decision-making process.

Hyperspectral multiphoton microscopy for in vivo visualization of multiple, spectrally overlapped fluorescent labels

The insensitivity of multiphoton microscopy to optical scattering enables high-resolution, high-contrast imaging deep into tissue, including in live animals. Scattering does, however, severely limit the use of spectral dispersion techniques to improve spectral resolution. In practice, this limited spectral resolution together with the need for multiple excitation wavelengths to excite different fluorophores limits multiphoton microscopy to imaging a few, spectrally-distinct fluorescent labels at a time, restricting the complexity of biological processes that can be studied. Here, we demonstrate a hyperspectral multiphoton microscope that utilizes three different wavelength excitation sources together with multiplexed fluorescence emission detection using angle-tuned bandpass filters. This microscope maintains scattering insensitivity, while providing high enough spectral resolution on the emitted fluorescence and capitalizing on the wavelength-dependent nonlinear excitation of fluorescent dyes to enable clean separation of multiple, spectrally overlapping labels, in vivo. We demonstrated the utility of this instrument for spectral separation of closely-overlapped fluorophores in samples containing ten different colors of fluorescent beads, live cells expressing up to seven different fluorescent protein fusion constructs, and in multiple in vivo preparations in mouse cortex and inflamed skin with up to eight different cell types or tissue structures distinguished.

In situ neutrophil efferocytosis shapes T cell immunity to influenza infection

Early recruitment of neutrophils from the blood to sites of tissue infection is a hallmark of innate immune responses. However, little is known about the mechanisms by which apoptotic neutrophils are cleared in infected tissues during resolution and the immunological consequences of in situ efferocytosis. Using intravital multiphoton microscopy, we show previously unrecognized motility patterns of interactions between neutrophils and tissue-resident phagocytes within the influenza-infected mouse airway. Newly infiltrated inflammatory monocytes become a chief pool of phagocytes and play a key role in the clearance of highly motile apoptotic neutrophils during the resolution phase. Apoptotic neutrophils further release epidermal growth factor and promote the differentiation of monocytes into tissue-resident antigen-presenting cells for activation of antiviral T cell effector functions. Collectively, these results suggest that the presence of in situ neutrophil resolution at the infected tissue is critical for optimal CD8+ T cell-mediated immune protection.

CD4+ T Cell Interstitial Migration Controlled by Fibronectin in the Inflamed Skin

The extracellular matrix (ECM) is extensively remodeled during inflammation providing essential guidance cues for immune cell migration and signals for cell activation and survival. There is increasing interest in the therapeutic targeting of ECM to mitigate chronic inflammatory diseases and enhance access to the tumor microenvironment. T cells utilize the ECM as a scaffold for interstitial migration, dependent on T cell expression of matrix-binding integrins αVβ1/αVβ3 and tissue display of the respective RGD-containing ligands. The specific ECM components that control T cell migration are unclear. Fibronectin (FN), a canonical RGD-containing matrix component, is heavily upregulated in inflamed tissues and in vitro can serve as a substrate for leukocyte migration. However, limited by lack of tools to intravitally visualize and manipulate FN, the specific role of FN in effector T cell migration in vivo is unknown. Here, we utilize fluorescently-tagged FN to probe for FN deposition, and intravital multiphoton microscopy to visualize T cell migration relative to FN in the inflamed ear dermis. Th1 cells were found to migrate along FN fibers, with T cells appearing to actively push or pull against flexible FN fibers. To determine the importance of T cell interactions with FN, we used a specific inhibitor of FN polymerization, pUR4. Intradermal delivery of pUR4 (but not the control peptide) to the inflamed skin resulted in a local reduction in FN deposition. We also saw a striking attenuation of Th1 effector T cell movement at the pUR4 injection site, suggesting FN plays a key role in T cell interstitial migration. In mechanistic studies, pUR4 incubation with FN in vitro resulted in enhanced tethering of T cells to FN matrix, limiting productive migration. In vivo, such tethering led to increased Th1 accumulation in the inflamed dermis. Enhanced Th1 accumulation exacerbated inflammation with increased Th1 activation and IFNγ cytokine production. Thus, our studies highlight the importance of ECM FN fibrils for T cell migration in inflamed tissues and suggest that manipulating local levels of ECM FN may prove beneficial in promoting T cell accumulation in tissues and enhancing local immunity to infection or cancer.

Retention and egress of Th1 cells in the inflamed skin

The inappropriate trafficking and retention of immune subsets in tissue is a hallmark of many diseases prevalent in the developed world. These issues include autoimmunity, chronic inflammation, and unbalanced tissue repair responses which represent a significant disease burden: diabetes, coronary artery disease, inflammatory bowel disease, hepatitis, rheumatoid arthritis, and asthma. Although knowledge about cell trafficking to inflamed sites has grown in the past decade, there is a current gap in knowledge in identifying when and how immune cells decide to stay or leave these tissues.

To determine phenotypic and functional differences between those Th1 cells that remain at the inflamed site and those that exit, we employed the photoconvertible Kaede protein system to enable the labelling of Th1 cells in time and space. Kaede+OT-II Th1 T cells were adoptively transferred into WT mice immunized with CFA/OVA. Th1 cells recruited to the inflamed ear skin were photoconverted from Kaede green to Kaede red. Kaede red Th1 cells retained at the inflamed site could be distinguished from newly recruited Kaede green cells, while Kaede red Th1 cells that subsequently exited the inflamed skin were readily detected in the draining lymph node. Using this system, we are able to analyze the phenotypic (multicolor flow cytometry) and transcriptional signatures (RNA-seq) of Th1 cells that are retained at the inflamed site and those Th1 cells that exit (tissue-migrants). Given the temporal component of our model, we are gaining new insight into when and where expression of conventional markers of trafficking and activation are regulated (CD62L, CD44, and CD69) and seek to identify new pathways that dictate T cell retention and egress at inflammatory sites.

CXCL10+ perivascular clusters nucleate Th1 cell tissue entry and activation in the inflamed skin

Efficient recruitment and correct positioning of T cells within infected peripheral tissues are crucial for pathogen clearance. Using a dual-chemokine reporter (Groom et Al. Immunity, 2012) mouse and intra-vital imaging, we identified a preferred tissue entry and accumulation site for effector Th1 cells, defined by clusters of CXCL9 and CXCL10 expressing hematopoietic cells, enriched with MHC-class-II+ cells, in restricted perivascular spaces within the inflamed dermis. Unbiased computational analysis of motility dynamics of antigen-specific Th1 cells within these clusters, revealed cell confinement characteristics. CXCR3 on Th1 cells and presence of cognate antigen were critical for persistence within clusters and T cell activation. Newly recruited Th1 cells strongly amplified CXCL9 and CXCL10 expression, mainly within skin monocyte-derived dendritic cells, in an antigen- and interferon-gamma-dependent manner, resulting in augmented chemokine clusters. Our data suggest a CXCR3-mediated mechanism of intrinsic amplification loop for optimal Th1 cell recruitment, positioning and activation in restricted antigen presentation sites within the inflamed skin.

In vivo tracking: Using a photoconvertible system to probe immune cell entry and exit in the context of inflammation

Chronic inflammatory diseases such as Crohn’s disease and rheumatoid arthritis affect millions of people worldwide. During an inflammatory response, immune cells enter the inflamed tissue to mediate pathogen clearance, however, their activation and overaccumulation in the tissue can lead to chronic inflammation and autoimmunity. Little is known about the signals that regulate immune cell entry into and exit from these inflamed tissues or about the retention cues that play a role in their accumulation. Therefore, a better understanding of these signals would inform the design of therapeutics intended to alleviate chronic inflammation. To study immune cell movement between a site of inflammation and lymphoid organs, our lab has acquired a mouse model that allows for the in vivo labeling of a subset of cells in time and space. The cells of Kaede transgenic mice constitutively express the photoconvertible fluorescent Kaede protein. CFA/OVA-inflamed ears were photoconverted from Kaede green to red by violet light exposure and the trafficking of Kaede red cells out of the skin to the draining lymph node was assessed over the course of 24 hours. Based on preliminary data, we found that, of the subsets assessed, neutrophils and CD4+ T cells made up the largest percentages of immune cells exiting the inflamed tissue on day 5 of inflammation. On the other hand, far fewer conventional dendritic cells and macrophages were seen to enter the draining lymph node from the inflamed tissue. The role of these tissue-emigrant immune cells is not yet clear, but cells such as neutrophils may play a role in shaping adaptive immune responses in the draining lymph node.

Programming of Distinct Chemokine-Dependent and -Independent Search Strategies for Th1 and Th2 Cells Optimizes Function at Inflamed Sites

T-helper (Th) cell differentiation drives specialized gene programs that dictate effector T cell function at sites of infection. Here, we have shown Th cell differentiation also imposes discrete motility gene programs that shape Th1 and Th2 cell navigation of the inflamed dermis. Th1 cells scanned a smaller tissue area in a G protein-coupled receptor (GPCR) and chemokine-dependent fashion, while Th2 cells scanned a larger tissue area independent of GPCR signals. Differential chemokine reliance for interstitial migration was linked to STAT6 transcription-factor-dependent programming of integrin α_Vβ₃ expression: Th2 cell differentiation led to high α_Vβ₃ expression relative to Th1 cells. Th1 and Th2 cell modes of motility could be switched simply by manipulating the amount of α_Vβ₃ on the cell surface. Deviating motility modes from those established during differentiation impaired effector function. Thus, programmed expression of α_Vβ₃ tunes effector T cell reliance on environmental cues for optimal exploration of inflamed tissues.

Innate Immune Cells Are Regulated by Axl in Hypertensive Kidney

The balance between adaptive and innate immunity in kidney damage in salt-dependent hypertension is unclear. We investigated early renal dysfunction and the influence of Axl, a receptor tyrosine kinase, on innate immune response in hypertensive kidney in mice with lymphocyte deficiency (Rag1^-/-). The data suggest that increased presence of CD11b⁺ myeloid cells in the medulla might explain intensified salt and water retention as well as initial hypertensive response in Rag1^-/- mice. Global deletion of Axl on Rag1^-/- background reversed kidney dysfunction and accumulation of myeloid cells in the kidney medulla. Chimeric mice that lack Axl in innate immune cells (in the absence of lymphocytes) significantly improved kidney function and abolished early hypertensive response. The bioinformatics analyses of Axl-related gene-gene interaction networks established tissue-specific variation in regulatory pathways. It was confirmed that complement C3 is important for Axl-mediated interactions between myeloid and vascular cells in hypertensive kidney. In summary, innate immunity is crucial for renal dysfunction in early hypertension, and is highly influenced by the presence of Axl.

CXCL9/10 producing cell clusters optimize Th1 cell positioning for interaction with antigen presenting cells

Intracellular pathogen clearance is dependent on successful T cell mediated immunity. Th1 cells must localize to the site of infection to be activated into IFN-γ producing effector T cells. How Th1 cells find and interact with antigen presenting cells (APC) in infected tissues is poorly understood. Preliminary data using intravital imaging and the REX3 mouse, in which CXCL9 and CXCL10 producing cells express red and blue fluorescent proteins (RFP/BFP) respectively, suggest that CXCL9/10-producing cells form dense perivascular clusters that attract and arrest CXCR3+ Th1 cells. We hypothesize that these CXCL10+ cell clusters are enriched for APC that optimize Th1 cell activation and IFN-γ production. To determine the phenotype of the immune cells within the chemokine-producing clusters we crossed the REX3 mouse with a photoactivatable green fluorescent protein (PA-GFP) mouse. This will enable selective labeling of cells within chemokine-rich clusters prior to flow cytometry. Using precise multiphoton delivery of 830 nm light, we have successfully photoactivated the CXCL10+ clustered regions in the inflamed ear and have developed a flow cytometry panel to distinguish between macrophages, cDCs, and monocyte-derived cells that may be positioned within or outside these chemokine-clusters. Additionally, we will determine if these cell clusters enhance Th1 function by sorting photoactivated (in clusters) and non-photoactivated (outside clusters) Th1 cells from the inflamed ear and analyzing IFN-γ expression. Understanding how Th1 cells are positioned and activated at sites of inflammation will allow the field to boost responses to infectious agents and therapeutically attenuate Th1 responses in autoimmunity.

CCL7 Is a Negative Regulator of Cutaneous Inflammation Following Leishmania major Infection

The chemokine CCL7 (MCP3) is known to promote the recruitment of many innate immune cell types including monocytes and neutrophils to sites of bacterial and viral infection and eosinophils and basophils to sites of allergic inflammation. CCL7 upregulation has been associated with many inflammatory settings including infection, cardiovascular disease, and the tumor microenvironment. CCL7's pleotropic effects are due in part to its ability to bind numerous chemokine receptors, namely CCR1, CCR2, CCR3, CCR5, and CCR10. CCL7-blockade or CCL7-deficiency is often marked by decreased inflammation and poor pathogen control. In the context of Leishmania major infection, CCL7 is specifically upregulated in the skin one-2 weeks after infection but its role in L. major control is unclear. To determine CCL7's impact on the response to L. major we infected WT and CCL7^-/- C57BL/6 mice. L. major infection of CCL7-deficient mice led to an unexpected increase in inflammation in the infected skin 2 weeks post-infection. A broad increase in immune cell subsets was observed but was dominated by enhanced neutrophilic infiltration. Increased neutrophil recruitment was associated with an enhanced IL-17 gene profile in the infected skin. CCL7 was shown to directly antagonize neutrophil migration in vitro and CCL7 add-back in vivo specifically reduced neutrophil influx into the infected skin revealing an unexpected role for CCL7 in limiting neutrophil recruitment during L. major infection. Enhanced neutrophilic infiltration in CCL7-deficient mice changed the balance of L. major infected host cells with an increase in the ratio of infected neutrophils over monocytes/macrophages. To determine the consequence of CCL7 deficiency on L. major control we analyzed parasite load cutaneously at the site of infection and viscerally in the draining LN and spleen. The CCL7^-/- mice supported robust cutaneous parasite control similar to their WT C57BL/6 counterparts. In contrast, CCL7-deficiency led to greater parasite dissemination and poor parasite control in the spleen. Our studies reveal a novel role for CCL7 in negatively regulating cutaneous inflammation, specifically neutrophils, early during L. major infection. We propose that CCL7-mediated dampening of the early immune response in the skin may limit the ability of the parasite to disseminate without compromising cutaneous control.

The Integrin LFA-1 Controls T Follicular Helper Cell Generation and Maintenance

T follicular helper (Tfh) cells are a CD4⁺ T cell subset critical for long-lived humoral immunity. We hypothesized that integrins play a decisive role in Tfh cell biology. Here we show that Tfh cells expressed a highly active form of leukocyte function-associated antigen-1 (LFA-1) that was required for their survival within the germinal center niche. In addition, LFA-1 promoted expression of Bcl-6, a transcriptional repressor critical for Tfh cell differentiation, and inhibition of LFA-1 abolished Tfh cell generation and prevented protective humoral immunity to intestinal helminth infection. Furthermore, we demonstrated that expression of Talin-1, an adaptor protein that regulates LFA-1 affinity, dictated Tfh versus Th2 effector cell differentiation. Collectively, our results define unique functions for LFA-1 in the Tfh cell effector program and suggest that integrin activity is important in lineage decision-making events in the adaptive immune system.

T Cell Interstitial Migration: Motility Cues from the Inflamed Tissue for Micro- and Macro-Positioning

Effector T cells exit the inflamed vasculature into an environment shaped by tissue-specific structural configurations and inflammation-imposed extrinsic modifications. Once within interstitial spaces of non-lymphoid tissues, T cells migrate in an apparent random, non-directional, fashion. Efficient T cell scanning of the tissue environment is essential for successful location of infected target cells or encounter with antigen-presenting cells that activate the T cell's antimicrobial effector functions. The mechanisms of interstitial T cell motility and the environmental cues that may promote or hinder efficient tissue scanning are poorly understood. The extracellular matrix (ECM) appears to play an important scaffolding role in guidance of T cell migration and likely provides a platform for the display of chemotactic factors that may help to direct the positioning of T cells. Here, we discuss how intravital imaging has provided insight into the motility patterns and cellular machinery that facilitates T cell interstitial migration and the critical environmental factors that may optimize the efficiency of effector T cell scanning of the inflamed tissue. Specifically, we highlight the local micro-positioning cues T cells encounter as they migrate within inflamed tissues, from surrounding ECM and signaling molecules, as well as a requirement for appropriate long-range macro-positioning within distinct tissue compartments or at discrete foci of infection or tissue damage. The central nervous system (CNS) responds to injury and infection by extensively remodeling the ECM and with the de novo generation of a fibroblastic reticular network that likely influences T cell motility. We examine how inflammation-induced changes to the CNS landscape may regulate T cell tissue exploration and modulate function.

Regulatory T Cell Numbers in Inflamed Skin Are Controlled by Local Inflammatory Cues That Upregulate CD25 and Facilitate Antigen-Driven Local Proliferation

CD4(+)Foxp3(+) regulatory T cells (Tregs) are key immune suppressors that regulate immunity in diverse tissues. The tissue and/or inflammatory signals that influence the magnitude of the Treg response remain unclear. To define signals that promote Treg accumulation, we developed a simple system of skin inflammation using defined Ags and adjuvants that induce distinct cytokine milieus: OVA protein in CFA, aluminum salts (Alum), and Schistosoma mansoni eggs (Sm Egg). Polyclonal and Ag-specific Treg accumulation in the skin differed significantly between adjuvants. CFA and Alum led to robust Treg accumulation, with >50% of all skin CD4(+) T cells being Foxp3(+) In contrast, Tregs accumulated poorly in the Sm Egg-inflamed skin. Surprisingly, we found no evidence of inflammation-specific changes to the Treg gene program between adjuvant-inflamed skin types, suggesting a lack of selective recruitment or adaptation to the inflammatory milieu. Instead, Treg accumulation patterns were linked to differences in CD80/CD86 expression by APC and the regulation of CD25 expression, specifically in the inflamed skin. Inflammatory cues alone, without cognate Ag, differentially supported CD25 upregulation (CFA and Alum > Sm Egg). Only in inflammatory milieus that upregulated CD25 did the provision of Ag enhance local Treg proliferation. Reduced IL-33 in the Sm Egg-inflamed environment was shown to contribute to the failure to upregulate CD25. Thus, the magnitude of the Treg response in inflamed tissues is controlled at two interdependent levels: inflammatory signals that support the upregulation of the important Treg survival factor CD25 and Ag signals that drive local expansion.

Role of Axl in T-Lymphocyte Survival in Salt-Dependent Hypertension

OBJECTIVE

Survival of immune and nonimmune cells relies on Axl, a receptor tyrosine kinase, which is implicated in hypertension. Activated T lymphocytes are involved in regulation of high blood pressure. The goal of the study was to investigate the role of Axl in T-lymphocyte functions and its contribution to salt-dependent hypertension.

APPROACH AND RESULTS

We report increased apoptosis in peripheral blood from Axl(-/-) mice because of lower numbers of white blood cells mostly lymphocytes. In vitro studies showed modest reduction in interferon gamma production in Axl(-/-) type 1 T helper cells. Axl did not affect basic proliferation capacity or production of interleukin 4 in Axl(-/-) type 2 T helper cells. However, competitive repopulation of Axl(-/-) bone marrow or adoptive transfer of Axl(-/-) CD4(+) T cells to Rag1(-/-) mice showed robust effect of Axl on T lymphocyte expansion in vivo. Adoptive transfer of Axl(-/-) CD4(+) T cells was protective in a later phase of deoxycorticosterone-acetate and salt hypertension. Reduced numbers of CD4(+) T cells in circulation and in perivascular adventitia decreased vascular remodeling and increased vascular apoptosis in the late phase of hypertension.

CONCLUSIONS

These findings suggest that Axl is critical for survival of T lymphocytes, especially during vascular remodeling in hypertension.

Imaging CD4 T Cell Interstitial Migration in the Inflamed Dermis

The ability of CD4 T cells to carry out effector functions is dependent upon the rapid and efficient migration of these cells in inflamed peripheral tissues through an as-yet undefined mechanism. The application of multiphoton microscopy to the study of the immune system provides a tool to measure the dynamics of immune responses within intact tissues. Here we present a protocol for non-invasive intravital multiphoton imaging of CD4 T cells in the inflamed mouse ear dermis. Use of a custom imaging platform and a venous catheter allows for the visualization of CD4 T cell dynamics in the dermal interstitium, with the ability to interrogate these cells in real-time via the addition of blocking antibodies to key molecular components involved in motility. This system provides advantages over both in vitro models and surgically invasive imaging procedures. Understanding the pathways used by CD4 T cells for motility may ultimately provide insight into the basic function of CD4 T cells as well as the pathogenesis of both autoimmune diseases and pathology from chronic infections.

Abstract 005: Axl Controls Survival of the CD4+ T Lymphocytes in Salt-dependent Hypertension

Introduction: Axl, a receptor tyrosine kinase, is required for vascular and immune cell survival. We sought to investigate the effects of Axl on T lymphocyte survival during deoxycorticosterone acetate (DOCA)-salt hypertension in mice.

Methods and Results: We found significant reduction in systolic blood pressure (BP) after 5-6 weeks of DOCA-salt in RAG1-/- mice after adoptive transfer of CD4+ T cells from Axl knockout (Axl-/- →RAG1-/-) compared to transferred CD4+ T cells from wild type (Axl+/+ →RAG1-/-) mice. Media area of the mesenteric artery was significantly lower in Axl-/- →RAG1-/- (4.2±0.7x10 3 m 2 ) vs. Axl+/+ →RAG1-/- (6.0±0.9x10 3 m 2 ) or Axl+/+ (6.8±0.6x10 3 m 2 ) mice. There was significant decrease in interferon gamma production by the T cells from Axl-/- (396±23 ng/mL) compared to Axl+/+ (512±42 ng/mL) after T h 1-priming. The number of carboxyfluorescein succinimidyl ester-positive cells in 6 th division was dramatically declined in Axl-/- (~0.3%) vs. Axl+/+ (~1.8%) in culture. Accordingly, we found lower number of lymphocytes in blood from Axl-/- (4.5±0.7x10 9 ) compared to Axl+/+ (7.8±0.7x10 9 ) mice. Blood leukocyte apoptosis was 2.5-fold higher in Axl-/- mice. We next investigated repopulation capacities of the hematopoietic cells from Axl-/- vs. Axl+/+ mice. There was significant decrease in Axl-/- CD3+ T cells (21±3 %) than Axl+/+ (49±3 %) in spleen after 8 weeks of competitive repopulation of bone marrow-derived cells. However, we found even greater reduction of Axl-/- T lymphocytes (15±1 %) vs. Axl+/+ T lymphocytes (52±6 %) in peripheral blood after 8 weeks of competitive repopulation. Finally, percentage of apoptotic cells was the greatest in the media (20±7 %) and adventitia (13±5 %) from Axl-/- →RAG1-/- mice compared to vascular apoptosis (6-14 % in media; and 6-9 % in adventitia) in other groups after 6 weeks of DOCA-salt.

Conclusions: Our data suggest that Axl-dependent survival of the T lymphocytes is crucial for the late increase in BP in DOCA-salt hypertension.

Abstract 001: Axl Expression by CD4+ T Lymphocytes Promotes Salt-Dependent Hypertension

Introduction: Our laboratory has shown that Axl, a receptor tyrosine kinase, is important in both vascular and immune functions during deoxycorticosterone acetate (DOCA)-salt hypertension. We hypothesized that Axl activity specifically in T lymphocytes could explain the dependence of hypertension on Axl.

Methods and Results: We did adoptive transfers of either Axl+/+ or Axl-/- CD4+ T cells to RAG1-/- mice that lack mature T cells. Once CD4+ T cell repopulations were confirmed, we induced DOCA-salt hypertension for 6 weeks. Systolic blood pressure (BP, mmHg) increased by 20±5 in Axl+/+RAG-/- mice after DOCA-salt, but Axl-/- RAG-/- mice had increases in BP by only 6+3 after 6 weeks of DOCA-salt. We isolated naïve CD4+ T cells from both Axl+/+ and Axl-/- littermates and primed them under either Th1 or Th2 polarizing conditions in culture. Production of interferon gamma (IFN-γ ng/mL) was significantly decreased (-23%, p<0.05) in Axl-/- (396±23) compared to Axl+/+ (512±42) under Th1-priming. However, Axl had no effect on interleukin 4 (IL-4, ng/mL) production under Th2 polarizing conditions. Intracellular staining of the Th1/Th2 cells with IFN-γ and IL-4 antibodies by flow cytometry confirmed expression of cytokines in culture media. Complete blood counts showed that Axl-/- mice had significantly lower white blood cells due to decreased numbers of lymphocytes (4.5±0.7x10 9 ) compared to Axl+/+ mice (7.8±0.7x10 9 ). We found a higher population of AnnexinV (marker of early apoptosis)-positive peripheral leukocytes in Axl-/- mice (10±1%) compared to Axl+/+ (4±1%) by flow cytometry; while the percentages of dead cells (~10%) were similar between Axl+/+ and Axl-/- mice.

Conclusions: Altogether we show that expression of Axl by T cells drives salt-induced hypertension. The mechanism of Axl-dependent effects on T cells occurs via T-cell-dependent expression of the pro-inflammatory cytokine IFN-γ. In addition, Axl plays a role in inhibiting lymphocyte apoptosis in the circulation. Future work will focus on how Axl expression in T cells affects T cell-dependent vascular remodeling during hypertension.

Platelet factor 4 limits Th17 differentiation and cardiac allograft rejection

Th cells are the major effector cells in transplant rejection and can be divided into Th1, Th2, Th17, and Treg subsets. Th differentiation is controlled by transcription factor expression, which is driven by positive and negative cytokine and chemokine stimuli at the time of T cell activation. Here we discovered that chemokine platelet factor 4 (PF4) is a negative regulator of Th17 differentiation. PF4-deficient and platelet-deficient mice had exaggerated immune responses to cardiac transplantation, including increased numbers of infiltrating Th17 cells and increased plasma IL-17. Although PF4 has been described as a platelet-specific molecule, we found that activated T cells also express PF4. Furthermore, bone marrow transplantation experiments revealed that T cell-derived PF4 contributes to a restriction in Th17 differentiation. Taken together, the results of this study demonstrate that PF4 is a key regulator of Th cell development that is necessary to limit Th17 differentiation. These data likely will impact our understanding of platelet-dependent regulation of T cell development, which is important in many diseases, in addition to transplantation.

IL-4 attenuates Th1-associated chemokine expression and Th1 trafficking to inflamed tissues and limits pathogen clearance

Interleukin 4 (IL-4) plays a central role in the orchestration of Type 2 immunity. During T cell activation in the lymph node, IL-4 promotes Th2 differentiation and inhibits Th1 generation. In the inflamed tissue, IL-4 signals promote innate and adaptive Type-2 immune recruitment and effector function, positively amplifying the local Th2 response. In this study, we identify an additional negative regulatory role for IL-4 in limiting the recruitment of Th1 cells to inflamed tissues. To test IL-4 effects on inflammation subsequent to Th2 differentiation, we transiently blocked IL-4 during ongoing dermal inflammation (using anti-IL-4 mAb) and analyzed changes in gene expression. Neutralization of IL-4 led to the upregulation of a number of genes linked to Th1 trafficking, including CXCR3 chemokines, CCL5 and CCR5 and an associated increase in IFNγ, Tbet and TNFα genes. These gene expression changes correlated with increased numbers of IFNγ-producing CD4+ T cells in the inflamed dermis. Moreover, using an adoptive transfer approach to directly test the role of IL-4 in T cell trafficking to the inflamed tissues, we found IL-4 neutralization led to an early increase in Th1 cell recruitment to the inflamed dermis. These data support a model whereby IL-4 dampens Th1-chemokines at the site of inflammation limiting Th1 recruitment. To determine biological significance, we infected mice with Leishmania major, as pathogen clearance is highly dependent on IFNγ-producing CD4+ T cells at the infection site. Short-term IL-4 blockade in established L. major infection led to a significant increase in the number of IFNγ-producing CD4+ T cells in the infected ear dermis, with no change in the draining LN. Increased lymphocyte influx into the infected tissue correlated with a significant decrease in parasite number. Thus, independent of IL-4's role in the generation of immune effectors, IL-4 attenuates lymphocyte recruitment to the inflamed/infected dermis and limits pathogen clearance.

Inflammation-induced interstitial migration of effector CD4⁺ T cells is dependent on integrin αV

Leukocytes must traverse inflamed tissues to effectively control local infection. Although motility in dense tissues appears to be integrin-independent actin-myosin based, during inflammation changes to the extracellular matrix (ECM) may necessitate distinct motility requirements. Indeed, we found that T cell interstitial motility was critically dependent on RGD-binding integrins in the inflamed dermis. Inflammation-induced deposition of fibronectin was functionally linked to increased α_v integrin expression on effector CD4⁺ T cells. Using intravital multi-photon imaging, we found that CD4⁺ T cell motility was dependent on α_v expression. Selective α_v blockade or knockdown arrested T_H1 motility in the inflamed tissue and attenuated local effector function. These data show a context-dependent specificity of lymphocyte movement in inflamed tissues that is essential for protective immunity.

Role of Axl in early kidney inflammation and progression of salt-dependent hypertension

The Gas6/Axl pathway regulates many cell functions and is implicated in hypertension. In this study, we aimed to investigate the role of Axl in immune cells on initiation and progression of salt-dependent hypertension. Deoxycorticosterone acetate (75 mg/60 days release)-salt hypertension was induced for 1 week or 6 weeks in Axl chimeras generated by bone marrow transplant to restrict Axl deficiency to hematopoietic or nonhematopoietic compartments. Depletion of Axl in hematopoietic cells (Axl(-/-) →Axl(+/+)) reduced (133 ± 2 mm Hg) increase in systolic blood pressure compared with other Axl chimeras (≈150 mm Hg) 1 week after deoxycorticosterone acetate-salt. Urine protein and renal oxidative stress were lowest in Axl(-/-) →Axl(+/+) at 1 week after deoxycorticosterone acetate-salt. Compensatory increase in Gas6 in kidneys of recipient Axl(-/-) may affect kidney function and blood pressure in early phase of hypertension. Flow cytometry on kidneys from Axl(-/-) →Axl(+/+) showed increase in total leukocytes, B, and dendritic cells and decrease in macrophages compared with Axl(+/+) →Axl(+/+). These immune changes were associated with decrease in proinflammatory gene expression, in particular interferon γ. Systolic blood pressure returned to baseline in Axl(-/-) →Axl(+/+) and Axl(-/-) →Axl(-/-) but remained increased in Axl(+/+) →Axl(+/+) and Axl(+/+) →Axl(-/-) chimeras after 6 weeks of deoxycorticosterone acetate-salt. Vascular apoptosis was increased in the global Axl(-/-) chimeras in the late phase of hypertension. In summary, we found that expression of Axl in hematopoietic cells is critical for kidney pathology in early phase of salt-dependent hypertension. However, Axl in both hematopoietic and nonhematopoietic lineages contributes to the late phase of hypertension.

Abstract 175: The Role of Axl in Accumulation of Immune Cells in Kidney and the Onset of Hypertension

Introduction: Gas6/Axl pathway contributes to elevation of blood pressure. Immune cells are implicated in initiation and maintenance of hypertension. In this study we aimed to investigate the role of Axl in immune cells on kidney injury and initiation of hypertension.

Methods and Results: Deoxycorticosterone-acetate (DOCA; 75mg, 60days release) and salt hypertension was induced for 1wk or 6wks in four groups of Axl chimeras (n=4-5) that were generated by bone marrow (BM) transplant. Multi parameter flow cytometry was used to quantify five major immune cell subsets in digested kidneys from Axl chimeras. Systolic blood pressure (SBP) increased by 30mmHg in Axl+/+ →Axl+/+, Axl-/- →Axl-/- and Axl+/+ →Axl-/- mice after 1wk of DOCA-salt. However, chimeras that lack Axl in the BM cells (Axl-/- →Axl+/+) showed reduction in early increase in SBP (16+2mmHg). We observed a significant decrease in urine protein levels in Axl-/- →Axl+/+ (0.3+0.1μg/μl) compared to other Axl chimeras (∼0.7μg/μl) after 1wk of DOCA-salt. Kidney glomeruli areas were reduced in Axl-/- →Axl+/+ (4,143+229μm 2 ) compared to other Axl chimeras (∼6,000μm 2 ) after 6wks of DOCA-salt. Kidneys from Axl-/- →Axl-/- showed an increase in total leukocytes (8 vs. 4%), B cells (29 vs. 12%) and decrease in monocytes/macrophages (16 vs. 22%) and dendritic cells (5 vs. 10%) compared to Axl+/+ →Axl+/+. Moreover, Axl-/- →Axl+/+ showed further increase in leukocytes (17%), B (39%) and dendritic (13%) cells in kidneys compared to other Axl chimeras. In addition a small percentage of wild type T cells was increased in the kidneys from Axl-/- →Axl+/+ chimeras.

Conclusions: These findings suggest that Axl expression in BM-derived cells is critical for kidney injury in DOCA-salt hypertension. Axl-dependent pathways regulate immune cell populations in the kidneys during initiation of hypertension. This study was supported by HL105623 grant (VAK)

Uropod elongation is a common final step in leukocyte extravasation through inflamed vessels

Uropod elongation occurs during leukocyte extravasation.

The efficient trafficking of immune cells into peripheral nonlymphoid tissues is key to enact their protective functions. Despite considerable advances in our understanding of cell migration in secondary lymphoid organs, real-time leukocyte recruitment into inflamed tissues is not well characterized. The conventional multistep paradigm of leukocyte extravasation depends on CD18 integrin–mediated events such as rapid arrest and crawling on the surface of the endothelium and transmigration through the endothelial layer. Using enhanced three-dimensional detection of fluorescent CD18 fusion proteins in a newly developed knockin mouse, we report that extravasating leukocytes (neutrophils, monocytes, and T cells) show delayed uropod detachment and become extremely elongated before complete transmigration across the endothelium. Additionally, these cells deposit CD18⁺ microparticles at the subendothelial layer before retracting the stretched uropod. Experiments with knockout mice and blocking antibodies reveal that the uropod elongation and microparticle formation are the result of LFA-1–mediated adhesion and VLA-3–mediated cell migration through the vascular basement membrane. These findings suggest that uropod elongation is a final step in the leukocyte extravasation cascade, which may be important for precise regulation of leukocyte recruitment into inflamed tissues.

Leishmania induces survival, proliferation and elevated cellular dNTP levels in human monocytes promoting acceleration of HIV co-infection

Leishmaniasis is a parasitic disease that is widely prevalent in many tropical and sub-tropical regions of the world. Infection with Leishmania has been recognized to induce a striking acceleration of Human Immunodeficiency Virus Type 1 (HIV-1) infection in coinfected individuals through as yet incompletely understood mechanisms. Cells of the monocyte/macrophage lineage are the predominant cell types coinfected by both pathogens. Monocytes and macrophages contain extremely low levels of deoxynucleoside triphosphates (dNTPs) due to their lack of cell cycling and S phase, where dNTP biosynthesis is specifically activated. Lentiviruses, such as HIV-1, are unique among retroviruses in their ability to replicate in these non-dividing cells due, at least in part, to their highly efficient reverse transcriptase (RT). Nonetheless, viral replication progresses more efficiently in the setting of higher intracellular dNTP concentrations related to enhanced enzyme kinetics of the viral RT. In the present study, in vitro infection of CD14+ peripheral blood-derived human monocytes with Leishmania major was found to induce differentiation, marked elevation of cellular p53R2 ribonucleotide reductase subunit and R2 subunit expression. The R2 subunit is restricted to the S phase of the cell cycle. Our dNTP assay demonstrated significant elevation of intracellular monocyte-derived macrophages (MDMs) dNTP concentrations in Leishmania-infected cell populations as compared to control cells. Infection of Leishmania-maturated MDMs with a pseudotyped GFP expressing HIV-1 resulted in increased numbers of GFP+ cells in the Leishmania-maturated MDMs as compared to control cells. Interestingly, a sub-population of Leishmania-maturated MDMs was found to have re-entered the cell cycle, as demonstrated by BrdU labeling. In conclusion, Leishmania infection of primary human monocytes promotes the induction of an S phase environment and elevated dNTP levels with notable elevation of HIV-1 expression in the setting of coinfection.

Leishmaniasis is a parasitic disease that infects several human host immune cells, including neutrophils, monocytes, and macrophages. Moreover, while HIV-1 infects monocytes and macrophages, only the infected macrophages productively release viral progenies. Importantly, patients coinfected with both pathogens progress more rapidly to AIDS. In this study, we examine how Leishmania major changes the cellular environment of monocytes in vitro. We found that Leishmania-infected monocytes actively mature into macrophages in the absence of GM-CSF, and that these cells up-regulate the expression of ribonucleotide reductase, an enzyme that catalyzes the formation of deoxynucleoside triphosphates (dNTPs). We confirmed the elevation of dNTP concentrations using a very sensitive dNTP assay for monocytes and monocyte-maturated macrophages. Collectively, these data support a model in which infection of monocytes with Leishmania elevates the intracellular dNTP pools, which is one of the natural anti-viral blocks to HIV-1 infection in monocytes and macrophages in patients.

Itk controls the spatiotemporal organization of T cell activation

During T cell activation by antigen-presenting cells (APCs), the diverse spatiotemporal organization of components of T cell signaling pathways modulates the efficiency of activation. Here, we found that loss of the tyrosine kinase interleukin-2 (IL-2)-inducible T cell kinase (Itk) in mice altered the spatiotemporal distributions of 14 of 16 sensors of T cell signaling molecules in the region of the interface between the T cell and the APC, which reduced the segregation of signaling intermediates into distinct spatiotemporal patterns. Activation of the Rho family guanosine triphosphatase Cdc42 at the center of the cell-cell interface was impaired, although the total cellular amount of active Cdc42 remained intact. The defect in Cdc42 localization resulted in impaired actin accumulation at the T cell-APC interface in Itk-deficient T cells. Reconstitution of cells with active Cdc42 that was specifically directed to the center of the interface restored actin accumulation in Itk-deficient T cells. Itk also controlled the central localization of the guanine nucleotide exchange factor SLAT [Switch-associated protein 70 (SWAP-70)-like adaptor of T cells], which may contribute to the activation of Cdc42 at the center of the interface. Together, these data illustrate how control of the spatiotemporal organization of T cell signaling controls critical aspects of T cell function.

Cutting edge: Regulatory T cells selectively attenuate, not terminate, T cell signaling by disrupting NF-κB nuclear accumulation in CD4 T cells

A key consequence of regulatory T cell (Treg) suppression of CD4 T cells is the inhibition of IL-2 production, yet how Tregs attenuate IL-2 has not been defined. Current models predict a termination of TCR signaling, by disrupting T-APC contacts, or TCR signal modification, through mechanisms such as cAMP. To directly define Treg effects on TCR signaling in CD4 T cell targets, we visualized changes in nuclear accumulation of transcription factors at time points when IL-2 was actively suppressed. Nuclear accumulation of NFAT was highly dependent on sustained TCR signaling in the targets. However, in the presence of Tregs, NFAT and AP-1 signals were sustained in the target cells. In contrast, NF-κB p65 was selectively attenuated. Thus, Tregs do not generally terminate TCR signals. Rather, Tregs selectively modulate TCR signals within hours of contact with CD4 targets, independent of APCs, resulting in the specific loss of NF-κB p65 signals.

Uncoupling of proliferation and cytokines from suppression within the CD4+CD25+Foxp3+ T-cell compartment in the 1st year of human type 1 diabetes

OBJECTIVE

The mechanistic basis for the breakdown of T-cell tolerance in type 1 diabetes is unclear and could result from a gain of effector function and/or loss of regulatory function. In humans, the CD4+CD25+Foxp3+ T-cell compartment contains both effector and regulatory T cells, and it is not known how their relative proportions vary in disease states.

RESEARCH DESIGN AND METHODS

We performed a longitudinal study of CD4+CD25+ T-cell function in children with type 1 diabetes at onset and throughout the 1st year of disease. Function was assessed using single-cell assays of proliferation, cytokine production, and suppression. Type 1 diabetic individuals were compared with age-matched control subjects, and suppression was directly assessed by coculture with control T-cell targets.

RESULTS

We identify novel functional changes within the type 1 diabetes CD4+CD25+ compartment. Type 1 diabetic CD4+CD25+ cells exhibited a striking increase in proliferative capacity in coculture with CD4 T cells that was present at onset and stable 9-12 months from diagnosis. Elevated type 1 diabetes CD4+CD25+ cell proliferation correlated with increased inflammatory cytokines interleukin 17 and tumor necrosis factor-α but not γ-interferon. Type 1 diabetes CD4+CD25+ cytokine production occurred coincident with suppression of the same cytokines in the control targets. Indeed, enhanced proliferation/cytokines by CD4+CD25+ cells was uncoupled from their suppressive ability. Longitudinally, we observed a transient defect in type 1 diabetes CD4+CD25+ suppression that unexpectedly correlated with measures of improved metabolic function.

CONCLUSIONS

Type 1 diabetes onset, and its subsequent remission period, is associated with two independent functional changes within the CD4+CD25+ T-cell compartment: a stable increase in effector function and a transient decrease in regulatory T-cell suppression.

Critical requirement for the Wiskott-Aldrich syndrome protein in Th2 effector function

Patients with Wiskott-Aldrich syndrome (WAS) have numerous immune cell deficiencies, but it remains unclear how abnormalities in individual cell types contribute to the pathologies of WAS. In T cells, the WAS protein (WASp) regulates actin polymerization and transcription, and plays a role in the dynamics of the immunologic synapse. To examine how these events influence CD4 function, we isolated the WASp deficiency to CD4(+) T cells by adoptive transfer into wild-type mice to study T-cell priming and effector function. WAS(-/-) CD4(+) T cells mediated protective T-helper 1 (Th1) responses to Leishmania major in vivo, but were unable to support Th2 immunity to Nippostrongylus brasiliensis or L major. Mechanistically, WASp was not required for Th2 programming but was required for Th2 effector function. WAS(-/-) CD4(+) T cells up-regulated IL-4 and GATA3 mRNA and secreted IL-4 protein during Th2 differentiation. In contrast, cytokine transcription was uncoupled from protein production in WAS(-/-) Th2-primed effectors. WAS(-/-) Th2s failed to produce IL-4 protein on restimulation despite elevated IL-4/GATA3 mRNA. Moreover, dominant-negative WASp expression in WT effector T cells blocked IL-4 production, but had no effect on IFNgamma. Thus WASp plays a selective, posttranscriptional role in Th2 effector function.

CTLA-4 is required by CD4+CD25+ Treg to control CD4+ T-cell lymphopenia-induced proliferation

CTLA-4 is constitutively expressed by CD4(+)CD25(+)Foxp3(+) Treg but its precise role in Treg function is not clear. Although blockade of CTLA-4 interferes with Treg function, studies using CTLA-4-deficient Treg have failed to reveal an essential requirement for CTLA-4 in Treg suppression in vivo. Conditional deletion of CTLA-4 in Foxp3(+) T cells disrupts immune homeostasis in vivo but the immune processes disrupted by CTLA-4 deletion have not been determined. We demonstrate that Treg expression of CTLA-4 is essential for Treg control of lymphopenia-induced CD4 T-cell expansion. Despite IL-10 expression, CTLA-4-deficient Treg were unable to control the expansion of CD4(+) target cells in a lymphopenic environment. Moreover, unlike their WT counterparts, CTLA-4-deficient Treg failed to inhibit cytokine production associated with homeostatic expansion and were unable to prevent colitis. Thus, while Treg developing in the absence of CTLA-4 appear to acquire some compensatory suppressive mechanisms in vitro, we identify a non-redundant role for CTLA-4 in Treg function in vivo.

Signals for the execution of Th2 effector function

Appropriate control of infection depends on the generation of lymphocytes armed with a particular array of cytokine and chemokine effector molecules. The differentiation of naïve T cells into functionally distinct effector subsets is regulated by signals from the T cell receptor (TCR) and cytokine receptors. Using gene knock-out approaches, the initiation of discrete effector programs appears differentially sensitive to the loss of individual TCR signaling components; likely due to differences in the transcription factors needed to activate individual cytokine genes. Less well understood however, are the signal requirements for the execution of effector function. With a focus on Th2 cells and the kinase ITK, we review recent observations that point to differences between the signals needed for the initiation and implementation of cytokine programs in CD4+ T cells. Indeed, Th2 effector cells signal differently from both their naïve counterparts and from Th1 effectors suggesting they may transduce activation signals differently or may be selectively receptive to different activation signals. Potential regulation points for effector function lie at the level of transcription and translation of cytokine genes. We also discuss how provision of these execution signals may be spatially segregated in vivo occurring at tissue sites of inflammation and subject to modulation by the pathogen itself.

Pathogen-imposed skewing of mouse chemokine and cytokine expression at the infected tissue site

Compartmentalization of immunity ensures tight regulation of T cell activation in the LN and precise effector T cell delivery to inflamed sites. Herein we show that the tissue-specific accumulation of effector T cells can be subverted by a pathogen at the infection site. Using the Leishmania major mouse model of dermal infection, we observed a restricted chemokine profile at the infection site, i.e., the expression of Th2 cell-attracting CCL7 but not of Th1 cell-attracting chemokines. Consistent with these chemokine expression data, recruitment of cytokine-producing T cells to the infection site was also selective. Both IL-4- and IFN-gamma-producing effector T cells homed to inflamed OVA/CFA-immunized dermis, but only IL-4-producing cells homed to L. major-infected dermis. The narrowing of the cytokine repertoire at the site of infection with L. major was driven, in part, by pathogen-induced CCL7. Inflammatory signals failed to disrupt the early restrictive L. major infection site, which suggests that L. major dominantly modifies the local milieu. We have highlighted an emerging principle in pathogen-host interactions: that the cytokine repertoire at the infection site and the LN draining the infection site can be different because of the ability of the pathogen to modify the chemokine profile at the infection site. Thus, pathogens may edit the LN cytokine repertoire through differential recruitment of cytokine-producing cells.

Mechanisms of regulatory T-cell suppression - a diverse arsenal for a moving target

Naturally-occurring regulatory T cells (Tregs) are emerging as key regulators of immune responses to self-tissues and infectious agents. Insight has been gained into the cell types and the cellular events that are regulated by Tregs. Indeed, Tregs have been implicated in the control of initial activation events, proliferation, differentiation and effector function. However, the mechanisms by which Tregs disable their cellular targets are not well understood. Here we review recent advances in the identification of distinct mechanisms of Treg action and of signals that enable cellular targets to escape regulation. Roles for inhibitory cytokines, cytotoxic molecules, modulators of cAMP and cytokine competition have all been demonstrated. The growing number of inhibitory mechanisms ascribed to Tregs suggests that Tregs take a multi-pronged approach to immune regulation. It is likely that the relative importance of each inhibitory mechanism is context dependent and modulated by the inflammatory milieu and the magnitude of the immune response. In addition, the target cell may be differentially susceptible or resistant to distinct Treg mechanisms depending on their activation or functional status at the time of the Treg encounter. Understanding when and where each suppressive tool is most effective will help to fine tune therapeutic strategies to promote or constrain specific arms of Treg suppression.

The development of effector T cell subsets in murine Leishmania major infection

Leishmania major infection has proven an exceptional model for CD4+ subset development in inbred mice. Most strains contain infection coincident with the appearance of T helper 1 (Th1) cells that produce gamma-interferon (IFN-gamma) required for macrophage activation. In contrast, mice on the BALB background are unable to control infection due to the development of Th2 cells that produce counter-regulatory cytokines, particularly interleukin 4 (IL-4), capable of abrogating the effects of IFN-gamma. Selective gene disruption studies in mice have illustrated critical components of the host response to L. major. Mice deficient in beta 2 microglobulin, which have no major histocompatibility complex (MHC) class I or CD8+ T cells, control infection as well as wild-type mice, whereas mice deficient in MHC class II (and CD4+ T cells) suffer fatal infection. Mice with disruption of the gene coding IFN-gamma are also incapable of containing infection, reflecting absolute requirements for this cytokine. A number of interventions have been demonstrated to abrogate Th2 cell development in BALB mice, enabling these mice to control infection. Each of these--IL-12, anti-IL-4, anti-IL-2, anti-CD4 and CTLA4-Ig--has in common the capacity to make IL-4 rate limiting at the time of CD4+ cell priming.

A key role for Itk in both IFN gamma and IL-4 production by NKT cells

NKT cells rapidly secrete cytokines upon TCR stimulation and thus may modulate the acquired immune response. Recent studies suggest that signaling for development and effector function in NKT cells may differ from conventional T cells. The tyrosine kinase Itk is activated downstream of the TCR, and its absence in CD4(+) T cells results in impaired Th2, but not Th1 responses. In this study, we investigated NKT cell function in the absence of Itk as impaired type 2 responses in vivo could be manifest through IL-4 defects in a number of cell types. We show that Itk-deficient NKT cells up-regulate IL-4 mRNA in the thymus and express constitutive IL-4 and IFN-gamma transcripts in peripheral organs. Thus, Itk is not required for the developmental activation of cytokine loci in NKT cells. Nevertheless, Itk-deficient NKT cells are severely impaired in IL-4 protein production. Strikingly, unlike conventional CD4(+) T cells, Itk-deficient NKT cells also have profound defects in IFN-gamma production. Furthermore, both IL-4 and IFN-gamma production were markedly impaired following in vivo challenge with alpha-galactosyl ceramide. Function can be restored in Itk-deficient NKT cells by provision of calcium signals using ionomycin. These results suggest that NKT cells are highly dependent on Itk for IL-4- and IFN-gamma-mediated effector function. Thus, the pattern of cytokine genes that are affected by Itk deficiency appears to be cell lineage-specific, likely reflecting differences in activation threshold between immune effectors. The severe defect in NKT cell function may underlie a number of the Th1 and Th2 immune defects in Itk-deficient mice.

Distinct molecular program imposed on CD4+ T cell targets by CD4+CD25+ regulatory T cells

CD4+CD25+ regulatory T cells (Tregs) are key modulators of immunity, but their mechanism of action is unclear. To elucidate the molecular consequences of Treg encounter, we analyzed changes in gene expression in CD4+ T cell targets activated in the presence or absence of CD4+CD25+ Tregs. Tregs did not alter the early activation program of CD4+ T cells, but had reversed many of the activation-induced changes by 36 h. It is not known whether Tregs simply induce a set of transcriptional changes common to other nonproliferative states or whether instead Tregs mediate a distinct biological activity. Therefore, we compared the gene profile of T cells following Treg encounter with that of T cells made anergic, TGF-beta-treated, or IL-2-deprived; all possible modes of Treg action. Strikingly, all genes down-regulated in suppressed cells were indeed common to these nonproliferative states. In contrast, Treg encounter led to elevated expression of a unique set of genes in the target T cells. Although different from the nonproliferative states tested, the Treg-imposed gene program is exemplified by expression of many genes associated with growth arrest or inhibition of proliferation. We suggest that Tregs function by the induction of a distinct set of negative regulatory factors that initiate or maintain target T cells in a nonproliferative state.

T regulatory and primed uncommitted CD4 T cells express CD73, which suppresses effector CD4 T cells by converting 5'-adenosine monophosphate to adenosine

CD73 (5'-ectonucleotidase) is expressed by two distinct mouse CD4 T cell populations: CD25+ (FoxP3+) T regulatory (Treg) cells that suppress T cell proliferation but do not secrete IL-2, and CD25- uncommitted primed precursor Th (Thpp) cells that secrete IL-2 but do not suppress in standard Treg suppressor assays. CD73 on both Treg and Thpp cells converted extracellular 5'-AMP to adenosine. Adenosine suppressed proliferation and cytokine secretion of Th1 and Th2 effector cells, even when target cells were activated by anti-CD3 and anti-CD28. This represents an additional suppressive mechanism of Treg cells and a previously unrecognized suppressive activity of Thpp cells. Infiltration of either Treg or Thpp cells at inflammatory sites could potentially convert 5'-AMP generated by neutrophils or dying cells into the anti-inflammatory mediator adenosine, thus dampening excessive immune reactions.

Cutting edge: Itk-dependent signals required for CD4+ T cells to exert, but not gain, Th2 effector function

The TCR signals for the release of CD4 effector function are poorly understood. Itk plays an essential role in Th2, but not Th1, responses. However, when Itk is required during Th2 development is unclear. We followed the fate of Itk-deficient T cells during Th2 development in vitro and in vivo using an IL-4/GFP reporter. Surprisingly, a similar frequency of itk(-/-) CD4(+) cells differentiated and committed to the Th2 lineage as wild-type cells. However, Itk-deficient Th2 cells failed to exert effector function upon TCR triggering. Loss of function was marked by defective transcriptional enhancement of Th2 cytokines and GATA3. IL-4 production in itk(-/-) Th2s could be rescued by the expression of kinase-active Itk. Thus, Itk is necessary for the release, but not gain, of Th2 function. We suggest that the liberation of effector function is tightly controlled through qualitative changes in TCR signals, facilitating postdifferentiation regulation of cytokine responses.

Early kinetic window of target T cell susceptibility to CD25+ regulatory T cell activity

Peripheral tolerance is maintained in part by thymically derived CD25+CD4+ T cells (regulatory T cells (Tregs)). Their mechanism of action has not been well characterized. Therefore, to get a better understanding of Treg action, we investigated the kinetics of murine Treg activity in vitro. Tregs were suppressive within a surprisingly narrow kinetic window: necessary and sufficient only in the first 6-10 h of culture. Visualization of this time frame, using a sensitive single-cell assay for IL-2, revealed the early elaboration of target cell IL-2 producers in the first 6 h despite the presence of CD25+CD4+ Tregs. However, after 6 h, a rapid rise in the number of IL-2 producers in the absence of Tregs was dramatically abrogated by the presence of Tregs. Importantly, the timing of suppression was dictated by the kinetics of target T cell activation suggesting that early target T cell signals may alter susceptibility to suppression. Modulating target T cell activation signals with provision of CD28, IL-2, or high Ag dose all abrogated suppression of proliferation late in culture. However, only CD28 signals enabled target T cells to resist the early Treg-induced down-regulation of IL-2. Therefore the quality of early target T cell activation signals, in particular engagement of CD28, represents an important control point in the balance between vulnerability and resistance to Treg suppression.

Cutting edge: selective requirement for the Wiskott-Aldrich syndrome protein in cytokine, but not chemokine, secretion by CD4+ T cells

The mechanism of cytokine secretion is not well understood, but cytokines appear to be synthesized and released in a polarized fashion toward an Ag-specific target cell. In this study, we demonstrate that the Wiskott-Aldrich syndrome protein (WASp) is an essential component of the cytokine secretory pathway in CD4(+) T cells. Murine WASp-deficient CD4(+) T cells fail to polarize cytokines toward a target and show an unexpected and striking block in cytokine secretion. In contrast, chemokine secretion and trafficking of plasma membrane proteins, transported via the constitutive secretory pathway, are unaffected by the lack of WASp. These results suggest that CD4(+) T cell cytokines require a specialized, WASp-dependent pathway for cellular traffic and/or vesicle release that is distinct from that required for chemokine release. We propose that the use of different secretory pathways for cytokines and chemokines enables CD4(+) T cell activity to be further fine-tuned to serve specialized effector functions.