Transcriptional regulation of CD4+ TH cells that mediate tissue inflammation

Acquired and genetic immunodeficiencies have revealed an indispensable role for CD4⁺ T cells in the induction of protective host immune responses against a myriad of microbial pathogens. Influenced by the cytokines present in the microenvironment, activated CD4⁺ T cells may differentiate into several highly-specialized helper subsets defined by the production of distinct signature cytokines tailored to combat diverse classes of pathogens. The process of specification and differentiation is controlled by networks of core, master, and accessory transcription factors, which ensure that CD4⁺ T helper (T_H ) cell responses mounted against an invading microbe are of the correct specificity and type. However, aberrant activation or inactivation of transcription factors can result in sustained and elevated expression of immune-related genes, leading to chronic activation of CD4⁺ T_H  cells and organ-specific autoimmunity. In this review, we provide an overview of the molecular basis of CD4⁺ T_H  cell differentiation and examine how combinatorial expression of transcription factors, which promotes genetic plasticity of CD4⁺ T_H  cells, can contribute to immunological dysfunction of CD4⁺ T_H responses. We also discuss recent studies which highlight the potential of exploiting the genetic plasticity of CD4⁺ T_H  cells in the treatment of autoimmune and other immune-mediated disorders.

Notch in T Cell Differentiation: All Things Considered

Differentiation of naïve T cells into effector cells is required for optimal protection against different classes of microbial pathogen and for the development of immune memory. Recent findings have revealed important roles for the Notch signaling pathway in T cell differentiation into all known effector subsets, raising the question of how this pathway controls such diverse differentiation programs. Studies in preclinical models support the therapeutic potential of manipulating the Notch pathway to alleviate immune pathology, highlighting the importance of understanding the mechanisms through which Notch regulates T cell differentiation and function. We review these findings here, and outline both unifying principles involved in Notch-mediated T cell fate decisions and cell type- and context-specific differences that may present the most suitable points for therapeutic intervention.

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.

T helper cell differentiation more than just cytokines

CD4(+) T helper (T(H)) cells play a critical role in orchestrating a pleiotropy of immune activities against a large variety of pathogens. It is generally thought that this is achieved through the acquisition of highly specialized functions after activation followed by the differentiation into various functional subsets. The differentiation process of naive precursor T(H) cells into defined effector subsets is controlled by cells of the innate immune system and their complex array of effector molecules such as secreted cytokines and membrane bound costimulatory molecules. These provide a unique quantitative or qualitative signal initiating T(H) development, which is subsequently reinforced via T cell-mediated feedback signals and selective survival and proliferative cues, ultimately resulting in the predominance of a particular T cell subset. In recent years, the number of defined T(H)cell subsets has expanded and the once rigid division of labor among them has been blurred with reports of plasticity among the subsets. In this chapter, we summarize and speculate on the current knowledge of the differentiation requirements of T(H) cell lineages, with particular focus on the T(H)17 subset.

Alternatively activated macrophages inhibit T-cell proliferation by Stat6-dependent expression of PD-L2

Alternatively activated macrophages (AAM) accumulate in tissues during Th2-associated immune responses like helminth infections and allergic disorders. These cells differentiate in response to interleukin 4 (IL-4)/IL-13-mediated activation of Stat6 and possess potent inhibitory activity against T cells. The molecular mechanism that leads to T-cell suppression remains unclear and could involve soluble factors or inhibitory ligands. Microarray analysis revealed that the inhibitory ligand, programmed death ligand 2 (PD-L2) was strongly induced by IL-4 in macrophages from wild-type but not Stat6-deficient mice. PD-L2 expression correlated with other established markers for AAM-like Relm-α/Fizz1, arginase1, or Ym1 and thereby serves as useful surface marker to identify and isolate AAM from tissues. Antibodies against PD-L2 blocked the inhibitory activity of AAM and retroviral expression of PD-L2 in macrophages from Stat6(-/-) mice was sufficient to inhibit T-cell proliferation, which demonstrates that PD-L2 mediates potent and nonredundant inhibition of T cells independently of other Stat6-regulated genes. Infection of conditional IL-4/IL-13-deficient mice with the helminth Nippostrongylus brasiliensis further showed that PD-L2 expression was dependent on IL-4/IL-13 from Th2 cells. In vivo blockade of PD-L2 during N brasiliensis infection caused an enhanced Th2 response in the lung, indicating that AAM inhibit Th2 cells by expression of PD-L2.

The lung is an important site for priming CD4 T-cell-mediated protective immunity against gastrointestinal helminth parasites

The rodent hookworm Nippostrongylus brasiliensis typically infects its host by penetrating the skin and rapidly migrating to the lungs and gut. Following primary infection, immunocompetent mice become highly protected from reinfection with N. brasiliensis, with the numbers of worms gaining access to the lungs and gut being reduced by up to 90%. We used green fluorescent protein/interleukin-4 (IL-4) reporter mice and truncated infection studies to identify both the tissue site and mechanism(s) by which the host protects itself from reinfection with N. brasiliensis. Strikingly, we demonstrated that the lung is an important site for priming immune protection. Furthermore, a lung-initiated, CD4 T-cell-dependent, and IL-4- and STAT6-dependent response was sufficient to confer protection against reinfection. In conclusion, vaccination strategies which seek to break the cycle of reinfection and egg production by helminths such as hookworms can include strategies which directly stimulate Th2 responses in the lung.

Regulation of pathogenesis and immunity in helminth infections

Helminths are multicellular eukaryotic parasites that infect over one quarter of the world's population. Through coevolution with the human immune system, these organisms have learned to exploit immunoregulatory pathways, resulting in asymptomatic tolerance of infections in many individuals. When infections and the resulting immune responses become dysregulated, however, acute and chronic pathologies often develop. A recent international meeting focused on how these parasites modulate host immunity and how control of parasitic and immunopathological disease might be achieved.

The different faces of Notch in T-helper-cell differentiation

Interleukin-12 (IL-12) and IL-4 induce T helper 1 (T(H)1)- and T(H)2-cell differentiation, respectively, in vitro. However, not all T(H)1-cell responses require IL-12 in vivo, and T(H)2-cell responses are remarkably independent of IL-4-receptor signalling, suggesting that other polarizing signals must exist. Accumulating evidence indicates that Notch is a candidate receptor that might mediate these signals. However, contrasting roles for Notch have been proposed: some evidence shows that Notch promotes T(H)1-cell differentiation, whereas other evidence supports a prominent role for Notch in T(H)2-cell differentiation. In this Review, we discuss recent findings that help to reconcile this discrepancy and highlight the accumulating evidence for the role of Notch in T-cell-mediated diseases.

How are T(H)1 and T(H)2 effector cells made?

Differentiation of T(H)1 and T(H)2 effector cells proceeds through several phases: First, naïve CD4(+) precursor cells are instructed to differentiate as appropriate to optimally fight the infectious threat encountered. This process is governed by the IL12 and IL4 cytokines, as well as by signaling through the Notch receptor. In response to these signals, transcription is initiated of lineage specific cytokine genes including the Ifngamma and Il4 genes as well as of genes encoding transcriptional regulators, such as T-bet and Gata3. The respective differentiation programs are reinforced by both positive and negative feedback mechanisms. Furthermore, epigenetic modifications of the lineage specific genes result in the emergence of regulatory elements, which control high level lineage restricted expression by both intrachromosomal and interchromosomal associations. Together, these mechanisms ensure stable inheritance of the differentiated fate in the numerous progeny of the original naïve CD4(+) T cells.

Recent findings on the pathogenesis of bronchial asthma. Part I. Asthma as a neurohumoral disorder, a pathological vago-vagal axon reflex

The novel data on the pathogenesis of asthma are summarized in this three-part review. Its immunological background is well established but it is more than an immunological disorder. Multiple lines indicate that both peripheral and central neural mechanisms are also involved in the pathogenesis of asthma. In the present first part of the review asthma is described as vago-vagal axon reflex brought about by multiple positive feed-back mechanisms, receptor upregulation, wind-up, phenotypic switch and formation of a pathological conditioned reflex. In the coming second part the main dispositional (mostly hormonal) and external contributing factors are reviewed, while the third part deals with the role of inheritance, i.e., with gene alleles leading to enhanced production of mediators of asthma.

CD11b+ myeloid cells are the key mediators of Th2 cell homing into the airway in allergic inflammation

STAT6-mediated chemokine production in the lung is required for Th2 lymphocyte and eosinophil homing into the airways in allergic pulmonary inflammation, and thus is a potential therapeutic target in asthma. However, the critical cellular source of STAT6-mediated chemokine production has not been defined. In this study, we demonstrate that STAT6 in bone marrow-derived myeloid cells was sufficient for the production of CCL17, CCL22, CCL11, and CCL24 and for Th2 lymphocyte and eosinophil recruitment into the allergic airway. In contrast, STAT6 in airway-lining cells did not mediate chemokine production or support cellular recruitment. Selective depletion of CD11b(+) myeloid cells in the lung identified these cells as the critical cellular source for the chemokines CCL17 and CCL22. These data reveal that CD11b(+) myeloid cells in the lung help orchestrate the adaptive immune response in asthma, in part, through the production of STAT6-inducible chemokines and the recruitment of Th2 lymphocytes into the airway.