Macrophages display remarkable plasticity and can change their physiology in response to environmental cues. These changes can give rise to different populations of cells with distinct functions. In this Review we suggest a new grouping of macrophage populations based on three different homeostatic activities - host defence, wound healing and immune regulation. We propose that similarly to primary colours, these three basic macrophage populations can blend into various other 'shades' of activation. We characterize each population and provide examples of macrophages from specific disease states that have the characteristics of one or more of these populations.

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Exploring the full spectrum of macrophage activation.

On activation, T cells undergo distinct developmental pathways, attaining specialized properties and effector functions. T-helper (T(H)) cells are traditionally thought to differentiate into T(H)1 and T(H)2 cell subsets. T(H)1 cells are necessary to clear intracellular pathogens and T(H)2 cells are important for clearing extracellular organisms. Recently, a subset of interleukin (IL)-17-producing T (T(H)17) cells distinct from T(H)1 or T(H)2 cells has been described and shown to have a crucial role in the induction of autoimmune tissue injury. In contrast, CD4+CD25+Foxp3+ regulatory T (T(reg)) cells inhibit autoimmunity and protect against tissue injury. Transforming growth factor-beta (TGF-beta) is a critical differentiation factor for the generation of T(reg) cells. Here we show, using mice with a reporter introduced into the endogenous Foxp3 locus, that IL-6, an acute phase protein induced during inflammation, completely inhibits the generation of Foxp3+ T(reg) cells induced by TGF-beta. We also demonstrate that IL-23 is not the differentiation factor for the generation of T(H)17 cells. Instead, IL-6 and TGF-beta together induce the differentiation of pathogenic T(H)17 cells from naive T cells. Our data demonstrate a dichotomy in the generation of pathogenic (T(H)17) T cells that induce autoimmunity and regulatory (Foxp3+) T cells that inhibit autoimmune tissue injury.

Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells.

The orphan nuclear receptor RORgammat directs the differentiation program of proinflammatory IL-17+ T helper cells.

CD4(+) T cells producing interleukin 17 (IL-17) are associated with autoimmunity, although the precise mechanisms that control their development are undefined. Here we present data that challenge the idea of a shared developmental pathway with T helper type 1 (T(H)1) or T(H)2 lineages and instead favor the idea of a distinct effector lineage we call 'T(H)-17'. The development of T(H)-17 cells from naive precursor cells was potently inhibited by interferon-gamma (IFN-gamma) and IL-4, whereas committed T(H)-17 cells were resistant to suppression by T(H)1 or T(H)2 cytokines. In the absence of IFN-gamma and IL-4, IL-23 induced naive precursor cells to differentiate into T(H)-17 cells independently of the transcription factors STAT1, T-bet, STAT4 and STAT6. These findings provide a basis for understanding how inhibition of IFN-gamma signaling enhances development of pathogenic T(H)-17 effector cells that can exacerbate autoimmunity.

Interleukin 17–producing CD4 + effector T cells develop via a lineage distinct from the T helper type 1 and 2 lineages

CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-β plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORγt, and RORα) involved in the development of Th17 cells have just been identified. The participation of TGF-β in the differentiation of Th17 cells places ...

IL-17 and Th17 Cells.

Interleukin (IL)-23 is a heterodimeric cytokine composed of a unique p19 subunit, and a common p40 subunit shared with IL-12. IL-12 is important for the development of T helper (Th)1 cells that are essential for host defense and tumor suppression. In contrast, IL-23 does not promote the development of interferon-γ–producing Th1 cells, but is one of the essential factors required for the expansion of a pathogenic CD4+ T cell population, which is characterized by the production of IL-17, IL-17F, IL-6, and tumor necrosis factor. Gene expression analysis of IL-23–driven autoreactive T cells identified a unique expression pattern of proinflammatory cytokines and other novel factors, distinguishing them from IL-12–driven T cells. Using passive transfer studies, we confirm that these IL-23–dependent CD4+ T cells are highly pathogenic and essential for the establishment of organ-specific inflammation associated with central nervous system autoimmunity.

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IL-23 drives a pathogenic T cell population that induces autoimmune inflammation

Interleukin (IL) 23 is a heterodimeric cytokine composed of a p19 subunit and the p40 subunit of IL-12. IL-23 affects memory T cell and inflammatory macrophage function through engagement of a novel receptor (IL-23R) on these cells. Recent analysis of the contribution of IL-12 and IL-23 to central nervous system autoimmune inflammation demonstrated that IL-23 rather than IL-12 was the essential cytokine. Using gene-targeted mice lacking only IL-12 (p35−/−) or IL-23 (p19−/−), we show that the specific absence of IL-23 is protective, whereas loss of IL-12 exacerbates collagen-induced arthritis. IL-23 gene-targeted mice did not develop clinical signs of disease and were completely resistant to the development of joint and bone pathology. Resistance correlated with an absence of IL-17–producing CD4+ T cells despite normal induction of collagen-specific, interferon-γ–producing T helper 1 cells. In contrast, IL-12–deficient p35−/− mice developed more IL-17–producing CD4+ T cells, as well as elevated mRNA expression of proinflammatory tumor necrosis factor, IL-1β, IL-6, and IL-17 in affected tissues of diseased mice. The data presented here indicate that IL-23 is an essential promoter of end-stage joint autoimmune inflammation, whereas IL-12 paradoxically mediates protection from autoimmune inflammation.

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Yi Yi Chen

Papers

IL-23 drives a pathogenic T cell population that induces autoimmune inflammation

Divergent Pro- and Antiinflammatory Roles for IL-23 and IL-12 in Joint Autoimmune Inflammation

Efficacy and safety of pembrolizumab monotherapy in EGFR-mutant squamous cell lung cancer with PD-L1 over-expression: A case report