Efficient execution of apoptotic cell death followed by efficient clearance mediated by professional macrophages is a key mechanism in maintaining tissue homeostasis. Removal of apoptotic cells usually involves three central elements: (1) attraction of phagocytes via soluble `find me' signals, (2) recognition and phagocytosis via cell surface presenting `eat me' signals, and (3) suppression or initiation of inflammatory responses depending on additional innate immune stimuli. Suppression of inflammation involves both direct inhibition of pro-inflammatory cytokine production and release of anti-inflammatory factors, which all contribute to the resolution of inflammation. In the present study, using wild type and adenosine A2A receptor (A2AR) null mice, we investigated whether A2ARs, known to mediate anti-inflammatory signals in macrophages, participate in the apoptotic cell-mediated immunosuppression. We found that macrophages engulfing apoptotic cells release adenosine in sufficient amount to trigger A2ARs, and simultaneously increase the expression of A2ARs, possibly via activation of activation of liver X receptor and peroxisome proliferators activated receptor δ. In macrophages engulfing apoptotic cells, stimulation of A2ARs suppresses the NO-dependent formation of neutrophil migration factors, such as macrophage inflammatory protein-2, using the adenylate cyclase / protein kinase A pathway. As a result, loss of A2ARs results in elevated chemoattractant secretion. This was evident as pronounced neutrophil migration upon exposure of macrophages to apoptotic cells in an in vivo peritonitis model. Altogether our data indicate that adenosine is one of the soluble mediators released by macrophages that mediate engulfment-dependent apoptotic cell suppression of inflammation.

Transglutaminase 2 (TG2) is a protein crosslinking enzyme with several additional biochemical functions. Loss of TG2 in vivo results in impaired phagocytosis of apoptotic cells and altered proinflammatory cytokine production by macrophages engulfing apoptotic cells leading to autoimmunity. It has been proposed that TG2 acts as an integrin β(3) coreceptor in the engulfment process, while altered proinflammatory cytokine production is related to the lack of latent TGFβ activation by TG2 null macrophages. Here we report that TG2 null macrophages respond to lipopolysaccharide treatment by elevated IL-6 and TNFα production. Though TGFβ has been proposed to act as a feed back regulator of proinflammatory cytokine production in LPS-stimulated macrophages, this phenomenon is not related to the lack of active TGFβ production. Instead, in the absence of TG2 integrin β(3) maintains an elevated basal Src family kinase activity in macrophages, which leads to enhanced phosphorylation and degradation of the IκBα. Low basal levels of IκBα explain the enhanced sensitivity of TG2 null macrophages to signals that regulate NF-κB. Our data suggest that TG2 null macrophages bear a proinflammatory phenotype, which might contribute to the enhanced susceptibility of these mice to develop autoimmunity and atherosclerosis.

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Transglutaminase 2 (TG2) has been known for a long time to be associated with the in vivo apoptosis program of various cell types, including T cells. Though the expression of the enzyme is strongly induced in mouse thymocytes following apoptosis induction in vivo, no significant induction of TG2 can be detected, when thymocytes are induced to die by the same stimuli in vitro indicating that signals arriving from the tissue environment are required for the proper in vivo induction of the enzyme. Previous studies from our laboratory have demonstrated that two of these signals, transforming growth factor-β (TGF-β) and retinoids, are produced by macrophages engulfing apoptotic cells. However, in addition to TGF-β and retinoids, engulfing macrophages produce adenosine as well. Here, we show that in vitro adenosine, adenosine, and retinoic acid or adenosine, TGF-β and retinoic acids together can significantly enhance the TG2 mRNA expression in dying thymocytes. The effect of adenosine is mediated via adenosine A2A receptors (A2ARs) and the A2AR-triggered adenylate cyclase signaling pathway. In accordance, loss of A2ARs in A2AR null mice significantly attenuates the in vivo induction of TG2 following apoptosis induction in the thymus indicating that adenosine indeed contributes in vivo to the apoptosis-related appearance of the enzyme. We also demonstrate that adenosine is produced extracellularly during engulfment of apoptotic thymocytes, partly from adenine nucleotides released via thymocyte pannexin-1 channels. Our data reveal a novel crosstalk between macrophages and apoptotic cells, in which apoptotic cell uptake-related adenosine production contributes to the appearance of TG2 in the dying thymocytes.

A bőr többi sejttípusához képest a faggyúsejt viszonylag kevés bőrbetegségben érintett. Hogy ennek oka a figyelmünk hiánya vagy pedig valóban a faggyúmirigy atavisztikus, az emberben funkcióját tekintve jelentősen korlátozott jellege-e, továbbra is megválaszolandó kérdés. Az utóbbi bő évtizedben ugyanakkor számos eredmény látott napvilágot, mely bár döntően alapkutatási, de mégis felveti annak a lehetőségét, hogy a faggyúmirigyek nem csupán mellékes függelékei a bőrnek, de annak homeosztázisához, patológiás működéséhez egyaránt képesek hozzájárulni. Cikkünkkel e kutatási eredményeket kívánjuk összefoglalni és értékelni, mind betegség mind pedig terápiás szempontból, segítve ezáltal elhelyezni a fagygyúsejteket nem csak a bőrgyógyászatban, de a gondolkodásunkban is.