Jan Kisielow

IL-21R on T Cells Is Critical for Sustained Functionality and Control of Chronic Viral Infection

Controlling Chronic Viral Infections Chronic viral infections such as HIV and hepatitis B and C viruses are major public health concerns. T cell—mediated immune responses are critical for controlling viral infections. In contrast to acute infections, chronic viral infections are characterized by “exhausted” cytotoxic CD8+ T cells, cells which exhibit reduced proliferative capacity, cytokine secretion, and cytotoxicity. Treatments that reverse exhaustion result in increased viral control. Despite their exhaustion, these CD8+ T cells eventually help to control chronic infections by killing virally infected cells, and require CD4+ T cell help to do so. How do CD4+ T cells provide help to CD8+ T cells during chronic infection (see the Perspective by Johnson and Jameson)? Elsaesser et al. (p. 1569, published online 7 May), Yi et al. (p. 1572, published online 14 May), and Fröhlich et al. (p. 1576, published online 28 May) now show that the cytokine, interleukin-21 (IL-21), known to be critical for the differentiation of certain CD4+ T cell effector subsets, is an essential factor produced by CD4+ T cells that helps CD8+ T cells to control chronic lymphocytic choriomeningitis virus infection in mice. Acute and chronic infections resulted in differing amounts of IL-21 production by virus-specific CD4+ T cells. CD8+ T cells required IL-21 directly, and when CD8+ T cells were unable to signal through IL-21 or IL-21 was not available, they were reduced in number, exhibited a more exhausted phenotype, and were not able to control the virus. In contrast, the absence of IL-21–dependent signaling did not affect primary CD8+ T cell responses to acute infection or responses to a viral rechallenge, suggesting that differentiation of memory CD8+ T cells is independent of IL-21. Thymus cells of the immune system require the cytokine interleukin-21 to control chronic viral infections. Chronic viral infection is often associated with the dysfunction of virus-specific T cells. Our studies using Il21r-deficient (Il21r–/–) mice now suggest that interleukin-21 (IL-21) is critical for the long-term maintenance and functionality of CD8+ T cells and the control of chronic lymphocytic choriomeningitis virus infection in mice. Cell-autonomous IL-21 receptor (IL-21R)–dependent signaling by CD8+ T cells was required for sustained cell proliferation and cytokine production during chronic infection. Il21r–/– mice showed normal CD8+ T cell expansion, effector function, memory homeostasis, and recall responses during acute and after resolved infection with several other nonpersistent viruses. These data suggest that IL-21R signaling is required for the maintenance of polyfunctional T cells during chronic viral infections and have implications for understanding the immune response to other persisting antigens, such as tumors.

Psoriasiform dermatitis is driven by IL-36-mediated DC-keratinocyte crosstalk.

Psoriasis is a chronic inflammatory disorder of the skin affecting approximately 2% of the worlds population. Accumulating evidence has revealed that the IL-23/IL-17/IL-22 pathway is key for development of skin immunopathology. However, the role of keratinocytes and their crosstalk with immune cells at the onset of disease remains poorly understood. Here, we show that IL-36R-deficient (Il36r-/-) mice were protected from imiquimod-induced expansion of dermal IL-17-producing γδ T cells and psoriasiform dermatitis. Furthermore, IL-36R antagonist-deficient (Il36rn-/-) mice showed exacerbated pathology. TLR7 ligation on DCs induced IL-36-mediated crosstalk with keratinocytes and dermal mesenchymal cells that was crucial for control of the pathological IL-23/IL-17/IL-22 axis and disease development. Notably, mice lacking IL-23, IL-17, or IL-22 were less well protected from disease compared with Il36r-/- mice, indicating an additional distinct activity of IL-36 beyond induction of the pathological IL-23 axis. Moreover, while the absence of IL-1R1 prevented neutrophil infiltration, it did not protect from acanthosis and hyperkeratosis, demonstrating that neutrophils are dispensable for disease manifestation. These results highlight a central and unique IL-1-independent role for IL-36 in control of the IL-23/IL-17/IL-22 pathway and development of psoriasiform dermatitis.

Nrf2 is essential for cholesterol crystal-induced inflammasome activation and exacerbation of atherosclerosis.

Oxidative stress and inflammation — two components of the natural host response to injury — constitute important etiologic factors in atherogenesis. The pro‐inflammatory cytokine interleukin (IL)‐1 significantly enhances atherosclerosis, however, the molecular mechanisms of IL‐1 induction within the artery wall remain poorly understood. Here we have identified the oxidative stress‐responsive transcription factor NF‐E2‐related 2 (Nrf2) as an essential positive regulator of inflammasome activation and IL‐1‐mediated vascular inflammation. We show that cholesterol crystals, which accumulate in atherosclerotic plaques, represent an endogenous danger signal that activates Nrf2 and the NLRP3 inflammasome. The resulting vigorous IL‐1 response critically depended on expression of Nrf2, and Nrf2‐deficient apolipoprotein E (Apoe)−/− mice were highly protected against diet‐induced atherogenesis. Importantly, therapeutic neutralization of IL‐1α and IL‐1β reduced atherosclerosis in Nrf2+/−Apoe−/− but not in Nrf2−/−Apoe−/− mice, suggesting that the pro‐atherogenic effect of Nrf2‐signaling was primarily mediated by its permissive role in IL‐1 production. Our studies demonstrate a role for Nrf2 in inflammasome activation, and identify cholesterol crystals as disease‐relevant triggers of the NLRP3 inflammasome and potent pro‐atherogenic cytokine responses. These findings suggest a common pathway through which oxidative stress and metabolic danger signals converge and mutually perpetuate the chronic vascular inflammation that drives atherosclerosis.

IL-21 and IL-21R are not required for development of Th17 cells and autoimmunity in vivo

Th17 cells have been recognized as the central effectors in organ‐related autoimmune diseases. IL‐6 is a key factor that reciprocally regulates Th17 and Foxp3+ Treg differentiation by inhibition of TGF‐β induced Foxp3 and induction of RORγt, a Th17 lineage‐specific transcription factor. Recently IL‐21 has been suggested to induce RORγt and Th17 development in the absence of IL‐6. However, the relevance of IL‐21 for Th17‐dependent inflammatory responses in vivo remains unclear. In this study, we demonstrate that differentiation of IL‐17‐producing CD4 T cells, their recruitment to inflamed organs, and the development of autoimmune disease was not affected in il21R–/– and il21–/– mice in models of myelin oligodendrocyte glycoprotein‐induced autoimmune encephalitis and autoimmune myocarditis. IL‐6 induced Th17 differentiation independent of and much more potently than IL‐21 in vitro. These data suggest that IL‐6 is sufficient to drive Th17 development and associated autoimmunity in vivo in the absence of IL‐21 or IL‐21R.

Expression of lymphocyte activation gene 3 (LAG-3) on B cells is induced by T cells

Lymphocyte activation gene 3 (LAG‐3/CD223) is a CD4 homolog known to be selectively expressed in activated T and NK cells. It is thought to have a negative regulatory function in T cells. With the help of new monoclonal antibodies against mouse LAG‐3, we show that LAG‐3 surface expression is not limited to activated T and NK cells but is also found on activated B cells. Induction of B cell surface expression is T cell dependent and mediated by a soluble factor. The majority of LAG‐3 on B cell surface is endogenously produced, even though soluble LAG‐3 is present in the culture supernatants and can be passively absorbed. As B cells express LAG‐3 in a T cell dependent manner and not when activated by Toll‐like‐receptor agonists alone, we propose LAG‐3 as a new marker of T cell induced B cell activation.

The kinase activity of Rip2 determines its stability and consequently Nod1- and Nod2-mediated immune responses.

Rip2 (RICK, CARD3) has been identified as a key effector molecule downstream of the pattern recognition receptors, Nod1 and Nod2; however, its mechanism of action remains to be elucidated. In particular, it is unclear whether its kinase activity is required for signaling or for maintaining protein stability. We have investigated the expression level of different retrovirally expressed kinase-dead Rip2 mutants and the role of Rip2 kinase activity in the signaling events that follow Nod1 and Nod2 stimulation. We show that in primary cells expressing kinase-inactive Rip2, protein levels were severely compromised, and stability could not be reconstituted by the addition of a phospho-mimetic mutation in its autophosphorylation site. Consequently, inflammatory cytokine production in response to Nod1 and Nod2 ligands was abrogated both in vitro and in vivo in the absence of Rip2 kinase activity. Our results highlight the central role that Rip2 kinase activity plays in conferring stability to the protein and thus in the preservation of Nod1- and Nod2-mediated innate immune responses.

Bacterial-induced protection against allergic inflammation through a multicomponent immunoregulatory mechanism

Background Airborne microbial products have been reported to promote immune responses that suppress asthma, yet how these beneficial effects take place remains controversial and poorly understood. Methods We exposed mice to the bacterium Escherichia coli and subsequently induced allergic airway inflammation through sensitization and intranasal challenge with ovalbumin. Results Pulmonary exposure to the bacterium Escherichia coli leads to a suppression of allergic airway inflammation. This immune modulation was neither mediated by the induction of a T helper 1 (Th1) response nor regulatory T cells; however, it was dependent on Toll-like receptor 4 (TLR4) but did not involve TLR desensitisation. Dendritic cell migration to the draining lymph nodes and activation of T cells was unaffected by prior exposure to E.coli, while dendritic cells in the lung displayed a less activated phenotype and had impaired antigen presentation capacity. Consequently, in situ Th2 cytokine production was abrogated. The suppression of airway hyper-responsiveness was mediated through the recruitment of gd T cells; however, the suppression of dendritic cells and T cells was mediated through a distinct mechanism that could not be overcome by the local administration of activated dendritic cells, or by the in vivo administration of tumour necrosis factor a. Conclusion Our data reveal a localized immunoregulatory pathway that acts to protect the airways from allergic inflammation.

TARPP, a novel protein that accompanies TCR gene rearrangement and thymocyte education.

Studies on thymic T cell development have usually concentrated on cell surface molecules. However, intracellular proteins expressed only in thymocytes have never been described. Here we reportthe discovery of a novel thymocyte‐specific protein, named TARPP, which represents a high molecular mass (∼ 100 kDa) variant of the previously identified protein ARPP‐21 (∼ 21 kDa). TARPP is a cytosolic protein that is expressed at high levels in immature thymocytes. It appears concomitant with the commitment to T cell lineage, and its expression is switched off as a consequence of TCR engagement during positive selection. Such an expression pattern, correlating with the rearrangement of the TCR genes and thymocyte education, suggests a role for TARPP during this important phase of thymocyte development.

IL-21 induces death of marginal zone B cells during chronic inflammation

Interleukin-2 (IL-2) and IL-21 share activities in the control of T- and B-cell maturation, proliferation, function, and survival. However, opposing roles for IL-2 and IL-21 have been reported in the development of regulatory T cells. To dissect unique, redundant, and opposing activities of IL-2 and IL-21, we compared T- and B-cell development and function in mice lacking both IL-2 receptor α (IL-2Rα) and IL-21R (double knockouts [DKO]) with single knockout and wild-type (WT) mice. Similarly to il2ra(-/-) mice, DKO showed reduced numbers of regulatory T cells and, consequently, hyper-activation and proliferation of T cells associated with inflammatory disease (ie, colitis), weight loss, and reduced survival. The absence of IL-2Rα resulted in overproduction of IL-21 by IFN-γ-producing CD4(+) T cells, which induced apoptosis of marginal zone (MZ) B cells. Hence, MZ B cells and MZ B-cell immunoglobulin M antibody responses to Streptococcus pneumoniae phosophorylcholine were absent in il2ra(-/-) mice but were completely restored in DKO mice. Our results highlight key roles of IL-2 in inhibiting IL-21 production by CD4(+) T cells and of IL-21 in negatively regulating MZ B-cell survival and antibody production.

Evidence for the divergence of innate and adaptive T-cell precursors before commitment to the αβ and γδ lineages

In addition to adaptive T cells, the thymus supports the development of unconventional T cells such as natural killer T (NKT) and CD8αα intraepithelial lymphocytes (IELs), which have innate functional properties, particular antigenic specificities, and tissue localization. Both conventional and innate T cells are believed to develop from common precursors undergoing instructive, TCR-mediated lineage fate decisions, but innate T cells are proposed to undergo positive instead of negative selection in response to agonistic TCR signals. In the present study, we show that, in contrast to conventional αβT cells, innate αβT cells are not selected against functional TCRγ rearrangements and express TCRγ mRNA. Likewise, in contrast to the majority of γδT cells, thymic innate γδT cells are not efficiently selected against functional TCRβ chains. In precursors of conventional T cells, autonomous TCR signals emanating from the pre-TCR or γδTCR in the absence of ligand mediate selection against the TCR of the opposite isotype and αβ/γδ lineage commitment. Our data suggest that developing innate T cells ignore such signals and rely solely on agonistic TCR interactions. Consistently, most innate T cells reacted strongly against autologous thymocytes. These results suggest that innate and adaptive T-cell lineages do not develop from the same pool of precursors and potentially diverge before αβ/γδ lineage commitment.