Endogenous IFN-γ facilitates Pneumocystis infection and downregulates carbohydrate receptors in CD4+ T cell-depleted mice.

IFN-γ plays a critical role in host defense against intracellular pathogens. IFN-γ is produced in the bronchoalveolar lavage fluid of mice infected with Pneumocystis, but the role of IFN-γ in host defense against Pneumocystis remains controversial. It has been previously reported that although exogenous IFN-γ has beneficial effects on eradication of Pneumocystis, endogenous IFN-γ has a negative impact on innate immunity in immunocompromised hosts. Surprisingly, CD4+ T cell-depleted IFN-γ deficient (GKO) mice exhibit resistance to Pneumocystis. Alveolar macrophages (AM) from GKO mice exhibit higher expression of macrophage mannose receptor (MMR) and Dectin-1. Concomitantly, they exhibited greater ability to phagocytize Pneumocystis, and this activity was suppressed by inhibitors of these receptors. Incubation with IFN-γ resulted in a reduction in both the expression of these receptors on AM and their Pneumocystis-phagocytic activity. These results indicate that endogenous IFN-γ facilitates Pneumocystis to escape from host innate immunity by attenuating the phagocytic activity of AM via downregulation of MMR and Dectin-1.

Interleukin-3-dependent potentiation of IgE responsiveness in mouse basophils.

Basophils produce interleukins (IL)-4 in response to various stimuli and may contribute to type 2 immune responses to various infections and allergens. We found that resting basophils freshly isolated from mice produce IL-4 in response to IL-3 but not to high-affinity Fc receptor (FcεRI) cross-linking (CL), yet both required the immunoreceptor tyrosine-based activation motif (ITAM) containing adaptor Fc receptor γ-chain (FcRγ), while basophils activated in vitro by IL-3 become responsive to FcεRI CL. Acquisition of responsiveness to FcεRI CL occurred upon infection with Trichinella spiralis or administration of superantigen. Because cultured basophils return to a quiescent state upon starvation with IL-3 with surface FcεRI levels unchanged, this acquisition is reversible and probably reflects intracellular events requiring protein synthesis. Interestingly, similar activation-associated acquisition was observed for responsiveness to other stimuli, including CD200R3 CL, which is known to signal via DAP-12, and the allergen protease papain. This acquisition of responsiveness to FcεRI CL was inhibited by Jak inhibitor. Thus, the IL-3 signal bifurcates downstream of Jak, into two distinct pathway, one leading to IL-4 production and the other to render basophils competent to respond to stimuli dependent on ITAM-containing adaptors DAP12 and FcRγ for IL-4 production.

Interferon regulatory factor-2 is required for the establishment of the gut intraepithelial T-cell compartment.

CD8αα+ intestinal intraepithelial lymphocytes (iIELs) are known for their unique role in keeping the integrity of the intestinal epithelial barrier, but factors affecting the development of these cells have not been thoroughly understood. Here, we found that the transcriptional regulator interferon regulatory factor-2 (IRF-2) plays a cell-intrinsic, indispensable role in establishing iIEL populations. CD8αα+, but not CD8αß+, iIELs bearing TCRαß or TCRγδ were severely reduced in numbers in mice lacking this factor (Irf2-/- mice). Moreover, the majority of residual CD8αα+TCRαß+ iIELs in these mice was immature as judged from their Thy1.2high phenotype and inefficient T-bet expression. Thymic IEL precursors isolated from Irf2-/- mice failed to efficiently generate CD8αα+TCRαß+ and TCRγδ+ IELs upon transfer in vivo and CD8αα+TCRαß+ cells in response to IL-15 in vitro. Double mutant mice lacking both interleukin-15 (IL-15) and IRF-2 showed an even more severe iIEL defect than in mice lacking IL-15 alone. Upon increasing agonistic TCR signal strength through OT-II TCR transgenesis, CD8αα+TCRαß+ iIELs became more abundant but remained immature on the Irf2-/- background. Our current observations, thus, revealed the unique bimodal role that IRF-2 plays in promoting not only generation of IEL progenitors in the thymus but also maturation of iIELs in the periphery in IL-15-dependent and -independent manners.

TAK1 Limits Death Receptor Fas-Induced Proinflammatory Cell Death in Macrophages.

Fas, a member of the death receptor family, plays a central role in initiating cell death, a biological process crucial for immune homeostasis. However, the immunological and pathophysiological impacts to which enhanced Fas signaling gives rise remain to be fully understood. Here we demonstrate that TGF-ß-activated kinase 1 (TAK1) works as a negative regulator of Fas signaling in macrophages. Upon Fas engagement with high concentrations of FasL, mouse primary macrophages underwent cell death, and, surprisingly, Fas stimulation led to proteolytic cleavage of gasdermin (GSDM) family members GSDMD and GSDME, a hallmark of pyroptosis, in a manner dependent on caspase enzymatic activity. Remarkably, TAK1-deficient macrophages were highly sensitive to even low concentrations of FasL. Mechanistically, TAK1 negatively modulated RIPK1 kinase activity to protect macrophages from excessive cell death. Intriguingly, mice deficient for TAK1 in macrophages (TAK1mKO mice) spontaneously developed tissue inflammation, and, more important, the emergence of inflammatory disease symptoms was markedly diminished in TAK1mKO mice harboring a catalytically inactive RIPK1. Taken together, these findings not only revealed an unappreciated role of TAK1 in Fas-induced macrophage death but provided insight into the possibility of perturbation of immune homeostasis driven by aberrant cell death.

Circulating T cells and resident non-T cells restrict type 2 innate lymphoid cell expansion in the small intestine.

Interaction among various adaptive, circulating cells and tissue-resident cells including innate lymphocytes during the establishment and maintenance of the barrier-tissue immune system has only recently started to be explored. Here, we show that the cellular crosstalk with circulating T cells and other resident cells regulated the population size of type 2 innate lymphoid cells (ILC2s) in the small intestine lamina propria. Rag1-/- mice had excessive numbers of both ILC2s and ILC3s, and such an over-expansion was corrected by establishing parabiosis with wild type mice or by adoptively transferring wild type CD4+ T cells. In contrast, anti-CD3 antibody-mediated T cell depletion in wild type mice increased ILC2 but not ILC3 numbers. Unconventional CD4-CD8- αß T and γδ T cells could also restrict ILC2 expansion as the numbers of ILC2s were not altered even in mice treated with anti-CD4/anti-CD8 antibodies. ILC3 restriction seemed to be through the control of proliferation, but that for ILC2s did not. In addition, elevation in ILC2 numbers seen in mice lacking the transcription factors RORγt and STAT6 was found to be T cell-independent. Our current findings altogether uncovered the homeostatic 'quota' restriction imposed on intestinal ILC2s in the steady state by resident non-T cells via RORγt- and STAT6-dependent mechanisms as well as by conventional and nonconventional T cells.

2-Deoxy-d-glucose induces deglycosylation of proinflammatory cytokine receptors and strongly reduces immunological responses in mouse models of inflammation.

Anti-proinflammatory cytokine therapies against interleukin (IL)-6, tumor necrosis factor (TNF)-α, and IL-1 are major advancements in treating inflammatory diseases, especially rheumatoid arthritis. Such therapies are mainly performed by injection of antibodies against cytokines or cytokine receptors. We initially found that the glycolytic inhibitor 2-deoxy-d-glucose (2-DG), a simple monosaccharide, attenuated cellular responses to IL-6 by inhibiting N-linked glycosylation of the IL-6 receptor gp130. Aglycoforms of gp130 did not bind to IL-6 or activate downstream intracellular signals that included Janus kinases. 2-DG completely inhibited dextran sodium sulfate-induced colitis, a mouse model for inflammatory bowel disease, and alleviated laminarin-induced arthritis in the SKG mouse, an experimental model for human rheumatoid arthritis. These diseases have been shown to be partially dependent on IL-6. We also found that 2-DG inhibited signals for other proinflammatory cytokines such as TNF-α, IL-1ß, and interferon -γ, and accordingly, prevented death by another inflammatory disease, lipopolysaccharide (LPS) shock. Furthermore, 2-DG prevented LPS shock, a model for a cytokine storm, and LPS-induced pulmonary inflammation, a model for acute respiratory distress syndrome of coronavirus disease 2019 (COVID-19). These results suggest that targeted therapies that inhibit cytokine receptor glycosylation are effective for treatment of various inflammatory diseases.

Cutting Edge: TAK1 Safeguards Macrophages against Proinflammatory Cell Death.

TGF-ß-activated kinase 1 (TAK1) is known to play vital roles for innate and adaptive immunity; however, little is known about its potential role in limiting biological responses such as inflammation. In this study, we report that macrophage TAK1 participates in negatively regulating inflammation by restraining proinflammatory cell death. Macrophages from TAK1-deficient mice underwent cell death in response to LPS and poly(I:C), which took place in a manner dependent on TLR/TRIF-induced active Caspase8-mediated cleavage of gasdermin D, known as an executioner of pyroptosis. Likewise, TNF-α induced Caspase8-dependent gasdermin D processing following cell death in TAK1-deficient macrophages. Importantly, we demonstrated that this type of proinflammatory macrophage death is linked to susceptibility to septic shock in mice lacking TAK1 in macrophages in a TNF-α-independent fashion. Taken together, our data revealed that TAK1 acts as a signaling checkpoint to protect macrophages from unique proinflammatory cell death, ensuring the maintenance of innate immune homeostasis.

Staphylococcal superantigen-like 12 activates murine bone marrow derived mast cells.

Staphylococcal superantigen-like (SSL) protein is a family of exotoxins that consists of 14 SSLs, and the roles of several SSLs in immune evasion of the cocci have been revealed. However little is known whether they act as immune activators and are involved in inflammatory disorders such as atopic dermatitis. In this study we examined whether SSLs activate mast cells, the key player of local inflammation. SSL12 evoked the release of a granule enzyme ß-hexosaminidase from bone marrow derived mast cells (BMMCs) in the absence of IgE. The release of the granule enzyme caused by SSL12 was not accompanied with the leakage of a cytosolic enzyme lactate dehydrogenase (LDH), unlike staphylococcal δ-toxin that was reported to induce both the release of ß-hexosaminidase and the leakage of LDH from the cells, suggesting that SSL12 evokes the degranulation of mast cells without cell membrane damage. Furthermore SSL12 induced IL-6 and IL-13 in both mRNA and protein levels indicating that SSL12 induces de novo synthesis of the cytokines. Evans blue extravasation was elevated by the intradermal injection of SSL12, suggesting that SSL12 is also able to evoke local inflammation in vivo. These findings indicate the novel mast cell activating activity of SSLs, and SSL12 is likely an important factor in both initiation phase and effector phase of allergic and immune responses.

Generation of a common innate lymphoid cell progenitor requires interferon regulatory factor 2.

Innate lymphoid cells (ILCs), composed of heterogeneous populations of lymphoid cells, contribute critically to immune surveillance at mucosal surfaces. ILC subsets develop from common lymphoid progenitors through stepwise lineage specification. However, the composition and temporal regulation of the transcription factor network governing such a process remain incompletely understood. Here, we report that deletion of the transcription factor interferon regulatory factor 2 (IRF-2), known also for its importance in the maturation of conventional NK cells, resulted in an impaired generation of ILC1, ILC2 and ILC3 subsets with lymphoid tissue inducer (LTi)-like cells hardly affected. In IRF-2-deficient mice, PD-1hi ILC precursors (ILCPs) that generate these three ILCs but not LTi-like cells were present at normal frequency, while their sub-population expressing high amounts of PLZF, another marker for ILCPs, was severely reduced. Notably, these IRF-2-deficient ILCPs contained normal quantities of PLZF-encoding Zbtb16 messages, and PLZF expression in developing invariant NKT cells within the thymus was unaffected in these mutant mice. These results point to a unique, cell-type selective role for IRF-2 in ILC development, acting at a discrete step critical for the generation of functionally competent ILCPs.

Staphylococcal α-hemolysin does not induce cell damage in murine mast cells but it augments the degranulation induced by FcεRI cross-linking and ionomycin.

Staphylococcal α-hemolysin (Hla) is a principal small ß-barrel pore forming toxin. It targets a variety of mammalian cells including immune cells; however little is known about its effects on mast cells. In this study, we examined whether Hla affects the degranulation of mast cells. Although Hla bound to the surface of bone marrow-derived mast cells (BMMCs) and formed SDS-stable oligomers on the cells, Hla alone induced neither cytotoxicity nor obvious release of a granule enzyme, ß-hexosaminidase. However, Hla more than doubled the releases of ß-hexosaminidase from BMMCs induced by FcεRI cross-linking or treatment with ionomycin. The augmentation of the enzyme release by rHla was impaired in the presence of 130 mM of extracellular KCl. The mutants of Hla that lacked pore-formation did not augment the release of the enzyme. These findings demonstrate that Hla is able to enhance the degranulation of mast cells induced by FcεRI cross-linking and ionomycin, although it alone does not induce the degranulation, and the pore-formation of Hla followed by potassium efflux is involved in the augmentation. These findings propose a previously unrecognized role for Hla in S. aureus-associated allergic and inflammatory processes via augmentation of mast cell responses.

Conditional Deletion of TAK1 in T Cells Reveals a Pivotal Role of TCRαß+ Intraepithelial Lymphocytes in Preventing Lymphopenia-Associated Colitis.

The kinase TAK is required for the development of conventional and regulatory T cells. We previously reported that mice with conditional deletion of TAK1 in T cells (Lck-cre:TAK1fl/fl mice) exhibited severe T lymphopenia, and were nevertheless predisposed to spontaneous colitis with unknown etiology. Here we focused on the immunopathological mechanism in colitic Lck-cre:TAK1fl/fl mice. We found that 'leaky' CD4+ T cells retaining TAK1 acquired inflammatory phenotypes that contribute to disease onset in Lck-cre:TAK1fl/fl mice. Furthermore, the gut microbiota-triggered signaling was also a key event leading to the pathogenesis. We discovered that Lck-cre:TAK1fl/fl mice were almost completely devoid of TCRαß+CD8α+ intestinal intraepithelial lymphocytes (IELs) and this was largely due to the developmental defect of the thymic precursors by TAK1 deficiency. Remarkably, transfer of TCRαß+CD8α+ IELs from wild-type mice ameliorated colitis in Lck-cre:TAK1fl/fl mice. Taken together, our current study highlighted the emerging role of TAK1 in configuring the gut-specialized T cell subset, which regulates mucosal homeostasis under lymphopenic conditions.

IRF-2 regulates B-cell proliferation and antibody production through distinct mechanisms.

Interferon regulatory factor (IRF)-2 is a transcription factor involved in type I (IFN- α/ß) signaling. It has been reported that IRF-2 deficiency results in various immune dysfunctions. However, the role of IRF-2 in B-cell functions needs to be elucidated. Unlike wild-type (WT) B cells, IRF-2(-/-) B2 cells were refractory to anti-IgM, but not LPS. Such a defect in proliferation was dependent on IFN- α/ß receptor (IFNAR). Marginal zone B cells increased in the proportion relative to B2 cells in IRF-2(-/-) mice produced IgM normally to LPS stimulation. However, IRF-2(-/-) B2 cells were defective in IgM production in an IFNAR-independent manner, although both B-cell subsets differentiated phenotypically to plasma cells at elevated efficiencies. Class switch recombination of IRF-2(-/-) B2 cells by LPS plus IL-4 was also impaired. Their reduced IgM production was conceivably due to an inefficient up-regulation of Blimp-1. Consistent with these in vitro observations, specific antibody production in vivo to a T-dependent antigen by B2 cells was severely impaired in IRF-2(-/- )mice. However, a low, but significant, level of IgG was detected at a late time point, and this IgG exhibited comparable binding affinity to that in WT mice. Follicular helper T-cell development and germinal center formation were normal. A similar tendency was observed when µ chain(-/-) mice were reconstituted with IRF-2(-/- )B cells. These results revealed a multi-faceted role of IRF-2 in the function of B cells, particularly B2 cells, through regulating proliferation in an IFNAR-dependent manner and antibody production via up-regulation of Blimp-1.

Differential requirements for IRF-2 in generation of CD1d-independent T cells bearing NK cell receptors.

NK cell receptors (NKRs) such as NK1.1, NKG2D, and Ly49s are expressed on subsets of CD1d-independent memory phenotype CD8(+) and CD4(-)CD8(-) T cells. However, the mechanism for the generation and functions of these NKR(+) T cells remained elusive. In this study, we found that CD1d-independent Ly49(+) T cells were reduced severely in the spleen, bone marrow, and liver, but not thymus, in mice doubly deficient for IFN regulatory factor-2 (IRF-2) and CD1d, in which the overall memory phenotype T cell population was contrastingly enlarged. Because a large fraction of Ly49(+) T cells coexpressed NK1.1 or NKG2D, the reduction of Ly49(+) T cells resulted indirectly in underrepresentation of NK1.1(+) or NKG2D(+) cells. Ly49(+) T cell deficiency was observed in IRF-2(-/-) mice additionally lacking IFN-α/ßR α-chain (IFNAR1) as severely as in IRF-2(-/-) mice, arguing against the involvement of the accelerated IFN-α/ß signals due to IRF-2 deficiency. Rather, mice lacking IFN-α/ßR alone also exhibited relatively milder Ly49(+) T cell reduction, and IL-2 could expand Ly49(+) T cells from IFNAR1(-/-), but not from IRF-2(-/-), spleen cells in vitro. These results together indicated that IRF-2 acted in Ly49(+) T cell development in a manner distinct from that of IFN-α/ß signals. The influence of IRF-2 deficiency on Ly49(+) memory phenotype T cells observed in this study suggested a unique transcriptional program for this T cell population among other NKR(+) T and memory phenotype T cells.

IL-15-high-responder developing NK cells bearing Ly49 receptors in IL-15-/- mice.

In mice lacking IL-15, NK cell development is arrested at immature stages, providing an opportunity to investigate the earliest developing NK cells that would respond to IL-15. We show in this study that immature NK cells were present in the spleen as well as bone marrow (BM) and contained IL-15-high-responder cells. Thus, mature NK cells were generated more efficiently from IL-15(-/-) than from control donor cells in radiation BM chimeras, and the rate of IL-15-induced cell division in vitro was higher in NK cells in the spleen and BM from IL-15(-/-) mice than in those from wild-type mice. Phenotypically, NK cells developed in IL-15(-/-) mice up to the minor but discrete CD11b(-)CD27(+)DX5(hi)CD51(dull)CD127(dull)CD122(hi) stage, which contained the majority of Ly49G2(+) and D(+) NK cells both in the spleen and BM. Even among wild-type splenic NK cells, IL-15-induced proliferation was most prominent in CD11b(-)DX5(hi) cells. Notably, IL-15-mediated preferential expansion (but not conversion from Ly49(-) cells) of Ly49(+) NK cells was observed in vitro only for NK cells in the spleen. These observations indicated the uneven distribution of NK cells of different developing stages with variable IL-15 responsiveness in these lymphoid organs. Immature NK cells in the spleen may contribute, as auxiliaries to those in BM, to the mature NK cell compartment through IL-15-driven extramarrow expansion under steady-state or inflammatory conditions.

Critical role of Th17 cells in inflammation and neovascularization after ischaemia.

AIMS: Increasing evidence suggests that CD4(+) T cells contribute to neovascularization in ischaemic tissue. However, the T cell subset responsible for neovascularization after ischaemia remains to be determined. Here, we investigated the role of Th17 cells secreting interleukin (IL)-17, a newly identified subset of CD4(+) T cells, in the neovascularization after murine hindlimb ischaemia. METHODS AND RESULTS: Unilateral hindlimb ischaemia was produced in wild-type (WT) C57BL/6 mice. Depletion of CD4(+) T cells resulted in significantly reduced blood flow perfusion in the ischaemic limbs. The expression of IL-17 and retinoic acid receptor-related orphan receptor γt (RORγt) was up-regulated in the ischaemic limbs. IL-17-deficient mice showed a significant reduction in blood flow perfusion, inflammatory cell infiltration, and production of angiogenic cytokines in the ischaemic limbs compared with WT mice. In bone marrow transplantation experiments, the absence of IL-17 specifically in bone marrow cells diminished the neovascularization after ischaemia. Furthermore, IL-17-deficient CD4(+) T cells transferred into the ischaemic limbs of T cell-deficient athymic nude mice evoked a significantly limited neovascularization compared with WT CD4(+) T cells. CONCLUSION: These findings identify Th17 cells as a new angiogenic T cell subset and provide new insight into the mechanism by which T cells promote neovascularization after ischaemia.

Potentiation of TLR9 responses for human naïve B-cell growth through RP105 signaling.

Toll-like receptor 9 (TLR9) signals induce important pathways in the early defense against microbial pathogens. Although TLR9 signaling can activate memory B cells directly, efficient naïve B cell responses seem to require additional, but as yet unidentified, signals. We explored the effects of RP105 (CD180) on CpG DNA-activated naïve and memory B cells from normal controls and patients with common variable immunodeficiency (CVID). RP105 dramatically enhanced CpG DNA-induced proliferation/survival by naïve B cells but not by memory B cells. This enhancement was mediated by TLR9 upregulation induced by RP105, leading to Akt activation and sustained NF-kappaB activation. CpG DNA-activated CVID B cells showed enhancement of proliferation/survival by RP105 and produced specific IgM antibody to Streptococcus pneumoniae polysaccharides in response to interleukin-21 stimulation. Thus, RP105 strongly affects expansion of the naïve B-cell pool, and suggests that the putative RP105 ligand (s) upon cytokine stimulation facilitates antibody-mediated acute pathogen clearance.

Distinct response in maintenance of human naive and memory B cells via IL-21 receptor and TCL1/Akt pathways.

The molecular mechanisms involving in B-cell survival/proliferation are poorly understood. Here we investigated the molecules affecting the survival of human naïve and memory B cells. Without stimulation, naïve B cells survived longer than memory B cells. Moreover, the viability of memory B cells decreased more rapidly than that of naïve B cells following with Staphylococcus aureus Cowan strain (SAC), anti-immunoglobulin (Ig), or anti-CD40 stimulation, but displayed the same levels of survival following CpG DNA stimulation. We analyzed the transcriptional differences between B-cell subsets by gene expression profiling, and identified 15 genes significantly correlated to survival/proliferation. Among them, IL-21 receptor (IL-21R) and T-cell leukemia 1 (TCL1) proto-oncogene were highly expressed in naïve B cells. IL-21 induced the proliferation of both naïve and memory B cells. Marked phosphorylation of Akt was found in naïve B cells compared with memory B cells. This study suggests that naive and memory B cells are regulated by several distinct molecules, and the IL-21R and TCL1/Akt pathways might play crucial roles in naïve B cells for their maintenance.

Fc receptor gamma-chain, a constitutive component of the IL-3 receptor, is required for IL-3-induced IL-4 production in basophils.

The Fc receptor common gamma-chain (FcRgamma) is a widely expressed adaptor bearing an immunoreceptor tyrosine-based activation motif (ITAM) that transduces activation signals from various immunoreceptors. We show here that basophils lacking FcRgamma developed normally and proliferated efficiently in response to interleukin 3 (IL-3) but were very impaired in IL-3-induced production of IL-4 and in supporting T helper type 2 differentiation. Through its transmembrane portion, FcRgamma associated constitutively with the common beta-chain of the IL-3 receptor and signaled by recruiting the kinase Syk. Retrovirus-mediated complementation demonstrated the essential function of the ITAM of FcRgamma in IL-3 signal transduction. Our results identify a previously unknown mechanism whereby FcRgamma functions to 'route' selective cytokine-triggered signals into the ITAM-mediated IL-4 production pathway.

STAT6 signalling is important in CD8 T-cell activation and defence against Toxoplasma gondii infection in the brain.

Signal transducer and activator of transcription (STAT) 6 is a molecule involved in interleukin (IL)-4 and -13 signalling. We investigated the role of STAT6 signalling in Toxoplasma gondii-infected mice using STAT6-deficient (STAT6(-/-)) and wild-type (WT) mice. A significantly larger number of cysts were recovered from the brain in STAT6(-/-) than in WT mice on days 28 and 56 post-infection. CD8(+) T cells in cerebrospinal fluid and spleen stimulated with T. gondii antigen produced higher levels of interferon (IFN)-gamma in WT than in STAT6(-/-) mice. CD8(+) T-cell function, estimated by expression of CD25 and cytotoxic activity, was lower in STAT6(-/-) than in WT mice. Transfer of CD8(+) but not CD4(+) T cells, purified from infected WT mice, into STAT6(-/-) mice successfully prevented formation of cysts in the brain. However, transfer of naïve CD8(+) T cells from WT into STAT6(-/-) mice did not show either activation of CD8(+) T cells or a decrease in the number of cysts in the brain. Transfer of splenic adherent cells from WT into STAT6(-/-) mice induced activation of CD8(+) T cells and decreased the number of cysts in the brain. Expression of CD86 on splenic dendritic cells and IL-12 p40 production were weaker in STAT6(-/-) than in WT mice after T. gondii infection. These results indicate that STAT6 signalling is important in CD8(+) T-cell activation, possibly through regulation of antigen-presenting cells, which could suppress T. gondii infection in the brain.

IL-15 inhibits pre-B cell proliferation by selectively expanding Mac-1+B220+ NK cells.

Natural killer (NK) cells are the cells critical for inhibition of repopulation of allogenic bone marrow cells. However, it is not well known if NK cells affect autologous lymphopoiesis. Here, we observed that NK cells could inhibit pre-B cell proliferation in vitro driven by interleukin (IL)-7 in a manner dependent on IL-15. Interestingly, the great majority of expanding NK cells were Mac-1(+)B220(+), a recently identified potent interferon (IFN)-gamma producer. Indeed, IFN-gamma was produced in those cultures, and pre-B cells lacking IFN-gamma receptors, but not those lacking type I IFN receptors, were resistant to such an inhibition. Furthermore, even NK cells from mice lacking beta2-microglobulin, which were known to be functionally dampened, inhibited pre-B cell proliferation as well. Thus, activated NK cells, which were expanded selectively by IL-15, could potentially regulate B lymphopoiesis through IFN-gamma beyond the selection imposed upon self-recognition.