VCAM1/VLA4 interaction mediates Ly6Clow monocyte recruitment to the brain in a TNFR signaling dependent manner during fungal infection

Partial funding for Open Access provided by the UMD Libraries' Open Access Publishing Fund.Monocytes exist in two major populations, termed Ly6C^hi and Ly6C^low monocytes. Compared to Ly6C^hi monocytes, less is known about Ly6C^low monocyte recruitment and mechanisms involved in the recruitment of this subset. Furthermore, the role of Ly6C^low monocytes during infections is largely unknown. Here, using intravital microscopy, we demonstrate that Ly6C^low monocytes are predominantly recruited to the brain vasculature following intravenous infection with Cryptococcus neoformans, a fungal pathogen causing meningoencephalitis. The recruitment depends primarily on the interaction of VCAM1 expressed on the brain endothelium with VLA4 expressed on Ly6C^low monocytes. Furthermore, TNFR signaling is essential for the recruitment through enhancing VLA4 expression on Ly6C^low monocytes. Interestingly, the recruited Ly6C^low monocytes internalized C. neoformans and carried the organism while crawling on and adhering to the luminal wall of brain vasculature and migrating to the brain parenchyma. Our study reveals a substantial recruitment of Ly6C^low monocytes to the brain and highlights important properties of this subset during infection.https://doi.org/10.1371/journal.ppat.100836

The immunoregulation of interleukin-27 in African trypanosome infection

Interleukin (IL)-27 is a cytokine with diverse impacts on regulation of vertebrate T helper Type 1 (Th1) responses. Initially, it was predicted as a promoter of Th1 responses. However, it was lately identified as a potent negative regulator of T cell responses in a variety of disease models, including infection with viruses, bacteria, and intracellular parasites. The extracellular protozoan parasites, African trypanosomes, cause a chronic debilitating disease associated with persistent inflammation. Using this infection model, we aim to identify novel immunoregulatory functions of IL-27 on innate and adaptive immunity. Here we demonstrate that IL-27 receptor deficient (IL-27R-/-) mice infected with African trypanosomes display excessive production of IFN-γ by CD4+ T cells, exacerbated liver pathology, and dramatically shortened survival as compared with infected wild-type mice. Depletion of CD4+ T cells or neutralization of IFN-γ ameliorates the liver pathology and extends the survival of infected IL-27R-/- mice. Our further interest is in deciphering the mechanisms of how CD4+ T cells and IFN-γ shape the monocyte-featured innate immunity in African trypanosome infected IL-27R-/- mice. Blood monocytes typically consist of a heterogenous population of Ly6C+ and Ly6C- monocytes. Ly6C+ monocytes can give rise to inflammatory TNF-α/iNOS producing dendritic cells (Tip-DCs) and anti-inflammatory macrophages. Here we find that IL-27R-/- mice exhibit a higher frequency of Ly6C+ monocytes recruitment to the liver, where they preferentially differentiate into Tip-DCs. This is coincided with impaired development of Ly6C- monocytes and macrophages in the liver. Depletion of CD4+ T cells or neutralization of IFN-γ in infected IL-27R-/- mice diminishes the recruitment of Ly6C+ monocytes, and their differentiation into Tip-DCs in the liver. This is accompanied by the greatly enhanced counts of Ly6C- monocytes and macrophages following antibody treatments. Further evidences show that 1) IFN-γ produced by CD4+ T cells induces cell death of Ly6C- monocytes which perturb the development of Tip-DCs in infected IL-27R-/- mice and 2) cell intrinsic IFN-γ signaling drives Ly6C+ monocytes to differentiate into Tip-DCs in infected IL-27R-/- mice. Thus, our data identify IL-27 signaling as a novel immunoregulator to prevent Ly6C+ monocytes from differentiation into Tip-DCs through suppressing CD4+ T cells to secrete IFN-γ

Interferon Gamma in African Trypanosome Infections: Friends or Foes?

African trypanosomes cause fatal infections in both humans and livestock. Interferon gamma (IFN-γ) plays an essential role in resistance to African trypanosomes. However, increasing evidence suggests that IFN-γ, when excessively synthesized, also induces immunopathology, enhancing susceptibility to the infection. Thus, production of IFN-γ must be tightly regulated during infections with African trypanosomes to ensure that a robust immune response is elicited without tissue destruction. Early studies have shown that secretion of IFN-γ is downregulated by interleukin 10 (IL-10). More recently, IL-27 has been identified as a negative regulator of IFN-γ production during African trypanosome infections. In this review, we discuss the current state of our understanding of the role of IFN-γ in African trypanosome infections. We have focused on the cellular source of IFN-γ, its beneficial and detrimental effects, and mechanisms involved in regulation of its production, highlighting some recent advances and offering some perspectives on future directions

CXCR6+CD4+ T cells promote mortality during Trypanosoma brucei infection

Liver macrophages internalize circulating bloodborne parasites. It remains poorly understood how this process affects the fate of the macrophages and T cell responses in the liver. Here, we report that infection by Trypanosoma brucei induced depletion of macrophages in the liver, leading to the repopulation of CXCL16-secreting intrahepatic macrophages, associated with substantial accumulation of CXCR6+CD4+ T cells in the liver. Interestingly, disruption of CXCR6 signaling did not affect control of the parasitemia, but significantly enhanced the survival of infected mice, associated with reduced inflammation and liver injury. Infected CXCR6 deficient mice displayed a reduced accumulation of CD4+ T cells in the liver; adoptive transfer experiments suggested that the reduction of CD4+ T cells in the liver was attributed to a cell intrinsic property of CXCR6 deficient CD4+ T cells. Importantly, infected CXCR6 deficient mice receiving wild-type CD4+ T cells survived significantly shorter than those receiving CXCR6 deficient CD4+ T cells, demonstrating that CXCR6+CD4+ T cells promote the mortality. We conclude that infection of T. brucei leads to depletion and repopulation of liver macrophages, associated with a substantial influx of CXCR6+CD4+ T cells that mediates mortality.https://doi.org/10.1371/journal.ppat.100996

IL-27 Signaling Promotes Th1 Responses and Is Required to Inhibit Fungal Growth in the Lung during Repeated Exposure to Aspergillus fumigatus

Partial funding for Open Access provided by the UMD Libraries' Open Access Publishing Fund.Aspergillus fumigatus is an opportunistic fungal pathogen that causes a wide spectrum of diseases in humans, including life-threatening invasive infections as well as several hypersensitivity respiratory disorders. Disease prevention is predicated on the host’s ability to clear A. fumigatus from the lung while also limiting inflammation and preventing allergic responses. IL-27 is an important immunoregulatory cytokine, but its role during A. fumigatus infection remains poorly understood. In contrast to most infection settings demonstrating that IL-27 is anti-inflammatory, in this study we report that this cytokine plays a proinflammatory role in mice repeatedly infected with A. fumigatus. We found that mice exposed to A. fumigatus had significantly enhanced secretion of IL-27 in their lungs. Genetic ablation of IL-27Rα in mice resulted in significantly higher fungal burdens in the lung during infection. The increased fungal growth in IL-27Rα−/− mice was associated with reduced secretion of IL-12, TNF-α, and IFN-γ, diminished T-bet expression, as well as a reduction in CD4+ T cells and their activation in the lung, demonstrating that IL-27 signaling promotes Th1 immune responses during repeated exposure to A. fumigatus. In addition, infected IL-27Rα−/− mice displayed reduced accumulation of dendritic cells and exudate macrophages in their lungs, and these cells had a lower expression of MHC class II. Collectively, this study suggests that IL-27 drives type 1 immunity and is indispensable for inhibiting fungal growth in the lungs of mice repeatedly exposed to A. fumigatus, highlighting a protective role for this cytokine during fungal infection.https://doi.org/10.4049/immunohorizons.210011

IL-27 Negatively Regulates Tip-DC Development during Infection

Partial funding for Open Access provided by the UMD Libraries' Open Access Publishing Fund.Tumor necrosis factor (TNF)/inducible nitric oxide synthase (iNOS)-producing dendritic cells (Tip-DCs) have profound impacts on host immune responses during infections. The mechanisms regulating Tip-DC development remain largely unknown. Here, using a mouse model of infection with African trypanosomes, we show that a deficiency in interleukin-27 receptor (IL-27R) signaling results in escalated intrahepatic accumulation of Ly6C-positive (Ly6C1) monocytes and their differentiation into Tip-DCs. Blocking Tip-DC development significantly ameliorates liver injury and increases the survival of infected IL-27R2/2 mice. Mechanistically, Ly6C1 monocyte differentiation into pathogenic Tip-DCs in infected IL-27R2/2 mice is driven by a CD41 T cell-interferon gamma (IFN-g) axis via cell-intrinsic IFN-g signaling. In parallel, hyperactive IFN-g signaling induces cell death of Ly6C-negative (Ly6C2) monocytes in a cell-intrinsic manner, which in turn aggravates the development of pathogenic Tip-DCs due to the loss of the negative regulation of Ly6C2 monocytes on Ly6C1 monocyte differentiation into Tip-DCs. Thus, IL-27 inhibits the dual-track exacerbation of Tip-DC development induced by a CD41 T cell–IFN-g axis. We conclude that IL-27 negatively regulates Tip-DC development by preventing the cell-intrinsic effects of IFN-g and that the regulation involves CD41 T cells and Ly6C2 monocytes. Targeting IL-27 signaling may manipulate Tip-DC development for therapeutic intervention.https://doi.org/10.1128/mBio.03385-2

Depletion of CD4⁺, but not CD8⁺, T cells significantly reduces the production of inflammatory cytokines and the serum activities of ALT, and enhances the survival of IL-27R^-/- (WSX-1^-/-) mice infected with <i>T</i>. <i>congolense</i>.

<p>IL-27R^-/- mice were infected with <i>T</i>. <i>congolense</i>, and treated with 0.5 mg rat anti-mouse CD4 mAb, rat anti-mouse CD8 mAb, or rat IgG on day 0, 2, 4, and 6 after infection, respectively. (A) Parasitemia and survival of the IL-27R^-/- mice (n = 4–5) infected with <i>T</i>. <i>congolense</i>. (B) Serum ALT activities were assessed in IL-27R^-/- mice (n = 4) on day 7 after infection with <i>T</i>. <i>congolense</i>. (C) Plasma levels of IFN-γ, IL-12p40, and TNF-α of IL-27R^-/- mice (n = 4) on day 7 after infection with <i>T</i>. <i>congolense</i>. Data are presented as the mean ± SEM. The results presented are representative of 2 separate experiments.</p

IL-27 signaling plays a crucial role in dampening Th1 mediated immune responses, allowing prolonged survival of mice infected with <i>T</i>. <i>brucei</i>.

<p>(A) mRNA expression levels of IL-27p28, EBI3 and WSX-1 in the liver of wild-type mice infected with <i>T</i>. <i>brucei</i> on day 6 and 9 versus day 0 (uninfected). (B) Parasitemia and survival of IL-27R^-/- (WSX-1^-/-) and wild-type mice (n = 6–7) infected with <i>T</i>. <i>brucei</i>. (C) Production of IFN-γ detected on day 6 in the plasma and supernatant fluids of cultured spleen cells and serum activities of ALT examined on day 6 and 9 in IL-27R^-/- and wild-type mice after infection with <i>T</i>. <i>brucei</i>. (D) Survival of IL-27R^-/- mice (n = 5–6) infected with <i>T</i>. <i>brucei</i>, following administration of 0.5 mg rat anti-mouse CD4 mAb, rat anti-mouse CD8 mAb, or rat IgG on day 0, 2, 4, and 6 after infection, respectively. Data are presented as the mean ± SEM. The results presented are representative of 2–3 separate experiments.</p

IL-27 signaling is required to prevent liver immunopathology during infection with <i>T</i>. <i>congolense</i>.

<p>(A) Macroscopic examination of liver on day 10 after infection with <i>T</i>. <i>congolense</i> revealed the presence of extensive pale geographic areas in IL-27R^-/- (WSX-1^-/-), but not wild-type mice (n = 4). (B) Hematoxylin and eosin staining showing loss of hepatocyte cellular architecture in the liver of IL-27R^-/-, but not wild-type mice (n = 4) on day 10 after infection with <i>T</i>. <i>congolense</i> (original magnification ×40). (C) Serum ALT activities were assessed in IL-27R^-/- and wild-type mice (n = 4) on day 7 and 10 after infection with <i>T</i>. <i>congolense</i>. Data are presented as the mean ± SEM. The results presented are representative of 2 separate experiments.</p