Specific depletion of Ly6C(hi) inflammatory monocytes prevents immunopathology in experimental cerebral malaria

Protocols for circulating neutrophil depletion in neonatal C57Bl/6 mice

Murine neonatal neutrophil depletion strategies have problems achieving deep neutrophil clearance and accurate residual neutrophil fraction detection. An isotype switch method can achieve profound neutrophil clearance using a combination of anti-Ly6G and antirat κ Ig light chain antibodies in adult C57Bl/6 mice, proven by extra- and intracellular Ly6G detection by flow cytometry. We adapted this technique to neonatal mice, testing four neutrophil depletion strategies in the peripheral circulation, bone marrow, and spleen. Four protocols were tested: P3 Ly6G and P1-3 Ly6G (anti-Ly6G on postnatal days [P] 3 and 1-3, respectively), and P3 Dual and P1-3 Dual (anti-Ly6G and antirat κ Ig light chain on P3 and P1-3, respectively). Intracellular and extracellular Ly6G presence was detected using flow cytometry. Isotype control antibodies were used as controls. P1-3 Dual protocol achieved significantly better neutrophil depletion than the P1-3 Ly6G or P3 Ly6G protocols (97% vs 74% and 97% vs 50%, respectively) in the peripheral circulation. The P3 Dual protocol alone was enough to achieve significantly better neutrophil clearance (93%) than any of the Ly6G alone protocols. The Ly6G alone protocols led to near-total elimination of extracellular Ly6G. However, there was a significant presence of intracellular Ly6G in the CD45+ cell population, evading detection by extracellular Ly6G antibody-based detection methods. P3 protocols perform better than P1-3 protocols for bone marrow and splenic neutrophil clearance. Thus, the P3 Dual protocol might be the most effective and ethical protocol to induce profound neutrophil depletion in neonatal mice, an alternative to daily anti-Ly6G injections.

Transient Macrophage Depletion Circumvents Scavenging and Redirects Biodistribution of mRNA-Lipid Nanoparticles

The mononuclear phagocytic system is recognized as a major scavenger of mRNA-lipid nanoparticles (LNPs), clearing and redirecting these particles away from their intended targets and thus diminishing their delivery efficacy. Understanding the mechanism by which mRNA-LNPs interact with phagocytes and how this interaction affects the mRNA transfection is critical to enhancing the delivery of mRNA. In this study, we temporarily depleted both circulating and resident macrophages (MF) and evaluated the transfection efficiency and biodistribution of mRNA-LNPs. We first demonstrated the enhanced liver expression using two liver-tropic formulations and the significant improvement of the in vivo gene editing efficiency of CRISPR-Cas9 in the Ai14 mouse model after MF depletion, providing a versatile strategy for enhanced mRNA delivery to the liver regardless of the formulation employed. We then extended our investigations to lung-tropic and lymphoid-tropic LNP formulations and discovered that MF depletion abolishes the targeting capacities of these non-liver-tropic formulations, providing insights into the organ targeting of LNPs. Finally, we screened and compared various clinically relevant MF depletion methods, providing the translation potential of this method on enhanced hepatic delivery of mRNA.

The spatiotemporal transcriptional profiling of murine brain during cerebral malaria progression and after artemisinin treatment

Cerebral malaria (CM) is a severe encephalopathy caused by Plasmodium parasite infection, resulting in thousands of annual deaths and neuro-cognitive sequelae even after anti-malarial drugs treatment. Despite efforts to dissect the mechanism, the cellular transcriptomic reprogramming within the spatial context remains elusive. Here, we constructed single-cell and spatial transcriptome atlases of experimental CM (ECM) male murine brain tissues with or without artesunate (ART) treatment. We identified activated inflammatory endothelial cells during ECM, characterized by a disrupted blood-brain barrier, increased antigen presentation, and leukocyte adhesion. We also observed that inflammatory microglia enhance antigen presentation pathway such as MHC-I to CD8+ cytotoxic T cells. The latter underwent an inflammatory state transition with up-regulated cytokine expression and cytotoxic activity. Multi-omics analysis revealed that the activated interferon-gamma response of injured neurons during ECM and persisted after ART treatment. Overall, our research provides valuable resources for understanding malaria parasite-host interaction mechanisms and adjuvant therapy development.

Lung immune incompetency after mild peritoneal sepsis and its partial restoration by type 1 interferon: a mouse model study

BACKGROUND

Sepsis is commonly associated with acute respiratory distress syndrome (ARDS). Although the exaggerated inflammation may damage intact lung tissues, a percentage of patients with ARDS are reportedly immunocompromised, with worse outcomes. Herein, using a murine sepsis model, time-course immune reprogramming after sepsis was evaluated to explore whether the host is immunocompromised. Leukocyte kinetics in the lung tissue were evaluated in a male C57/BL6 mouse model of mild peritoneal sepsis induced by cecal ligation and puncture, with the survival rate exceeds 90%. Lung immune reactivity was evaluated by intratracheal instillation of lipopolysaccharide (LPS; 30 µg). Furthermore, the effect of interferon (IFN)-β in vivo and ex vivo was evaluated.

RESULTS

Four days after sepsis, the lung water content remained high, even among mice in clinical recovery. While monocytes and neutrophils gradually accumulated in the lung interstitium, the inflammatory cytokine/chemokine expression levels in the lungs continued to decline. Intratracheal LPS instillation induced more leukocyte trafficking and protein leakage into the alveoli in the septic lung, indicating more severe lung injury. However, LPS stimulation-associated mRNA expression of tnf, il6, ccl2, and cxcl1 was suppressed. Intra-alveolar expression of tumor necrosis factor (TNF)-α, interleukin (IL)-6, monocyte chemoattractant protein (MCP)-1, and keratinocyte-derived cytokine (KC) was also suppressed. Monocytes isolated from the lung tissue showed an impaired response in il6, ccl2, and cxcl1 to LPS. Systemic IFN-β restored the above impaired regulator function of monocytes, as did coculturing these cells from lung tissue with IFN-β.

CONCLUSIONS

Histologically accelerated inflammation and paradoxically suppressed immunological regulator signaling were observed in the early recovery phase of sepsis. This observation may provide a model for the immunologically irresponsive state that occurs in some patients with sepsis. Systemic IFN-β partly restored the post-septic immunocompromised state, indicating its therapeutic potential for the immunosuppressive state seen in some patients with sepsis/ARDS.

Exposure to bacterial PAMPs before RSV infection exacerbates innate inflammation and disease via IL-1α and TNF-α

Respiratory syncytial virus (RSV) can cause severe lower respiratory tract infections. Understanding why some individuals get more serious disease may help with diagnosis and treatment. One possible risk factor underlying severe disease is bacterial exposure before RSV infection. Bacterial exposure has been associated with increased respiratory viral-induced disease severity but the mechanism remains unknown. Respiratory bacterial infections or exposure to their pathogen associated molecular patterns (PAMPs) trigger innate immune inflammation, characterised by neutrophil and inflammatory monocyte recruitment and the production of inflammatory cytokines. We hypothesise that these changes to the lung environment alter the immune response and disease severity during subsequent RSV infection. To test this, we intranasally exposed mice to LPS, LTA or Acinetobacter baumannii (an airway bacterial pathogen) before RSV infection and observed an early induction of disease, measured by weight loss, at days 1-3 after infection. Neutrophils or inflammatory monocytes were not responsible for driving this exacerbated weight loss. Instead, exacerbated disease was associated with increased IL-1α and TNF-α, which orchestrated the recruitment of innate immune cells into the lung. This study shows that exposure to bacterial PAMPs prior to RSV infection increases the expression of IL-1α and TNF-α, which dysregulate the immune response resulting in exacerbated disease.

Protocols for circulating neutrophil depletion in neonatal C57Bl/6 mice

Murine neonatal neutrophil depletion strategies have problems achieving deep neutrophil clearance and accurate residual neutrophil fraction detection. An isotype switch method can achieve profound neutrophil clearance using a combination of anti-Ly6G and anti-rat κ Ig light chain antibodies in adult C57Bl/6 mice, proven by extra- and intracellular Ly6G detection by flow cytometry. We adapted this technique to neonatal mice, testing four neutrophil depletion strategies in the peripheral circulation, bone marrow, and spleen. Four protocols were tested: P3 Ly6G and P1-3 Ly6G (anti-Ly6G on postnatal days (P) 3 and 1-3 respectively), and P3 Dual and P1-3 Dual (anti-Ly6G and anti-rat κ Ig light chain on P3 and P1-3 respectively). Intracellular and extracellular Ly6G presence was detected using flow cytometry. Isotype control antibodies were used as controls. P1-3 Dual protocol achieved significantly better neutrophil depletion than the P1-3 Ly6G or P3 Ly6G protocols (97% vs. 74% and 97% vs. 50%, respectively) in the peripheral circulation. The P3 Dual protocol alone was enough to achieve significantly better neutrophil clearance (93%) than any of the Ly6G alone protocols. The Ly6G alone protocols led to near-total elimination of extracellular Ly6G. However, there was a significant presence of intracellular Ly6G in the CD45+ cell population, evading detection by extracellular Ly6G antibody-based detection methods. P3 protocols perform better than P1-3 protocols for bone marrow and splenic neutrophil clearance. Thus, the P3 Dual protocol might be the most effective and ethical protocol to induce profound neutrophil depletion in neonatal mice, an alternative to daily anti-Ly6G injections.

Summary Sentence

Dual antibody-based neutrophil depletion effectively induces circulating neutrophil clearance in neonatal mice.

Immunological clues to sex differences in parasitic diseases

The effect of sex on the prevalence and severity of parasitic diseases is an emerging area of research. Several factors underlie sex-based differences, including sociocultural influences that affect exposure to parasites, and physiological disparities linked to biological sex. Hence, human studies must be interpreted cautiously; however, studies conducted under controlled laboratory conditions are important to validate findings in humans. Such research can more effectively elucidate the role of sex-determining physiological factors (particularly their impact on immune responses), as well as the role of sex-specific differences in resistance to, or severity of, parasitic diseases. This review focuses on the overarching impact of biological sex variables on immunity. Both human and rodent experimental data are discussed, with a focus on selected protozoan and helminth infections.

Plasmodium yoelii Infection Enhances the Expansion of Myeloid-Derived Suppressor Cells via JAK/STAT3 Pathway

Myeloid-derived suppressor cells (MDSCs), the negative immune regulators, have been demonstrated to be involved in immune responses to a variety of pathological conditions, such as tumors, chronic inflammation, and infectious diseases. However, the roles and mechanisms underlying the expansion of MDSCs in malaria remain unclear. In this study, the phenotypic and functional characteristics of splenic MDSCs during Plasmodium yoelii NSM infection are described. Furthermore, we provide compelling evidence that the sera from P. yoelii-infected C57BL/6 mice containing excess IL-6 and granulocyte-macrophage colony-stimulating factor promote the accumulation of MDSCs by inducing Bcl2 expression. Serum-induced MDSCs exert more potent suppressive effects on T cell responses than control MDSCs within both in vivo P. yoelii infection and in vitro serum-treated bone marrow cells experiments. Serum treatment increases the MDSC inhibitory effect, which is dependent on Arg1 expression. Moreover, mechanistic studies reveal that the serum effects are mediated by JAK/STAT3 signaling. By inhibiting STAT3 phosphorylation with the JAK inhibitor JSI-124, effects of serum on MDSCs are almost eliminated. In vivo depletion of MDSCs with anti-Gr-1 or 5-fluorouracil significantly reduces the parasitemia and promotes Th1 immune response in P. yoelii-infected C57BL/6 mice by upregulating IFN-γ expression. In summary, this study indicates that P. yoelii infection facilitates the accumulation and function of MDSCs by upregulating the expression of Bcl2 and Arg1 via JAK/STAT3 signaling pathway in vivo and in vitro. Manipulating the JAK/STAT3 signaling pathway or depleting MDSCs could be promising therapeutic interventions to treat malaria.

Unravelling mysteries at the perivascular space: a new rationale for cerebral malaria pathogenesis

Cerebral malaria (CM) is a severe neurological complication caused by Plasmodium falciparum parasites; it is characterized by the sequestration of infected red blood cells within the cerebral microvasculature. New findings, combined with a better understanding of the central nervous system (CNS) barriers, have provided greater insight into the players and events involved in CM, including site-specific T cell responses in the human brain. Here, we review the updated roles of innate and adaptive immune responses in CM, with a focus on the role of the perivascular macrophage-endothelium unit in antigen presentation, in the vascular and perivascular compartments. We suggest that these events may be pivotal in the development of CM.

Monocytes as primary defenders against Toxoplasma gondii infection

Monocytes are recruited from the bone marrow to sites of infection where they release cytokines and chemokines, function in antimicrobial immunity, and differentiate into macrophages and dendritic cells to control infection. Although many studies have focused on monocyte-derived macrophages and dendritic cells, recent work has examined the unique roles of monocytes during infection to promote immune defense. We focus on the effector functions of monocytes during infection with the parasite Toxoplasma gondii, and discuss the signals that mobilize monocytes to sites of infection, their production of inflammatory cytokines and antimicrobial mediators, their ability to shape the adaptive immune response, and their immunoregulatory functions. Insights from other infections, including Plasmodium and Listeria are also included for comparison and context.

Increased Neutrophil Percentage and Neutrophil-T Cell Ratio Precedes Clinical Onset of Experimental Cerebral Malaria

Newly emerging data suggest that several neutrophil defense mechanisms may play a role in both aggravating and protecting against malaria. These exciting findings suggest that the balance of these cells in the host body may have an impact on the pathogenesis of malaria. To fully understand the role of neutrophils in severe forms of malaria, such as cerebral malaria (CM), it is critical to gain a comprehensive understanding of their behavior and functions. This study investigated the dynamics of neutrophil and T cell responses in C57BL/6 and BALB/c mice infected with Plasmodium berghei ANKA, murine models of experimental cerebral malaria (ECM) and non-cerebral experimental malaria, respectively. The results demonstrated an increase in neutrophil percentage and neutrophil-T cell ratios in the spleen and blood before the development of clinical signs of ECM, which is a phenomenon not observed in the non-susceptible model of cerebral malaria. Furthermore, despite the development of distinct forms of malaria in the two strains of infected animals, parasitemia levels showed equivalent increases throughout the infection period evaluated. These findings suggest that the neutrophil percentage and neutrophil-T cell ratios may be valuable predictive tools for assessing the dynamics and composition of immune responses involved in the determinism of ECM development, thus contributing to the advancing of our understanding of its pathogenesis.

F4/80+Ly6Chigh Macrophages Lead to Cell Plasticity and Cancer Initiation in Colitis

BACKGROUND & AIMS

Colorectal cancer is a leading cause of cancer death, and a major risk factor is chronic inflammation. Despite the link between colitis and cancer, the mechanism by which inflammation leads to colorectal cancer is not well understood.

METHODS

To investigate whether different forms of inflammation pose the same risk of cancer, we compared several murine models of colitis (dextran sodium sulfate [DSS], 2,4,6-trinitrobenzene sulfonic acid, 4-ethoxylmethylene-2-phenyloxazol-5-one, Citrobacter rodentium, Fusobacterium nucleatum, and doxorubicin) with respect to their ability to lead to colonic tumorigenesis. We attempted to correlate the severity of colitis and inflammatory profile with the risk of tumorigenesis in both azoxymethane-dependent and Dclk1/APCfl/fl murine models of colitis-associated cancer.

RESULTS

DSS colitis reproducibly led to colonic tumors in both mouse models of colitis-associated cancer. In contrast, all other forms of colitis did not lead to cancer. When compared with the colitis not associated with tumorigenesis, DSS colitis was characterized by significantly increased CD11b+F4/80+Ly6Chigh macrophages and CD11b+Ly6G+ neutrophils. Interestingly, depletion of the CD11b+F4/80+Ly6Chigh macrophages inhibited tumorigenesis, whereas depletion of CD11b+Ly6G+ neutrophils had no effect on tumorigenesis. Furthermore, the macrophage-derived cytokines interleukin-1β, tumor necrosis factor-α, and interleukin-6 were significantly increased in DSS colitis and promoted stemness of Dclk1+ tuft cells that serve as the cellular origin of cancer.

CONCLUSIONS

We have identified CD11b+F4/80+Ly6Chigh macrophages as key mediators of cancer initiation in colitis-associated cancer. Development of new therapies that target these cells may provide an effective preventative strategy for colitis-associated cancer.

Who Knew? Dopamine Transporter Activity Is Critical in Innate and Adaptive Immune Responses

The dopamine transporter (DAT) regulates the dimension and duration of dopamine transmission. DAT expression, its trafficking, protein-protein interactions, and its activity are conventionally studied in the CNS and within the context of neurological diseases such as Parkinson's Diseases and neuropsychiatric diseases such as drug addiction, attention deficit hyperactivity and autism. However, DAT is also expressed at the plasma membrane of peripheral immune cells such as monocytes, macrophages, T-cells, and B-cells. DAT activity via an autocrine/paracrine signaling loop regulates macrophage responses to immune stimulation. In a recent study, we identified an immunosuppressive function for DAT, where blockade of DAT activity enhanced LPS-mediated production of IL-6, TNF-α, and mitochondrial superoxide levels, demonstrating that DAT activity regulates macrophage immune responses. In the current study, we tested the hypothesis that in the DAT knockout mice, innate and adaptive immunity are perturbed. We found that genetic deletion of DAT (DAT^-/-) results in an exaggerated baseline inflammatory phenotype in peripheral circulating myeloid cells. In peritoneal macrophages obtained from DAT^-/- mice, we identified increased MHC-II expression and exaggerated phagocytic response to LPS-induced immune stimulation, suppressed T-cell populations at baseline and following systemic endotoxemia and exaggerated memory B cell expansion. In DAT^-/- mice, norepinephrine and dopamine levels are increased in spleen and thymus, but not in circulating serum. These findings in conjunction with spleen hypoplasia, increased splenic myeloid cells, and elevated MHC-II expression, in DAT^-/- mice further support a critical role for DAT activity in peripheral immunity. While the current study is only focused on identifying the role of DAT in peripheral immunity, our data point to a much broader implication of DAT activity than previously thought. This study is dedicated to the memory of Dr. Marc Caron who has left an indelible mark in the dopamine transporter field.

Increased circulating myeloid-derived suppressor cells in vivax malaria and severe falciparum malaria

BACKGROUND

Circulating myeloid-derived-suppressor-cells (MDSC) with immunosuppressive function are increased in human experimental Plasmodium falciparum infection, but have not been studied in clinical malaria.

METHODS

Using flow-cytometry, circulating polymorphonuclear-MDSC were evaluated in cryopreserved samples from patients with uncomplicated Plasmodium vivax (n = 8) and uncomplicated (n = 4) and severe (n = 16) falciparum malaria from Papua, Indonesia.

RESULTS

The absolute number of circulating polymorphonuclear-MDSC were significantly elevated in severe falciparum malaria patients compared to controls (n = 10). Polymorphonuclear-MDSC levels in uncomplicated vivax malaria were also elevated to levels comparable to that seen in severe falciparum malaria.

CONCLUSION

Control of expansion of immunosuppressive MDSC may be important for development of effective immune responses in falciparum and vivax malaria.

Neutrophils in malaria: A double-edged sword role

Neutrophils are the most abundant leukocytes in human peripheral blood. They form the first line of defense against invading foreign pathogens and might play a crucial role in malaria. According to World Health Organization (WHO), malaria is a globally significant disease caused by protozoan parasites from the Plasmodium genus, and it's responsible for 627,000 deaths in 2020. Neutrophils participate in the defense response against the malaria parasite via phagocytosis and reactive oxygen species (ROS) production. Neutrophils might also be involved in the pathogenesis of malaria by the release of toxic granules and the release of neutrophil extracellular traps (NETs). Intriguingly, malaria parasites inhibit the anti-microbial function of neutrophils, thus making malaria patients more susceptible to secondary opportunistic Salmonella infections. In this review, we will provide a summary of the role of neutrophils during malaria infection, some contradicting mouse model neutrophil data and neutrophil-related mechanisms involved in malaria patients' susceptibility to bacterial infection.

Immunopathology and Trypanosoma congolense parasite sequestration cause acute cerebral trypanosomiasis

Trypanosoma congolense causes a syndrome of variable severity in animals in Africa. Cerebral trypanosomiasis is a severe form, but the mechanism underlying this severity remains unknown. We developed a mouse model of acute cerebral trypanosomiasis and characterized the cellular, behavioral, and physiological consequences of this infection. We show large parasite sequestration in the brain vasculature for long periods of time (up to 8 hr) and extensive neuropathology that associate with ICAM1-mediated recruitment and accumulation of T cells in the brain parenchyma. Antibody-mediated ICAM1 blocking and lymphocyte absence reduce parasite sequestration in the brain and prevent the onset of cerebral trypanosomiasis. Here, we establish a mouse model of acute cerebral trypanosomiasis and we propose a mechanism whereby parasite sequestration, host ICAM1, and CD4⁺ T cells play a pivotal role.

An update on cerebral malaria for therapeutic intervention

BACKGROUND

Cerebral malaria is often pronounced as a major life-threatening neurological complication of Plasmodium falciparum infection. The complex pathogenic landscape of the parasite and the associated neurological complications are still not elucidated properly. The growing concerns of drugresistant parasite strains along with the failure of anti-malarial drugs to subdue post-recovery neuro-cognitive dysfunctions in cerebral malaria patients have called for a demand to explore novel biomarkers and therapeutic avenues. Due course of the brain infection journey of the parasite, events such as sequestration of infected RBCs, cytoadherence, inflammation, endothelial activation, and blood-brain barrier disruption are considered critical.

METHODS

In this review, we briefly summarize the diverse pathogenesis of the brain-invading parasite associated with loss of the blood-brain barrier integrity. In addition, we also discuss proteomics, transcriptomics, and bioinformatics strategies to identify an array of new biomarkers and drug candidates.

CONCLUSION

A proper understanding of the parasite biology and mechanism of barrier disruption coupled with emerging state-of-art therapeutic approaches could be helpful to tackle cerebral malaria.

IL-6 dependent expansion of inflammatory MDSCs (CD11b+ Gr-1+) promote Th-17 mediated immune response during experimental cerebral malaria

Myeloid derived suppressor cells (MDSCs) are a group of heterogeneous cell populations that can suppress T cell responses. Various aspects of MDSCs in regulating immune responses in several cancer and infectious diseases have been reported till date. But the role and regulation of MDSCs have not been systematically studied in the context of malaria. This study depicts the phenotypic and functional characteristics of splenic MDSCs and how they regulate Th-17 mediated immune response during Experimental Cerebral Malaria (ECM). Flow cytometric analysis reveals that MDSCs in the spleen and bone marrow expand at 8 dpi during ECM. Among subtypes of MDSCs, PMN-MDSCs show significant expansion in the spleen but M-MDSCs remain unaltered. Functional analysis of sorted MDSCs from spleens of Plasmodium berghei ANKA (PbA) infected mice shows suppressive nature of these cells and high production of Nitric oxide (NO). Besides, MDSCs were also found to express various inflammatory markers during ECM suggesting the M1 type phenotype of these cells. In-vivo depletion of MDSCs by the use of Anti Gr-1 increases mice survival but doesn't significantly alter the parasitemia. Previously, it has been reported that Treg/Th-17 balance in the spleen is skewed towards Th-17 during ECM. Depletion of MDSCs was found to regulate Th-17 percentages to homeostatic levels and subvert various inflammatory changes in the spleen. Among different factors, IL-6 was found to play an important role in the expansion of MDSCs and expression of inflammatory markers on MDSCs in a STAT3-dependent manner. These findings provide a unique insight into the role of IL-6 in the expansion of the MDSC population which causes inflammatory changes and increased Th-17 responses during ECM.

Reduced Neutrophil Extracellular Trap Formation During Ischemia Reperfusion Injury in C3 KO Mice: C3 Requirement for NETs Release

Complement C3 plays a prominent role in inflammatory processes, and its increase exacerbates ischemia reperfusion injury (IRI)-induced acute kidney injury (AKI). Infiltrated neutrophils can be stimulated to form neutrophil extracellular traps (NETs), leading to renal injury. However, the relationship between the increase of C3 and the release of NETs in AKI was not clear. Here we found that IRI in the mouse kidney leads to increased neutrophils infiltration and NET formation. Furthermore, neutrophils depletion by anti-Ly6G IgG (1A8) did not reduce C3 activation but reduced kidney injury and inflammation, indicating a link between neutrophils infiltration and renal tissue damage. Pretreatment with 1A8 suppressed ischemia-induced NET formation, proving that extracellular traps (ETs) in renal tissue were mainly derived from neutrophils. Renal ischemia injury also leads to increased expression of C3. Moreover, C3 KO mice (C3 KO) with IRI exhibited attenuated kidney damage and decreased neutrophils and NETs. In vitro, C3a primed neutrophils to form NETs, reflected by amorphous extracellular DNA structures that colocalized with CitH3 and MPO. These data reveal that C3 deficiency can ameliorate AKI by reducing the infiltration of neutrophils and the formation of NETs. Targeting C3 activation may be a new therapeutic strategy for alleviating the necroinflammation of NETs in AKI.

Infection-Derived Monocytic MDSCs Require TGF-β to Suppress Filarial-Specific IFN-γ But Not IL-13 Release by Filarial-Specific CD4+ T Cells In Vitro

Lymphatic filariasis (LF) remains a major health problem with severe economic repercussions in endemic communities of Sub-saharan Africa, South-East Asia and South America. The rodent-specific nematode Litomosoides sigmodontis (Ls) is used to study the immunomodulatory potential of filariae and research has elucidated pathways involving regulatory T cells (Tregs), IL-10 producing cells and alternatively activated macrophages (AAMs) and that CD4+ T cells play a paramount role during infection. Myeloid-derived suppressor cells (MDSCs) have been identified and characterised in man in cancer and other pathologies. The hallmark of MDSC populations is the suppression of T and B cell responses using various mechanisms, which are mostly specific to the pathology or setting. However, until now, it remains unclear whether they play a role in filarial-specific responses. We report here that monocytic MDSCs (Mo-MDSCs, CD11b+Ly6C+Ly6G-) and polymorphonuclear MDSCs (PMN-MDSCs, CD11b+Ly6Cint/loLy6G+) expanded in the thoracic cavity (TC, the site of infection) and correlated positively with filarial life-stages in Ls-infected BALB/c mice. In vitro, only infection-derived Mo-MDSCs showed a suppressive nature by preventing IL-13 and IFN-γ secretion from filarial-specific CD4+ T cells upon co-culture with soluble worm extract. This suppression was not mediated by IL-10, IL-6 or TNF-α, and did not require cell-contact, nitric oxide (NO), IL-4/IL-5 signalling pathways or CCR2. Interestingly, neutralizing TGF-β significantly rescued IFN-γ but not IL-13 production by filarial-specific CD4+ T cells. In comparison to naive cells, PCR array data showed an overall down-regulation of inflammatory pathways in both infection-derived Mo-MDSCs and PMN-MDSCs. In conclusion, these primary data sets show activity and expansion of MDSCs during Ls infection adding this regulatory cell type to the complex milieu of host responses during chronic helminth infections.

Neutrophils in malaria: the good, the bad or the ugly?

Neutrophils are the most abundant circulating leukocytes in human peripheral blood. They are often the first cells to respond to an invading pathogen and might therefore play an important role in malaria. Malaria is a globally important disease caused by Plasmodium parasites, responsible for more than 400 000 deaths each year. Most of these deaths are caused by complications, including cerebral malaria, severe malarial anemia, placental malaria, renal injury, metabolic problems and malaria-associated acute respiratory distress syndrome. Neutrophils contribute in the immune defense against malaria, through clearance of parasites via phagocytosis, production of reactive oxygen species and release of neutrophil extracellular traps (NETs). However, Plasmodium parasites diminish antibacterial functions of neutrophils, making patients more susceptible to other infections. Neutrophils might also be involved in the development of malaria complications, for example via the release of toxic granules and NETs. However, technical pitfalls in the determination of the roles of neutrophils have caused contradicting results. Further investigations need to consider these pitfalls, in order to elucidate the role of neutrophils in malaria complications.

Eosinophils Suppress the Migration of T Cells Into the Brain of Plasmodium berghei-Infected Ifnar1-/- Mice and Protect Them From Experimental Cerebral Malaria

Cerebral malaria is a potentially lethal disease, which is caused by excessive inflammatory responses to Plasmodium parasites. Here we use a newly developed transgenic Plasmodium berghei ANKA (PbA_Ama1OVA) parasite that can be used to study parasite-specific T cell responses. Our present study demonstrates that Ifnar1^-/- mice, which lack type I interferon receptor-dependent signaling, are protected from experimental cerebral malaria (ECM) when infected with this novel parasite. Although CD8⁺ T cell responses generated in the spleen are essential for the development of ECM, we measured comparable parasite-specific cytotoxic T cell responses in ECM-protected Ifnar1^-/- mice and wild type mice suffering from ECM. Importantly, CD8⁺ T cells were increased in the spleens of ECM-protected Ifnar1^-/- mice and the blood-brain-barrier remained intact. This was associated with elevated splenic levels of CCL5, a T cell and eosinophil chemotactic chemokine, which was mainly produced by eosinophils, and an increase in eosinophil numbers. Depletion of eosinophils enhanced CD8⁺ T cell infiltration into the brain and increased ECM induction in PbA_Ama1OVA-infected Ifnar1^-/- mice. However, eosinophil-depletion did not reduce the CD8⁺ T cell population in the spleen or reduce splenic CCL5 concentrations. Our study demonstrates that eosinophils impact CD8⁺ T cell migration and proliferation during PbA_Ama1OVA-infection in Ifnar1^-/- mice and thereby are contributing to the protection from ECM.

CCR2 Signaling Promotes Brain Infiltration of Inflammatory Monocytes and Contributes to Neuropathology during Cryptococcal Meningoencephalitis

Cryptococcal meningoencephalitis (CM) is a leading cause of central nervous system (CNS) infection-related mortality worldwide, with surviving patients often developing neurological deficiencies. While CNS inflammation has been implicated in the pathogenesis of CM, little is known about the relative contribution of the specific inflammatory/immune pathways to CNS pathology versus fungal clearance. Increased cerebrospinal fluid level of C-C chemokine receptor 2 (CCR2) ligand CCL2 is associated with disease deterioration in patients with CM. Using a murine model, we investigated the role of the CCR2 pathway in the development of CNS inflammation and pathology during CM. We found that CCR2-deficient mice exhibited improved 28-day survival and alleviated neurological disease scores despite a brain fungal burden higher than that of the WT mice. Reduced CM pathology in CCR2-deficient mice was accompanied by markedly decreased neuronal cell death around cryptococcal microcysts and restored expression of genes involved in neurotransmission, connectivity, and neuronal cell structure in the brains. Results show that CCR2 axis is the major pathway recruiting CD45hiCD11b+Ly6C+ inflammatory monocyte to the brain and indirectly modulates the accumulation of CD4+ T cells and CD8+ T cells. In particular, CCR2 axis promotes recruitment of interferon gamma (IFN-γ)-producing CD4+ T cells and classical activation of myeloid cells. In this context, CCR2 deletion limits the immune network dysregulation we see in CM and attenuates neuropathology. Thus, the CCR2 axis is a potential target for interventions aimed to limit inflammatory CNS pathology in CM patients. IMPORTANCE Cryptococcal meningoencephalitis (CM) causes nearly 200,000 deaths worldwide each year, and survivors frequently develop long-lasting neurological sequelae. The high rate of mortality and neurologic sequelae in CM patients indicate that antifungal therapies alone are often insufficient to control disease progression. Here, we reveal that CM disease progression in mice is accompanied by inflammatory monocytes infiltration at the periphery of the infected foci that overlap locally perturbed neuronal function and death. Importantly, we identified that CCR2 signaling is a critical pathway driving neuroinflammation, especially inflammatory monocyte recruitment, as well as CNS pathology and mortality in CM mice. Our results imply that targeting the CCR2 pathway may be beneficial as a therapy complementary to antifungal drug treatment, helping to reduce CNS damage and mortality in CM patients.

Perivascular macrophages create an intravascular niche for CD8+ T cell localisation prior to the onset of fatal experimental cerebral malaria

Objectives

The immunologic events that build up to the fatal neurological stage of experimental cerebral malaria (ECM) are incompletely understood. Here, we dissect immune cell behaviour occurring in the central nervous system (CNS) when Plasmodium berghei ANKA (PbA)‐infected mice show only minor clinical signs.

Methods

A 2‐photon intravital microscopy (2P‐IVM) brain imaging model was used to study the spatiotemporal context of early immunological events in situ during ECM.

Results

Early in the disease course, antigen‐specific CD8⁺ T cells came in contact and arrested on the endothelium of post‐capillary venules. CD8⁺ T cells typically adhered adjacent to, or were in the near vicinity of, perivascular macrophages (PVMs) that line post‐capillary venules. Closer examination revealed that CD8⁺ T cells crawled along the inner vessel wall towards PVMs that lay on the abluminal side of large post‐capillary venules. ‘Activity hotspots’ in large post‐capillary venules were characterised by T‐cell localisation, activated morphology and clustering of PVM, increased abutting of post‐capillary venules by PVM and augmented monocyte accumulation. In the later stages of infection, when mice exhibited neurological signs, intravascular CD8⁺ T cells increased in number and changed their behaviour, actively crawling along the endothelium and displaying frequent, short‐term interactions with the inner vessel wall at hotspots.

Conclusion

Our study suggests an active interaction between PVM and CD8⁺ T cells occurs across the blood–brain barrier (BBB) in early ECM, which may be the initiating event in the inflammatory cascade leading to BBB alteration and neuropathology.

K18-hACE2 mice develop respiratory disease resembling severe COVID-19

SARS-CoV-2 emerged in late 2019 and resulted in the ongoing COVID-19 pandemic. Several animal models have been rapidly developed that recapitulate the asymptomatic to moderate disease spectrum. Now, there is a direct need for additional small animal models to study the pathogenesis of severe COVID-19 and for fast-tracked medical countermeasure development. Here, we show that transgenic mice expressing the human SARS-CoV-2 receptor (angiotensin-converting enzyme 2 [hACE2]) under a cytokeratin 18 promoter (K18) are susceptible to SARS-CoV-2 and that infection resulted in a dose-dependent lethal disease course. After inoculation with either 104 TCID50 or 105 TCID50, the SARS-CoV-2 infection resulted in rapid weight loss in both groups and uniform lethality in the 105 TCID50 group. High levels of viral RNA shedding were observed from the upper and lower respiratory tract and intermittent shedding was observed from the intestinal tract. Inoculation with SARS-CoV-2 resulted in upper and lower respiratory tract infection with high infectious virus titers in nasal turbinates, trachea and lungs. The observed interstitial pneumonia and pulmonary pathology, with SARS-CoV-2 replication evident in pneumocytes, were similar to that reported in severe cases of COVID-19. SARS-CoV-2 infection resulted in macrophage and lymphocyte infiltration in the lungs and upregulation of Th1 and proinflammatory cytokines/chemokines. Extrapulmonary replication of SARS-CoV-2 was observed in the cerebral cortex and hippocampus of several animals at 7 DPI but not at 3 DPI. The rapid inflammatory response and observed pathology bears resemblance to COVID-19. Additionally, we demonstrate that a mild disease course can be simulated by low dose infection with 102 TCID50 SARS-CoV-2, resulting in minimal clinical manifestation and near uniform survival. Taken together, these data support future application of this model to studies of pathogenesis and medical countermeasure development.

Integrative analysis of microRNA and mRNA expression profiles of monocyte-derived dendritic cells differentiation during experimental cerebral malaria

Heterogeneity and high plasticity are common features of cells from the mononuclear phagocyte system: monocytes (MOs), macrophages, and dendritic cells (DCs). Upon activation by microbial agents, MO can differentiate into MO-derived DCs (MODCs). In previous work, we have shown that during acute infection with Plasmodium berghei ANKA (PbA), MODCs become, transiently, the main CD11b+ myeloid population in the spleen (SP) and once recruited to the brain play an important role in the development of experimental cerebral malaria (ECM). Here, we isolated 4 cell populations: bone marrow (BM) MOs (BM-MOs) and SP-MOs from uninfected mice; BM inflammatory MOs (BM-iMOs) and SP-MODCs from PbA-infected mice and used a system biology approach to a holistic transcriptomic comparison and provide an interactome analysis by integrating differentially expressed miRNAs (DEMs) and their differentially expressed gene targets (DEGs) data. The Jaccard index (JI) was used for gauging the similarity and diversity among these cell populations. Whereas BM-MOs, BM-iMOs, and SP-MOs presented high similarity of DEGs, SP-MODCs distinguished by showing a greater number of DEGs. Moreover, functional analysis identified an enrichment in canonical pathways, such as DC maturation, neuroinflammation, and IFN signaling. Upstream regulator analysis identified IFNγ as the potential upstream molecule that can explain the observed DEMs-Target DEGs intersections in SP-MODCs. Finally, directed target analysis and in vivo/ex vivo assays indicate that SP-MODCs differentiate in the SP and IFNγ is a main driver of this process.

CXCR4 and MIF are required for neutrophil extracellular trap release triggered by Plasmodium-infected erythrocytes

Neutrophil extracellular traps (NETs) evolved as a unique effector mechanism contributing to resistance against infection that can also promote tissue damage in inflammatory conditions. Malaria infection can trigger NET release, but the mechanisms and consequences of NET formation in this context remain poorly characterized. Here we show that patients suffering from severe malaria had increased amounts of circulating DNA and increased neutrophil elastase (NE) levels in plasma. We used cultured erythrocytes and isolated human neutrophils to show that Plasmodium-infected red blood cells release macrophage migration inhibitory factor (MIF), which in turn caused NET formation by neutrophils in a mechanism dependent on the C-X-C chemokine receptor type 4 (CXCR4). NET production was dependent on histone citrullination by peptidyl arginine deiminase-4 (PAD4) and independent of reactive oxygen species (ROS), myeloperoxidase (MPO) or NE. In vitro, NETs functioned to restrain parasite dissemination in a mechanism dependent on MPO and NE activities. Finally, C57/B6 mice infected with P. berghei ANKA, a well-established model of cerebral malaria, presented high amounts of circulating DNA, while treatment with DNAse increased parasitemia and accelerated mortality, indicating a role for NETs in resistance against Plasmodium infection.

Androgens predispose males to monocyte-mediated immunopathology by inducing the expression of leukocyte recruitment factor CXCL1

Hepatic amebiasis, predominantly occurring in men, is a focal destruction of the liver due to the invading protozoan Entamoeba histolytica. Classical monocytes as well as testosterone are identified to have important functions for the development of hepatic amebiasis in mice, but a link between testosterone and monocytes has not been identified. Here we show that testosterone treatment induces proinflammatory responses in human and mouse classical monocytes. When treated with 5α-dihydrotestosterone, a strong androgen receptor ligand, human classical monocytes increase CXCL1 production in the presence of Entamoeba histolytica antigens. Moreover, plasma testosterone levels of individuals undergoing transgender procedure correlate positively with the TNF and CXCL1 secretion from their cultured peripheral blood mononuclear cells following lipopolysaccharide stimulation. Finally, testosterone substitution of castrated male mice increases the frequency of TNF/CXCL1-producing classical monocytes during hepatic amebiasis, supporting the hypothesis that the effects of androgens may contribute to an increased risk of developing monocyte-mediated pathologies.

The Suppressive Attitude of Inflammatory Monocytes in Antiviral Antibody Responses

Inflammatory monocytes play important functions in antiviral immune responses, including release of inflammatory cytokines and antigen presentation to T lymphocytes. Depending on the pathological context, these functions might translate into beneficial or detrimental effects in the resolution of the disease. Recent literature has highlighted a role for inflammatory monocytes also in direct suppression of B cell responses. In this review, we will briefly discuss research showing the relationship between inflammatory monocytes and B lymphocytes, its functional consequences on antiviral antibody responses, and possible implications in the design of future vaccination strategies.

Protection of Batf3-deficient mice from experimental cerebral malaria correlates with impaired cytotoxic T-cell responses and immune regulation

Excessive inflammatory immune responses during infections with Plasmodium parasites are responsible for severe complications such as cerebral malaria (CM) that can be studied experimentally in mice. Dendritic cells (DCs) activate cytotoxic CD8+ T-cells and initiate immune responses against the parasites. Batf3-/- mice lack a DC subset, which efficiently induces strong CD8 T-cell responses by cross-presentation of exogenous antigens. Here we show that Batf3-/- mice infected with Plasmodium berghei ANKA (PbA) were protected from experimental CM (ECM), characterized by a stable blood-brain barrier (BBB) and significantly less infiltrated peripheral immune cells in the brain. Importantly, the absence of ECM in Batf3-/- mice correlated with attenuated responses of cytotoxic T-cells, as their parasite-specific lytic activity as well as the production of interferon gamma and granzyme B were significantly decreased. Remarkably, spleens of ECM-protected Batf3-/- mice had elevated levels of regulatory immune cells and interleukin 10. Thus, protection from ECM in PbA-infected Batf3-/- mice was associated with the absence of strong CD8+ T-cell activity and induction of immunoregulatory mediators and cells.

Dissociation of neonatal and adult mice brain for simultaneous analysis of microglia, astrocytes and infiltrating lymphocytes by flow cytometry

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Recovery of neural cells is higher with 30 % Percoll gradient than 30–70 %.

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Papain enhances combined extraction of microglia, astrocytes and lymphocytes.

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Dispase II potentiates papain action only in adult brain.

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Mechanical dissociation isolates neonatal and adult astrocytes better than enzymes.

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Papain + dispase II alows cell cytometry quantification of glial activation by LPS.

The technical difficulty to isolate microglia, astrocytes and infiltrating immune cells from mouse brain is nowadays a limiting factor in the study of neuroinflammation. Brain isolation requirements are cell-type and animal-age dependent, but current brain dissociation procedures are poorly standardized. This lack of comprehensive studies hampers the selection of optimized methodologies. Thus, we present here a comparative analysis of dissociation methods and Percoll-based separation to identify the most efficient procedure for the combined isolation of healthy microglia, astrocytes and infiltrated leukocytes; distinguishing neonatal and adult mouse brain. Gentle mechanical dissociation and DNase I incubation was supplemented with papain or collagenase II. Dispase II digestion was also used alone or in combination. In addition, cell separation efficiency of 30 % and 30–70 % Percoll gradients was compared. In these experiments, cell yield and integrity of freshly dissociated cells was measured by flow cytometry. We found that papain digestion in combination with dispase II followed by 30 % Percoll separation is the most balanced method to obtain a mixture of microglia, astrocytes and infiltrated immune cells; while addition of dispase II was not an advantage for neonatal brain. These dissociation conditions allowed flow cytometry detection of a slight glial activation triggered by sublethal LPS injection. In conclusion, the enzymes and Percoll density gradients tested here affected differently resting microglia, activated microglia/macrophages, astrocytes and infiltrated lymphocytes. Also, newborn and adult brain showed contrasting reactions to digestion. Our study highlights the strength of flow cytometry for the simultaneous analysis of neuroimmune cell populations once extraction is optimized.

A quantitativeex vivostudy of the interactions between reconstituted high-density lipoproteins and human leukocytes

Knowledge of the interactions between nanoparticles and immune cells is required for optimal design of nanoparticle-based drug delivery systems, either when aiming to avoid phagocytic clearance of the nanoparticles or promote an immune response by delivering therapeutic agents to specific immune cells. Several studies have suggested that reconstituted high-density lipoproteins (rHDL) are attractive drug delivery vehicles. However, detailed studies of rHDL interactions with circulating leukocytes are limited. Here, we evaluated the association of discoidal rHDL with leukocytes in human whole blood (HWB) using quantitative approaches. We found that while the rHDL of various lipid compositions associated preferentially with monocytes, the degree of association depended on the lipid composition. However, consistent with the long circulation half-life of rHDL, we show that only a minor fraction of the rHDL associated with the leukocytes. Furthermore, we used three-dimensional fluorescence microscopy and imaging flow cytometry to evaluate the possible internalization of rHDL cargo into the cells, and we show increased internalization of rHDL cargo in monocytes relative to granulocytes. The preferential rHDL association with monocytes and the internalization of rHDL cargo could possibly be mediated by the scavenger receptor class B type 1 (SR-BI), which we show is expressed to a higher extent on monocytes than on the other major leukocyte populations. Our work implies that drug-loaded rHDL can deliver its cargo to monocytes in circulation, which could lead to some off-target effects when using rHDL for systemic drug delivery, or it could pave the way for novel immunotherapeutic treatments aiming to target the monocytes.

We used novel quantitative methods to study the interactions between reconstituted high-density lipoproteins (rHDL) and human leukocytes – showing that rHDL cargo are preferentially taken up by monocytes.

NFAT1 Regulates Ly6Chi Monocyte Recruitment to the CNS and Plays an Essential Role in Resistance to Toxoplasma gondii Infection

Monocytes play key roles in the maintenance of homeostasis and in the control of the infection. Monocytes are recruited from the bone marrow to inflammatory sites and are essential for antimicrobial activity to limit tissue damage and promote adaptive T cell responses. Here, we investigated the role of Nuclear Factor of Activated T cells 1 (NFAT1) in the regulation of Ly6C^hi inflammatory monocyte recruitment to the CNS upon T. gondii infection. We show that NFAT-1-deficient monocytes are unable to migrate to the CNS of T. gondii-infected mice. Moreover, NFAT1^−/− mice are highly susceptible to chronic T. gondii infection due to a failure to control parasite replication in the CNS. The inhibition of Ly6C^hi inflammatory monocyte recruitment to the CNS severely blocked CXCL10 production and consequently the migration of IFN-γ-producing CD4⁺ T cells. Moreover, the transfer of Ly6C^hi monocytes to infected NFAT1^−/− mice favored CD4⁺ T cell migration to the CNS and resulted in the inhibition of parasite replication and host defense. Together, these results demonstrated for the first time the contribution of NFAT1 to the regulation of Ly6C^hi monocyte recruitment to the CNS and to resistance during chronic T. gondii infection.

Monocyte-derived dendritic cells in malaria

The pathogenesis of malaria is a multifactorial syndrome associated with a deleterious inflammatory response that is responsible for many of the clinical manifestations. While dendritic cells (DCs) play a critical role in initiating acquired immunity and host resistance to infection, they also play a pathogenic role in inflammatory diseases. In our recent studies, we found in different rodent malaria models that the monocyte-derived DCs (MO-DCs) become, transiently, a main DC population in spleens and inflamed non-lymphoid organs. These studies suggest that acute infection with Plasmodium berghei promotes the differentiation of splenic monocytes into inflammatory monocytes (iMOs) and thereafter into MO-DCs that play a pathogenic role by promoting inflammation and tissue damage. The recruitment of MO-DCs to the lungs and brain are dependent on expression of CCR4 and CCR5, respectively, and expression of respective chemokine ligands in each organ. Once they reach the target organ the MO-DCs produce the CXCR3 ligands (CXCL9 and CXCL10), recruit CD8⁺ T cells, and produce toxic metabolites that play an important role in the development of experimental cerebral malaria (ECM) and acute respiratory distress syndrome (ARDS).

Differential Immune Activation in Fetal Macrophage Populations

Distinct macrophage subsets populate the developing embryo and fetus in distinct waves. However little is known about the functional differences between in utero macrophage populations or how they might contribute to fetal and neonatal immunity. Here we tested the innate immune response of mouse macrophages derived from the embryonic yolk sac and from fetal liver. When isolated from liver or lung, CD11bHI fetal liver derived macrophages responded to the TLR4 agonist LPS by expressing and releasing inflammatory cytokines. However F4/80HI macrophages from the yolk sac did not respond to LPS treatment. While differences in TLR4 expression did not appear to explain these data, F4/80HI macrophages had much lower NLRP3 inflammasome expression compared to CD11bHI macrophages. Gene expression profiling also demonstrated LPS-induced expression of inflammatory genes in CD11bHI macrophages, but not in F4/80HI cells. Genes expressed in LPS-treated CD11bHI macrophages were more likely to contain predicted NF-κB binding sites in their promoter regions. Our data show that CD11bHI macrophages derived from fetal liver are the major pro-inflammatory cells in the developing fetus. These findings could have important implications in better understanding the fetal inflammatory response and the unique features of neonatal immunity.

KCC1 Activation protects Mice from the Development of Experimental Cerebral Malaria

Plasmodium falciparum malaria causes half a million deaths per year, with up to 9% of this mortality caused by cerebral malaria (CM). One of the major processes contributing to the development of CM is an excess of host inflammatory cytokines. Recently K+ signaling has emerged as an important mediator of the inflammatory response to infection; we therefore investigated whether mice carrying an ENU induced activation of the electroneutral K+ channel KCC1 had an altered response to Plasmodium berghei. Here we show that Kcc1^M935K/M935K mice are protected from the development of experimental cerebral malaria, and that this protection is associated with an increased CD4+ and TNFa response. This is the first description of a K+ channel affecting the development of experimental cerebral malaria.

The Ins and Outs of Cerebral Malaria Pathogenesis: Immunopathology, Extracellular Vesicles, Immunometabolism, and Trained Immunity

Complications from malaria parasite infections still cost the lives of close to half a million people every year. The most severe is cerebral malaria (CM). Employing murine models of CM, autopsy results, in vitro experiments, neuroimaging and microscopic techniques, decades of research activity have investigated the development of CM immunopathology in the hope of identifying steps that could be therapeutically targeted. Yet important questions remain. This review summarizes recent findings, primarily mechanistic insights on the essential cellular and molecular players involved gained within the murine experimental cerebral malaria model. It also highlights recent developments in (a) cell-cell communication events mediated through extracellular vesicles (EVs), (b) mounting evidence for innate immune memory, leading to “trained“ increased or tolerised responses, and (c) modulation of immune cell function through metabolism, that could shed light on why some patients develop this life-threatening condition whilst many do not.

Unravelling the roles of innate lymphoid cells in cerebral malaria pathogenesis

Cerebral malaria (CM) is one complication of Plasmodium parasite infection that can lead to strong inflammatory immune responses in the central nervous system (CNS), accompanied by lung inflammation and anaemia. Here, we focus on the role of the innate immune response in experimental cerebral malaria (ECM) caused by blood-stage murine Plasmodium berghei ANKA infection. While T cells are important for ECM pathogenesis, the role of innate lymphoid cells (ILCs) is only emerging. The role of ILCs and non-lymphoid cells, such as neutrophils and platelets, contributing to the host immune response and leading to ECM and human cerebral malaria (HCM) is reviewed.

Macrophage-Mediated Phagocytosis and Dissolution of Amyloid-Like Fibrils in Mice, Monitored by Optical Imaging

Light chain-associated amyloidosis is characterized by the extracellular deposition of amyloid fibrils in abdominothoracic organs, skin, soft tissue, and peripheral nerves. Phagocytic cells of the innate immune system appear to be ineffective at clearing the material; however, human light chain amyloid extract, injected subcutaneously into mice, is rapidly cleared in a process that requires neutrophil activity. To better elucidate the phagocytosis of light chain fibrils, a potential method of cell-mediated dissolution, amyloid-like fibrils were labeled with the pH-sensitive dye pHrodo red and a near infrared fluorophore. After injecting this material subcutaneously in mice, optical imaging was used to quantitatively monitor phagocytosis and dissolution of fibrils concurrently. Histologic evaluation of the residual fibril masses revealed the presence of CD68⁺, F4/80⁺, ionized calcium binding adaptor molecule 1^- macrophages containing Congo red-stained fibrils as well as neutrophil-associated proteins with no evidence of intact neutrophils. These data suggest an early infiltration of neutrophils, followed by extensive phagocytosis of the light chain fibrils by macrophages, leading to dissolution of the mass. Optical imaging of this novel murine model, coupled with histologic evaluation, can be used to study the cellular mechanisms underlying dissolution of synthetic amyloid-like fibrils and human amyloid extracts. In addition, it may serve as a test bed to evaluate investigational opsonizing agents that might serve as therapeutic agents for light chain-associated amyloidosis.

Experimental severe malaria is resolved by targeting newly-identified monocyte subsets using immune-modifying particles combined with artesunate

Current treatment of severe malaria and associated cerebral malaria (CM) and respiratory distress syndromes are directed primarily at the parasite. Targeting the parasite has only partial efficacy in advanced infection, as neurological damage and respiratory distress are due to accumulation of host blood cells in the brain microvasculature and lung interstitium. Here, computational analysis identifies Ly6C^lo monocytes as a major component of the immune infiltrate in both organs in a preclinical mouse model. Specifically targeting Ly6C^lo monocyte precursors, identified by adoptive transfer, with immune-modifying particles (IMP) prevents experimental CM (ECM) in 50% of Plasmodium berghei ANKA-infected mice in early treatment protocols. Furthermore, treatment at onset of clinical ECM with 2 doses of a novel combination of IMP and anti-malarial drug artesunate results in 88% survival. This combination confers protection against ECM and mortality in late stage severe experimental malaria and provides a viable advance on current treatment regimens.

The role of myeloid-derived suppressor cells in chronic infectious diseases and the current methodology available for their study

An effective pathogen has the ability to evade the immune response. The strategies used to achieve this may be based on the direct action of virulence factors or on the induction of host factors. Myeloid-derived suppressor cells (MDSCs) are immune cells with an incredible ability to suppress the inflammatory response, which makes them excellent targets to be exploited by pathogenic bacteria, viruses, or parasites. In this review, we describe the origin and suppressive mechanisms of MDSCs, as well as their role in chronic bacterial, viral, and parasitic infections, where their expansion seems to be essential in the chronicity of the disease. We also analyze the disadvantages of current MDSC depletion strategies and the different in vitro generation methods, which can be useful tools for the deeper study of these cells in the context of microbial infections.

N-glycosylation converts non-glycoproteins into mannose receptor ligands and reveals antigen-specific T cell responses in vivo

N-glycosylation is generally accepted to enhance the immunogenicity of antigens because of two main reasons. First, the attachment of glycans enables recognition by endocytic receptors like the mannose receptor (MR) and hence increased uptake by dendritic cells (DCs). Second, foreign glycans are postulated to be immunostimulatory and their recognition could induce DC activation. However, a direct comparison between the immunogenicity of N-glycosylated vs. de-glycosylated proteins in vivo and a direct effect of N-glycosylated antigens on the intrinsic capacity of DCs to activate T cells have not been assessed so far.To analyze whether enforced N-glycosylation is a suited strategy to enhance the immunogenicity of non-glycosylated antigens for vaccination studies, we targeted non-glycoproteins towards the MR by introduction of artificial N-glycosylation using the methylotrophic yeast Komagataella phaffii (previously termed Pichia pastoris). We could demonstrate that the introduction of a single N-X-S/T motif was sufficient for efficient MR-binding and internalization. However, addition of N-glycosylated proteins neither influenced DC maturation nor their general capacity to activate T cells, pointing out that enforced N-glycosylation does not increase the immunogenicity of the antigen per se. Additionally, increased antigen-specific cytotoxic T cell responses in vivo after injection of N-glycosylated compared to de-glycosylated proteins were observed but this effect strongly depended on the epitope tested. A beneficial effect of N-glycosylation on antibody production could not be detected, which might be due to MR-cross-linking on DCs and to concomitant differences in IL-6 production by CD4+ T cells.These observations point out that the effect of N-glycosylation on antigen immunogenicity can vary between different antigens and therefore might have important implications for the development of vaccines using K. phaffii.

Do we know enough to find an adjunctive therapy for cerebral malaria in African children?

Cerebral malaria is the deadliest complication of malaria, a febrile infectious disease caused by Plasmodium parasite. Any of the five human Plasmodium species can cause disease, but, for unknown reasons, in approximately 2 million cases each year P. falciparum progresses to severe disease, ultimately resulting in half a million deaths. The majority of these deaths are in children under the age of five. Currently, there is no way to predict which child will progress to severe disease and there are no adjunctive therapies to halt the symptoms after onset. Herein, we discuss what is known about the disease mechanism of one form of severe malaria, cerebral malaria, and how we might exploit this understanding to rescue children in the throes of cerebral disease.

Effects of Acyclovir and IVIG on Behavioral Outcomes after HSV1 CNS Infection

Herpes simplex virus 1 (HSV) encephalitis (HSE) has serious neurological complications, involving behavioral and cognitive impairments that cause significant morbidity and a reduced quality of life. We showed that HSE results from dysregulated central nervous system (CNS) inflammatory responses. We hypothesized that CNS inflammation is casually involved in behavioral abnormalities after HSE and that treatment with ACV and pooled human immunoglobulin (IVIG), an immunomodulatory drug, would improve outcomes compared to mice treated with phosphate buffered saline (PBS) or ACV alone. Anxiety levels were high in HSV-infected PBS and ACV-treated mice compared to mice treated with ACV + IVIG, consistent with reports implicating inflammation in anxiety induced by lipopolysaccharide (LPS) or stress. Female, but not male, PBS-treated mice were cognitively impaired, and unexpectedly, ACV was protective, while the inclusion of IVIG surprisingly antagonized ACV's beneficial effects. Distinct serum proteomic profiles were observed for male and female mice, and the antagonistic effects of ACV and IVIG on behavior were paralleled by similar changes in the serum proteome of ACV- and ACV + IVIG-treated mice. We conclude that inflammation and other factors mediate HSV-induced behavioral impairments and that the effects of ACV and IVIG on behavior involve novel mechanisms.

Inflammatory monocytes regulate Th1 oriented immunity to CpG adjuvanted protein vaccines through production of IL-12

Due to their capacity to skew T cell responses towards Th1 oriented immunity, oligonucleotides containing unmethylated CpG motifs (CpG) have emerged as interesting adjuvants for vaccination. Whereas the signalling pathways in response to CpG mediated TLR9 activation have been extensively documented at the level of the individual cell, little is however known on the precise identity of the innate immune cells that govern T cell priming and polarisation to CpG adjuvanted protein antigens in vivo. In this study, we demonstrate that optimal induction of Th1 oriented immunity to CpG adjuvanted protein vaccines requires the coordinated actions of conventional DCs and of monocytes. Whilst conventional DCs were required for antigen presentation and initial T cell priming, monocytes constitute the main source of the Th1 polarising cytokine IL-12.

Small molecule-based inhibition of MEK1/2 proteins dampens inflammatory responses to malaria, reduces parasite load, and mitigates pathogenic outcomes

Malaria infections cause several systemic and severe single- or multi-organ pathologies, killing hundreds of thousands of people annually. Considering the existing widespread resistance of malaria parasites to anti-parasitic drugs and their high propensity to develop drug resistance, alternative strategies are required to manage malaria infections. Because malaria is a host immune response-driven disease, one approach is based on gaining a detailed understanding of the molecular and cellular processes that modulate malaria-induced innate and adaptive immune responses. Here, using a mouse cerebral malaria model and small-molecule inhibitors, we demonstrate that inhibiting MEK1/2, the upstream kinases of ERK1/2 signaling, alters multifactorial components of the innate and adaptive immune responses, controls parasitemia, and blocks pathogenesis. Specifically, MEK1/2 inhibitor treatment up-regulated B1 cell expansion, IgM production, phagocytic receptor expression, and phagocytic activity, enhancing parasite clearance by macrophages and neutrophils. Further, the MEK1/2 inhibitor treatment down-regulated pathogenic pro-inflammatory and helper T cell 1 (Th1) responses and up-regulated beneficial anti-inflammatory cytokine responses and Th2 responses. These inhibitor effects resulted in reduced granzyme B expression by T cells, chemokine and intracellular cell adhesion molecule 1 (ICAM-1) expression in the brain, and chemokine receptor expression by both myeloid and T cells. These bimodal effects of the MEK1/2 inhibitor treatment on immune responses contributed to decreased parasite biomass, organ inflammation, and immune cell recruitment, preventing tissue damage and death. In summary, we have identified several previously unrecognized immune regulatory processes through which a MEK1/2 inhibitor approach controls malaria parasitemia and mitigates pathogenic effects on host organs.

Inflammatory monocytes require type I interferon receptor signaling to activate NK cells via IL-18 during a mucosal viral infection

Although type I interferon is critical for NK cell activation, the underlying mechanism is under debate and is unknown during a mucosal infection. Lee et al. have determined that type I interferon induces inflammatory monocytes to produce IL-18 to directly activate NK cells to combat viral infections.

The requirement of type I interferon (IFN) for natural killer (NK) cell activation in response to viral infection is known, but the underlying mechanism remains unclear. Here, we demonstrate that type I IFN signaling in inflammatory monocytes, but not in dendritic cells (DCs) or NK cells, is essential for NK cell function in response to a mucosal herpes simplex virus type 2 (HSV-2) infection. Mice deficient in type I IFN signaling, Ifnar^−/− and Irf9^−/− mice, had significantly lower levels of inflammatory monocytes, were deficient in IL-18 production, and lacked NK cell–derived IFN-γ. Depletion of inflammatory monocytes, but not DCs or other myeloid cells, resulted in lower levels of IL-18 and a complete abrogation of NK cell function in HSV-2 infection. Moreover, this resulted in higher susceptibility to HSV-2 infection. Although Il18^−/− mice had normal levels of inflammatory monocytes, their NK cells were unresponsive to HSV-2 challenge. This study highlights the importance of type I IFN signaling in inflammatory monocytes and the induction of the early innate antiviral response.

Monocyte-Derived Dendritic Cells Impair Early Graft Function Following Allogeneic Islet Transplantation

Islet transplantation can cure type 1 diabetes but is limited by lack of donor organs and early graft dysfunction, such that many patients require multiple transplants to achieve insulin independence. Monocyte-derived dendritic cells (moDCs) arise during inflammation and allograft encounters where they can promote various innate and adaptive immune responses. To determine whether moDCs impair early graft function following allogeneic islet transplantation, we transplanted MHC-mismatched BALB/c (H-2d) islets into diabetic C57BL/6-CCR2.DTR recipients (H-2b) treated with either saline (control) or diphtheria toxin (DT) to deplete moDCs. Graft function was assessed by blood glucose (BG) measurement. DT treatment resulted in specific depletion of graft site moDCs posttransplant. Despite equivalent pretransplant BG levels [27.0 ± 1.3 vs. 29.6 ± 1.1 mM, not significant (ns)], DT recipients achieved lower posttransplant BG levels and better rates of normoglycemia than control recipients (11.0 ± 1.9 vs. 19.1 ± 1.4 mM, p = 0.004) at 1 day posttransplant in diabetic recipients. When a suboptimal donor dose of 200 islets was transplanted, DT-induced moDC depletion resulted in normoglycemia in 78% compared to 25% of control recipients (p = 0.03). As well as amelioration of graft dysfunction in the immediate peritransplant period, prolonged DT administration (15 days posttransplant) resulted in improved graft survival (21 vs. 11 days, p = 0.005). moDCs impair early graft function post-allogeneic islet transplantation. moDC depletion may allow for improved early graft function, permit transplantation with lower islet masses, and enhance graft survival.

MARCH1 E3 Ubiquitin Ligase Dampens the Innate Inflammatory Response by Modulating Monocyte Functions in Mice

Ubiquitination was recently identified as a central process in the pathogenesis and development of numerous inflammatory diseases, such as obesity, atherosclerosis, and asthma. Treatment with proteasomal inhibitors led to severe side effects because ubiquitination is heavily involved in a plethora of cellular functions. Thus, new players regulating ubiquitination processes must be identified to improve therapies for inflammatory diseases. In addition to their role in adaptive immunity, endosomal MHC class II (MHCII) molecules were shown to modulate innate immune responses by fine tuning the TLR4 signaling pathway. However, the role of MHCII ubiquitination by membrane associated ring-CH-type finger 1 (MARCH1) E3 ubiquitin ligase in this process remains to be assessed. In this article, we demonstrate that MARCH1 is a key inhibitor of innate inflammation in response to bacterial endotoxins. The higher mortality of March1^-/- mice challenged with a lethal dose of LPS was associated with significantly stronger systemic production of proinflammatory cytokines and splenic NK cell activation; however, we did not find evidence that MARCH1 modulates LPS or IL-10 signaling pathways. Instead, the mechanism by which MARCH1 protects against endotoxic shock rests on its capacity to promote the transition of monocytes from Ly6C^Hi to Ly6C^+/- Moreover, in competitive bone marrow chimeras, March1^-/- monocytes and polymorphonuclear neutrophils outcompeted wild-type cells with regard to bone marrow egress and homing to peripheral organs. We conclude that MARCH1 exerts MHCII-independent effects that regulate the innate arm of immunity. Thus, MARCH1 might represent a potential new target for emerging therapies based on ubiquitination reactions in inflammatory diseases.