Systemic inflammatory response syndrome triggered by blood-borne pathogens induces prolonged dendritic cell paralysis and immunosuppression.

Blood-borne pathogens can cause systemic inflammatory response syndrome (SIRS) followed by protracted, potentially lethal immunosuppression. The mechanisms responsible for impaired immunity post-SIRS remain unclear. We show that SIRS triggered by pathogen mimics or malaria infection leads to functional paralysis of conventional dendritic cells (cDCs). Paralysis affects several generations of cDCs and impairs immunity for 3-4 weeks. Paralyzed cDCs display distinct transcriptomic and phenotypic signatures and show impaired capacity to capture and present antigens in vivo. They also display altered cytokine production patterns upon stimulation. The paralysis program is not initiated in the bone marrow but during final cDC differentiation in peripheral tissues under the influence of local secondary signals that persist after resolution of SIRS. Vaccination with monoclonal antibodies that target cDC receptors or blockade of transforming growth factor ß partially overcomes paralysis and immunosuppression. This work provides insights into the mechanisms of paralysis and describes strategies to restore immunocompetence post-SIRS.

A hepatic network of dendritic cells mediates CD4 T cell help outside lymphoid organs.

While CD4+ T cells are a prerequisite for CD8+ T cell-mediated protection against intracellular hepatotropic pathogens, the mechanisms facilitating the transfer of CD4-help to intrahepatic CD8+ T cells are unknown. Here, we developed an experimental system to investigate cognate CD4+ and CD8+ T cell responses to a model-antigen expressed de novo in hepatocytes and reveal that after initial priming, effector CD4+ and CD8+ T cells migrate into portal tracts and peri-central vein regions of the liver where they cluster with type-1 conventional dendritic cells. These dendritic cells are locally licensed by CD4+ T cells and expand the number of CD8+ T cells in situ, resulting in larger effector and memory CD8+ T cell pools. These findings reveal that CD4+ T cells promote intrahepatic immunity by amplifying the CD8+ T cell response via peripheral licensing of hepatic type-1 conventional dendritic cells and identify intrahepatic perivascular compartments specialized in facilitating effector T cell-dendritic cell interactions.

mRNA vaccine against malaria tailored for liver-resident memory T cells.

Malaria is caused by Plasmodium species transmitted by Anopheles mosquitoes. Following a mosquito bite, Plasmodium sporozoites migrate from skin to liver, where extensive replication occurs, emerging later as merozoites that can infect red blood cells and cause symptoms of disease. As liver tissue-resident memory T cells (Trm cells) have recently been shown to control liver-stage infections, we embarked on a messenger RNA (mRNA)-based vaccine strategy to induce liver Trm cells to prevent malaria. Although a standard mRNA vaccine was unable to generate liver Trm or protect against challenge with Plasmodium berghei sporozoites in mice, addition of an agonist that recruits T cell help from type I natural killer T cells under mRNA-vaccination conditions resulted in significant generation of liver Trm cells and effective protection. Moreover, whereas previous exposure of mice to blood-stage infection impaired traditional vaccines based on attenuated sporozoites, mRNA vaccination was unaffected, underlining the potential for such a rational mRNA-based strategy in malaria-endemic regions.

Interplay between liver and blood stages of Plasmodium infection dictates malaria severity via γδ T cells and IL-17-promoted stress erythropoiesis.

Plasmodium replicates within the liver prior to reaching the bloodstream and infecting red blood cells. Because clinical manifestations of malaria only arise during the blood stage of infection, a perception exists that liver infection does not impact disease pathology. By developing a murine model where the liver and blood stages of infection are uncoupled, we showed that the integration of signals from both stages dictated mortality outcomes. This dichotomy relied on liver stage-dependent activation of Vγ4+ γδ T cells. Subsequent blood stage parasite loads dictated their cytokine profiles, where low parasite loads preferentially expanded IL-17-producing γδ T cells. IL-17 drove extra-medullary erythropoiesis and concomitant reticulocytosis, which protected mice from lethal experimental cerebral malaria (ECM). Adoptive transfer of erythroid precursors could rescue mice from ECM. Modeling of γδ T cell dynamics suggests that this protective mechanism may be key for the establishment of naturally acquired malaria immunity among frequently exposed individuals.

Complexing CpG adjuvants with cationic liposomes enhances vaccine-induced formation of liver TRM cells.

Tissue resident memory T cells (TRM cells) can provide effective tissue surveillance and can respond rapidly to infection. Vaccination strategies aimed at generating TRM cells have shown promise against a range of pathogens. We have previously shown that the choice of adjuvant critically influences CD8+ TRM cell formation in the liver. However, the range of adjuvants tested was limited. Here, we assessed the ability of a broad range of adjuvants stimulating membrane (TLR4), endosomal (TLR3, TLR7 and TLR9) and cytosolic (cGAS, RIG-I) pathogen recognition receptors for their capacity to induce CD8+ TRM formation in a subunit vaccination model. We show that CpG oligodeoxynucleotides (ODN) remain the most efficient inducers of liver TRM cells among all adjuvants tested. Moreover, their combination with the cationic liposome DOTAP further enhances the potency, particularly of the class B ODN CpG 1668 and the human TLR9 ligand CpG 2006 (CpG 7909). This study informs the design of efficient liver TRM-based vaccines for their potential translation.

Using Full-Spectrum Flow Cytometry to Phenotype Memory T and NKT Cell Subsets with Optimized Tissue-Specific Preparation Protocols.

Full-spectrum flow cytometry is now routinely used in many laboratories internationally, and the demand for this technology is rapidly increasing. With capacity to use larger and more complex staining panels, standardized protocols are required for optimal panel design and analysis. Importantly, for ex vivo analysis, tissue preparation methods also need to be optimized to ensure samples are truly representative of tissues in situ. This is particularly relevant given the recent interest in adaptive immune cells that form residency in specific organs. Here we provide optimized protocols for tissue processing and phenotyping of memory T cells and natural killer T (NKT) cell subsets from liver, lung, spleen, and lymph node using full-spectrum flow cytometry. We provide a 21-color antibody panel for identification of different memory subsets, including tissue-resident memory T (TRM ) cells, which are increasingly regarded as important effectors in adaptive immunity. We show that processing procedures can affect outcomes, with liver TRM cells particularly sensitive to heat, such that accurate evaluation requires fast processing at defined temperatures. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Processing mouse liver for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 2: Processing mouse spleen for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 3: Processing mouse lungs for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 4: Processing mouse lymph nodes for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 5: Staining and flow cytometric analysis of samples for memory T and NKT cell subsets Support Protocol: Obtaining cell counts from flow cytometry data.

6″-Modifed α-GalCer-peptide conjugate vaccine candidates protect against liver-stage malaria.

Self-adjuvanting vaccines consisting of peptide epitopes conjugated to immune adjuvants are a powerful way of generating antigen-specific immune responses. We previously showed that a Plasmodium-derived peptide conjugated to a rearranged form of α-galactosylceramide (α-GalCer) could stimulate liver-resident memory T (TRM) cells that were effective killers of liver-stage Plasmodium berghei ANKA (Pba)-infected cells. To investigate if similar or even superior TRM responses can be induced by modifying the α-GalCer adjuvant, we created new conjugate vaccine cadidates by attaching an immunogenic Plasmodium-derived peptide antigen to 6″-substituted α-GalCer analogues. Vaccine synthesis involved developing an efficient route to α-galactosylphytosphingosine (α-GalPhs), from which the prototypical iNKT cell agonist, α-GalCer, and its 6″-deoxy-6″-thio and -amino analogues were derived. Attaching a cathepsin B-cleavable linker to the 6″-modified α-GalCer created pro-adjuvants bearing a pendant ketone group available for peptide conjugation. Optimized reaction conditions were developed that allow for the efficient conjugation of peptide antigens to the pro-adjuvants via oxime ligation to create new glycolipid-peptide (GLP) conjugate vaccines. A single dose of the vaccine candidates induced acute NKT and Plasmodium-specific CD8+ T cell responses that generated potent hepatic TRM responses in mice. Our findings demonstrate that attaching antigenic peptides to 6″-modifed α-GalCer generates powerful self-adjuvanting conjugate vaccine candidates that could potentially control hepatotropic infections such as liver-stage malaria.

The liver contains distinct interconnected networks of CX3CR1+ macrophages, XCR1+ type 1 and CD301a+ type 2 conventional dendritic cells embedded within portal tracts.

Portal tracts are key intrahepatic structures where leukocytes accumulate during immune responses. They contain the blood inflow, which includes portal blood from the gut, and lymphatic and biliary outflow of the liver, and as such represent a key interface for potential pathogen entry to the liver. Myeloid cells residing in the interstitium of the portal tract might play an important role in the surveillance or prevention of pathogen dissemination; however, the exact composition and localization of this population has not been explored fully. Our in-depth characterization of portal tract myeloid cells revealed that in addition to T lymphocytes, portal tracts contain a heterogeneous population of MHCIIhigh myeloid cells with potential antigen presenting cell (APC) function. These include a previously unreported subset of CSF1R-dependent CX3CR1+ macrophages that phenotypically and morphologically resemble liver capsular macrophages, as well as the two main dendritic cell subsets (cDC1 and cDC2). These cells are not randomly distributed, but each subset forms interconnected networks intertwined with specific components of the portal tract. The CX3CR1+ cells were preferentially detected along the outer border of the portal tracts, and also in the portal interstitium adjacent to the portal vein, bile duct, lymphatic vessels and hepatic artery. cDC1s abounded along the lymphatic vessels, while cDC2s mostly surrounded the biliary tree. The specific distributions of these discrete subsets predict that they may serve distinct functions in this compartment. Overall, our findings suggest that portal tracts and their embedded cellular networks of myeloid cells form a distinctive lymphoid compartment in the liver that has the potential to orchestrate immune responses in this organ.

Differential location of NKT and MAIT cells within lymphoid tissue.

Natural Killer T (NKT) cells and Mucosal-Associated Invariant T (MAIT) cells are innate-like T cells that express semi-invariant αß T cell receptors (TCRs) through which they recognise CD1d and MR1 molecules, respectively, in complex with specific ligands. These cells play important roles in health and disease in many organs, but their precise intra-organ location is not well established. Here, using CD1d and MR1 tetramer staining techniques, we describe the precise location of NKT and MAIT cells in lymphoid and peripheral organs. Within the thymus, NKT cells were concentrated in the medullary side of the corticomedullary junction. In spleen and lymph nodes, NKT cells were mainly localised within T cell zones, although following in vivo activation with the potent NKT-cell ligand α-GalCer, they expanded throughout the spleen. MAIT cells were clearly detectable in Vα19 TCR transgenic mice and were rare but detectable in lymphoid tissue of non-transgenic mice. In contrast to NKT cells, MAIT cells were more closely associated with the B cell zone and red pulp of the spleen. Accordingly, we have provided an extensive analysis of the in situ localisation of NKT and MAIT cells and suggest differences between the intra-organ location of these two cell types.

Marginal zone B cells acquire dendritic cell functions by trogocytosis.

Marginal zone (MZ) B cells produce broad-spectrum antibodies that protect against infection early in life. In some instances, antibody production requires MZ B cells to display pathogen antigens bound to major histocompatibility complex class II (MHC II) molecules to T cells. We describe the trogocytic acquisition of these molecules from conventional dendritic cells (cDCs). Complement component 3 (C3) binds to murine and human MHC II on cDCs. MZ B cells recognize C3 with complement receptor 2 (CR2) and trogocytose the MHC II-C3 complexes, which become exposed on their cell surface. The ubiquitin ligase MARCH1 limits the number of MHC II-C3 complexes displayed on cDCs to prevent their elimination through excessive trogocytosis. Capture of C3 by MHC II thus enables the transfer of cDC-like properties to MZ B cells.

The Batman and Robin of liver-stage immunity to malaria.

Malaria parasites replicate within the liver shortly after infection. This stage can be controlled by CD8 T cells, but which subsets undertake this function is unclear. Lefebvre et al. now elegantly show that effector memory T (TEM) cells are avid participants, working as a dynamic duo with liver tissue-resident memory T (TRM) cells to combat infection.

Type I interferon production elicits differential CD4+ T-cell responses in mice infected with Plasmodium berghei ANKA and P. chabaudi.

Plasmodium parasites that infect humans are highly polymorphic, and induce various infections ranging from an asymptomatic state to life-threatening diseases. However, how the differences between the parasites affect host immune responses during blood-stage infection remains largely unknown. We investigated the CD4+ T-cell immune responses in mice infected with P. berghei ANKA (PbA) or P. chabaudi chabaudi AS (Pcc) using PbT-II cells, which recognize a common epitope of these parasites. In the acute phase of infection, CD4+ T-cell responses in PbA-infected mice showed a lower involvement of Th1 cells and a lower proportion of Ly6Clo effector CD4+ T cells than those in Pcc-infected mice. Transcriptome analysis of PbT-II cells indicated that type I interferon (IFN)-regulated genes were expressed at higher levels in both Th1- and Tfh-type PbT-II cells from PbA-infected mice than those from Pcc-infected mice. Moreover, IFN-α levels were considerably higher in PbA-infected mice than in Pcc-infected mice. Inhibition of type I IFN signaling increased PbT-II and partially reversed the Th1 over Tfh bias of the PbT-II cells in both PbA- and Pcc-infected mice. In the memory phase, PbT-II cells in PbA-primed mice maintained higher numbers and exhibited a better recall response to the antigen. However, recall responses were not significantly different between the infection groups after re-challenge with PbA, suggesting the effect of the inflammatory environment by the infection. These observations suggest that the differences in Plasmodium-specific CD4+ T-cell responses between PbA- and Pcc-infected mice were associated with the difference in type I IFN production during the early phase of the infection.

Sphingosine 1-phosphate receptor 5 (S1PR5) regulates the peripheral retention of tissue-resident lymphocytes.

Tissue-resident memory T (TRM) cells provide long-lasting immune protection. One of the key events controlling TRM cell development is the local retention of TRM cell precursors coupled to downregulation of molecules necessary for tissue exit. Sphingosine-1-phosphate receptor 5 (S1PR5) is a migratory receptor with an uncharted function in T cells. Here, we show that S1PR5 plays a critical role in T cell infiltration and emigration from peripheral organs, as well as being specifically downregulated in TRM cells. Consequentially, TRM cell development was selectively impaired upon ectopic expression of S1pr5, whereas loss of S1pr5 enhanced skin TRM cell formation by promoting peripheral T cell sequestration. Importantly, we found that T-bet and ZEB2 were required for S1pr5 induction and that local TGF-ß signaling was necessary to promote coordinated Tbx21, Zeb2, and S1pr5 downregulation. Moreover, S1PR5-mediated control of tissue residency was conserved across innate and adaptive immune compartments. Together, these results identify the T-bet-ZEB2-S1PR5 axis as a previously unappreciated mechanism modulating the generation of tissue-resident lymphocytes.

The unexpected contribution of conventional type 1 dendritic cells in driving antibody responses.

Antibodies are hallmarks of most effective vaccines. For successful T-dependent antibody responses, conventional dendritic cells (cDC) have been largely attributed the role of priming T cells. By contrast, follicular dendritic cells and macrophages have been seen as responsible for B cell activation, due to their strategic location within secondary lymphoid tissues and capacity to present native antigen to B cells. This review summarizes the mounting evidence that cDC can also present native antigen to B cells. cDC2 have been the main subset linked to humoral responses, based largely on their favorable location, capacity to prime CD4+ T cells, and ability to present native antigen to B cells. However, studies using strategies to deliver antigen to receptors on cDC1, reveal this subset can also contribute to naïve B cell activation, as well as T cell priming. cDC1 location within lymphoid tissues reveals their juxtaposition to B cell follicles, with ready access to B cells for presentation of native antigen. These findings support the view that both cDC1 and cDC2 are capable of initiating humoral responses provided antigen is captured by relevant surface receptors attuned to this process. Such understanding is fundamental for the development of innovative humoral vaccination approaches.

High expression of CD38 and MHC class II on CD8+ T cells during severe influenza disease reflects bystander activation and trogocytosis.

OBJECTIVES: Although co-expression of CD38 and HLA-DR reflects T-cell activation during viral infections, high and prolonged CD38+HLA-DR+ expression is associated with severe disease. To date, the mechanism underpinning expression of CD38+HLA-DR+ is poorly understood. METHODS: We used mouse models of influenza A/H9N2, A/H7N9 and A/H3N2 infection to investigate mechanisms underpinning CD38+MHC-II+ phenotype on CD8+ T cells. To further understand MHC-II trogocytosis on murine CD8+ T cells as well as the significance behind the scenario, we used adoptively transferred transgenic OT-I CD8+ T cells and A/H3N2-SIINKEKL infection. RESULTS: Analysis of influenza-specific immunodominant DbNP366 +CD8+ T-cell responses showed that CD38+MHC-II+ co-expression was detected on both virus-specific and bystander CD8+ T cells, with increased numbers of both CD38+MHC-II+CD8+ T-cell populations observed in immune organs including the site of infection during severe viral challenge. OT-I cells adoptively transferred into MHC-II-/- mice had no MHC-II after infection, suggesting that MHC-II was acquired via trogocytosis. The detection of CD19 on CD38+MHC-II+ OT-I cells supports the proposition that MHC-II was acquired by trogocytosis sourced from B cells. Co-expression of CD38+MHC-II+ on CD8+ T cells was needed for optimal recall following secondary infection. CONCLUSIONS: Overall, our study demonstrates that both virus-specific and bystander CD38+MHC-II+ CD8+ T cells are recruited to the site of infection during severe disease, and that MHC-II presence occurs via trogocytosis from antigen-presenting cells. Our findings highlight the importance of the CD38+MHC-II+ phenotype for CD8+ T-cell recall.

Elucidating the Motif for CpG Oligonucleotide Binding to the Dendritic Cell Receptor DEC-205 Leads to Improved Adjuvants for Liver-Resident Memory.

DEC-205 is a cell-surface receptor that transports bound ligands into the endocytic pathway for degradation or release within lysosomal endosomes. This receptor has been reported to bind a number of ligands, including keratin, and some classes of CpG oligodeoxynucleotides (ODN). In this study, we explore in detail the requirements for binding ODNs, revealing that DEC-205 efficiently binds single-stranded, phosphorothioated ODN of ≥14 bases, with preference for the DNA base thymidine, but with no requirement for a CpG motif. DEC-205 fails to bind double-stranded phosphodiester ODN, and thus does not bind the natural type of DNA found in mammals. The ODN binding preferences of DEC-205 result in strong binding of B class ODN, moderate binding to C class ODN, minimal binding to P class ODN, and no binding to A class ODN. Consistent with DEC-205 binding capacity, induction of serum IL-12p70 or activation of B cells by each class of ODN correlated with DEC-205 dependence in mice. Thus, the greater the DEC-205 binding capacity, the greater the dependence on DEC-205 for optimal responses. Finally, by covalently linking a B class ODN that efficiently binds DEC-205, to a P class ODN that shows poor binding, we improved DEC-205 binding and increased adjuvancy of the hybrid ODN. The hybrid ODN efficiently enhanced induction of effector CD8 T cells in a DEC-205-dependent manner. Furthermore, the hybrid ODN induced robust memory responses, and was particularly effective at promoting the development of liver tissue-resident memory T cells.

Hemozoin-mediated inflammasome activation limits long-lived anti-malarial immunity.

During acute malaria, most individuals mount robust inflammatory responses that limit parasite burden. However, long-lived sterilizing anti-malarial memory responses are not efficiently induced, even following repeated Plasmodium exposures. Using multiple Plasmodium species, genetically modified parasites, and combinations of host genetic and pharmacologic approaches, we find that the deposition of the malarial pigment hemozoin directly limits the abundance and capacity of conventional type 1 dendritic cells to prime helper T cell responses. Hemozoin-induced dendritic cell dysfunction results in aberrant Plasmodium-specific CD4 T follicular helper cell differentiation, which constrains memory B cell and long-lived plasma cell formation. Mechanistically, we identify that dendritic cell-intrinsic NLRP3 inflammasome activation reduces conventional type 1 dendritic cell abundance, phagocytosis, and T cell priming functions in vivo. These data identify biological consequences of hemozoin deposition during malaria and highlight the capacity of the malarial pigment to program immune evasion during the earliest events following an initial Plasmodium exposure.

Discrete tissue microenvironments instruct diversity in resident memory T cell function and plasticity.

Tissue-resident memory T (TRM) cells are non-recirculating cells that exist throughout the body. Although TRM cells in various organs rely on common transcriptional networks to establish tissue residency, location-specific factors adapt these cells to their tissue of lodgment. Here we analyze TRM cell heterogeneity between organs and find that the different environments in which these cells differentiate dictate TRM cell function, durability and malleability. We find that unequal responsiveness to TGFß is a major driver of this diversity. Notably, dampened TGFß signaling results in CD103- TRM cells with increased proliferative potential, enhanced function and reduced longevity compared with their TGFß-responsive CD103+ TRM counterparts. Furthermore, whereas CD103- TRM cells readily modified their phenotype upon relocation, CD103+ TRM cells were comparatively resistant to transdifferentiation. Thus, despite common requirements for TRM cell development, tissue adaptation of these cells confers discrete functional properties such that TRM cells exist along a spectrum of differentiation potential that is governed by their local tissue microenvironment.

CD8+ and CD4+ T Cells Infiltrate into the Brain during Plasmodium berghei ANKA Infection and Form Long-Term Resident Memory.

In the Plasmodium berghei ANKA mouse model of malaria, accumulation of CD8+ T cells and infected RBCs in the brain promotes the development of experimental cerebral malaria (ECM). In this study, we used malaria-specific transgenic CD4+ and CD8+ T cells to track evolution of T cell immunity during the acute and memory phases of P. berghei ANKA infection. Using a combination of techniques, including intravital multiphoton and confocal microscopy and flow cytometric analysis, we showed that, shortly before onset of ECM, both CD4+ and CD8+ T cell populations exit the spleen and begin infiltrating the brain blood vessels. Although dominated by CD8+ T cells, a proportion of both T cell subsets enter the brain parenchyma, where they are largely associated with blood vessels. Intravital imaging shows these cells moving freely within the brain parenchyma. Near the onset of ECM, leakage of RBCs into areas of the brain can be seen, implicating severe damage. If mice are cured before ECM onset, brain infiltration by T cells still occurs, but ECM is prevented, allowing development of long-term resident memory T cell populations within the brain. This study shows that infiltration of malaria-specific T cells into the brain parenchyma is associated with cerebral immunopathology and the formation of brain-resident memory T cells. The consequences of these resident memory populations is unclear but raises concerns about pathology upon secondary infection.