Paired immunoglobulin-like receptors and their MHC class I recognition

Innate Allorecognition in Transplantation: Ancient Mechanisms With Modern Impact

Through the effective targeting of the adaptive immune system, solid organ transplantation became a life-saving therapy for organ failure. However, beyond 1 y of transplantation, there is little improvement in transplant outcomes. The adaptive immune response requires the activation of the innate immune system. There are no modalities for the specific targeting of the innate immune system involvement in transplant rejection. However, the recent discovery of innate allorecognition and innate immune memory presents novel targets in transplantation that will increase our understanding of organ rejection and might aid in improving transplant outcomes. In this review, we look at the latest developments in the study of innate allorecognition and innate immune memory in transplantation.

CaSSiDI: novel single-cell "Cluster Similarity Scoring and Distinction Index" reveals critical functions for PirB and context-dependent Cebpb repression

PirB is an inhibitory cell surface receptor particularly prominent on myeloid cells. PirB curtails the phenotypes of activated macrophages during inflammation or tumorigenesis, but its functions in macrophage homeostasis are obscure. To elucidate PirB-related functions in macrophages at steady-state, we generated and compared single-cell RNA-sequencing (scRNAseq) datasets obtained from myeloid cell subsets of wild type (WT) and PirB-deficient knockout (PirB KO) mice. To facilitate this analysis, we developed a novel approach to clustering parameter optimization called "Cluster Similarity Scoring and Distinction Index" (CaSSiDI). We demonstrate that CaSSiDI is an adaptable computational framework that facilitates tandem analysis of two scRNAseq datasets by optimizing clustering parameters. We further show that CaSSiDI offers more advantages than a standard Seurat analysis because it allows direct comparison of two or more independently clustered datasets, thereby alleviating the need for batch-correction while identifying the most similar and different clusters. Using CaSSiDI, we found that PirB is a novel regulator of Cebpb expression that controls the generation of Ly6Clo patrolling monocytes and the expansion properties of peritoneal macrophages. PirB's effect on Cebpb is tissue-specific since it was not observed in splenic red pulp macrophages (RPMs). However, CaSSiDI revealed a segregation of the WT RPM population into a CD68loIrf8+ "neuronal-primed" subset and an CD68hiFtl1+ "iron-loaded" subset. Our results establish the utility of CaSSiDI for single-cell assay analyses and the determination of optimal clustering parameters. Our application of CaSSiDI in this study has revealed previously unknown roles for PirB in myeloid cell populations. In particular, we have discovered homeostatic functions for PirB that are related to Cebpb expression in distinct macrophage subsets.

Immune receptors and aging brain

Aging brings about a myriad of degenerative processes throughout the body. A decrease in cognitive abilities is one of the hallmark phenotypes of aging, underpinned by neuroinflammation and neurodegeneration occurring in the brain. This review focuses on the role of different immune receptors expressed in cells of the central and peripheral nervous systems. We will discuss how immune receptors in the brain act as sentinels and effectors of the age-dependent shift in ligand composition. Within this 'old-age-ligand soup,' some immune receptors contribute directly to excessive synaptic weakening from within the neuronal compartment, while others amplify the damaging inflammatory environment in the brain. Ultimately, chronic inflammation sets up a positive feedback loop that increases the impact of immune ligand-receptor interactions in the brain, leading to permanent synaptic and neuronal loss.

Immunoglobulin-like receptors in chickens: identification, functional characterization, and renaming to cluster homolog of immunoglobulin-like receptors

The cluster homolog of immunoglobulin-like receptors (CHIRs), previously known as the "chicken homolog of immunogloublin-like receptors," represents is a large group of transmembrane glycoproteins that direct the immune response. However, the full repertoire of putatively activating, inhibitory, or dual function CHIRA, CHIRB, and CHIRAB on chickens' immune responses is poorly understood. Herein, the study objective was to determine the genes encoding CHIR proteins and predict their function by searching canonical protein structure. A bioinformatics pipeline based on previous work was employed to search for the CHIRs from the newly updated broiler and layer genomes. The categorization into CHIRA, CHIRB, and CHIRAB types was assigned through motif searches, multiple sequence alignment, and phylogeny. In total, 150 protein-encoding genes on Chromosome 31 were identified as CHIRs. Gene members of each functional group (CHIRA, CHIRB, CHIRAB) were classified in accordance with previously recognized proteins. The genes were renamed to "cluster homolog of immunoglobulin-like receptors" (CHIRs) to allow for the naming of orthologous genes in other avian species. Additionally, expression analysis of the classified CHIRs across various reinforces their importance as immune regulators and activation in inflammatory tissues. Furthermore, over 1,000 diverse and rare CHIRs variants associated with differential Marek's disease response (P < 0.05) emphasize the impact of CHIRs on shaping avian immune responses in diverse contexts. The practical applications of these findings encompass advancing immunology, improving poultry health management, optimizing breeding programs for disease resistance, and enhancing overall animal health through a deeper understanding of the roles and functions of CHIRA, CHIRB, and CHIRAB types in avian immune responses.

Epithelial recognition and elimination against aberrant cells

Epithelial cells, which are non-immune cells, not only function as a physical defence barrier but also continuously monitor and eliminate aberrant epithelial cells in their vicinity. In other words, it has become evident that epithelial cells possess immune cell-like functions. In fact, recent research has revealed that epithelial cells recognise the Major Histocompatibility Complex I (MHC-I) of aberrant cells as a mechanism for surveillance. This cellular defence mechanism of epithelial cells probably detects aberrant cells more promptly than the conventional immune response, making it a novel and primary biological defence. Furthermore, there is the potential for this new immune-like biological defence mechanism to establish innovative treatment for disease prevention, leading to increasing anticipation for its future medical applications. In this review, we aim to summarise the recognition and attack mechanisms of aberrant cells by epithelial cells in mammals, with a particular focus on the field of cancer. Additionally, we discuss the potential therapeutic applications of epithelial cell-based defence against cancer, including novel prophylactic treatment methods based on molecular mechanisms.

Human leukocyte immunoglobulin-like receptors in health and disease

Human leukocyte immunoglobulin (Ig)-like receptors (LILR) are a family of 11 innate immunomodulatory receptors, primarily expressed on lymphoid and myeloid cells. LILRs are either activating (LILRA) or inhibitory (LILRB) depending on their associated signalling domains (D). With the exception of the soluble LILRA3, LILRAs mediate immune activation, while LILRB1-5 primarily inhibit immune responses and mediate tolerance. Abnormal expression and function of LILRs is associated with a range of pathologies, including immune insufficiency (infection and malignancy) and overt immune responses (autoimmunity and alloresponses), suggesting LILRs may be excellent candidates for targeted immunotherapies. This review will discuss the biology and clinical relevance of this extensive family of immune receptors and will summarise the recent developments in targeting LILRs in disease settings, such as cancer, with an update on the clinical trials investigating the therapeutic targeting of these receptors.

Alloantigen Infusion Activates the Transcriptome of Type 2 Conventional Dendritic Cells

Recent studies have revealed novel molecular mechanisms by which innate monocytic cells acutely recognize and respond to alloantigen with significance to allograft rejection and tolerance. What remains unclear is the single-cell heterogeneity of the innate alloresponse, particularly the contribution of dendritic cell (DC) subsets. To investigate the response of these cells to exposure of alloantigen, C57BL/6J mice were administered live allogenic BALB/cJ splenic murine cells versus isogenic cells. In parallel, we infused apoptotic allogenic and isogenic cells, which have been reported to modulate immunity. Forty-eight hours after injection, recipient spleens were harvested, enriched for DCs, and subjected to single-cell mRNA sequencing. Injection of live cells induced a greater transcriptional change across DC subsets compared with apoptotic cells. In the setting of live cell infusion, type 2 conventional DCs (cDC2s) were most transcriptionally responsive with a Ccr2+ cDC2 subcluster uniquely responding to the presence of alloantigen compared with the isogenic control. In vitro experimentation confirmed unique activation of CCR2+ cDC2s following alloantigen exposure. Candidate receptors of allorecognition in other innate populations were interrogated and A type paired Ig-like receptors were found to be increased in the cDC2 population following alloexposure. These results illuminate previously unclear distinctions between therapeutic infusions of live versus apoptotic allogenic cells and suggest a role for cDC2s in innate allorecognition. More critically, these studies allow for future interrogation of the transcriptional response of immune cells in the setting of alloantigen exposure in vivo, encouraging assessment of novel pathways and previously unexamined receptors in this setting.

Microglial MHC-I induction with aging and Alzheimer's is conserved in mouse models and humans

Major histocompatibility complex I (MHC-I) CNS cellular localization and function is still being determined after previously being thought to be absent from the brain. MHC-I expression has been reported to increase with brain aging in mouse, rat, and human whole tissue analyses, but the cellular localization was undetermined. Neuronal MHC-I is proposed to regulate developmental synapse elimination and tau pathology in Alzheimer's disease (AD). Here, we report that across newly generated and publicly available ribosomal profiling, cell sorting, and single-cell data, microglia are the primary source of classical and non-classical MHC-I in mice and humans. Translating ribosome affinity purification-qPCR analysis of 3-6- and 18-22-month-old (m.o.) mice revealed significant age-related microglial induction of MHC-I pathway genes B2m, H2-D1, H2-K1, H2-M3, H2-Q6, and Tap1 but not in astrocytes and neurons. Across a timecourse (12-23 m.o.), microglial MHC-I gradually increased until 21 m.o. and then accelerated. MHC-I protein was enriched in microglia and increased with aging. Microglial expression, and absence in astrocytes and neurons, of MHC-I-binding leukocyte immunoglobulin-like (Lilrs) and paired immunoglobin-like type 2 (Pilrs) receptor families could enable cell -autonomous MHC-I signaling and increased with aging in mice and humans. Increased microglial MHC-I, Lilrs, and Pilrs were observed in multiple AD mouse models and human AD data across methods and studies. MHC-I expression correlated with p16INK4A, suggesting an association with cellular senescence. Conserved induction of MHC-I, Lilrs, and Pilrs with aging and AD opens the possibility of cell-autonomous MHC-I signaling to regulate microglial reactivation with aging and neurodegeneration.

LILRB2/PirB mediates macrophage recruitment in fibrogenesis of nonalcoholic steatohepatitis

Inhibition of immunocyte infiltration and activation has been suggested to effectively ameliorate nonalcoholic steatohepatitis (NASH). Paired immunoglobulin-like receptor B (PirB) and its human ortholog receptor, leukocyte immunoglobulin-like receptor B (LILRB2), are immune-inhibitory receptors. However, their role in NASH pathogenesis is still unclear. Here, we demonstrate that PirB/LILRB2 regulates the migration of macrophages during NASH by binding with its ligand angiopoietin-like protein 8 (ANGPTL8). Hepatocyte-specific ANGPTL8 knockout reduces MDM infiltration and resolves lipid accumulation and fibrosis progression in the livers of NASH mice. In addition, PirB^-/- bone marrow (BM) chimeras abrogate ANGPTL8-induced MDM migration to the liver. And yet, PirB ectodomain protein could ameliorate NASH by sequestering ANGPTL8. Furthermore, LILRB2-ANGPTL8 binding-promoted MDM migration and inflammatory activation are also observed in human peripheral blood monocytes. Taken together, our findings reveal the role of PirB/LILRB2 in NASH pathogenesis and identify PirB/LILRB2-ANGPTL8 signaling as a potential target for the management or treatment of NASH.

Transcriptional and spatial profiling of the kidney allograft unravels a central role for FcyRIII+ innate immune cells in rejection

Rejection remains the main cause of premature graft loss after kidney transplantation, despite the use of potent immunosuppression. This highlights the need to better understand the composition and the cell-to-cell interactions of the alloreactive inflammatory infiltrate. Here, we performed droplet-based single-cell RNA sequencing of 35,152 transcriptomes from 16 kidney transplant biopsies with varying phenotypes and severities of rejection and without rejection, and identified cell-type specific gene expression signatures for deconvolution of bulk tissue. A specific association was identified between recipient-derived FCGR3A+ monocytes, FCGR3A+ NK cells and the severity of intragraft inflammation. Activated FCGR3A+ monocytes overexpressed CD47 and LILR genes and increased paracrine signaling pathways promoting T cell infiltration. FCGR3A+ NK cells overexpressed FCRL3, suggesting that antibody-dependent cytotoxicity is a central mechanism of NK-cell mediated graft injury. Multiplexed immunofluorescence using 38 markers on 18 independent biopsy slides confirmed this role of FcγRIII+ NK and FcγRIII+ nonclassical monocytes in antibody-mediated rejection, with specificity to the glomerular area. These results highlight the central involvement of innate immune cells in the pathogenesis of allograft rejection and identify several potential therapeutic targets that might improve allograft longevity.

Microglial MHC-I induction with aging and Alzheimer's is conserved in mouse models and humans

Major Histocompatibility Complex I (MHC-I) CNS cellular localization and function is still being determined after previously being thought to be absent from the brain. MHC-I expression has been reported to increase with brain aging in mouse, rat, and human whole tissue analyses but the cellular localization was undetermined. Neuronal MHC-I is proposed to regulate developmental synapse elimination and tau pathology in Alzheimer's disease (AD). Here we report that across newly generated and publicly available ribosomal profiling, cell sorting, and single-cell data, microglia are the primary source of classical and non-classical MHC-I in mice and humans. Translating Ribosome Affinity Purification-qPCR analysis of 3-6 and 18-22 month old (m.o.) mice revealed significant age-related microglial induction of MHC-I pathway genes B2m , H2-D1 , H2-K1 , H2-M3 , H2-Q6 , and Tap1 but not in astrocytes and neurons. Across a timecourse (12-23 m.o.), microglial MHC-I gradually increased until 21 m.o. and then accelerated. MHC-I protein was enriched in microglia and increased with aging. Microglial expression, and absence in astrocytes and neurons, of MHC-I binding Leukocyte Immunoglobulin-like (Lilrs) and Paired immunoglobin-like type 2 (Pilrs) receptor families could enable cell-autonomous MHC-I signaling and increased with aging in mice and humans. Increased microglial MHC-I, Lilrs, and Pilrs were observed in multiple AD mouse models and human AD data across methods and studies. MHC-I expression correlated with p16INK4A , suggesting an association with cellular senescence. Conserved induction of MHC-I, Lilrs, and Pilrs with aging and AD opens the possibility of cell-autonomous MHC-I signaling to regulate microglial reactivation with aging and neurodegeneration.

Comparative genomics of the Leukocyte Receptor Complex in carnivores

Background

The mammalian Leukocyte Receptor Complex (LRC) chromosomal region may contain gene families for the killer cell immunoglobulin-like receptor (KIR) and/or leukocyte immunoglobulin-like receptor (LILR) collections as well as various framing genes. This complex region is well described in humans, mice, and some domestic animals. Although single KIR genes are known in some Carnivora, their complements of LILR genes remain largely unknown due to obstacles in the assembly of regions of high homology in short-read based genomes.

Methods

As part of the analysis of felid immunogenomes, this study focuses on the search for LRC genes in reference genomes and the annotation of LILR genes in Felidae. Chromosome-level genomes based on single-molecule long-read sequencing were preferentially sought and compared to representatives of the Carnivora.

Results

Seven putatively functional LILR genes were found across the Felidae and in the Californian sea lion, four to five genes in Canidae, and four to nine genes in Mustelidae. They form two lineages, as seen in the Bovidae. The ratio of functional genes for activating LILRs to inhibitory LILRs is slightly in favor of inhibitory genes in the Felidae and the Canidae; the reverse is seen in the Californian sea lion. This ratio is even in all of the Mustelidae except the Eurasian otter, which has a predominance of activating LILRs. Various numbers of LILR pseudogenes were identified.

Conclusions

The structure of the LRC is rather conservative in felids and the other Carnivora studied. The LILR sub-region is conserved within the Felidae and has slight differences in the Canidae, but it has taken various evolutionary paths in the Mustelidae. Overall, the process of pseudogenization of LILR genes seems to be more frequent for activating receptors. Phylogenetic analysis found no direct orthologues across the Carnivora which corroborate the rapid evolution of LILRs seen in mammals.

Paired immunoglobulin-like receptor B is an entry receptor for mammalian orthoreovirus

Mammalian orthoreovirus (reovirus) infects most mammals and is associated with celiac disease in humans. In mice, reovirus infects the intestine and disseminates systemically to cause serotype-specific patterns of disease in the brain. To identify receptors conferring reovirus serotype-dependent neuropathogenesis, we conducted a genome-wide CRISPRa screen and identified paired immunoglobulin-like receptor B (PirB) as a receptor candidate. Ectopic expression of PirB allowed reovirus binding and infection. PirB extracelluar D3D4 region is required for reovirus attachment and infectivity. Reovirus binds to PirB with nM affinity as determined by single molecule force spectroscopy. Efficient reovirus endocytosis requires PirB signaling motifs. In inoculated mice, PirB is required for maximal replication in the brain and full neuropathogenicity of neurotropic serotype 3 (T3) reovirus. In primary cortical neurons, PirB expression contributes to T3 reovirus infectivity. Thus, PirB is an entry receptor for reovirus and contributes to T3 reovirus replication and pathogenesis in the murine brain.

Involvement of Paired Immunoglobulin-Like Receptor B in Cognitive Dysfunction Through Hippocampal-Dependent Synaptic Plasticity Impairments in Mice Subjected to Chronic Sleep Restriction

Sleep loss is often associated with cognitive dysfunction. Alterations in the structure and function of synapses in the hippocampus are thought to underlie memory storage. Paired immunoglobulin-like receptor B (PirB) plays a negative role in various neurological diseases by inhibiting axon regeneration and synaptic plasticity. However, the contributions of PirB to the mechanisms underlying the changes in synaptic plasticity after sleep loss that ultimately promote deficits in cognitive function have not been well elucidated. Here, we showed that chronic sleep restriction (CSR) mice displayed cognitive impairment and synaptic deficits accompanied by upregulation of PirB expression in the hippocampus. Mechanistically, PirB caused the dysregulation of actin through the RhoA/ROCK2/LIMK1/cofilin signalling pathway, leading to abnormal structural and functional plasticity, which in turn resulted in cognitive dysfunction. PirB knockdown alleviated synaptic deficits and cognitive impairment after CSR by inhibiting the RhoA/ROCK2/LIMK1/cofilin signalling pathway. Moreover, we found that fasudil, a widely used ROCK2 inhibitor, could mimic the beneficial effect of PirB knockdown and ameliorate synaptic deficits and cognitive impairment, further demonstrating that PirB induced cognitive dysfunction after CSR via the RhoA/ROCK2/LIMK1/cofilin signalling pathway. Our study sheds new light on the role of PirB as an important mediator in modulating the dysfunction of synaptic plasticity and cognitive function via the RhoA/ROCK2/LIMK1/cofilin signalling pathway, which indicated that hippocampal PirB is a promising therapeutic target for counteracting cognitive impairment after CSR. This illustration depicts the signalling pathway by PirB in mediating cognitive impairment and synaptic deficits in CSR mice. In the hippocampus of CSR mice, the expression level of PirB was significantly increased. In addition, CSR increases RhoA and ROCK2 levels and reduces levels of both LIMK1 and cofilin phosphorylation. PirB knockdown reverses cognitive impairment and synaptic plasticity disorders caused by CSR through the RhoA/ROCK2/LIMK1/cofilin signalling pathway.

LOTUS suppresses amyloid β-induced dendritic spine elimination through the blockade of amyloid β binding to PirB

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disease worldwide but has no effective treatment. Amyloid beta (Aβ) protein, a primary risk factor for AD, accumulates and aggregates in the brain of patients with AD. Paired immunoglobulin-like receptor B (PirB) has been identified as a receptor of Aβ and Aβ-PirB molecular interactions that cause synapse elimination and synaptic dysfunction. PirB deletion has been shown to suppress Aβ-induced synaptic dysfunction and behavioral deficits in AD model mice, implying that PirB mediates Aβ-induced AD pathology. Therefore, inhibiting the Aβ-PirB molecular interaction could be a successful approach for combating AD pathology. We previously showed that lateral olfactory tract usher substance (LOTUS) is an endogenous antagonist of type1 Nogo receptor and PirB and that LOTUS overexpression promotes neuronal regeneration following damage to the central nervous system, including spinal cord injury and ischemic stroke. Therefore, in this study, we investigated whether LOTUS inhibits Aβ-PirB interaction and Aβ-induced dendritic spine elimination.

METHODS

The inhibitory role of LOTUS against Aβ-PirB (or leukocyte immunoglobulin-like receptor subfamily B member 2: LilrB2) binding was assessed using a ligand-receptor binding assay in Cos7 cells overexpressing PirB and/or LOTUS. We assessed whether LOTUS inhibits Aβ-induced intracellular alterations and synaptotoxicity using immunoblots and spine imaging in a primary cultured hippocampal neuron.

RESULTS

We found that LOTUS inhibits the binding of Aβ to PirB overexpressed in Cos7 cells. In addition, we found that Aβ-induced dephosphorylation of cofilin and Aβ-induced decrease in post-synaptic density-95 expression were suppressed in cultured hippocampal neurons from LOTUS-overexpressing transgenic (LOTUS-tg) mice compared with that in wild-type mice. Moreover, primary cultured hippocampal neurons from LOTUS-tg mice improved the Aβ-induced decrease in dendritic spine density. Finally, we studied whether human LOTUS protein inhibits Aβ binding to LilrB2, a human homolog of PirB, and found that human LOTUS inhibited the binding of Aβ to LilrB2 in a similar manner.

CONCLUSIONS

This study implied that LOTUS improved Aβ-induced synapse elimination by suppressing Aβ-PirB interaction in rodents and inhibited Aβ-LilrB2 interaction in humans. Our findings revealed that LOTUS may be a promising therapeutic agent in counteracting Aβ-induced AD pathologies.

Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps

Xenotransplantation has the potential to solve the shortfall of human organ donors. Genetically modified pigs have been considered as potential animal donors for human xenotransplantation and have been widely used in preclinical research. The genetic modifications aim to prevent the major species-specific barriers, which include humoral and cellular immune responses, and physiological incompatibilities such as complement and coagulation dysfunctions. Genetically modified pigs can be created by deleting several pig genes related to the synthesis of various pig specific antigens or by inserting human complement- and coagulation-regulatory transgenes. Finally, in order to reduce the risk of infection, genes related to porcine endogenous retroviruses can be knocked down. In this review, we focus on genetically modified pigs and comprehensively summarize the immunological mechanism of xenograft rejection and recent progress in preclinical and clinical studies. Overall, both genetically engineered pig-based xenografts and technological breakthroughs in the biomedical field provide a promising foundation for pig-to-human xenotransplantation in the future.

Placing human gene families into their evolutionary context

Following the draft sequence of the first human genome over 20 years ago, we have achieved unprecedented insights into the rules governing its evolution, often with direct translational relevance to specific diseases. However, staggering sequence complexity has also challenged the development of a more comprehensive understanding of human genome biology. In this context, interspecific genomic studies between humans and other animals have played a critical role in our efforts to decode human gene families. In this review, we focus on how the rapid surge of genome sequencing of both model and non-model organisms now provides a broader comparative framework poised to empower novel discoveries. We begin with a general overview of how comparative approaches are essential for understanding gene family evolution in the human genome, followed by a discussion of analyses of gene expression. We show how homology can provide insights into the genes and gene families associated with immune response, cancer biology, vision, chemosensation, and metabolism, by revealing similarity in processes among distant species. We then explain methodological tools that provide critical advances and show the limitations of common approaches. We conclude with a discussion of how these investigations position us to gain fundamental insights into the evolution of gene families among living organisms in general. We hope that our review catalyzes additional excitement and research on the emerging field of comparative genomics, while aiding the placement of the human genome into its existentially evolutionary context.

VCAM1 confers innate immune tolerance on haematopoietic and leukaemic stem cells

Haematopoietic stem cells (HSCs) home to the bone marrow via, in part, interactions with vascular cell adhesion molecule-1 (VCAM1)^1-3. Once in the bone marrow, HSCs are vetted by perivascular phagocytes to ensure their self-integrity. Here we show that VCAM1 is also expressed on healthy HSCs and upregulated on leukaemic stem cells (LSCs), where it serves as a quality-control checkpoint for entry into bone marrow by providing 'don't-eat-me' stamping in the context of major histocompatibility complex class-I (MHC-I) presentation. Although haplotype-mismatched HSCs can engraft, Vcam1 deletion, in the setting of haplotype mismatch, leads to impaired haematopoietic recovery due to HSC clearance by mononuclear phagocytes. Mechanistically, VCAM1 'don't-eat-me' activity is regulated by β2-microglobulin MHC presentation on HSCs and paired Ig-like receptor-B (PIR-B) on phagocytes. VCAM1 is also used by cancer cells to escape immune detection as its expression is upregulated in multiple cancers, including acute myeloid leukaemia (AML), where high expression associates with poor prognosis. In AML, VCAM1 promotes disease progression, whereas VCAM1 inhibition or deletion reduces leukaemia burden and extends survival. These results suggest that VCAM1 engagement regulates a critical immune-checkpoint gate in the bone marrow, and offers an alternative strategy to eliminate cancer cells via modulation of the innate immune tolerance.

Immunoglobulin-like receptors and the generation of innate immune memory

Host immunity is classically divided into "innate" and "adaptive." While the former has always been regarded as the first, rapid, and antigen-nonspecific reaction to invading pathogens, the latter represents the more sophisticated and antigen-specific response that has the potential to persist and generate memory. Recent work however has challenged this dogma, where murine studies have successfully demonstrated the ability of innate immune cells (monocytes and macrophages) to acquire antigen-specific memory to allogeneic major histocompatibility complex (MHC) molecules. The immunoreceptors so far identified that mediate innate immune memory are the paired immunoglobulin-like receptors (PIRs) in mice, which are orthologous to human leukocyte immunoglobulin-like receptors (LILRs). These receptor families are mainly expressed by the myelomonocytic cell lineage, suggesting an important role in the innate immune response. In this review, we will discuss the role of immunoglobulin-like receptors in the development of innate immune memory across species.

Allorecognition and the spectrum of kidney transplant rejection

Detection of mismatched human leukocyte antigens by adaptive immune cells is considered as the main cause of transplant rejection, leading to either T-cell mediated rejection or antibody-mediated rejection. This canonical view guided the successful development of immunosuppressive therapies and shaped the diagnostic Banff classification for kidney transplant rejection that is used in clinics worldwide. However, several observations have recently emerged that question this dichotomization between T-cell mediated rejection and antibody-mediated rejection, related to heterogeneity in the serology, histology, and prognosis of the rejection phenotypes. In parallel, novel insights were obtained concerning the dynamics of donor-specific anti-human leukocyte antigen antibodies, the immunogenicity of donor-recipient non-human leukocyte antigen mismatches, and the autoreactivity against self-antigens. Moreover, the potential of innate allorecognition was uncovered, as exemplified by natural killer cell-mediated microvascular inflammation through missing self, and by the emerging evidence on monocyte-driven allorecognition. In this review, we highlight the gaps in the current classification of rejection, provide an overview of the expanding insights into the mechanisms of allorecognition, and critically appraise how these could improve our understanding and clinical approach to kidney transplant rejection. We argue that consideration of the complex interplay of various allorecognition mechanisms can foster a more integrated view of kidney transplant rejection and can lead to improved risk stratification, targeted therapies, and better outcome after kidney transplantation.

Epithelial cells remove precancerous cells by cell competition via MHC class I-LILRB3 interaction

Epithelial cells have an ability termed 'cell competition', which is an immune surveillance-like function that extrudes precancerous cells from the epithelial layer, leading to apoptosis and clearance. However, it remains unclear how epithelial cells recognize and extrude transformed cells. Here, we discovered that a PirB family protein, leukocyte immunoglobulin-like receptor B3 (LILRB3), which is expressed on non-transformed epithelial cells, recognizes major histocompatibility complex class I (MHC class I) that is highly expressed on transformed cells. MHC class I interaction with LILRB3 expressed on normal epithelial cells triggers an SHP2-ROCK2 pathway that generates a mechanical force to extrude transformed cells. Removal of transformed cells occurs independently of natural killer (NK) cell or CD8⁺ cytotoxic T cell-mediated activity. This is a new mechanism in that the immunological ligand-receptor system generates a mechanical force in non-immune epithelial cells to extrude precancerous cells in the same epithelial layer.

Enhancing motor learning by increasing the stability of newly formed dendritic spines in the motor cortex

Dendritic spine dynamics are thought to be substrates for motor learning and memory, and altered spine dynamics often lead to impaired performance. Here, we describe an exception to this rule by studying mice lacking paired immunoglobulin receptor B (PirB^-/-). Pyramidal neuron dendrites in PirB^-/- mice have increased spine formation rates and density. Surprisingly, PirB^-/- mice learn a skilled reaching task faster than wild-type (WT) littermates. Furthermore, stabilization of learning-induced spines is elevated in PirB^-/- mice. Mechanistically, single-spine uncaging experiments suggest that PirB is required for NMDA receptor (NMDAR)-dependent spine shrinkage. The degree of survival of newly formed spines correlates with performance, suggesting that increased spine stability is advantageous for learning. Acute inhibition of PirB function in M1 of adult WT mice increases the survival of learning-induced spines and enhances motor learning. These results demonstrate that there are limits on motor learning that can be lifted by manipulating PirB, even in adulthood.

The Activating Receptors of Natural Killer Cells and Their Inter-Switching Potentials

The global incidence of cancer is on the increase and researchers are prospecting for specific and non-selective therapies derived from the immune system. The killer activating receptors of NK cells are known to be involved in immunosurveillance against tumor and virally-infected cells. These receptors belong to two main categories, namely the immunoglobulin like and C-lectin like families. Though they have different signal pathways, all the killer activating receptors have similar effector functions which include direct cytotoxicity and the release of inflammatory cytokines such as IFN-gamma and TNF-alpha. To transduce signals that exceed the activation threshold for cytotoxicity, most of these receptors require synergistic effort. This review profiles 21 receptors: 13 immunoglobulin-like, 5 lectin-like, and 3 others. It critically explores their structural uniqueness, role in disease, respective transduction signal pathways and their status as current and prospective targets for cancer immunotherapy. While the native ligands of most of these receptors are known, much work is required to prospect for specific antibodies, peptides and multi-target small molecules with high binding affinities.

Axon regeneration after optic nerve injury in rats can be improved via PirB knockdown in the retina

BACKGROUND

In the central nervous system (CNS), three types of myelin-associated inhibitors (MAIs) exert major inhibitory effects on nerve regeneration: Nogo-A, myelin-associated glycoprotein (MAG), and oligodendrocyte-myelin glycoprotein (OMgp). MAIs have two co-receptors, Nogo receptor (NgR) and paired immunoglobulin-like receptor B (PirB). Existing studies confirm that inhibiting NgR only exerted a modest disinhibitory effect in CNS. However, the inhibitory effects of PirB on nerve regeneration after binding to MAIs are controversial too. We aimed to further investigate the effect of PirB knockdown on the neuroprotection and axonal regeneration of retinal ganglion cells (RGCs) after optic nerve injury in rats.

METHODS

The differential expression of PirB in the retina was observed via immunofluorescence and western blotting after 1, 3, and 7 days of optic nerve injury (ONI). The retina was locally transfected with adeno-associated virus (AAV) PirB shRNA, then, the distribution of virus in tissues and cells was observed 21 days after AAV transfection to confirm the efficiency of PirB knockdown. Level of P-Stat3 and expressions of ciliary neurotrophic factor (CNTF) were detected via western blotting. RGCs were directly labeled with cholera toxin subunit B (CTB). The new axons of the optic nerve were specifically labeled with growth associated protein-43 (GAP43) via immunofluorescence. Flash visual evoked potential (FVEP) was used to detect the P1 and N1 latency, as well as N1-P1, P1-N2 amplitude to confirm visual function.

RESULTS

PirB expression in the retina was significantly increased after ONI. PirB knockdown was successful and significantly promoted P-Stat3 level and CNTF expression in the retina. PirB knockdown promoted the regeneration of optic nerve axons and improved the visual function indexes such as N1-P1 and P1-N2 amplitude.

CONCLUSIONS

PirB is one of the key molecules that inhibit the regeneration of the optic nerve, and inhibition of PirB has an excellent effect on promoting nerve regeneration, which allows the use of PirB as a target molecule to promote functional recovery after ONI.

Turning enemies into allies-reprogramming tumor-associated macrophages for cancer therapy

Checkpoint blockade therapies that target inhibitory receptors on T cells have revolutionized clinical oncology. Antibodies targeting CTLA-4 or the PD-1/PD-L1 axis are now successfully used alone or in combination with chemotherapy for numerous tumor types. Despite the clinical success of checkpoint blockade therapies, tumors exploit multiple mechanisms to escape or subvert the anti-tumor T cell response. Within the tumor microenvironment, tumor-associated macrophages (TAM) can suppress T cell responses and facilitate tumor growth in various ways, ultimately debilitating clinical responses to T cell checkpoint inhibitors. There is therefore significant interest in identifying biologicals and drugs that target immunosuppressive TAM within the tumor microenvironment and can be combined with immune checkpoint inhibitors. Here we review approaches that are currently being evaluated to convert immunosuppressive TAM into immunostimulatory macrophages that promote T cell responses and tumor elimination. Tumor-associated macrophages (TAMs) are a major component of the tumor microenvironment that impact anti-tumor immune responses and susceptibility to checkpoint blockade. TAMs are very heterogeneous and can be either immunosuppressive or immunostimulatory. Here, Molgora and Colonna review current strategies that aim to reprogram TAMs to enhance rather than inhibit immune responses.

A screened PirB antagonist peptide antagonizes Aβ42-mediated inhibition of neurite outgrowth in vitro

Alzheimer's disease (AD) is a type of progressive neurodegenerative disease, and amyloid β-protein 42 (Aβ₄₂) serves an important role in the pathological process of development of AD. Paired immunoglobulin-like receptor B (PirB) is a functional receptor for myelin inhibitors of neuron regeneration in the CNS, and it has also been identified to function as a high-affinity receptor for Aβ. Here, we used a phage display to identify a specific PirB antagonist peptide 11(PAP11, PFRLQLS), which could reverse Aβ₄₂-induced neurotoxicity and promote neurite outgrowth in vitro. Immunofluorescence analysis showed that PAP11 colocalized with PirB on the membrane of cortical neurons. Horseradish peroxidase-streptavidin-biotin assay further proved that PAP11 directly binds to PirB and the dissociation constant (K_d) was 0.128μM. PAP11 functionally antagonized the neurite outgrowth inhibitory effect induced by Aβ₄₂ in cortical neurons, and the underlying mechanism was associated with a PirB-ROCK2/CRMP2 signaling pathway. The novel PirB antagonist peptide PAP11 may be a promising candidate therapeutic agent for the treatment of AD and other neurodegenerative diseases. KEY POINTS: • PAP11 was the first PirB antagonist peptide screened by phage display technology. • PAP11 could protect neurons by blocking the binding of Aβ₄₂ and PirB. • PAP11 reverse inhibitory effect of neurite outgrowth through ROCK2/CRMP2 pathway.

Innate immunity: Trained immunity and innate allorecognition against the allograft

The immune system response of transplant recipients is the main cause of allograft rejection; therefore, its suppression seems crucial. Nevertheless, immunosuppressive agents are largely ineffective against innate immune response. Innate immunity is immediately activated after transplantation and contribute to allograft inflammation and rejection. In this regard, understanding the mechanism of activation and targeting the components of innate immunity could improve allograft survival time. In this review, we discuss two scenarios in the innate immunity, i.e., danger and allogeneic signals in the context of both allogeneic and syngeneic graft. Moreover, the mechanisms of innate allorecognition (i.e., signal regulatory protein α-CD47 and paired immunoglobulin-like receptors-MHC I axis) are described, which can improve our clinical decisions to use a better therapeutic strategy.

Innate allorecognition in transplantation

Successful allogeneic transplantation has been made possible by suppressing activation of the adaptive immune system. Current immunosuppressive therapy prevents rejection by targeting T and B cells. Despite this effective treatment, it is the innate immune system, which includes dendritic cells, monocytes, natural killer cells, that is responsible for the initiation of the adaptive immune response. Recent work has described that the innate immune system is capable of recognizing allogeneic nonself and some of the mechanisms of innate allorecognition have been uncovered. Better understanding of the role of the innate immune system in initiation and maintenance of the allo-immune response has potential to lead to better treatment strategies for transplant patients, prolonging allograft survival. Here, we review advances in our understanding of innate allorecognition in transplantation.

Leukocyte immunoglobulin-like receptor subfamily B: therapeutic targets in cancer

Inhibitory leukocyte immunoglobulin-like receptors (LILRBs 1–5) transduce signals via intracellular immunoreceptor tyrosine-based inhibitory motifs that recruit phosphatases to negatively regulate immune activation. The activation of LILRB signaling in immune cells may contribute to immune evasion. In addition, the expression and signaling of LILRBs in cancer cells especially in certain hematologic malignant cells directly support cancer development. Certain LILRBs thus have dual roles in cancer biology—as immune checkpoint molecules and tumor-supporting factors. Here, we review the expression, ligands, signaling, and functions of LILRBs, as well as therapeutic development targeting them. LILRBs may represent attractive targets for cancer treatment, and antagonizing LILRB signaling may prove to be effective anti-cancer strategies.

Innate Allorecognition and Memory in Transplantation

Over the past few decades, we have witnessed a decline in the rates of acute rejection without significant improvement in chronic rejection. Current treatment strategies principally target the adaptive immune response and not the innate response. Therefore, better understanding of innate immunity in transplantation and how to target it is highly desirable. Here, we review the latest advances in innate immunity in transplantation focusing on the roles and mechanisms of innate allorecognition and memory in myeloid cells. These novel concepts could explain why alloimmune response do not abate over time and shed light on new molecular pathways that can be interrupted to prevent or treat chronic rejection.

Paired Immunoglobulin-like Receptor B Inhibition in Müller Cells Promotes Neurite Regeneration After Retinal Ganglion Cell Injury in vitro

In the central nervous system (CNS), three types of myelin-associated inhibitors (MAIs) have major inhibitory effects on nerve regeneration. They include Nogo-A, myelin-associated glycoprotein, and oligodendrocyte-myelin glycoprotein. MAIs possess two co-receptors, Nogo receptor (NgR) and paired immunoglobulin-like receptor B (PirB). Previous studies have confirmed that the inhibition of NgR only results in a modest increase in regeneration in the CNS; however, the inhibitory effects of PirB with regard to nerve regeneration after binding to MAIs remain controversial. In this study, we demonstrated that PirB is expressed in primary cultures of retinal ganglion cells (RGCs), and the inhibitory effects of the three MAIs on the growth of RGC neurites are not significantly decreased after direct PirB knockdown using adenovirus PirB shRNA. Interestingly, we found that retinal Müller cells expressed PirB and that its knockdown enhanced the regeneration of co-cultured RGC neurites. PirB knockdown also activated the JAK/Stat3 signaling pathway in Müller cells and upregulated ciliary neurotrophic factor levels. These findings indicate that PirB plays a novel role in retinal Müller cells and that its action in these cells may indirectly affect the growth of RGC neurites. The results also reveal that PirB in Müller cells affects RGC neurite regeneration. Our findings provide a novel basis for the use of PirB as a target molecule to promote nerve regeneration.

Assessing the Expression of Major Histocompatibility Complex Class I on Primary Murine Hippocampal Neurons by Flow Cytometry

Increasing evidence supports the hypothesis that neuro-immune interactions impact nervous system function in both homeostatic and pathologic conditions. A well-studied function of major histocompatibility complex class I (MHCI) is the presentation of cell-derived peptides to the adaptive immune system, particularly in response to infection. More recently it has been shown that the expression of MHCI molecules on neurons can modulate activity-dependent changes in the synaptic connectivity during normal development and neurologic disorders. The importance of these functions to the brain health supports the need for a sensitive assay that readily detects MHCI expression on neurons. Here we describe a method for primary culture of murine hippocampal neurons and then assessment of MHCI expression by flow cytometric analysis. Murine hippocampus is microdissected from prenatal mouse pups at the embryonic day 18. Tissue is dissociated into a single cell suspension using enzymatic and mechanical techniques, then cultured in a serum-free media that limits growth of non-neuronal cells. After 7 days in vitro, MHCI expression is stimulated by treating cultured cells pharmacologically with beta interferon. MHCI molecules are labeled in situ with a fluorescently tagged antibody, then cells are non-enzymatically dissociated into a single cell suspension. To confirm the neuronal identity, cells are fixed with paraformaldehyde, permeabilized, and labeled with a fluorescently tagged antibody that recognizes neuronal nuclear antigen NeuN. MHCI expression is then quantified on neurons by flow cytometric analysis. Neuronal cultures can easily be manipulated by either genetic modifications or pharmacologic interventions to test specific hypotheses. With slight modifications, these methods can be used to culture other neuronal populations or to assess expression of other proteins of interest.

PIRs mediate innate myeloid cell memory to nonself MHC molecules

Immunological memory specific to previously encountered antigens is a cardinal feature of adaptive lymphoid cells. However, it is unknown whether innate myeloid cells retain memory of prior antigenic stimulation and respond to it more vigorously on subsequent encounters. In this work, we show that murine monocytes and macrophages acquire memory specific to major histocompatibility complex I (MHC-I) antigens, and we identify A-type paired immunoglobulin-like receptors (PIR-As) as the MHC-I receptors necessary for the memory response. We demonstrate that deleting PIR-A in the recipient or blocking PIR-A binding to donor MHC-I molecules blocks memory and attenuates kidney and heart allograft rejection. Thus, innate myeloid cells acquire alloantigen-specific memory that can be targeted to improve transplant outcomes.

Nogo receptor antagonist LOTUS exerts suppression on axonal growth-inhibiting receptor PIR-B

Damaged axons in the adult mammalian central nervous system have a restricted regenerative capacity mainly because of Nogo protein, which is a major myelin-associated axonal growth inhibitor with binding to both receptors of Nogo receptor-1 (NgR1) and paired immunoglobulin-like receptor (PIR)-B. Lateral olfactory tract usher substance (LOTUS) exerts complete suppression of NgR1-mediated axonal growth inhibition by antagonizing NgR1. However, the regulation of PIR-B functions in neurons remains unknown. In this study, protein-protein interactions analyses found that LOTUS binds to PIR-B and abolishes Nogo-binding to PIR-B completely. Reverse transcription-polymerase chain reaction and immunocytochemistry revealed that PIR-B is expressed in dorsal root ganglions (DRGs) from wild-type and Ngr1-deficient mice (male and female). In these DRG neurons, Nogo induced growth cone collapse and neurite outgrowth inhibition, but treatment with the soluble form of LOTUS completely suppressed them. Moreover, Nogo-induced growth cone collapse and neurite outgrowth inhibition in Ngr1-deficient DRG neurons were neutralized by PIR-B function-blocking antibodies, indicating that these Nogo-induced phenomena were mediated by PIR-B. Our data show that LOTUS negatively regulates a PIR-B function. LOTUS thus exerts an antagonistic action on both receptors of NgR1 and PIR-B. This may lead to an improvement in the defective regeneration of axons following injury.

Myeloid immunosuppression and immune checkpoints in the tumor microenvironment

Tumor-promoting inflammation and the avoidance of immune destruction are hallmarks of cancer. While innate immune cells, such as neutrophils, monocytes, and macrophages, are critical mediators for sterile and nonsterile inflammation, persistent inflammation, such as that which occurs in cancer, is known to disturb normal myelopoiesis. This disturbance leads to the generation of immunosuppressive myeloid cells, such as myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs). Due to their potent suppressive activities against effector lymphocytes and their abundance in the tumor microenvironment, immunosuppressive myeloid cells act as a major barrier to cancer immunotherapy. Indeed, various therapeutic approaches directed toward immunosuppressive myeloid cells are actively being tested in preclinical and clinical studies. These include anti-inflammatory agents, therapeutic blockade of the mobilization and survival of myeloid cells, and immunostimulatory adjuvants. More recently, immune checkpoint molecules expressed on tumor-infiltrating myeloid cells have emerged as potential therapeutic targets to redirect these cells to eliminate tumor cells. In this review, we discuss the complex crosstalk between cancer-related inflammation and immunosuppressive myeloid cells and possible therapeutic strategies to harness antitumor immune responses.

Role of the Immune System in the Development of the Central Nervous System

The central nervous system (CNS) and the immune system are both intricate and highly organized systems that regulate the entire body, with both sharing certain common features in developmental mechanisms and operational modes. It is known that innate immunity-related molecules, such as cytokines, toll-like receptors, the complement family, and acquired immunity-related molecules, such as the major histocompatibility complex and antibody receptors, are also expressed in the brain and play important roles in brain development. Moreover, although the brain has previously been regarded as an immune-privileged site, it is known to contain lymphatic vessels. Not only microglia but also lymphocytes regulate cognition and play a vital role in the formation of neuronal circuits. This review provides an overview of the function of immune cells and immune molecules in the CNS, with particular emphasis on their effect on neural developmental processes.

Structure and flexibility of the extracellular region of the PirB receptor

Murine paired immunoglobulin receptor B (PirB) and its human ortholog leukocyte immunoglobulin-like receptor B2 (LILRB2) are widely expressed inhibitory receptors that interact with a diverse set of extracellular ligands and exert functions ranging from down-regulation of immune responses to inhibition of neuronal growth. However, structural information that could shed light on how PirB interacts with its ligands is lacking. Here, we report crystal structures of the PirB ectodomain; the first full ectodomain structure for a LILR family member, at 3.3-4.5 Å resolution. The structures reveal that PirB's six Ig-like domains are arranged at acute angles, similar to the structures of leukocyte immunoglobulin-like receptor (LILR) and killer-cell immunoglobulin-like receptor (KIR). We observe that this regular arrangement is followed throughout the ectodomain, resulting in an extended zigzag conformation. In two out of the five structures reported here, the repeating zigzag is broken by the first domain that can adopt two alternative orientations. Quantitative binding experiments revealed a 9 μm dissociation constant for PirB-myelin-associated glycoprotein (MAG) ectodomain interactions. Taken together, these structural findings and the observed PirB-MAG interactions are compatible with a model for intercellular signaling in which the PirB extracellular domains, which point away from the cell surface, enable interaction with ligands in trans.

Immunoglobulin-Like Receptors and Their Impact on Wiring of Brain Synapses

Synapse formation is mediated by a surprisingly large number and wide variety of genes encoding many different protein classes. One of the families increasingly implicated in synapse wiring is the immunoglobulin superfamily (IgSF). IgSF molecules are by definition any protein containing at least one Ig-like domain, making this family one of the most common protein classes encoded by the genome. Here, we review the emerging roles for IgSF molecules in synapse formation specifically in the vertebrate brain, focusing on examples from three classes of IgSF members: ( a) cell adhesion molecules, ( b) signaling molecules, and ( c) immune molecules expressed in the brain. The critical roles for IgSF members in regulating synapse formation may explain their extensive involvement in neuropsychiatric and neurodevelopmental disorders. Solving the IgSF code for synapse formation may reveal multiple new targets for rescuing IgSF-mediated deficits in synapse formation and, eventually, new treatments for psychiatric disorders caused by altered IgSF-induced synapse wiring.

Blocking immunoinhibitory receptor LILRB2 reprograms tumor-associated myeloid cells and promotes antitumor immunity

Tumor-associated myeloid cells maintain immunosuppressive microenvironments within tumors. Identification of myeloid-specific receptors to modulate tumor-associated macrophage and myeloid-derived suppressor cell (MDSC) functions remains challenging. The leukocyte immunoglobulin-like receptor B (LILRB) family members are negative regulators of myeloid cell activation. We investigated how LILRB targeting could modulate tumor-associated myeloid cell function. LILRB2 antagonism inhibited receptor-mediated activation of SHP1/2 and enhanced proinflammatory responses. LILRB2 antagonism also inhibited AKT and STAT6 activation in the presence of M-CSF and IL-4. Transcriptome analysis revealed that LILRB2 antagonism altered genes involved in cell cytoskeleton remodeling, lipid/cholesterol metabolism, and endosomal sorting pathways, as well as changed differentiation gene networks associated with inflammatory myeloid cells as opposed to their alternatively activated phenotype. LILRB2 blockade effectively suppressed granulocytic MDSC and Treg infiltration and significantly promoted in vivo antitumor effects of T cell immune checkpoint inhibitors. Furthermore, LILRB2 blockade polarized tumor-infiltrating myeloid cells from non-small cell lung carcinoma tumor tissues toward an inflammatory phenotype. Our studies suggest that LILRB2 can potentially act as a myeloid immune checkpoint by reprogramming tumor-associated myeloid cells and provoking antitumor immunity.

Ligand Recognition Determines the Role of Inhibitory B Cell Co-receptors in the Regulation of B Cell Homeostasis and Autoimmunity

B cells express various inhibitory co-receptors including CD22, CD72, and Siglec-G. These receptors contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in the cytoplasmic region. Although many of the inhibitory co-receptors negatively regulate BCR signaling by activating SH2-containing protein tyrosine phosphatase 1 (SHP-1), different inhibitory co-receptors have distinct functional properties. CD22, Siglec-G, and CD72 preferentially regulate tonic signaling in conventional B cells, B-1 cell homeostasis, and development of lupus-like disease, respectively. CD72 recognizes RNA-related lupus self-antigen Sm/RNP as a ligand. This ligand recognition recruits CD72 to BCR in Sm/RNP-reactive B cells thereby suppressing production of anti-Sm/RNP autoantibody involved in the pathogenesis of lupus. In contrast, Siglec-G recognizes α2,3 as well as α2,6 sialic acids whereas CD22 recognizes α2,6 sialic acid alone. Because glycoproteins including BCR are dominantly glycosylated with α2,3 sialic acids in B-1 cells, Siglec-G but not CD22 recruits BCR as a ligand specifically in B-1 cells, and regulates B-1 cell homeostasis by suppressing BCR signaling in B-1 cells. Thus, recognition of distinct ligands determines functional properties of different inhibitory B cell co-receptors.

Leukocyte Immunoglobulin-Like Receptors A2 and A6 are Expressed in Avian Macrophages and Modulate Cytokine Production by Activating Multiple Signaling Pathways

The activating leukocyte immunoglobulin-like receptors (LILRAs) play an important role in innate immunity. However, most of the LILRA members have not been characterized in avian species including chickens. The present study is the first attempt at cloning, structural analysis and functional characterization of two LILRAs (LILRA2 and LILRA6) in chickens. Multiple sequence alignments and construction of a phylogenetic tree of chicken LILRA2 and LILRA6 with mammalian proteins revealed high conservation between chicken LILRA2 and LILRA6 and a close relationship between the chicken and mammalian proteins. The mRNA expression of LILRA2 and LILRA6 was high in chicken HD11 macrophages and the small intestine compared to that in several other tissues and cells tested. To examine the function of LILRA2 and LILRA6 in chicken immunity, LILRA2 and LILRA6 were transfected into HD11 cells. Our findings indicated that LILRA2 and LILRA6 are associated with the phosphorylation of Src kinases and SHP2, which play a regulatory role in immune functions. Moreover, LILRA6 associated with and activated MHC class I, β2-microglobulin and induced the expression of transporters associated with antigen processing but LILRA2 did not. Furthermore, both LILRA2 and LILRA6 activated JAK-STAT, NF-κB, PI3K/AKT and ERK1/2 MAPK signaling pathways and induced Th1-, Th2- and Th17-type cytokines and Toll-like receptors. Collectively, this study indicates that LILRA2 and LILRA6 are essential for macrophage-mediated immune responses and they have the potential to complement the innate and adaptive immune system against pathogens.

Interleukin-6 is a key factor for immunoglobulin-like transcript-4-mediated immune injury in sepsis

Background

ILT4⁺ monocytes seem to be associated with poor prognosis of sepsis in humans, but the exact mechanisms are unknown. This study aimed to examine the biological behaviors and effects of immunoglobulin-like transcript-4 (ILT4) levels on monocytes during sepsis and on the prognosis of sepsis.

Methods

ILT4^+/+ (WT) and ILT4-knockout (ILT4^-/-) male BALB/c mice were used for sepsis modeling using cecal ligation puncture (CLP). Flow cytometry was used to measure the levels of ILT4 and major histocompatibility complex class II (MHC-II) on peripheral blood monocytes 24 h after CLP. ELISA was used to measure the serum levels of tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1β, IL-6, and IL-12 at 0, 6, 12, and 24 h after CLP. Survival and prognosis were monitored over the course of 168 h.

Results

ILT4 was highly expressed in peripheral blood monocytes of septic mice 24 h after CLP (1292.00 ± 143.70 vs. 193.50 ± 52.54, p < 0.05). MHC-II levels on peripheral blood monocytes in ILT4^-/- mice were significantly higher than those in WT mice (49.38 ± 5.66% vs. 24.25 ± 6.76%, p < 0.05). Serum IL-6 was significantly elevated 24 h after CLP (470.75 ± 88.03 vs. 54.25 ± 20.04, p < 0.05). The serum IL-6 levels were significantly lower in ILT4^-/- mice compared with those in WT mice after CLP (241.25 ± 45.10 vs. 470.75 ± 88.03, p < 0.05), but TNF-α, IL-1β, and IL-12 were not changed. The survival of ILT4^-/- mice was significantly better after CLP compared with that of WT mice.

Conclusions

High levels of ILT4 on monocytes were observed in peripheral blood during sepsis and found to be associated with high serum IL-6 levels and low MHC-II levels on monocytes, possibly associated with higher mortality. ILT-4-IL-6-MHC-II could be a potential signaling pathway involved in sepsis.

Neuronal PirB Upregulated in Cerebral Ischemia Acts as an Attractive Theranostic Target for Ischemic Stroke

BACKGROUND

Ischemic stroke is a complex disease with multiple etiologies and clinical manifestations. Paired immunoglobulin-like receptor B (PirB), which is originally thought to function exclusively in the immune system, is now also known to be expressed by neurons. A growing number of studies indicate that PirB can inhibit neurite outgrowth and restrict neuronal plasticity. The aim of the study is to investigate whether PirB can be an attractive theranostic target for ischemic stroke.

METHODS AND RESULTS

First, we investigated the spatial-temporal expression of PirB in multiple ischemic stroke models, including transient middle cerebral artery occlusion, photothrombotic cerebral cortex ischemia, and the neuronal oxygen glucose deprivation model. Then, anti-PirB immunoliposome nanoprobe was developed by thin-film hydration method and investigated its specific targeting in vitro and in vivo. Finally, soluble PirB ectodomain (sPirB) protein delivered by polyethylene glycol-modified nanoliposome was used as a therapeutic reagent for ischemic stroke by blocking PirB binding to its endogenous ligands. These results showed that PirB was significantly upregulated after cerebral ischemic injury in ischemic stroke models. Anti-PirB immunoliposome nanoprobe was successfully developed and specifically bound to PirB in vitro. There was accumulation of anti-PirB immunoliposome nanoprobe in the ischemic hemisphere in vivo. Soluble PirB ectodomains remarkably improved ischemic stroke model recovery by liposomal delivery system.

CONCLUSIONS

These data indicated that PirB was a significant element in the pathological process of cerebral ischemia. Therefore, PirB may act as a novel theranostic target for ischemic stroke.

Regulation of immune and neural function via leukocyte Ig-like receptors

Leukocyte Ig-like receptors (LILRs)/Ig-like transcripts (ILTs) are expressed on innate and adaptive immune cells and maintain immune homeostasis. LILRs consist of activating and inhibitory-type receptors that regulate adequate cellular functions. LILRs were firstly identified as MHC class I receptors, therefore expression and/or polymorphisms of LILRs are reported to associate with autoimmune disorders and transplant rejection; however, recent accumulating evidences have revealed that LILRs recognize with diverse ligands including bacteria and virus. In addition, inhibitory LILRB2 (ILT4) and murine relative paired Ig-like receptor (PIR)-B are expressed on neuron and is involved in the dysregulation of central nervous system via interaction with neuronal ligands including amyloid β-protein. In this review, we summarize recent discoveries on the functions of inhibitory MHC class I receptors, and discuss their regulatory roles in immune responses and neural functions.

Autoimmune arthritis induces paired immunoglobulin-like receptor B expression on CD4+ T cells from SKG mice

The chronic, destructive autoimmune arthritis in SKG mice, which closely resembles human rheumatoid arthritis, is the result of self-reactive T cells escaping thymic deletion. Since the inhibitory receptor LIR-1 is up-regulated on auto-reactive T cells in human rheumatoid arthritis, the role of its murine ortholog PIR-B was investigated. Peripheral CD4⁺ T cells from SKG mice were found to frequently express PIR-B, and this population produces more frequently IL-17 upon in vitro stimulation compared to PIR-B^- cells. A much larger fraction of PIR-B⁺ T cells, however, was found to secret no IL-17, but IFN-γ. With regards to the clinical course of the disease, high frequencies of PIR-B⁺ CD4⁺ T cells were found to be associated with a milder course of arthritis, suggesting that the net effect of PIR-B expression is suppression of autoreactive T cells. Our results indicate that overexpression of PIR-B on IL-17-producing SKG CD4⁺ T cells might represent an effective counter-regulatory mechanism against the destructive potential of those cells. More importantly, a major population of PIR-B⁺ T cells in SKG mice appears to play an inhibitory role by way of their IFN-γ production, since high frequencies of those cells ameliorate the disease.

LILRB receptor-mediated regulation of myeloid cell maturation and function

The leukocyte immunoglobulin-like receptor (LILR) family comprises a set of paired immunomodulatory receptors expressed among human myeloid and lymphocyte cell populations. While six members of LILR subfamily A (LILRA) associate with membrane adaptors to signal via immunoreceptor tyrosine-based activating motifs (ITAM), LILR subfamily B (LILRB) members signal via multiple cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIM). Ligand specificity of some LILR family members has been studied in detail, but new perspective into the immunoregulatory aspects of this receptor family in human myeloid cells has been limited. LILRB receptors and the murine ortholog, paired immunoglobulin-like receptor B (PIRB), have been shown to negatively regulate maturation pathways in myeloid cells including mast cells, neutrophils, dendritic cells, as well as B cells. Our laboratory further demonstrated in mouse models that PIRB regulated functional development of myeloid-derived suppressor cell and the formation of a tumor-permissive microenvironment. Based on observations from the literature and our own studies, our laboratory is focusing on how LILRs modulate immune homeostasis of human myeloid cells and how these pathways may be targeted in disease states. Integrity of this pathway in tumor microenvironments, for example, permits a myeloid phenotype that suppresses antitumor adaptive immunity. This review presents the evidence supporting a role of LILRs as myeloid cell regulators and ongoing efforts to understand the functional immunology surrounding this family.

Inhibition of PirB Activity by TAT-PEP Improves Mouse Motor Ability and Cognitive Behavior

Paired immunoglobulin-like receptor B (PirB), a functional receptor for myelin-associated inhibitory proteins, plays an important role in axon regeneration in injured brains. However, its role in normal brain function with age has not been previously investigated. Therefore in this study, we examined the expression level of PirB in the cerebral cortex, hippocampus and cerebellum of mice at 1 month, 3 months and 18 months of age. The results showed that the expression of PirB increased with age. We further demonstrated that overexpression of PirB inhibited neurite outgrowth in PC12 cells, and this inhibitory activity of PirB could be reversed by TAT-PEP, which is a recombinant soluble PirB ectodomain fused with TAT domain for blood-brain barrier penetration. In vivo study, intraperitoneal administration of TAT-PEP was capable of enhancing motor capacity and spatial learning and memory in mice, which appeared to be mediated through regulation of brain-derived neurotrophic factor (BDNF) secretion. Our study suggests that PirB is associated with aging and TAT-PEP may be a promising therapeutic agent for modulation of age-related motor and cognitive dysfunctions.

PirB regulates asymmetries in hippocampal circuitry

Left-right asymmetry is a fundamental feature of higher-order brain structure; however, the molecular basis of brain asymmetry remains unclear. We recently identified structural and functional asymmetries in mouse hippocampal circuitry that result from the asymmetrical distribution of two distinct populations of pyramidal cell synapses that differ in the density of the NMDA receptor subunit GluRε2 (also known as NR2B, GRIN2B or GluN2B). By examining the synaptic distribution of ε2 subunits, we previously found that β2-microglobulin-deficient mice, which lack cell surface expression of the vast majority of major histocompatibility complex class I (MHCI) proteins, do not exhibit circuit asymmetry. In the present study, we conducted electrophysiological and anatomical analyses on the hippocampal circuitry of mice with a knockout of the paired immunoglobulin-like receptor B (PirB), an MHCI receptor. As in β2-microglobulin-deficient mice, the PirB-deficient hippocampus lacked circuit asymmetries. This finding that MHCI loss-of-function mice and PirB knockout mice have identical phenotypes suggests that MHCI signals that produce hippocampal asymmetries are transduced through PirB. Our results provide evidence for a critical role of the MHCI/PirB signaling system in the generation of asymmetries in hippocampal circuitry.