Human plasma C3 is essential for the development of memory B, but not T, lymphocytes

Intracellular complement and immunometabolism: The advantages of compartmentalization

The complement system is a proteolytic cascade triggered by pathogen and danger-associated molecular patterns, with resultant outcomes of inflammation, cellular activation, and opsonization of material for removal by phagocytosis. While first discovered as an activity in serum, it is now recognized that complement components play important roles at local and individual cell-intrinsic levels. In particular, apart from the extracellular serum activities of complement, it is now believed that complement also acts intracellularly, as part of a cellular signal transduction cascade that can stimulate cellular survival and activation, and individual immune cell phenotypes, via effects on cellular metabolism. This review will describe what is currently known about how complement functions in intracellular signal transduction, and outline the functional advantages of a compartmentalized and intracellular complement system.

Complosome - the intracellular complement system

The complement system is a recognized pillar of host defence against infection and noxious self-derived antigens. Complement is traditionally known as a serum-effective system, whereby the liver expresses and secretes most complement components, which participate in the detection of bloodborne pathogens and drive an inflammatory reaction to safely remove the microbial or antigenic threat. However, perturbations in normal complement function can cause severe disease and, for reasons that are currently not fully understood, the kidney is particularly vulnerable to dysregulated complement activity. Novel insights into complement biology have identified cell-autonomous and intracellularly active complement - the complosome - as an unexpected central orchestrator of normal cell physiology. For example, the complosome controls mitochondrial activity, glycolysis, oxidative phosphorylation, cell survival and gene regulation in innate and adaptive immune cells, and in non-immune cells, such as fibroblasts and endothelial and epithelial cells. These unanticipated complosome contributions to basic cell physiological pathways make it a novel and central player in the control of cell homeostasis and effector responses. This discovery, together with the realization that an increasing number of human diseases involve complement perturbations, has renewed interest in the complement system and its therapeutic targeting. Here, we summarize the current knowledge about the complosome across healthy cells and tissues, highlight contributions from dysregulated complosome activities to human disease and discuss potential therapeutic implications.

The role of the complosome in health and disease

The complement system is one of the immune system's oldest defense mechanisms and is historically regarded as a liver-derived and serum-active innate immune system that 'complements' cell-mediated and antibody-mediated immune responses against pathogens. However, the complement system is now recognized as a central component of both innate and adaptive immunity at both the systemic and local tissue levels. More findings have uncovered novel activities of an intracellularly active complement system-the complosome-that have shifted established functional paradigms in the field. The complosome has been shown to play a critical function in regulating T cell responses, cell physiology (such as metabolism), inflammatory disease processes, and cancer, which has amply proved its immense research potential and informed us that there is still much to learn about this system. Here, we summarize current understanding and discuss the emerging roles of the complosome in health and disease.

T-cell receptor signaling in Schimke immuno-osseous dysplasia is SMARCAL1-independent

Schimke immuno-osseous dysplasia (SIOD) caused by mutations in SMARCAL1 is an ultra-rare disease characterized by specific facial features, skeletal dysplasia, and steroid-resistant nephrotic syndrome, which often leads to kidney failure and requires transplantation. Cellular (T-cell) deficiency, lymphopenia, and infections have been frequently reported, but whether they are due to T-cell-intrinsic defects in T-cell receptor (TCR) signaling associated with SMARCAL1 deficiency or to T-cell-extrinsic effects such as the impaired proliferation of hematopoietic precursors or T-cell-specific immunosuppression after renal transplantation remains unknown. We have explored the effects of SMARCAL1 deficiency on T-cell receptor signaling in primary and immortalized T cells from a 9-year-old SIOD patient under immunosuppression treatment when compared to healthy donors. Immortalized T cells recapitulated the SMARCAL1 deficiency of the patient, as judged by their impaired response to gamma irradiation. The results indicated that TCR-mediated signaling was normal in SIOD-derived immortalized T cells but strongly impaired in the primary T cells of the patient, although rescued with TCR-independent stimuli such as PMA + ionomycin, suggesting that SIOD-associated T-cell signaling is not intrinsically defective but rather the result of the impaired proliferation of hematopoietic precursors or of T-cell-specific immunosuppression. The lack of early thymic emigrants in our patients may support the former hypothesis.

Targeting the tissue-complosome for curbing inflammatory disease

Hyperactivated local tissue is a cardinal feature of immune-mediated inflammatory diseases of various organs such as the joints, the gut, the skin, or the lungs. Tissue-resident structural and stromal cells, which get primed during repeated or long-lasting bouts of inflammation form the basis of this sensitization of the tissue. During priming, cells change their metabolism to make them fit for the heightened energy demands that occur during persistent inflammation. Epigenetic changes and, curiously, an activation of intracellularly expressed parts of the complement system drive this metabolic invigoration and enable tissue-resident cells and infiltrating immune cells to employ an arsenal of inflammatory functions, including activation of inflammasomes. Here we provide a current overview on complement activation and inflammatory transformation in tissue-occupying cells, focusing on fibroblasts during arthritis, and illustrate ways how therapeutics directed at complement C3 could potentially target the complosome to unprime cells in the tissue and induce long-lasting abatement of inflammation.

Prognostic value of immune-related genes and immune cell infiltration analysis in the tumor microenvironment of head and neck squamous cell carcinoma

BACKGROUND

Head and neck squamous cell carcinoma (HNSCC) is one of the few malignant tumors that respond well to immunotherapy. We aimed to investigate the immune-related genes and immune cell infiltration of HNSCC and construct a predictive model for its prognosis.

METHODS

We calculated the stromal/immune scores of patients with HNSCC from The Cancer Genome Atlas using the Estimation of STromal and Immune cells in MAlignant Tumor tissues using Expression data algorithm and investigated the relationship between the scores and patients' prognosis. Three machine learning algorithms (LASSO, Random Forest, and Rbsurv) were performed to screen key immune-related genes and constructed a predictive model. The immune cell infiltrating was calculated by the Tumor Immune Estimation Resource algorithm.

RESULTS

The stromal and immune scores significantly correlated with prognosis. A 6-gene signature was selected and displayed a robust predictive effect. The expressions of key genes were associated with immune infiltrating. GSE65858 validated the results.

CONCLUSION

Our study comprehensively analyzed the tumor microenvironment of HNSCC and constructed a robust predictive model, providing a basis for further investigation of therapy.

Complement's favourite organelle-Mitochondria?

The complement system, well known for its central role in innate immunity, is currently emerging as an unexpected, cell‐autonomous, orchestrator of normal cell physiology. Specifically, an intracellularly active complement system—the complosome—controls key pathways of normal cell metabolism during immune cell homeostasis and effector function. So far, we know little about the exact structure and localization of intracellular complement components within and among cells. A common scheme, however, is that they operate in crosstalk with other intracellular immune sensors, such as inflammasomes, and that they impact on the activity of key subcellular compartments. Among cell compartments, mitochondria appear to have built a particularly early and strong relationship with the complosome and extracellularly active complement—not surprising in view of the strong impact of the complosome on metabolism. In this review, we will hence summarize the current knowledge about the close complosome–mitochondria relationship and also discuss key questions surrounding this novel research area.

Diapedesis-Induced Integrin Signaling via LFA-1 Facilitates Tissue Immunity by Inducing Intrinsic Complement C3 Expression in Immune Cells

Intrinsic complement C3 activity is integral to human T helper type 1 (Th1) and cytotoxic T cell responses. Increased or decreased intracellular C3 results in autoimmunity and infections, respectively. The mechanisms regulating intracellular C3 expression remain undefined. We identified complement, including C3, as among the most significantly enriched biological pathway in tissue-occupying cells. We generated C3-reporter mice and confirmed that C3 expression was a defining feature of tissue-immune cells, including T cells and monocytes, occurred during transendothelial diapedesis, and depended on integrin lymphocyte-function-associated antigen 1 (LFA-1) signals. Immune cells from patients with leukocyte adhesion deficiency type 1 (LAD-1) had reduced C3 transcripts and diminished effector activities, which could be rescued proportionally by intracellular C3 provision. Conversely, increased C3 expression by T cells from arthritis patients correlated with disease severity. Our study defines integrins as key controllers of intracellular complement, demonstrates that perturbations in the LFA-1-C3-axis contribute to primary immunodeficiency, and identifies intracellular C3 as biomarker of severity in autoimmunity.

New insights into the immune functions of complement

The recognition of microbial or danger-associated molecular patterns by complement proteins initiates a cascade of events that culminates in the activation of surface complement receptors on immune cells. Such signalling pathways converge with those activated downstream of pattern recognition receptors to determine the type and magnitude of the immune response. Intensive investigation in the field has uncovered novel pathways that link complement-mediated signalling with homeostatic and pathological T cell responses. More recently, the observation that complement proteins also act in the intracellular space to shape T cell fates has added a new layer of complexity. Here, we consider fundamental mechanisms and novel concepts at the interface of complement biology and immunity and discuss how these affect the maintenance of homeostasis and the development of human pathology.

Activated Hepatic Stellate Cells (HSCs) Exert Immunosuppressive Effects in Hepatocellular Carcinoma by Producing Complement C3

Objective

Hepatic stellate cells (HSCs) are the important players in liver cirrhosis and liver cancer. They also act as critical mediators of immunosuppression in hepatocellular carcinoma (HCC). In this study, we hypothesized that HSCs promote HCC progression via C3.

Methods

C3 in HSCs was knocked down using a shRNA retroviral plasmid. The conditioned medium from HSCs or shC3 HSCs (knockdown of C3 by shRNA in HSCs) was collected to detect their effects on bone marrow (BM) and T cells (including expansion and apoptosis) in vitro, and in an HCC in situ model in mice.

Results

We found that HSCs promoted T-cell apoptosis and decreased their proliferation, inhibited dendritic cell (DC) maturation, and induced myeloid-derived suppressor cell (MDSC) expansion through the C3 pathway in vitro. In addition, the knockdown of C3 suppressed HSC-promoted HCC development in the orthotopic transplantation tumor model of HCC in mice.

Conclusion

These findings provide more insights into the immunomodulatory roles of HSCs in HCC progression and indicate that modulation of the C3 pathway might be a novel therapeutic approach for liver cancer.

The multifaceted role of complement in kidney transplantation

Increasing evidence indicates an integral role for the complement system in the deleterious inflammatory reactions that occur during critical phases of the transplantation process, such as brain or cardiac death of the donor, surgical trauma, organ preservation and ischaemia-reperfusion injury, as well as in humoral and cellular immune responses to the allograft. Ischaemia is the most common cause of complement activation in kidney transplantation and in combination with reperfusion is a major cause of inflammation and graft damage. Complement also has a prominent role in antibody-mediated rejection (ABMR) owing to ABO and HLA incompatibility, which leads to devastating damage to the transplanted kidney. Emerging drugs and treatment modalities that inhibit complement activation at various stages in the complement cascade are being developed to ameliorate the damage caused by complement activation in transplantation. These promising new therapies have various potential applications at different stages in the process of transplantation, including inhibiting the destructive effects of ischaemia and/or reperfusion injury, treating ABMR, inducing accommodation and modulating the adaptive immune response.

Classical and Non-classical Presentations of Complement Factor I Deficiency: Two Contrasting Cases Diagnosed via Genetic and Genomic Methods

Deficiency of complement factor I is a rare immunodeficiency that typically presents with increased susceptibility to encapsulated bacterial infections. However, non-infectious presentations including rheumatological, dermatological and neurological disease are increasingly recognized and require a high-index of suspicion to reach a timely diagnosis. Herein, we present two contrasting cases of complement factor I deficiency: one presenting in childhood with invasive pneumococcal disease, diagnosed using conventional immunoassays and genetics and the second presenting in adolescence with recurrent sterile neuroinflammation, diagnosed via a genomic approach. Our report and review of the literature highlight the wide spectrum of clinical presentations associated with CFI deficiency and the power of genomic medicine to inform rare disease diagnoses.

Interaction of Serum-Derived and Internalized C3 With DNA in Human B Cells-A Potential Involvement in Regulation of Gene Transcription

Beside its classical role as a serum effector system of innate immunity, evidence is accumulating that complement has an intracellular repertoire of components that provides not only immune defense, but also functions to maintain cellular homeostasis. While complement proteins, mainly the central component C3, have been detected in B cells, their exact function and source remain largely unexplored. In this study, we investigated the expression and origin of intracellular C3 in human B cells together with its role in B cell homeostasis. Our data provide evidence that endogenous expression of C3 is very low in human B cells and, in accordance with the recent publication, the main origin of intracellular C3 is the serum. Interestingly, we found that both serum-derived and purified C3 are able to enter the nucleus of viable B cells, suggesting its potential involvement in regulation of gene transcription. ELISA, gel shift assay, confocal microscopy, and chromatin immunoprecipitation proved that C3 and C3a strongly bind to nuclear DNA, and among the interacting genes there are key factors of lymphocyte development and differentiation. The strong interaction of C3 with histone proteins and its potential ability to induce chromatin rearrangement suggest that C3/C3a might regulate DNA transcription via chromatin remodeling. Our data reveal a novel, hitherto undescribed role of C3 in immune cell homeostasis, which further extends the repertoire how complement links innate and adaptive immunity and regulates basic processes of the cells.

Cutaneous Vasculitis and Digital Ischaemia Caused by Heterozygous Gain-of-Function Mutation in C3

It is now increasingly recognized that some monogenic autoinflammatory diseases and immunodeficiencies cause vasculitis, although genetic causes of vasculitis are extremely rare. We describe a child of non-consanguineous parents who presented with cutaneous vasculitis, digital ischaemia and hypocomplementaemia. A heterozygous p.R1042G gain-of-function mutation (GOF) in the complement component C3 gene was identified as the cause, resulting in secondary C3 consumption and complete absence of alternative complement pathway activity, decreased classical complement activity, and low levels of serum C3 with normal C4 levels. The same heterozygous mutation and immunological defects were also identified in another symptomatic sibling and his father. C3 deficiency due GOF C3 mutations is thus now added to the growing list of monogenic causes of vasculitis and should always be considered in vasculitis patients found to have persistently low levels of C3 with normal C4.

Asparaginyl Endopeptidase (Legumain) Supports Human Th1 Induction via Cathepsin L-Mediated Intracellular C3 Activation

Autocrine activation of the complement receptors C3aR and CD46 by complement activation components C3a and C3b produced through C3 cleavage by the protease cathepsin L (CTSL) during T cell stimulation is a requirement for IFN-γ production and Th1 induction in human CD4⁺ T cells. Thus, lack of autocrine CD46 activation, such as in CD46-deficient patients, is associated with defective Th1 responses and recurrent infections. We have identified LGMN [the gene coding for legumain, also known as asparaginyl endopeptidase (AEP)] as one of the key genes induced by CD46 co-stimulation during human CD4⁺ T cell activation. AEP processes and activates a range of proteins, among those α1-thymosin and CTSL, which both drive intrinsically Th1 activity-but has so far not been described to be functionally active in human T cells. Here we found that pharmacological inhibition of AEP during activation of human CD4⁺ T cells reduced CTSL activation and the CTSL-mediated generation of intracellular C3a. This translated into a specific reduction of IFN-γ production without affecting cell proliferation or survival. In line with these findings, CD4⁺ T cells isolated from Lgmn ^-/- mice also displayed a specific defect in IFN-γ secretion and Th1 induction. Furthermore, we did not observe a role for AEP-driven autocrine α1-thymosin activation in T cell-derived IFN-γ production. These data suggest that AEP is an "upstream" activator of the CTSL-C3-IFN-γ axis in human CD4⁺ T cells and hence an important supporter of human Th1 induction.

Complement After Trauma: Suturing Innate and Adaptive Immunity

The overpowering effect of trauma on the immune system is undisputed. Severe trauma is characterized by systemic cytokine generation, activation and dysregulation of systemic inflammatory response complementopathy and coagulopathy, has been immensely instrumental in understanding the underlying mechanisms of the innate immune system during systemic inflammation. The compartmentalized functions of the innate and adaptive immune systems are being gradually recognized as an overlapping, interactive and dynamic system of responsive elements. Nonetheless the current knowledge of the complement cascade and its interaction with adaptive immune response mediators and cells, including T- and B-cells, is limited. In this review, we discuss what is known about the bridging effects of the complement system on the adaptive immune system and which unexplored areas could be crucial in understanding how the complement and adaptive immune systems interact following trauma.

Cutaneous Vasculitis and Recurrent Infection Caused by Deficiency in Complement Factor I

Cutaneous leukocytoclastic vasculitis arises from immune complex deposition and dysregulated complement activation in small blood vessels. There are many causes, including dysregulated host response to infection, drug reactions, and various autoimmune conditions. It is increasingly recognised that some monogenic autoinflammatory diseases cause vasculitis, although genetic causes of vasculitis are extremely rare. We describe a child of consanguineous parents who presented with chronic cutaneous leukocytoclastic vasculitis, recurrent upper respiratory tract infection, and hypocomplementaemia. A homozygous p.His380Arg mutation in the complement factor I (CFI) gene CFI was identified as the cause, resulting in complete absence of alternative complement pathway activity, decreased classical complement activity, and low levels of serum factor I, C3, and factor H. C4 and C2 levels were normal. The same homozygous mutation and immunological defects were also identified in an asymptomatic sibling. CFI deficiency is thus now added to the growing list of monogenic causes of vasculitis and should always be considered in vasculitis patients found to have persistently low levels of C3 with normal C4.

Unexpected Roles for Intracellular Complement in the Regulation of Th1 Responses

The complement system is generally recognized as an evolutionarily ancient and critical part of innate immunity required for the removal of pathogens that have breached the protective host barriers. It was originally defined as a liver-derived serum surveillance system that induces the opsonization and killing of invading microbes and amplifies the general inflammatory reactions. However, studies spanning the last four decades have established complement also as a vital bridge between innate and adaptive immunity. Furthermore, recent work on complement, and in particular its impact on human T helper 1 (Th1) responses, has led to the unexpected findings that the complement system also functions within cells and that it participates in the regulation of basic processes of the cell, including metabolism. These recent new insights into the unanticipated noncanonical activities of this ancient system suggest that the functions of complement extend well beyond mere host protection and into cellular physiology.