Giuseppina Arbore

T helper 1 immunity requires complement-driven NLRP3 inflammasome activity in CD4⁺ T cells.

Innate immune crosstalk in T cells The classical view of immune activation is that innate immune cells, such as macrophages and dendritic cells, recognize invading microbes and then alert adaptive immune cells, such as T cells, to respond. Arbore et al. now show that innate and adaptive immunity converge in human and mouse T cells. Activated T cells express components of the complement cascade, which in turn leads to the assembly of NLRP3 inflammasomes—both critical components of innate immunity that help hosts detect and eliminate microbes. In T cells, complement and inflammasomes work together to push T cells to differentiate into a specialized subset of T cells important for eliminating intracellular bacteria. Science, this issue p. 10.1126/science.aad1210 Complement and NLRP3 inflammasomes work together to promote T helper 1 cell differentiation. INTRODUCTION The inflammasomes and the complement system are traditionally viewed as quintessential components of innate immunity required for the detection and elimination of pathogens. Assembly of the NLRP3 inflammasome in innate immune cells controls the maturation of interleukin (IL)–1β, a proinflammatory cytokine critical to host defense, whereas activation of the liver-derived complement key components C3 and C5 in serum leads to opsonization and removal of microbes and induction of the inflammatory reaction. Recent studies, however, have highlighted an unanticipated direct role for complement C3 also in human T cell immunity: The anaphylatoxin C3a receptor (C3aR) and the complement regulator CD46 (which binds C3b) are critical checkpoints in human T cell lineage commitment, and they control initiation and resolution of T helper 1 (TH1) responses in an autocrine fashion via T cell–derived and intracellularly activated C3. We explored a novel functional cross-talk of complement with the NLRP3 inflammasome within CD4+ T cells and determined how the cooperation between these two “classically” innate systems directly affects interferon-γ (IFN-γ) production by adaptive immune cells. RATIONALE Given the critical role of intracellular C3 activation in human TH1 responses and the importance of C5 activation products in inflammation, we investigated whether human CD4+ T cells also harbor an “intracellular C5 activation system” and by what means this system may contribute to effector responses by using C5aR1 and C5aR2 agonists and antagonists, T cells from patients with cryopyrin-associated periodic syndromes (CAPS), and mouse models of infection and autoimmunity. RESULTS Human CD4+ T cells expressed C5 and generated increased intracellular C5a upon T cell receptor activation and CD46 autocrine costimulation. Subsequent engagement of the intracellular C5aR1 by C5a induced the generation of reactive oxygen species (ROS) and the unexpected assembly of a functional NLRP3 inflammasome in CD4+ T cells, whereas the surface-expressed C5aR2 negatively controlled this process. NLRP3 inflammasome–dependent autocrine IL-1β secretion and activity were required for optimal IFN-γ production by T cells; consequently, dysregulation of NLRP3 function in these cells affected their normal effector responses. For example, mutated, constitutively active NLRP3 in T cells from patients with CAPS induced hyperactive TH1 responses that could be normalized with a NLRP3 inhibitor. The in vivo importance of a T cell–intrinsic NLRP3 inflammasome was further supported by the finding that IFN-γ production by Nlrp3–/– CD4+ T cells was significantly reduced during viral infections in mice and that diminished TH1 induction due to lack of NLRP3 function in a CD4+ T cell transfer model of colitis led to uncontrolled TH17 infiltration and/or expansion in the intestine and aggravated disease. CONCLUSION Our results demonstrate that the regulated cross-talk between intracellularly activated complement components (the “complosome”) and the NLRP3 inflammasome is fundamental to human TH1 induction and regulation. The finding that established innate immune pathways are also operative in adaptive immune cells and orchestrate immunological responses contributes to our understanding of immunobiology and immune system evolution. In addition, the results suggest that the complement-NLRP3 axis in T cells represents a novel therapeutic target for the modulation of TH1 activity in autoimmunity and infection. An intrinsic complement-NLRP3 axis regulates human TH1 responses. T cell receptor activation and CD46 costimulation trigger NLRP3 expression and intracellular C5a generation. Subsequent intracellular C5aR1 engagement induces ROS production (and possibly IL1B gene transcription) and NLRP3 assembly, which in turn mediates IL-1β maturation. Autocrine IL-1β promotes TH1 induction (IFN-γ production) but restricts TH1 contraction (IL-10 coexpression). C5aR2 cell surface activation by secreted C5a negatively controls these events via undefined mechanisms. Dysfunction of this system contributes to impaired TH1 responses in infection or increased TH17 responses during intestinal inflammation. The NLRP3 inflammasome controls interleukin-1β maturation in antigen-presenting cells, but a direct role for NLRP3 in human adaptive immune cells has not been described. We found that the NLRP3 inflammasome assembles in human CD4+ T cells and initiates caspase-1–dependent interleukin-1β secretion, thereby promoting interferon-γ production and T helper 1 (TH1) differentiation in an autocrine fashion. NLRP3 assembly requires intracellular C5 activation and stimulation of C5a receptor 1 (C5aR1), which is negatively regulated by surface-expressed C5aR2. Aberrant NLRP3 activity in T cells affects inflammatory responses in human autoinflammatory disease and in mouse models of inflammation and infection. Our results demonstrate that NLRP3 inflammasome activity is not confined to “innate immune cells” but is an integral component of normal adaptive TH1 responses.

Complement Regulates Nutrient Influx and Metabolic Reprogramming during Th1 Cell Responses

Summary Expansion and acquisition of Th1 cell effector function requires metabolic reprogramming; however, the signals instructing these adaptations remain poorly defined. Here we found that in activated human T cells, autocrine stimulation of the complement receptor CD46, and specifically its intracellular domain CYT-1, was required for induction of the amino acid (AA) transporter LAT1 and enhanced expression of the glucose transporter GLUT1. Furthermore, CD46 activation simultaneously drove expression of LAMTOR5, which mediated assembly of the AA-sensing Ragulator-Rag-mTORC1 complex and increased glycolysis and oxidative phosphorylation (OXPHOS), required for cytokine production. T cells from CD46-deficient patients, characterized by defective Th1 cell induction, failed to upregulate the molecular components of this metabolic program as well as glycolysis and OXPHOS, but IFN-γ production could be reinstated by retrovirus-mediated CD46-CYT-1 expression. These data establish a critical link between the complement system and immunometabolic adaptations driving human CD4+ T cell effector function.

A novel "complement-metabolism-inflammasome axis" as a key regulator of immune cell effector function.

The inflammasomes are intracellular multiprotein complexes that induce and regulate the generation of the key pro‐inflammatory cytokines IL‐1β and IL‐18 in response to infectious microbes and cellular stress. The activation of inflammasomes involves several upstream signals including classic pattern or danger recognition systems such as the TLRs. Recently, however, the activation of complement receptors, such as the anaphylatoxin C3a and C5a receptors and the complement regulator CD46, in conjunction with the sensing of cell metabolic changes, for instance increased amino acid influx and glycolysis (via mTORC1), have emerged as additional critical activators of the inflammasome. This review summarizes recent advances in our knowledge about complement‐mediated inflammasome activation, with a specific focus on a novel “complement – metabolism – NLRP3 inflammasome axis.”

Intracellular complement − the complosome − in immune cell regulation

Abstract The complement system was defined over a century ago based on its ability to “complement” the antibody-mediated and cell-mediated immune responses against pathogens. Today our understanding of this ancient part of innate immunity has changed substantially and we know now that complement plays an undisputed pivotal role in the regulation of both innate and adaptive immunity. The complement system consists of over 50 blood-circulating, cell-surface expressed and intracellular proteins. It is key in the recognition and elimination of invading pathogens, also in the removal of self-derived danger such as apoptotic cells, and it supports innate immune responses and the initiation of the general inflammatory reactions. The long prevailing classic view of complement was that of a serum-operative danger sensor and first line of defence system, however, recent experimental and clinical evidences have demonstrated that “local” tissue and surprisingly intracellular complement (the complosome) activation impacts on normal cell physiology. This review will focus on novel aspects of intracellular complement activation and its unexpected roles in basic cell processes such as metabolism. We also discuss what the existence of the complosome potentially means for how the host handles intracellular pathogens such as viruses.

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

To the Editor: Primary C3 deficiency is an extremely rare autosomalrecessive disease, with fewer than 50 families described worldwide. Plasma and intracellular C3 are considered B-cell receptor (BCR) and T-cell receptor (TCR) costimulators, respectively, but their contribution to lymphocyte biology remains obscure, particularly in humans. Reduced plasma C3 can be caused not only by primary C3 deficiency, due to loss-of-function C3 mutations, but also by secondary C3 deficiency or C3 consumption, due to gain-of-function C3 mutations or due to mutations in C3 regulators such as complement Factor I (CFI). We reasoned that comparing Band T-cell differentiation and function in primary and secondary plasma C3 deficiency might help to understand the role of plasma and intracellular C3 in adaptive immunity. We report the immunological features of lymphocytes from 9 individuals with low plasma C3 belonging to 6 families, with mutations causing primary or secondary C3 deficiency and, in some cases, chronic kidney disease stages 1 to 3 (see Fig E1,A, and Tables E1 and E3 in this article’s Online Repository at www.jacionline.org).

Complement receptor CD46 co-stimulates optimal human CD8 + T cell effector function via fatty acid metabolism

The induction of human CD4+ Th1 cells requires autocrine stimulation of the complement receptor CD46 in direct crosstalk with a CD4+ T cell-intrinsic NLRP3 inflammasome. However, it is unclear whether human cytotoxic CD8+ T cell (CTL) responses also rely on an intrinsic complement-inflammasome axis. Here we show, using CTLs from patients with CD46 deficiency or with constitutively-active NLRP3, that CD46 delivers co-stimulatory signals for optimal CTL activity by augmenting nutrient-influx and fatty acid synthesis. Surprisingly, although CTLs express NLRP3, a canonical NLRP3 inflammasome is not required for normal human CTL activity, as CTLs from patients with hyperactive NLRP3 activity function normally. These findings establish autocrine complement and CD46 activity as integral components of normal human CTL biology, and, since CD46 is only present in humans, emphasize the divergent roles of innate immune sensors between mice and men.Complement, while serving to remove pathogens in the circulation, is also important for synergizing with inflammasomes to modulate CD4 T cell activation. Here the authors show that CD46, a complement receptor expressed only in humans, is essential for inducing optimal activation and effector functions of human CD8 T cells.