Keeping It All Going-Complement Meets Metabolism

Abstract

The complement system is an evolutionary old and crucial component of innate immunity, which is key to the detection and removal of invading pathogens. It was initially discovered as a liver-derived sentinel system circulating in serum, the lymph, and interstitial fluids that mediate the opsonization and lytic killing of bacteria, fungi, and viruses and the initiation of the general inflammatory responses. Although work performed specifically in the last five decades identified complement also as a critical instructor of adaptive immunity-indicating that complement's function is likely broader than initially anticipated-the dominant opinion among researchers and clinicians was that the key complement functions were in principle defined. However, there is now a growing realization that complement activity goes well beyond "classic" immune functions and that this system is also required for normal (neuronal) development and activity and general cell and tissue integrity and homeostasis. Furthermore, the recent discovery that complement activation is not confined to the extracellular space but occurs within cells led to the surprising understanding that complement is involved in the regulation of basic processes of the cell, particularly those of metabolic nature-mostly via novel crosstalks between complement and intracellular sensor, and effector, pathways that had been overlooked because of their spatial separation. These paradigm shifts in the field led to a renaissance in complement research and provide new platforms to now better understand the molecular pathways underlying the wide-reaching effects of complement functions in immunity and beyond. In this review, we will cover the current knowledge about complement's emerging relationship with the cellular metabolism machinery with a focus on the functional differences between serum-circulating versus intracellularly active complement during normal cell survival and induction of effector functions. We will also discuss how taking a closer look into the evolution of key complement components not only made the functional connection between complement and metabolism rather "predictable" but how it may also give clues for the discovery of additional roles for complement in basic cellular processes.

Keywords: T cells; complement; evolution; metabolic disease; metabolism.

Figure 1

Systemic complement activation. Serum-circulating complement can be activated via three pathways: the classical, lectin, and alternative pathways, which all cumulate in the formation of multiprotein complexes termed C3 convertases. The classical and lectin pathway C3 convertases (C4bC2a) and the alternative pathway C3 convertase (C3bBb) lead to cleavage of C3 into the opsonin C3b and the anaphylatoxin C3a. Properdin (P) is a stabilizator of the alternative C3 convertase. Upon subsequent generation of C5 convertase (C4bC2aC3b for the classical and lectin pathways, C3bBbC3b for the alternative pathway), C5b and the anaphylatoxin C5a are produced, with surface-bound C5b initiating the formation and insertion of the terminal complement pathway (TTC; or membrane attack complex, MAC) on pathogens (or other target membranes).

Figure 2

Autocrine and intracellular complement activation. (A) In resting T cells, C3 is processed intracellularly by the protease cathepsin L to generate bioactive C3a that sustains low-level mechanistic target of rapamycin (mTOR) activity via the engagement of the intracellular C3aR expressed on lysosomes and thus contributes to homeostatic survival of CD4⁺ T cells. (B) Local complement activation is triggered when activating signals [here, T cell receptor stimulation or toll-like receptors activation on antigen-presenting cells (not shown)] initiate secretion of preformed C3, C5, factor B, and factor D located in cellular storages, leading to C3 and C5 convertase formation in the extracellular space as well as on the cell surface—and the generation of C3a, C3b, C5b, and C5a (1). These complement fragments bind to their respective receptors on the T cell surface and induce Th1 induction with interferon (IFN)-γ secretion. C5 is also processed intracellularly by a yet unknown protease/convertase into C5a and C5b, and this process is increased through CD46-mediated signals. Intracellular C5a engages the intracellular C5aR1, which triggers NLRP3 inflammasome assembly and intrinsic IL-1β secretion that sustains Th1 induction in an autocrine fashion (2). Importantly, autocrine CD46 activation in conjunction with IL-2R signaling also induces IL-10 co-production in Th1 cells and the transition into a (self)regulative Th1 contraction phase (3). This “IL-10 switch” is accompanied by autocrine surface engagement of the C5aR2 via surface-shuttled C5a/C5a-desArg (3) through an as yet undefined signaling pathway (but possibly through direct suppression of intracellular C5aR1 activity, not shown).

Figure 3

The role of systemic complement in metabolism. Metabolically active proteins can activate systemic complement in the absence of infection. For example, adiponectin activates directly C1q, which results in the generation of C3a and C5a fragments via classical pathway activation. The anaphylatoxins are further processed by carboxypeptidase N (CPN) to C3a-desArg and C5a-desArg, which then impact on triglyceride (TG) clearance, increase insulin resistance, and alter adiposity by affecting signaling through the three anaphylatoxin receptors C3aR, C5aR1, and C5aR2 in several cell types such as adipocytes and pancreatic β-cells via diffusion into respective tissues. Importantly, although not yet formally proven, locally generated and activated complement also likely affects adipocytes activity as these cells are a major source of Factor D and can also synthesize and secrete other complement proteins such as Factor B, C3, and C5. These proteins, when secreted, form extracellular C3 and C5 convertases and generate locally C3a and C5a fragments, which engage their respective receptors and trigger AKT and NF-κB signaling that contributes to adipocyte development and activity.

Figure 4

Autocrine and intracellular complement drives key T cell metabolic pathways during T helper cell type 1 induction. T cell receptor and CD28-driven autocrine engagement of CD46 CYT-1 (by C3b) leads to upregulation of genes coding for the glucose transporter GLUT (SLC2A1), and the amino acid channel LAT1 (SLC7A5), allowing for increased influx of glucose and amino acids (AA) into the cell (2). In parallel, CD46 CYT-1-mediated signals induce increased expression of LAMTOR5, and via this assembly of the lysosome-based machinery enabling AA sensing via mTOR complex 1 (mTORC1), which then leads the induction of glycolysis and OXPHOS required for interferon (IFN)-γ production. Surface activation of the C3aR by C3a activates PI3K and AKT and further sustains mechanistic target of rapamycin (mTOR) activity. Intracellular C5aR1 stimulation (at a not yet defined cellular compartment) drives mitochondrial reactive oxygen species (ROS) production, which induces NLRP3 inflammasome activation and maintenance of IFN-γ secretion during T cell migration into the (inflamed) tissues.

Figure 5

Schematic depicting appearance of the “original” complement components during evolution. (A) Simplified phylogenic tree of species with emphasis on families and species important during evolution of the complement system (see text). (B) Cartoon depicting the appearance of complement(s) C3/C4/C5, factor B (FB), and earliest members of the MASP families during evolution. Color coding in the “title arrow” corresponds to specific key species in the phylogenetic tree under (A). Note that the appearance of fully functional TTC/MAC components and complement receptors and regulators are not depicted in this schematic.

Figure 6

“Metabolic domains” in modern and ancient C3 forms. The basic domain structure of C3 is not only conserved among species but also contains several domains that have high homology to proteins with key metabolic functions (see text for further details). (A) Compared to C3 found in “modern” higher vertebrates, C3 forms in (B) more ancient and basic organisms contained additional domains involved in core metabolic processes. These “lost metabolic” domains are shown in bright red (please see text for details). A, anaphylatoxin-like domain; A2M-N, domain with high homology to the N-terminal part of the A2M (alpha-2-macroglobulin) domain; different colors in the A2M domains indicate protein sequence differences among the A2M domains within one C3 form.