Complement — tapping into new sites and effector systems

Nature Reviews Immunology | Perspectives | Opinion

Martin Kolev,^{1, 2,} Gaelle Le Friec^{1, 2,} & Claudia Kemper^1,

Published online 14 November 2014

Abstract

Complement is traditionally known to be a system of serum proteins that provide protection against pathogens through direct cell lysis and the mobilization of innate and adaptive immunity. However, recent work indicates that the complement system has additional physiological roles beyond those in host defence. In this Opinion article, we describe the new modes and locations of complement activation that enable it to interact with other cell effector systems, such as growth factor receptors, inflammasomes and metabolic pathways. We propose that the location of complement activation dictates its function.

Introduction

 The complement system, which was discovered more than 100 years ago, is one of the oldest components of immunity and is central to the detection and destruction of invading pathogens^{1, 2, 3, 4, 5}. Complement is a system of fluid-phase proteins (found in the blood, lymph and interstitial fluids) and cell membrane-bound proteins. The serum-circulating proteins, which are generally synthesized in the liver, are mostly present in an inactive pro-enzyme state, and the membrane-bound proteins comprise receptors and regulators of complement activation fragments. The detection of microorganisms that have breached the host environmental barriers by fluid-phase complement components leads to the activation of the complement cascade and the elimination of the microbial target (Fig. 1a). The complement cascade can be activated by three pathways: the classical, alternative and lectin pathways^{1, 2, 3, 4, 5} (Fig. 1a). All activation pathways lead to the generation of the C3 and C5 convertase enzyme complexes, which cleave C3 into the anaphylatoxin C3a and the opsonin C3b, and C5 into the anaphylatoxin C5a and into C5b, respectively. Deposition of C5b onto a target initiates membrane attack complex (MAC) formation and target lysis¹. The opsonins and anaphylatoxins promote phagocytic uptake of pathogens by scavenger cells, and activate neutrophils, monocytes and mast cells, respectively^{1, 2, 3, 4, 5}. On the basis of these effector functions, complement has long been considered as an innate immune pathway.

 

Figure 1: Distinct location-directed functions of complement activation.

a | Liver-derived, systemically circulating complement forms the first line of defence against invading pathogens and can be activated through three pathways: the classical pathway, the lectin pathway and the alternative pathway, with the initial deposition of C3b on a surface also initiating a feedback amplification loop. Through the formation of C3 convertases (C4bC2a for the classical and lectin pathways, and C3bBb for the alternative pathway), these pathways culminate in the generation of the opsonin C3b and the anaphylatoxin C3a. Subsequent C5 convertase formation (C4bC2aC3b for the classical and lectin pathways, and C3bBbC3b for the alternative pathway) leads to C5b and anaphylatoxin C5a generation, with C5b initiating the formation of the membrane attack complex (MAC) and its insertion into target membranes. C3 and C5 can also be activated directly via activating proteases (see Box 1). Self tissue is protected from complement deposition through fluid-phase and cell-bound regulators; C1 inhibitor (C1-INH) inhibits the functions of C1r, C1s and mannan-binding lectin-associated serine protease 2 (MASP2). C3b (and C4b) are inactivated by the serine protease complement Factor I and one of several cofactor proteins (surface-bound CD46 and complement receptor type 1 (CR1) or fluid-phase Factor H and C4b-binding protein (C4BP)). Convertases are regulated through disassembly by regulators that have decay-accelerating activity — surface-bound CD55 and CR1 or fluid-phase Factor H and C4BP — and the formation of the MAC is controlled by CD59 and vitronectin (also known as S protein)¹¹³. b | Locally occurring complement activation is triggered when a cell-activating signal (such as T cell receptor (TCR) stimulation) initiates the generation and secretion of C3, C5, Factor B (FB) and Factor D (FD), leading to C3 and C5 convertase formation in the extracellular space and/or on the cell surface, and ultimately to the generation of the complement activation fragments C3a, C3b, C5b and C5a. C3a, C3b and C5a bind to their respective receptors on the T cell and induce cellular responses. Intracellular complement activation in resting CD4⁺ T cells (and possibly other cell types) occurs continuously through the action of the C3-cleaving protease cathepsin L. The resulting C3a fragment engages the intracellular lysosome-localized receptor C3aR, which sustains tonic mammalian target of rapamycin (mTOR) activation and T cell survival (resting T cells express C3aR only intracellularly). TCR activation induces cell-surface translocation (shuttling) of this intracellular C3 activation system (indicated by the dashed arrows), where engagement of surface C3aR and CD46 induce intracellular signalling events (for details on these signalling events, see Refs 11,35) that ultimately mediate upregulation of key growth factor receptors — including the receptors for interleukin-2 (IL-2), IL-7 and IL-12 (IL-2R, IL-7R and IL-12R, respectively) — as well as proliferation and the induction of effector function. Autocrine complement receptor activation in antigen-presenting cells (APCs) is triggered by Toll-like receptor (TLR) activation and mediates APC maturation and the expression of MHC class II and co-stimulatory molecules, as well as cytokine production. The sum of autocrine and paracrine effects of local complement activation during cognate APC and T cell interactions defines the functional outcome of T cell activity. Although not depicted here, the cell-surface expression of complement regulators affects these processes by regulating local complement activation^{9, 30, 36}. Furthermore, the C3 activation fragments inactive C3b (iC3b) and C3dg are deposited extracellularly on apoptotic cells and are then taken up by APCs; here, they regulate lysosomal fusion, processing of apoptotic cell debris and subsequent antigen presentation by an as yet undefined mechanism⁴⁸. MBL, mannose-binding lectin; P, properdin; T_H, T helper.

However, the discovery that receptors for complement activation fragments are expressed by almost all immune cells — including B cells and T cells — and that these cells can sense and convert the levels of complement activation into tailored responses⁶ led to the appreciation that complement directs both innate and adaptive immune responses. For example, complement receptor activation lowers the threshold for B cell activation, directs antigen handling by follicular dendritic cells (FDCs) and contributes to the maintenance of B cell tolerance and memory^{7, 8}. Similarly, complement has a non-redundant role in CD4⁺ and CD8⁺ T cell activation and function, either directly through stimulating complement receptor-mediated signalling events in T cells or indirectly through modulating antigen-presenting cell (APC) function^{9, 10, 11}. The appreciation of the role of complement in adaptive immunity coincided with the understanding that complement detects not only pathogenic microorganisms but also potentially harmful self molecules, such as those that are exposed by stressed, injured, apoptotic or necrotic tissues and cells⁴. The discovery that complement aids in the disposal of cellular debris and instructs the adaptive immune system provided the missing mechanistic explanations for the long-known but poorly understood finding that complement deficiencies predispose to autoimmune disease^{12, 13, 14, 15}.

 

Recent studies are also providing a new dimension to our understanding of complement. Unexpectedly, it was shown that complement can be activated not only at the cell surface, as traditionally thought, but also in intracellular compartments¹⁶. Moreover, it is now becoming clear that systemic serum complement has different functions from local immune cell-derived complement. Rather than being a mostly pro-inflammatory effector system, complement is emerging as a central player in cell and tissue development, homeostasis and repair. Studies of the molecular mechanisms underlying these new functions of complement have led to the discovery of new crosstalk between complement components and other cell effector systems, including growth factor receptors, inflammasomes, metabolic sensors and the Notch system. In this Opinion article, we propose a model to explain how the different locations of complement activation dictate its diverse functions and how complement engages other effector systems at these locations to regulate immune-related and non-immune-related processes.

 

Figure 2: Functional crosstalk between complement and other cell effector systems.

The functional crosstalk between the complement system and Toll-like receptors (TLRs) and the coagulation cascade has long been acknowledged. The recent developments in the field have led to the discovery of additional direct crosstalk with key effector systems, including the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome, carbohydrate receptors (such as dectin 1), Fc receptors for IgG (FcγRs), and cytokine and growth factor receptors, as well as the WNT and Notch systems. Cell populations in which this crosstalk occurs are indicated and, where identified, the signalling pathways driving the functional outcome of the crosstalk between complement and effector systems are shown. The regulation of the mammalian target of rapamycin (mTOR) metabolic sensing system by complement is not included here but the current knowledge about this crosstalk is summarized in Refs 23,39. '+' denotes upregulation; '−' denotes downregulation; AP-1, activator protein 1; APC, antigen-presenting cell; cAMP; cyclic AMP; DC, dendritic cell; DLL1, delta-like ligand 1; ER, endoplasmic reticulum; ERK, extracellular signal-regulated kinase; IL, interleukin; LRP, low-density lipoprotein receptor-related protein; MAC, membrane attack complex; MAPK, mitogen-activated protein kinase; MASP2, mannan-binding lectin-associated serine protease 2; MYD88, myeloid differentiation primary response protein 88; NF-κB, nuclear factor-κB; P2RX7, P2X purinoceptor 7; PI3K, phosphoinositide 3-kinase; SHIP, SH2 domain-containing inositol-5-phosphatase; SYK, spleen tyrosine kinase; R, receptor; SPAK, ST20/SPS1-related proline-alanine-rich protein kinase; T_H, T helper; TNF, tumour necrosis factor; T_Reg, regulatory T.

Figure 3: Complement at the nexus of the extensive crosstalk between cell effector systems.

The interaction between complement and other key cell effector systems involved in innate and adaptive immunity is multifactorial and in most cases bidirectional. Furthermore, the functional impact of complement on effector systems with primarily non-immune functions (for example, the Notch and WNT systems) is more substantial than previously thought, and indicates that complement contributes to normal development, and possibly to ageing and behaviour. We suggest that the emerging role of complement in core physiological metabolic pathways may be the crucial functional intersection point in this network. FcγR, Fc receptor for IgG; TLR, Toll-like receptor.

Conclusions and future perspectives

Complement has traditionally been defined as an innate and systemic system that functions in the defence against pathogens. However, it is now considered to be a central regulator of innate and adaptive immunity, with new functions that extend beyond protective immunity, including roles in cell generative, degenerative and regenerative processes. The finding that complement is activated within cells and not only engages intracellular complement receptors but also intersects with several other cell effector systems helps to explain its unexpectedly wide-reaching effects.

 

Nevertheless, there is still much to discover about this ancient system and key future questions include: how is intracellular complement generation and activation regulated? Does this novel pathway contribute to disease? Are additional complement components, including regulators, functionally active inside cells? In this regard, we have detected intracellular C5a (A. Fara and C. Kemper, unpublished observations) and several studies have reported intracellular expression of Factor D, complement receptor type 1 (CR1; also known as CD35) and the positive regulator properdin in resting cells^{89, 90}. Thus, one could envision the existence of an intracellular 'Complosome' — somewhat analogous to the inflammasome⁹¹ — that has novel functions in cell survival and activation. Furthermore, a unifying feature of the new roles and interactions for complement is their reliance on appropriate sensing of cellular integrity and balanced control of energy and substrate metabolism. Therefore, the emerging cooperation between complement and the metabolic pathway network may arise as a core intersection point for the diverse functions of complement in immunity and beyond (Fig. 3).