Complement: a key system for immune surveillance and homeostasis

Abstract

Nearly a century after the significance of the human complement system was recognized, we have come to realize that its functions extend far beyond the elimination of microbes. Complement acts as a rapid and efficient immune surveillance system that has distinct effects on healthy and altered host cells and foreign intruders. By eliminating cellular debris and infectious microbes, orchestrating immune responses and sending 'danger' signals, complement contributes substantially to homeostasis, but it can also take action against healthy cells if not properly controlled. This review describes our updated view of the function, structure and dynamics of the complement network, highlights its interconnection with immunity at large and with other endogenous pathways, and illustrates its multiple roles in homeostasis and disease.

Figure 1. Immune surveillance functions of complement

A constant low level of complement activation (i.e., tick-over) ensures occasional probing of healthy human cells (a), but the presence of surface-bound regulators and self-recognition by solution-based regulators prevents any amplification of the response. In the case of microbial intruders (b), however, a strong complement response is actively induced by various pattern recognition proteins and subsequently amplified in the absence of regulators. Opsonization by complement fragments and proinflammatory signaling via anaphylatoxins recruit macrophages and enable phagocytosis, and the formation of a lytic membrane attack complex on certain cells such as Gram-negative bacteria leads to cell death. Finally, complement degradation products stimulate downstream immune responses. Although pattern recognition proteins also recognize surfaces of apoptotic cells (c), residual and recruited complement regulators hold amplification and terminal pathways in check. Thus, opsonization facilitates elimination of the cell without triggering danger signals and further immune responses. While this fine-tuned interplay between complement recognition, activation, and regulation usually enables the differential reaction to healthy, apoptotic, and foreign cells, any imbalance between these events may lead to an attack on self-cells (d) and trigger a series of immune and inflammatory diseases. Although proinflammatory and immune signaling appears to be the driving force in most complement-mediated diseases, other processes like TCC-mediated lysis may be involved (e.g., of erythrocytes in the case of paroxysmal nocturnal hemoglobinuria).

Figure 2. Detailed view of complement activation, amplification, signaling, and regulation

(a) An intricate network of soluble and cell-surface- bound proteins enables the recognition, tagging, and elimination of microbial intruders and foreign cells and stimulates downstream immune responses. In the classical pathway (CP), C1q recognizes pathogen- or damage-associated patterns (such as IgG, IgM, or CRP) on foreign or apoptotic cells, inducing the formation of the CP C3 convertase (C4b2b) via cleavage of C2 and C4 by C1s. Detection of carbohydrate patches by MBL or ficolins associated with MASP via the lectin pathway enables the formation of the same convertase, which subsequently activates the abundant plasma protein C3, generating its active fragments C3a and C3b. Covalent deposition of C3b on nearby surfaces (i.e. opsonization) leads to the binding of factor B and conversion into the alternative pathway (AP) C3 convertase (C3bBb), which cleaves more C3 into C3b and thereby amplifies the complement response. In addition, a low level of complement activation is maintained in solution (i.e., tick-over), and resulting C3b or C3 convertases can be recruited to foreign surfaces and stabilized by properdin (P). Increasing densities of C3b on the surface lead to a gradual substrate shift of the convertases from C3 to C5. Cleavage of C5 into C5a and C5b initiates the assembly of the lytic terminal complement complex (TCC) on susceptible cells. Opsonization by C1q, C3b, and its degradation products iC3b, C3c, and C3d induces phagocytosis via a panel of complement receptors. In addition, the anaphylatoxins C3a and C5a cause strong proinflammatory signaling via their respective GPCR. C5a also co-regulates immunoglobulin (Ig)-mediated phagocytosis of immune complexes (IC) by modulating the differential expression of activating (FcγRI/III) and deactivating (FcγRIIB) Fc-gamma receptors. On B cells, binding of C3dg to the CR2/CD19 co-receptor complex lowers the threshold of activation by several orders of magnitude and plays an important role in their maturation. Close crosstalk between Toll-like receptors (TLR), complement receptors, and regulators modulates IL-12 in antigen-presenting cells (APC) and thereby influences activation and differentiation of T cells. (b) On healthy human cells, any complement activation or amplification is immediately attenuated by a panel of surface-bound regulators that either accelerate decay of the convertases (CR1, DAF), act as a cofactor for the factor I-mediated degradation of C3b and C4b (CR1, MCP), or prevent the formation of the TCC (CD59). In addition, solution-based regulators such as C4BP, FH, or FHL-1 recognize self-surface pattern-like glycosaminoglycans and further impair activation. Finally, a diverse set of regulators enables control at the level of initiation (C1-INH, MAP-1, sMAP, CRIT, FHR-4), the C5 convertases (FHR-1, CRIg) or TCC (FHR-1, VTN, clusterin).

Figure 3. Integrative role of complement in host defense and homeostasis

The traditional functions of complement (within the oval shape) and its orchestrating role in immunity and homeostasis are shown (key participating molecules are indicated next to the arrows): Complement regulates TLR signaling to coordinate innate defenses and potentiates coagulation to additionally provide a mechanical barrier against the spread of invading bacteria. It also facilitates IgG-mediated phagocytic killing of microbes by reducing the threshold for FcγR activation and promotes specific IgG antibody responses through B-cell activation. Complement regulation of APC TLRs impacts on the differentiation of T cells, which can also be affected directly by complement. Regulation of helper T cell differentiation by complement is a complex process; however, in the context of complement-TLR crosstalk, the anaphylatoxins may either promote or inhibit T_H1 or T_H17 development in vivo, depending on whether they act through DC or macrophages (MΦ) as APC. To replenish the immune system during infection or injury, complement regulates the mobilization of HSPC from the bone marrow. Most, if not all, mechanisms regulated by complement may also be involved in immunopathology, such as FcγR-mediated autoimmunity, disseminated intravascular coagulation (DIC), or inflammatory bone resorption (e.g., through sublytic C5b-9 signaling). The functional repertoire of complement includes a major role in the resolution of inflammation through induction of regulatory T cells (T_reg), non-inflammatory clearance of apoptotic cells (which permit complement activation through loss of complement regulatory proteins), and promotion of tissue repair. Most complement interactions are bidirectional, in that the production and activation of complement proteins and receptors is regulated by other receptors or system components (e.g., FcγRs, TLRs, thrombin).

Figure 4. Emerging roles of complement in health and disease

Besides the more `classical' roles of complement in the elimination of microbial intruders and clearance of apoptotic debris (as depicted in Fig. 1b,c), complement has important roles in cell homeostasis and several disease states. In the case of sepsis (a), high levels of infectious microorganisms in the blood cause excessive complement activation with release of C5a that contributes to a series of devastating effects ranging from immune depletion (e.g., by paralyzing neutrophils and inducing apoptosis in lymphocytes) to severe inflammation (by triggering a cytokine storm) and disseminated coagulation (partly via inducing the expression of tissue factor, TF), all of which may culminate in tissue damage, multi-organ failure, and death. Complement has also been shown to be important in synaptogenesis (b), eliminating weak or immature synapses. An unknown signal derived from immature astrocytes promotes recognition by C1q, which in turn leads to opsonization with C3b and iC3b and facilitates complement receptor (CR)-mediated phagocytosis by activated microglia. Although the role of the C5L2 receptor in the immune response is still a matter of debate, it appears to have an important role in lipid metabolism (c). Adipocytes are known to secrete C3, fB, and fD, and their expression may be promoted by stimuli such as insulin or lipids, leading to a higher turnover of the AP and generation of C5a, which is rapidly transformed into C5a^desArg. Also known as acetylation stimulating protein (ASP), this anaphylatoxin fragment has been reported to induce lipid clearance, glucose uptake, and triglyceride (TG) synthesis in adipocytes via C5L2 signaling. In cancer (d), complement is likely to have a dual role. On the one hand, it contributes to protection via direct activation of complement or as part of the complement-dependent cytotoxicity (CDC) of tumor-directed therapeutic antibodies. On the other hand, many tumors escape complement attack by expressing and secreting complement inhibitors that largely prevent amplification, TCC formation, or complement-mediated phagocytosis. Furthermore, recent research has shown that the generation of C5a in the tumor microenvironment may attract myeloid-derived suppressor cells (MDSC) and induce the generation of reactive oxygen and nitrogen species (ROS and RNS, respectively) via the C5a receptor (C5aR), which impairs the tumor-directed effect of T cells.