Complement, age-related macular degeneration and a vision of the future

Abstract

Age-related macular degeneration (AMD) is one of the most well-characterized late-onset, complex trait diseases. Remarkable advances in our understanding of the genetic and biological foundations of this disease were derived from a recent convergence of scientific and clinical data. Importantly, the more recent identification of AMD-associated variations in a number of complement pathway genes has provided strong support for earlier, paradigm-shifting studies that suggested that aberrant function of the complement system plays a key role in disease etiology. Collectively, this wealth of information has provided an impetus for the development of powerful tools to accurately diagnose disease risk and progression and complement-based therapeutics that will ultimately delay or prevent AMD. Indeed, we are poised to witness a new era of a personalized approach toward the assessment, management, and treatment of this debilitating, chronic disease.

Figure 1

Immunolocalization of complement factor H (CFH) and the membrane attack complex (MAC)/C5b-9 in the retinal pigment epithelium (RPE)/choroid (Chor) complex. A and B, Confocal immunofluorescence images from an 84-year-old man with atrophic age-related macular degeneration (AMD). Anti-CFH antibody labels substructural elements (arrows) in drusen (Dr) and the sub-RPE space (green; Cy2 channel). C and D, Localization of CFH in drusen and the sub-RPE space in an 83-year-old man with AMD (green). Drusen immunoreactivity (IR) is homogeneous; CFH IR is associated with the choriocapillaris and the sub-RPE space (arrowheads). E–L, The brown pigment in the RPE cytoplasm and Chor is melanin. E, Localization of CFH in drusen of a 79-year-old donor eye. Anti-CFH labeling (purple reaction product) is apparent in Dr, along the Bruch membrane (BM), and on the choroidal capillary walls (arrows). F, Control section of the same eye with no primary antibody; labeling is absent. G, Extensive labeling is present along the BM, the choroidal capillary walls, and intercapillary pillars (arrows) in a 78-year-old individual with AMD. H, Control section from the macula of a donor without AMD; much less labeling is apparent. Localization of C5b-9 in the RPE-Chor underlying the macula (I) and extramacula (J) in the same eye of an 81-year-old AMD donor. I, Intense anti-C5b-9 IR is associated with Dr, BM, and the choroidal capillary endothelium. J, Outside the macula, there is only sporadic labeling in the vicinity of the BM. Localization of C5b-9 in the macula from a donor with AMD (K) and from a second donor without AMD (L). K, Anti-C5b-9 labeling is associated primarily with the choroidal capillary walls (black arrowheads) and intercapillary pillars (white arrowheads). Labeling is much more intense in the AMD eye. Note the strong similarity to the anti-CFH labeling pattern in the macula from the same donor (G). Ret indicates retina. Reproduced with permission from Hageman et al.

Figure 2

Confocal microscopic localization of the membrane attack complex (C5b-9) in drusen (Dr) and in compromised retinal pigment epithelium (RPE) cells. A, Anti-C5b-9 labeling pattern at low magnification. Labeling in the choroid (Chor) (blue channel) is unremarkable except for staining in the Bruch membrane (BM) and around the choriocapillaris. Two RPE cells that may be the targets of complement attack are labeled (arrows). A nucleic acid-binding dye (YO-PRO; Invitrogen, Carlsbad, California) that also binds to the elastic layer of BM is used to visualize cell bodies on the green channel. No nucleic acid staining is evident in the anti-C5b-9–labeled RPE cells. B, Anti-C5b-9 labeling of Dr (blue channel). A condensed clump of lipofuscin granules (arrow) may have been expelled from the adjacent RPE cell. C, High magnification of a cell on the BM with anti-C5b-9 immunoreactivity on the basal surface. The only indication of its identity is the presence of a single red autofluorescent granule (arrow). Tightly packed vesicles in the cytoplasm are stained by the YO-PRO dye (green channel). D, C5b-9 immunoreactivity is present in the sub-RPE nodules that flank the Dr (blue channel). In this example, the nodule may have coalesced with the adjacent Dr. Reproduced with permission from Anderson et al.

Figure 3

The complement system is composed of proteins that are activated through a cascade of enzymatic cleavage. Precursor proteins are depicted in green, enzymes in red, byproducts in purple, and inactivated proteins in gray; red arrows indicate enzymatic reactions. Complement factor H (CFH) plays a key regulatory role by inhibiting the binding of complement component 3b (C3b) to complement factor B (CFB), thus preventing the formation of the alternative pathway's amplification loop C3 convertase (C3bBb) as well as acting as a cofactor for the complement factor I (CFI)–mediated degradation of the same convertase to inactive components iC3b and Bb. MAC indicates membrane attack complex; MBL, mannose-binding lectin.

Figure 4

A number of strategies for modulating the complement system are being considered for use in the treatment of various stages of age-related macular degeneration. These include, in general, approaches to (1) inhibit activation and the assembly of convertases, (2) promote the decay and proteolysis of macromolecular complexes, including the convertases, (3) block various effector molecules, such as C3a and C5a, and (4) reestablish control and homeostasis of the system, such as augmentation with protective complement factor H (CFH). This figure is a modified version of Figure 3; it depicts 4 potential targets within the complement pathway that are being considered for therapeutic intervention: C1-INH, which inhibits activation of the classical pathway; compstatin, which inhibits the activation of C3; sCR1, which promotes the degradation of the C3 convertase C3bBb; and eculizumab, which inhibits the system at the level of C5, thereby preventing formation of the membrane attack complex (MAC/C5b-9) and C5a. MAC indicates membrane attack complex; MBL, mannose-binding lectin.