Selective accumulation of the complement membrane attack complex in aging choriocapillaris

Abstract

The complement membrane attack complex (MAC) shows increased abundance in the choriocapillaris during normal aging and is especially prevalent in age-related macular degeneration (AMD). While perivascular MAC accumulation occurs in the choroid, it is not well understood whether similar deposition occurs in other aging tissues. In this study we examined the abundance of MAC across multiple human tissues. For studies on fixed tissues, paraffin sections were obtained from six human donor eyes and a commercially available tissue array containing 19 different tissues. Immunohistochemical labeling was performed using antibodies directed against the MAC and intercellular adhesion molecule-1 (ICAM-1), as well as the lectin Ulex europaeus agglutinin-I (UEA-I). The choriocapillaris was the only tissue with high levels of the MAC, which was not detected in any of the 38 additional samples from 19 tissues. ICAM-1 was abundantly expressed in the majority of tissues evaluated, and UEA-I labeled the vasculature in all tissues. A second experiment was performed using unfixed frozen sections of RPE-choroid and 7 extraocular tissues, which confirmed the relatively limited localization of the MAC to the choriocapillaris. In comparison to other tissues assessed, the restricted accumulation of MAC in the choriocapillaris may, in part, explain the specificity of AMD to the neural retina, RPE and choroid, and the relative absence of systemic pathology in this disease.

Keywords: Age-related macular degeneration; Choriocapillaris; Choroid; Complement; Microvasculature.

Figure 1

Choroid (A–D), stomach (E–H), small intestine (I–L), kidney (M–P), and pancreas (Q–T) tissue labeling for MAC (A, E, I, M, Q), UEA-I (B, F, J, N, R), ICAM-1 (C, G, K, O, S), and secondary control (D, H, L, P, T). Positive reactivity is indicated by purple reaction product. The choriocapillaris showed abundant MAC (A, arrow) and ICAM-1 (C, arrow) labeling, while the stomach had mild ICAM-1 (G) and no MAC labeling (E). Intracellular ICAM-1 labeling was present in the small intestine epithelium (K, arrow), while UEA-I was only present around the outer layer of the epithelium (J, white arrow) and around the vessels (J, arrow). Kidney tissue exhibited vascular staining with UEA-I (N, arrow) and ICAM-1 (O, arrow), while pancreatic pyramidal cells showed intracellular UEA-I (R) and ICAM-1 (S) staining. No MAC labeling was present for any of the tissues in the array (E, I, M, Q). Scale bar = 50 μm.

Figure 2

Immunofluorescent labeling of unfixed sections with antibodies directed against the MAC (green) and UEA-I (red) in choroid (A, A′), colon (B, B′), kidney (C, C′), liver (D, D′), lung (E, E′), skin (F, F′), tonsil (G, G′), and uterus (H, H′). Abundant MAC labeling was present in the choriocapillaris (A, A′, arrow). While mild MAC labeling was observed throughout the tissue, kidney glomerular capillaries were negative for MAC (C, C′). Mild MAC labeling was present surrounding the portal triads and central veins of the liver, leaving the microvessels unlabeled (D, D′). For all other tissues, MAC immunolabeling was absent (B, B′, E, E′, F, F′, G, G′, H, H′). Nonspecific labeling within the endometrial glands was visible in the stained uterus tissue (H, arrowhead), as well as the secondary control (not shown). Donor ages and gender are as follows: choroid = 62, F; colon = 75, F; kidney = 61, F; liver = 70, M; lung = 65, F; skin = 55, F; tonsil = 68, M; and uterus = 86, F. Scale bar in H = 200 μm, scale bar in H′ = 50 μm.