LAUSR

Abstract We report on a novel autoantigen expressed in human macular tissues, identified following an initial Western blot (WB)‐based screening of sera from subjects with age‐related macular degeneration (AMD) for circulating auto‐antibodies (AAbs) recognizing macular antigens. Immunoprecipitation, 2D‐gel electrophoresis (2D‐GE) and liquid chromatography‐tandem mass spectrometry (LC‐MS/MS), direct enzyme‐linked immunosorbent assays (ELISA), WBs, immunohistochemistry (IHC), human primary and ARPE‐19 immortalized cell cultures were used to characterize this novel antigen. An approximately 40‐kDa autoantigen in AMD was identified as the scavenger receptor CD5 antigen‐like protein (CD5L), also known as apoptosis inhibitor of macrophage (AIM). CD5L/AIM was localized to human RPE by IHC and WB methods and to retinal microglial cells by IHC. ELISAs with recombinant CD5L/AIM on a subset of AMD sera showed a nearly 2‐fold higher anti‐CD5L/AIM reactivity in AMD vs. Control sera (p = 0.000007). Reactivity ≥0.4 was associated with 18‐fold higher odds of having AMD (χ2 = 21.42, p = 0.00063). Circulating CD5L/AIM levels were also nearly 2‐fold higher in AMD sera compared to controls (p = 0.0052). The discovery of CD5L/AIM expression in the RPE and in retinal microglial cells adds to the known immunomodulatory roles of these cells in the retina. The discovery of AAbs recognizing CD5L/AIM identifies a possible novel disease biomarker and suggest a potential role for CD5L/AIM in the pathogenesis of AMD in situ. The possible mechanisms via which anti‐CD5L/AIM AAbs may contribute to AMD pathogenesis are discussed. In particular, since CD5L is known to stimulate autophagy and to participate in oxidized LDL uptake in macrophages, we propose that anti‐CD5L/AIM auto‐antibodies may play a role in drusen biogenesis and inflammatory RPE damage in AMD. Graphical abstract Sera of subjects with age‐related macular degeneration exhibit auto‐antibodies directed against an antigen that, following 2D gel and mass spectrometry experiments, was discovered to be the CD5‐like (CD5L) protein, a secreted scavenger receptor known to be produced by macrophages. Immunohistochemical experiments reveal that this antigen is expressed also throughout the cytoplasm and, more discretely, at the nuclear level (inset) by human retinal pigment epithelium cell lines. Confocal immunohistochemical microscopy experiments show also that CD5L reactivity in human macular neuroretinal tissue section colocalizes with reactivity for Iba1, a known microglial specific marker. Figure. No Caption available. HighlightsHuman retinal pigment epithelium and retinal microglia express the CD5 antigen‐like protein (CD5L).AMD patients exhibit autoantibodies against CD5L.Anti‐CD5L ELISA reactivity ≥0.4 is associated with 18‐fold higher odds of AMD.Anti‐CD5L autoreactivity could be a potential novel AMD biomarker with possible pathogenic potential.