We previously reported that FHR-3 is co-localized with activated macrophages/microglia cells in an aged and inflamed retina, but not detectable in a healthy retina (10). Due to our additional results, showing that RPE cells in response to stress increased the expression of complement components and pro-inflammatory factors (30), we proposed that FHR-3 could be a stress factor for the RPE in the aged retina promoting retinal degeneration (10). Consequently, we investigated here the cell-specific complement and inflammation-associated response of RPE cells exposed to FHR-3.
To exclude a genotype-primed reactivity of ARPE-19 cells and human primary RPE cells (hpRPE), used in this study, we characterized the most common AMD-associated SNPs within genes of the complement pathway (37). Homogenous AMD-risk SNPs could not be detected in the examined RPE cells, instead heterozygous SNPs were present in the CFH and C3 gene of ARPE-19 and CFH, C2/CFB, CFI and ARMS gene of hpRPE cells (Fig. S 1A).
We verified the epithelial phenotype of the used ARPE-19 cells, passaged for 38 times and cultivated under in vivo-like conditions, by staining tight junction protein zonula occludens 1 (ZO-1). A polarized monolayer could be detected for untreated and FHR3 treated cells, showing that FHR-3 had no effect on stable cell-cell contacts. We also excluded that ARPE-19 cell passaging had any influence on FHR-3 dependent tight junction formation by comparing ZO-1 stainings in passage number 38 (P38) with 25 (P25) (Fig. S 1D – G). Transepithelial resistance and cellular capacitance of the polarized ARPE19 cells were measured between 0.5–72 h of FHR-3-treatment. FHR-3 had no impact on cell barrier function and on cell membrane folding (Fig. S 1B). These characterizations resulted in a specific RPE phenotype, with a slight shift to mesenchymal characteristics established by mRNA expression analysis of EMT markers vimentin (VIM), α-smooth muscle actin (ACTA2) and collagen type 1 (COL1A1) (Fig. S 1H – J). FHR-3 treatment increased VIM expression in ARPE-19 cells P38 and P25 compared to untreated controls (Fig. S 1H), whereas ACTA2 and COL1A1 were only raised in ARPE-19 cells P25 (Fig. S 1I – J) indicating an early EMT caused by FHR-3 and already existing mesenchymal characteristics in high-passage ARPE-19 cells.
Due to the number of passages and the slight EMT, which is typical for aged human RPE cells (38), the investigated ARPE-19 cells P38 were termed senescent cells in this study.
FHR-3 was internalized by senescent ARPE-19 cells
Previous reports described an interaction of proteins of the FH-protein family with damaged cells: FH was internalized by apoptotic ARPE-19 cells (21), FHR-1 and FHR5 bound to necrotic ARPE19 cells (23), and FHR-1 interacted with a necrotic-type endothelial cell line (20). In this study, we showed for the first time that FHR-3 was bound to and internalized by senescent viable ARPE-19 cells (Fig. 1, Fig. S 1C), knowing that CFHR3 mRNA is not expressed in these cells (Fig. S 3). Interaction of FHR-3 with ARPE-19 cells was confirmed by immunofluorescence resulting in FHR-3-positive ARPE-19 cells following FHR-3 incubation (Fig. 1A, B). Further, in the supernatant of the ARPE-19 cells supplemented with FHR-3 only 30% of the added FHR-3 remained in the apical and 5% in the basal supernatant compared to the added complement control protein properdin (FP), which was stable to 92% in the apical and 3% in the basal supernatant after incubation with ARPE-19 cells (Fig. S 1C). To investigate whether the complement regulator was internalized, we labelled FHR-3 with pH-sensitive dye pHrodo (FHR-3-pHrodo). This dye is non-fluorescent outside the cell, but fluoresces in acidic, cellular compartments like endosomes, phagosomes or lysosomes. Added FHR-3-pHrodo was detected inside senescent ARPE-19 cells indicating a phagocytosis or receptor-mediated endocytosis (Fig. 1D). This shift in fluorescence activity could not be detected when cells were incubated with FH-pHrodo.
Just recently, it was shown that FHR-3 binds to malondialdehyde (MDA)-epitopes (19). These OSE are present on the surface of stressed ARPE-19 cells (16). We wondered, if these neoepitopes could be ligands for FHR-3 internalization (Fig. 1). We determined specific binding of FHR-3 and FH either to OSE 2(ωcarboxyethyl)pyrrole (CEP), MDA or malondialdehyde-acetaldehyde (MAA) (Fig. 2A, D, G). We detected a competitive binding of FHR-3 and FH to CEP, MDA and MAA. Interaction of FHR-3 and oxidative stress epitopes was not changed by additional incubation of FH (Fig. 2B, E, H), whereas a reduced FH interaction with CEP (38%), MDA (47%) and MAA (28%) could be observed when FHR-3 was added as a competitor (Fig. 2C, F, I). Our previously published anti-FHR-3 antibody RETC2 (10) decreased binding of FHR-3 to CEP (29%), MDA (44%) and MAA (24%) (Fig. 2B, E, H), and prevented the competitive effect of FHR-3 with FH for the interaction with CEP (47%) and MDA (87%). This impact could not be detected for MAA (Fig. 2C, F, I).
Here, we showed for the first time FHR-3 internalization by viable, senescent ARPE19 cells (Fig. 1), and determined peroxidation products CEP, MDA and MAA as potential ligands. FHR-3 bound to OSE and prevented their interaction with FH, which was reversed by anti-FHR-3 antibody RETC2 (10) (Fig. 2).
FHR-3 increased endogenous complement activation
Complement components are locally expressed by different retinal cell types and by ARPE-19 cells (27, 30, 31, 39). Here, we showed that the complement regulator FHR3 enhanced complement expression and secretion of RPE cells (Fig. 3 ‒ 5, Fig. S 2A). We demonstrated an increase of C3 mRNA expression in polarized senescent ARPE-19 cells and hpRPE cells after FHR-3 treatment, whereas no expression change was shown for FHR-1, FH or FP incubation (Fig. 3A, Fig. S 2A). On protein level, an increased C3 secretion after 24 h (Fig. 3B) and enhanced cell-associated C3 protein detection in cell lysates after 12 h FHR-3 incubation compared to untreated cells were examined (Fig. 3C). We visualized accumulated intracellular C3, co-localized with Golgi complex, increased in 12 h FHR-3-treated ARPE-19 cells (Fig. 3D, E).
In line with our previous published data, where we reported C3 activation fragments in ARPE-19 cells under oxidative stress (30), we proved that FHR-3 induced C3 cleavage in ARPE-19 cells (Fig. 4). We detected raised levels of cell-associated C3b (101 kDa) after 12 h of FHR-3 treatment (Fig. 4A). However, C3c as a marker for inactivated C3 was time-dependently reduced (5 h – 24 h) when ARPE-19 cells were incubated with FHR3 (Fig. 4B). Anaphylatoxin C3a was increased in FHR-3-treated cells and showed a translocation from the cytoplasm to the cell membrane after FHR-3 exposure (Fig. 4C).
Increased detection of CFB was also a consequence of FHR-3 addition to senescent ARPE-19 cells and hpRPE cells (Fig. 5, Fig. S 2B). CFB transcripts were highly elevated after 5 h and 24 h of FHR-3 incubation and no expression changes were shown for FHR-1, FH or FP treatment (Fig. 5A, Fig. S 2B). Enhanced CFB protein secretion after 24 h (Fig. 5B) and a time-dependent increase in cell-associated CFB protein expression in cell lysates from 5 h to 24 h of FHR-3 treatment were determined (Fig. 5C). CFB cleavage products Bb and Ba were also detected in ARPE-19 cells and raised in FHR-3-treated cells (Fig. S 1L). Using immunofluorescence, we showed accumulated intracellular CFB, partly co-localized with actin stress fibers in FHR-3 treated ARPE-19 cells (Fig. 5D, E).
Our data indicated a cell-associated complement-activating effect of FHR-3 by enhancing C3 and CFB expression as well as secretion and by anaphylatoxin C3a increase in immortal ARPE-19 cells (Fig. 3 ‒ 5, Fig. S 4) as well as cultivated, post-mitotic, hpRPE cells (Fig. S 2A, B).
FHR-3 altered complement receptor expression of ARPE-19 cells
ARPE-19 cells express a variety of complement receptors (30, 40). It has recently been reported, that oxidatively stressed ARPE-19 cells increased expression of complement receptors CR3 and C5aR1 (30). Here, we showed that FHR-3 modified complement receptor C3aR (Fig. 6, Fig. S 2C) and CD11b (Fig. 7, Fig. S 2D) expression on senescent ARPE-19 cells and hpRPE cells, independently from any systemic complement.
C3AR mRNA expression was time-dependently changed with decreased transcripts after 5 h (Fig. 6, Fig. S 2C) and elevated expression after 24 h of FHR-3 exposure (Fig. 6A). This was partly in accordance with protein data showing decreased cell-associated C3aR levels in Western blots (Fig. 6B) and immunohistochemically (Fig. 6C) 5 h after FHR-3 incubation. An increase in C3aR protein expression 24 h after treatment, as shown at the mRNA level, could not be verified. This might be explained by storage of C3AR RNA into RNA granules, either stress granules or processing bodies (41).
ARPE-19 cells express the α-chain CD11b of the integrin complement receptor CR3 (30). We detected very low mRNA expression in untreated-, and no expression in FH-treated senescent ARPE-19 cells. When cells were incubated with FHR-3, CD11B expression increased significantly could be shown in ARPE-19 and hpRPE cells, respectively (Fig. 7A, Fig. S 2D). Western blot analyses revealed a time-dependent upregulation of CD11b from 5 h – 24 h after FHR-3 incubation (Fig. 7B). Immunostainings of FHR-3 treated and untreated cells confirmed CD11b accumulation after 5 h and 24 h (Fig. 7C). We did not observe a change in C5aR1 mRNA transcript levels following FHR-3 treatment, and C5aR2 mRNA transcripts were not detected in ARPE-19 cells (Fig. S 3).
These results revealed an FHR-3 triggered alteration of complement receptor C3aR and CD11b expression in immortal ARPE-19 cells (Fig. 6, 7, Fig. S 4) as well as cultivated, post-mitotic, primary RPE cells (Fig. S 2C, D).
FHR-3 induced pro-inflammatory markers in ARPE-19 cells
NLRP3 inflammasome activation is considered as an additional hallmark for the development of AMD (42). It has already been shown that anaphylatoxins C3a and C5a, and oxidative stress lead to the priming of NLRP3 and secretion of pro-inflammatory cytokines in ARPE-19 cells (25, 30, 43). Here, we describe that FHR-3, a potential risk factor of AMD progression, induced inflammasome-associated pro-inflammation in ARPE-19 cells (Fig. 8A – D). NLRP3 mRNA expression was elevated 5 h and slightly decreased 24 h after FHR-3 incubation, whereas treatment with FHR-1, FH or FP did not show any expression changes (Fig. 8A). Accordingly, transcripts of IL1B were also upregulated 5 h and 24 h after FHR-3 incubation (Fig. 8B). In line with this, determination of cytokine secretion levels of FHR-3-treated ARPE-19 cells revealed an upregulation of apical secretion of IL-1ß (Fig. 8C) and IL-18 (Fig. 8D) compared to untreated cells (Fig. 8C – D, dotted line).
Inflammatory cellular micro-environments are shaped by further key players, e.g. FOXP3, an inflammation-associated transcription factor, which triggers secretion of anti-inflammatory cytokines in regulatory T-cells. Expression of FOXP3 was detected recently in ARPE-19 cells (30, 44, 45). We determined a time-dependent significant decrease in FOXP3 mRNA expression, after FHR-3 incubation (Fig. 8E), indicating a pro-inflammatory effect of FHR-3.
Additionally, the concentration of the pro-inflammatory cytokine IL-6 was elevated in both, apical and basal supernatants of FHR-3 incubated senescent ARPE-19 cells (Fig. 8F). A tendency to higher secretion levels of tumor necrosis factor (TNF)-α could be detected in apical supernatants of ARPE-19 cells incubated with FHR-3 (Fig. 8G). However, a regulation of pro-angiogenic marker vascular endothelial growth factor (VEGF)-α could not be determined after FHR-3 incubation (Fig. 8H).
Toll-like receptors (TLR), abundantly expressed in the retina, especially TLR1 and TLR3 in RPE cells (46, 47), are pattern recognition molecules of the innate immune system. Previous studies have shown, that TLRs protect RPE cells from oxidative stress, and that a dysfunction of TLR is associated with the development of AMD (46, 47). In this study, we described a complement-dependent regulation of TLR1 and TLR3 mRNA expression. Transcripts of TLR1 were significantly decreased in FHR-3-treated ARPE19 cells after 5 and 24 h, whereas TLR3 expression was time-dependently regulated with a respective decrease and increase after 5 h and 24 h of FHR-3 treatment compared to untreated ARPE-19 cells (Fig. 9A).
Increased expression of inflammation-associated genes has also been associated with an impaired ubiquitin-proteasome signalling pathway in RPE cells, which negatively interferes with RPE metabolism (48, 49). Here we show proteasome subunits PSME1, PSMA7, PSMB5 and PSMB8 to be time-dependently altered when ARPE-19 cells were incubated with FHR-3. We detected a downregulation after 5 h and an upregulation after 24 h FHR-3 treatment of all tested proteasome subunit transcripts (Fig. 9B).
In sum, these results proposed a pro-inflammatory role of FHR-3 on aged RPE cells, independent from blood-derived complement components, which may have a so far unknown impact on AMD progression (Fig. S 4).
RETC-2-ximab diminished the inflammatory effect of FHR-3
In our previous studies we generated a highly specific monoclonal mouse antibody (mAb) against human FHR-3, RETC-2. We showed that RETC-2 inhibits binding of FHR-3 to C3b and regained interaction of FH to C3b (10). Similar results could be observed regarding FHR-3 and FH binding to OSEs (CEP, MDA, MAA), as described afore (Fig. 2). To further investigate a therapeutic potential of RETC-2, chimerization was performed to replace complement-activating mouse regions of the mAb. The chimerized anti-FHR-3 mAb, called RETC-2-ximab, was tested in in vitro studies with polarized senescent ARPE-19 cells, which were treated apically for 24 h with FHR-3, FHR-3 and RETC-2-ximab or with FHR-3 and an antibody isotype control (control-ximab) (Fig. 10). Gene expression of C3 decreased significantly by 27% (Fig. 10A), CFB was reduced significantly by 48% (Fig. 10B), respectively, and C3AR mRNA expression was decreased by 21% (Fig. 10C) after combined treatment with FHR3/RETC-2-ximab compared to FHR-3 alone or with the unspecific control-ximab in aged ARPE19 cells.
These promising results could offer a potential recovery of local complement homeostasis and a reduced progression of retinal degeneration using RETC-2-ximab in the future.