In vitro phagocytosis of collagens by immortalised human retinal Muller cells

Purpose: This study is a first step to investigate phagocytosis of collagens by human retinal Muller cells, since Muller cells could be involved in remodelling of the vitreous and vitreoretinal interface in the human eye.

Methods: Muller cells in culture were exposed to 2.0 mu m fluorescent latex beads coated with BSA and human types I, II, and IV collagen and to non-coated beads for 2, 12, 24, and 48 h. To influence phagocytosis, cytochalasin B and anti-integrin subunits (alpha 1, alpha 2, and beta 1) were added to the cells. Phagocytosis was evaluated by flow cytometry, transmission electron microscopy (TEM) and confocal microscopy.

Results: Muller cells preferred to phagocytose beads coated with type II collagen compared with type IV collagen-, BSA- and non-coated beads. Phagocytosis of type I collagen- coated beads was intermediate. TEM and confocal microscopic evaluation confirmed phagocytosis of the beads. No significant differences were observed in phagocytosis of type II collagen- coated beads in the case of addition of cytochalasin B and anti-integrin subunits. Immunohistochemical analyses revealed that Muller cells were positive, under all tested circumstances, for vimentin and CRALBP. Less than 5% of the cells tested were GFAP positive.

Conclusions: Our observations demonstrate that human Muller cells in culture prefer to phagocytose type II collagen. In contrast, the phagocytosis of type IV collagen is comparable with the control coatings. We speculate that the relatively limited collagen phagocytosis by Muller cells supports a possible role for Muller cells in the slow process of vitreoretinal remodelling in adult human eyes.