Annexin-V binds subpopulation of immune cells altering its interpretation as an in vivo biomarker for apoptosis in the retina

In cells undergoing apoptosis phosphatidylserine, a major component of the plasma membrane, translocates to the outer leaflet where it provides eat-me signals for phagocytic recognition and is bound by annexin-V, an apoptotic marker. The need to track retinal ganglion cell death (RGC) in response to glaucomatous damage or optic neuropathy has led to the development of DARC (detection of apoptosing retinal cells) imaging, providing non-invasive, in vivo assessment of RGC death. Although the eye is an immune privileged site, resident and infiltrating immune cells are known to respond quickly to trauma or infection. Some immune cells have binding sites for annexin homologs; thus, their presence may confound estimates of apoptosis measured by annexin-V labeling. The purpose of this study was to re-examine the accuracy of annexin-V apoptotic labeling in the posterior eye and to temporally characterize contributions of non-apoptotic labeling in response to optic nerve (ON) injury. Here, we found annexin-V labeling consists of two phases. Initially, there is a rapid phase matching the time course of apoptotic cell death indicated by cleaved caspase-3 immunostaining observed ex vivo. This is followed by a sustained plateau phase that persists long after the peak of degeneration. We demonstrate that annexin-V binds to a specific subpopulation of myeloid cells in the retina, which were identified using simultaneous confocal scanning laser ophthalmoscopy. Optical coherence tomography and confocal imaging reveal these cells occupy the posterior hyaloid space above the retinal nerve fiber layer and at various retinal depths. Our results highlight the cellular morphological heterogeneity of non-apoptotic annexin-V labeling of retinal microglia. Accordingly, pharmacological depletion of microglia abolishes annexin-V labeling of elongated microglia in vivo revealing fainter labeling of round RGCs. Thus, consideration should be given to the time course of the immune response when interpreting fluorescently labeled annexin-V to visualize retinal cell apoptosis for clinical diagnosis.