Selected reports have documented microglia/immune responses to scatter
26–28 and focal
51,52 laser photocoagulation, but the high density of burns administered in the former studies and the excessive radiant exposures used in the latter studies, potentially render the data of limited relevance to DME treatment. In spite of differences in experimental regimes, however, common responses are evident when comparing the results of these studies with our own findings. These include rapid morphological transition/migration of microglia into the lesion site,
51,52 together with expression of immunological markers,
28 upregulation of leukocyte adhesion molecules,
26 and infiltration of macrophages and leukocytes.
27,28 It is likely these events are caused predominantly by injury to photoreceptors rather than to RPE cells, since vastly diminished outcomes were observed with the 2RT laser, which exclusively targets RPE cells (see below). The responses described above are characteristic of neuroinflammation, which is triggered primarily by microglia (the archetypal innate immune cell of the CNS), mediated via release of proinflammatory cytokines, and typically results in exacerbation of primary neuronal damage.
53–56 To shed more light on the inflammatory response to CW laser, we performed qPCR and immunohistochemistry for proinflammatory cytokines, with particular attention paid to IL-1β, “the master regulator of neuroinflammation.”
57 The results showed a striking upregulation of IL-1β, as well as TNFα, and a cellular pattern of IL-1β expression that was exclusive to microglia. It is well-documented that TNFα and IL-1β can stimulate trafficking of neutrophils/monocytes to sites of injury via upregulation of leukocyte adhesion molecules, chemokines, and MMP-9 on vascular endothelia.
58–61 Thus, our observations of these latter-named events after CW laser treatment are entirely consistent. By 3 days after laser, expression of proinflammatory cytokines had returned to baseline, while phagocytic microglia/macrophages were abundant in the subretinal space and denuded ONL. Interestingly, expression of osteopontin, a secreted glycoprotein protein with pleiotropic properties, was maximal at this time point. The function of osteopontin after laser treatment is unknown, but may involve promotion of photoreceptor survival
62 and remodeling of the extracellular matrix.
63