CD40 in Retinal Müller Cells Induces P2X7-Dependent Cytokine Expression in Macrophages/Microglia in Diabetic Mice and Development of Early Experimental Diabetic Retinopathy

Abstract

Müller cells and macrophages/microglia are likely important for the development of diabetic retinopathy; however, the interplay between these cells in this disease is not well understood. An inflammatory process is linked to the onset of experimental diabetic retinopathy. CD40 deficiency impairs this process and prevents diabetic retinopathy. Using mice with CD40 expression restricted to Müller cells, we identified a mechanism by which Müller cells trigger proinflammatory cytokine expression in myeloid cells. During diabetes, mice with CD40 expressed in Müller cells upregulated retinal tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), intracellular adhesion molecule 1 (ICAM-1), and nitric oxide synthase (NOS2), developed leukostasis and capillary degeneration. However, CD40 did not cause TNF-α or IL-1β secretion in Müller cells. TNF-α was not detected in Müller cells from diabetic mice with CD40+ Müller cells. Rather, TNF-α was upregulated in macrophages/microglia. CD40 ligation in Müller cells triggered phospholipase C–dependent ATP release that caused P2X7-dependent production of TNF-α and IL-1β by macrophages. P2X7−/− mice and mice treated with a P2X7 inhibitor were protected from diabetes-induced TNF-α, IL-1β, ICAM-1, and NOS2 upregulation. Our studies indicate that CD40 in Müller cells is sufficient to upregulate retinal inflammatory markers and appears to promote experimental diabetic retinopathy and that Müller cells orchestrate inflammatory responses in myeloid cells through a CD40-ATP-P2X7 pathway.

Ligation of CD40 in Human Müller Cells Induces P2X7 Receptor–Dependent Death of Retinal Endothelial Cells

“…Indeed, expression of CD40 restricted to Müller cells was sufficient to cause capillary degeneration in diabetic transgenic mice. 46 Long-term studies in diabetic P2X 7 −/− mice may help elucidate the in vivo role of this receptor in capillary degeneration. We recently observed that P2X 7 receptor also promotes expression of proinflammatory cytokines in the diabetic retina.…”

“…We recently observed that P2X 7 receptor also promotes expression of proinflammatory cytokines in the diabetic retina. 46 Ligation of CD40 in human and mouse Müller cell lines caused secretion of ATP that in turn induced P2X 7 receptor–dependent IL-1β and TNF-α secretion by bystander myeloid cells. 46 Moreover, diabetic transgenic mice that expressed CD40 restricted to Müller cells exhibited retinal upregulation of IL-1β and TNF-α was dependent on P2X 7 receptor.…”

“…46 Ligation of CD40 in human and mouse Müller cell lines caused secretion of ATP that in turn induced P2X 7 receptor–dependent IL-1β and TNF-α secretion by bystander myeloid cells. 46 Moreover, diabetic transgenic mice that expressed CD40 restricted to Müller cells exhibited retinal upregulation of IL-1β and TNF-α was dependent on P2X 7 receptor. 46 These results together with the present study indicate that the CD40-ATP-P2X 7 receptor pathway plays an important role in induction of inflammatory responses and death of RECs, events considered pathogenic in the development of diabetic retinopathy.…”

“…CD40 enhances retinal leukostasis in diabetic mice, upregulates NOS2, increases retinal protein nitration and upregulates TNF-α and IL-1β in the retinas of diabetic mice. 15,46 All these phenomena have been linked to PCD of RECs and retinal capillary degeneration. 3,7,11–14,47 …”

Nrf2 Activation Is a Potential Therapeutic Approach to Attenuate Diabetic Retinopathy

“…37 Overall, these findings are consistent with previous studies in other retinal diseases showing that Nrf2 influences the integrity of the BRB. 32 Increasing evidence indicates that DR is a pro-inflammatory condition, and IL-6 and TNF-a as well as the chemokine MCP-1 and the leukocyte adhesion molecule, ICAM-1 are implicated in the pathogenesis of DR. 30,[38][39][40] Müller cells are recognized as an important mediator and source of the increased levels of these pro-inflammatory factors in DR. 10,11,41 The ability of dh404 to reduce the increased production of IL-6, TNF-a, MCP-1, and ICAM-1 in the retina of diabetic rats as well as Müller cells exposed to hyperglycemia highlights the importance of Müller cells in DR and the powerful anti-inflammatory capacity of Nrf2 activation.…”