IL1A enhances TNF-induced retinal ganglion cell death
Introduction: A growing body of literature suggests a role for neuroinflammation in retinal ganglion cell (RGC) death in glaucoma. For instance, deficiency of three proinflammatory cytokines, complement component 1, subcomponent q (C1q), interleukin 1 alpha (Il1a), and tumor necrosis factor (Tnf), resulted in significant protection of RGCs after glaucoma-relevant insults. While TNF and C1Q have been extensively investigated in glaucoma-relevant model systems, the role of IL1A in RGCs is not well defined.  Methods: Eyes of 2-4 month-old C57BL/6J mice or mice deficient in either Jun or Sarm1 were intravitreally injected with IL1A alone, TNF alone, or IL1A and TNF together. Retinal flat mounts were assessed for RGC survival using immunostaining of RBPMS. Bulk RNA-sequencing and differential expression analyses of retinal tissue was performed to determine molecular changes in response to IL1A, TNF, and IL1A combined with TNF within C57BL/6J and Sarm1 deficient mice.  Results: Intravitreal injection of IL1A did not result in RGC death at either 14 days or 12 weeks. Consistent with previous studies, TNF injection did not cause significant RGC loss at 14 days but did after 12 weeks. Together, IL1A+TNF resulted in a relatively rapid RGC death, driving significant loss two weeks after injection. We identified molecular changes which occur in response to IL1A and to combined IL1A+TNF treatment with limited changes identified in TNF alone treated eyes. Using mice deficient in Jun or Sarm1, we showed RGC loss after IL1A+TNF insult is JUN-independent and SARM1-dependent. Furthermore, RNA-seq analysis showed Sarm1 deficiency does not stop the neuroinflammatory response to IL1A+TNF.  Discussion: We identified a novel role of IL1A, we found that IL1A acted as a sensitizer to TNF-induced death. Co-injection of IL1A and TNF resulted in rapid RGC death, with significant RGC loss 14 days after injection. TNF+IL1A-induced RGC death did not depend on JUN activation and was rather SARM1 dependent.  Also, RNA-seq analyses indicated that while Sarm1 deficiency protected from IL1A+TNF induced RGC loss it did not significantly alter microglia and astrocyte responses. Altogether, these findings indicate that IL1A potentiates SARM1-dependent TNF-induced RGC death in vivo. Overall design: Eyes of 2-4 month-old C57BL/6J mice or mice deficient in either Jun or Sarm1 were intravitreally injected with IL1A alone, TNF alone, or IL1A and TNF together.  Retinas were dissected after 2, 14, or 35 days and subjected to tissue processing and RNA-sequencing.