Endocrine Abstracts

Background: Diabetes is characterized by a prolonged latency between the onset of metabolic abnormalities and the clinical appearance of microvascular disease, consistent with the occurrence of vascular repair in the early stages. Patrolling monocytes (PMo), a subset of circulating monocytes, survey microvessels by crawling on the endothelium; their functional and biosynthetic characteristics suggest they play a role in the protection of the endothelium from vascular stress.

Hypothesis: In early diabetes, PMo adhere to retinal microvessels and exert protective functions, a potential mechanism for the latency that precedes the appearance of retinopathy.

Methods: Streptozotocin-diabetes was induced in male NR4A1-deficient mice (KO) that lack PMo, and in age-matched C57BL6/J controls (WT). To study retinal leukostasis, whole-mounted retinas from mice perfused intracardially were immunostained for CD45 (pan-leukocytic) and CD16.2 (PMo-specific) to identify and enumerate cells firmly adherent to microvessels. Retinal microangiopathy was assessed by counting acellular capillaries (AC) on retinal trypsin digests. To study their biosynthetic characteristics, RNA was isolated from circulating PMo sorted from mice after 5 months of diabetes using anti-CD45, CD11b, CD3, CD19, NK1.1, Ly6G, CD115, and Ly6C; and analyzed by RNAseq and qPCR.

Results: In WT mice, 3 months of diabetes increased firmly adherent leukocytes (Controls (C): 58±23/retina, Diabetes (DM): 93±18, P<0.001)) as well as PMo (C: 10±4/retina, DM: 37±6, P<0.001). Conversely, diabetes did not increase firmly adherent leukocytes in KO mice. AC were not increased after 4 months of diabetes in WT, nor KO mice. In WT mice, 6 months of diabetes increased AC (C: 5±1/mm² retina, DM: 7±1; P<0.01); the AC increase was more pronounced in KO-DM (12±3) when compared to WT-DM and KO-C (5±1) (P<0.001 vs both). RNAseq and qPCR of PMo from diabetic mice showed a pro-adhesive, anti-inflammatory, anti-apoptotic, vasculo-protective signature.

Conclusion: These data demonstrate several features of the role of PMo in diabetes including (i) more severe retinal microangiopathy in mice lacking PMo after 6 months of diabetes, (ii) increased PMo adherence to retinal microvessels at early time points of diabetes; and (iii) a vasculo-protective biosynthetic program of circulating PMo. Collectively, the findings suggest that in early diabetes PMo deliver to retinal microvessels protective/healing activities that counteract the damaging effects of diabetes. Leukostasis may thus represent, at least in part, a mechanism for healing, rather than a pro-inflammatory event as currently proposed.