Nerve-Langerhans cell interactions in diabetes and aging

Structural Changes of Cutaneous Immune Cells in Patients With Type 1 Diabetes and Their Relationship With Diabetic Polyneuropathy

BACKGROUND AND OBJECTIVES

Diabetic polyneuropathy (DPN) is a complication of diabetes characterized by pain or lack of peripheral sensation, but the underlying mechanisms are not yet fully understood. Recent evidence showed increased cutaneous macrophage infiltration in patients with type 2 diabetes and painful DPN, and this study aimed to understand whether the same applies to type 1 diabetes.

METHODS

The study included 104 participants: 26 healthy controls and 78 participants with type 1 diabetes (participants without DPN [n = 24], participants with painless DPN [n = 29], and participants with painful DPN [n = 25]). Two immune cells, dermal IBA1⁺ macrophages and epidermal Langerhans cells (LCs, CD207⁺), were visualized and quantified using immunohistological labeling and stereological counting methods on skin biopsies from the participants. The IBA1⁺ macrophage infiltration, LC number density, LC soma cross-sectional area, and LC processes were measured in this study.

RESULTS

Significant difference in IBA1⁺ macrophage expression was seen between the groups (p = 0.003), with lower expression of IBA1 in participants with DPN. No differences in LC morphologies (LC number density, soma cross-sectional area, and process level) were found between the groups (all p > 0.05). In addition, IBA1⁺ macrophages, but not LCs, correlated with intraepidermal nerve fiber density, Michigan neuropathy symptom inventory, (questionnaire and total score), severity of neuropathy as assessed by the Toronto clinical neuropathy score, and vibration detection threshold in the whole study cohort.

DISCUSSION

This study showed expressional differences of cutaneous IBA1⁺ macrophages but not LC in participants with type 1 diabetes-induced DPN compared with those in controls. The study suggests that a reduction in macrophages may play a role in the development and progression of autoimmune-induced diabetic neuropathy.

Lost in Translation? Measuring Diabetic Neuropathy in Humans and Animals

The worldwide diabetes epidemic is estimated to currently afflict almost 500 million persons. Long-term diabetes damages multiple organ systems with the blood vessels, eyes, kidneys and nervous systems being particularly vulnerable. These complications of diabetes reduce lifespan, impede quality of life and impose a huge social and economic burden on both the individual and society. Peripheral neuropathy is a debilitating complication that will impact over half of all persons with diabetes. There is no treatment for diabetic neuropathy and a disturbingly long history of therapeutic approaches showing promise in preclinical studies but failing to translate to the clinic. These failures have prompted re-examination of both the animal models and clinical trial design. This review focuses on the functional and structural parameters used as indices of peripheral neuropathy in preclinical and clinical studies and the extent to which they share a common pathogenesis and presentation. Nerve conduction studies in large myelinated fibers have long been the mainstay of preclinical efficacy screening programs and clinical trials, supplemented by quantitative sensory tests. However, a more refined approach is emerging that incorporates measures of small fiber density in the skin and cornea alongside these traditional assays at both preclinical and clinical phases.

Rats bred for low and high running capacity display alterations in peripheral tissues and nerves relevant to neuropathy and pain

INTRODUCTION

Diet and activity are recognized as modulators of nervous system disease, including pain. Studies of exercise consistently reveal a benefit on pain. This study focused on female rats to understand differences related to metabolic status and peripheral nerve function in females.

METHODS

Here, we investigated parameters of peripheral nerve function relevant to pain in rats selectively bred for high (high-capacity runners; HCR) or low endurance exercise capacity (low-capacity runners; LCR) resulting in divergent intrinsic aerobic capacities and susceptibility for metabolic conditions.

RESULTS

LCR female rats have reduced mechanical sensitivity, higher intraepidermal nerve fiber density and TrkA-positive epidermal axons, increased numbers of Langerhans and mast cells in cutaneous tissues, and a higher fat content despite similar overall body weights compared to female HCR rats. Sensory and motor nerve conduction velocities, thermal sensitivity, and mRNA expression of selected genes relevant to peripheral sensation were not different.

CONCLUSIONS

These results suggest that aerobic capacity and metabolic status influence sensory sensitivity and aspects of inflammation in peripheral tissues that could lead to poor responses to tissue damage and painful stimuli. The LCR and HCR rats should prove useful as models to assess how the metabolic status impacts pain.

Peripheral Neuropathy in Mouse Models of Diabetes

Peripheral neuropathy is a frequent complication of chronic diabetes that most commonly presents as a distal degenerative polyneuropathy with sensory loss. Around 20% to 30% of such patients may also experience neuropathic pain. The underlying pathogenic mechanisms are uncertain, and therapeutic options are limited. Rodent models of diabetes have been used for more than 40 years to study neuropathy and evaluate potential therapies. For much of this period, streptozotocin-diabetic rats were the model of choice. The emergence of new technologies that allow relatively cheap and routine manipulations of the mouse genome has prompted increased use of mouse models of diabetes to study neuropathy. In this article, we describe the commonly used mouse models of type 1 and type 2 diabetes, and provide protocols to phenotype the structural, functional, and behavioral indices of peripheral neuropathy, with a particular emphasis on assays pertinent to the human condition. © 2016 by John Wiley & Sons, Inc.

Langerhans cells regulate cutaneous innervation density and mechanical sensitivity in mouse footpad

Langerhans cells are epidermal dendritic cells responsible for antigen presentation during an immune response. Langerhans cells associate intimately with epidermal sensory axons. While there is evidence that Langerhans cells may produce neurotrophic factors, a role in regulating cutaneous innervation has not been established. We used genetically engineered mice in which the diphtheria toxin (DT) receptor is targeted to Langerhans cells (Lang-DTR mice) to assess sensory axon-dendritic cell interactions. Diphtheria toxin administration to wild type mice did not affect epidermal structure, Langerhans cell content, or innervation density. A DT administration regimen supramaximal for completely ablating epidermal Langerhans cells in Lang-DTR mice reduced PGP 9.5-immunoreactive total innervation and calcitonin gene related peptide-immunoreactive peptidergic nociceptor innervation. Quantitative real-time polymerase chain reaction showed that epidermal gene expression of brain derived neurotrophic factor was unchanged, but nerve growth factor and glial cell line-derived neurotrophic factor mRNAs were reduced. Behavioral testing showed that, while thermal sensitivity was unaffected, mice depleted of Langerhans cells displayed mechanical hypersensitivity. These findings provide evidence that Langerhans cells play an important role in determining cutaneous sensory innervation density and mechanical sensitivity. This may involve alterations in neurotrophin production by Langerhans or other epidermal cells, which in turn may affect mechanical sensitivity directly or as a result of neuropathic changes.

Angiotensin II receptor type 2 activation is required for cutaneous sensory hyperinnervation and hypersensitivity in a rat hind paw model of inflammatory pain

Many pain syndromes are associated with abnormal proliferation of peripheral sensory fibers. We showed previously that angiotensin II, acting through its type 2 receptor (AT2), stimulates axon outgrowth by cultured dorsal root ganglion neurons. In this study, we assessed whether AT2 mediates nociceptor hyperinnervation in the rodent hind paw model of inflammatory pain. Plantar injection of complete Freund's adjuvant (CFA), but not saline, produced marked thermal and mechanical hypersensitivity through 7 days. This was accompanied by proliferation of dermal and epidermal PGP9.5-immunoreactive (ir) and calcitonin gene-related peptide-immunoreactive (CGRP-ir) axons, and dermal axons immunoreactive for GFRα2 but not tyrosine hydroxylase or neurofilament H. Continuous infusion of the AT2 antagonist PD123319 beginning with CFA injection completely prevented hyperinnervation as well as hypersensitivity over a 7-day period. A single PD123319 injection 7 days after CFA also reversed thermal hypersensitivity and partially reversed mechanical hypersensitivity 3 hours later, without affecting cutaneous innervation. Angiotensin II-synthesizing proteins renin and angiotensinogen were largely absent after saline but abundant in T cells and macrophages in CFA-injected paws with or without PD123319. Thus, emigrant cells at the site of inflammation apparently establish a renin-angiotensin system, and AT2 activation elicits nociceptor sprouting and heightened thermal and mechanical sensitivity.

PERSPECTIVE

Short-term AT2 activation is a potent contributor to thermal hypersensitivity, whereas long-term effects (such as hyperinnervation) also contribute to mechanical hypersensitivity. Pharmacologic blockade of AT2 signaling represents a potential therapeutic strategy aimed at biologic mechanisms underlying chronic inflammatory pain.