Probing in vivo origins of mononuclear phagocytes by conditional ablation and reconstitution

ILC3 in Axial Spondyloarthritis: the Gut Angle

PURPOSE OF REVIEW

A growing body of evidence supports the relevance of the interleukin-23/interleukin-17 (IL-23/IL-17) pathway for the pathogenesis of axial spondyloarthritis (axSpA) and its treatment. Recently, innate lymphoid cells (ILC), a heterogeneous family of immune effector cells, have been identified as a relevant contributor in tissue homeostasis, partially via IL-23/IL-17 axis. This review describes the biology and the origins of the group 3 ILCs (ILC3s) in humans, focusing on their role in the pathogenesis of axSpA.

RECENT FINDINGS

Clinical trials showed the effectiveness of IL23/IL-17 axis inhibition in both spondyloarthritis (SpA) and Inflammatory Bowel Disease (IBD). Recent findings confirm the high prevalence of subclinical gut inflammation in patients with SpA. Translational data in humans have demonstrated an increase in the number of ILC3s responsive to IL-23 and producing either IL-22 or IL-17 in the gut of SpA patients. The observation of gut-derived ILC3s in circulation and at inflamed tissues in patients with SpA suggest a recirculation of ILCs from mucosal site to lymphoid tissues and possibly enthesis and joints. Multiple observations demonstrate the expansion of IL-17- and IL-22-producing ILC3 in the subclinically inflamed gut of SpA patients. These innate immune cells, also observed in normal entheses, seem to be able to re-circulate from the gut to inflamed tissues of SpA patients, thus contributing to the disease perpetuation. The development of tools that can provide access to diseased tissue from sacroiliac joint and spinal entheses will provide valuable knowledge on the role of ILC3 in axSpA pathogenesis.

Paired immunoglobulin-like receptor A is an intrinsic, self-limiting suppressor of IL-5-induced eosinophil development

Eosinophilia is a hallmark characteristic of T helper type 2 (TH2) cell-associated diseases and is critically regulated by the central eosinophil growth factor interleukin 5 (IL-5). Here we demonstrate that IL-5 activity in eosinophils was regulated by paired immunoglobulin-like receptors PIR-A and PIR-B. Upon self-recognition of β₂-microglobulin (β₂M) molecules, PIR-B served as a permissive checkpoint for IL-5-induced development of eosinophils by suppressing the proapoptotic activities of PIR-A, which were mediated by the Grb2-Erk-Bim pathway. PIR-B-deficient bone marrow eosinophils underwent compartmentalized apoptosis, resulting in decreased blood eosinophilia in naive mice and in mice challenged with IL-5. Subsequently, Pirb(-/-) mice displayed impaired aeroallergen-induced lung eosinophilia and induction of lung TH2 cell responses. Collectively, these data uncover an intrinsic, self-limiting pathway regulating IL-5-induced expansion of eosinophils, which has broad implications for eosinophil-associated diseases.

Coordinate regulation of tissue macrophage and dendritic cell population dynamics by CSF-1

CSF-1 drives the homeostatic expansion of macrophages within the growing myometrium of pregnant mice by stimulating in situ proliferation and inducing monocyte precursor recruitment from the blood.

Tissue macrophages (Mϕs) and dendritic cells (DCs) play essential roles in tissue homeostasis and immunity. How these cells are maintained at their characteristic densities in different tissues has remained unclear. Aided by a novel flow cytometric technique for assessing relative rates of blood-borne precursor recruitment, we examined Mϕ and DC population dynamics in the pregnant mouse uterus, where rapid tissue growth facilitated a dissection of underlying regulatory mechanisms. We demonstrate how Mϕ dynamics, and thus Mϕ tissue densities, are locally controlled by CSF-1, a pleiotropic growth factor whose in situ level of activity varied widely between uterine tissue layers. CSF-1 acted in part by inducing Mϕ proliferation and in part by stimulating the extravasation of Ly6C^hi monocytes (Mos) that served as Mϕ precursors. Mo recruitment was dependent on the production of CCR2 chemokine receptor ligands by uterine Mϕs in response to CSF-1. Unexpectedly, a parallel CSF-1–regulated, but CCR2-independent pathway influenced uterine DC tissue densities by controlling local pre-DC extravasation rates. Together, these data provide cellular and molecular insight into the regulation of Mϕ tissue densities under noninflammatory conditions and reveal a central role for CSF-1 in the coordination of Mϕ and DC homeostasis.

Monocyte and macrophage dynamics during atherogenesis

Vascular inflammation is associated with and in large part driven by changes in the leukocyte compartment of the vessel wall. Here, we focus on monocyte influx during atherosclerosis, the most common form of vascular inflammation. Although the arterial wall contains a large number of resident macrophages and some resident dendritic cells, atherosclerosis drives a rapid influx of inflammatory monocytes (Ly-6C(+) in mice) and other monocytes (Ly-6C(-) in mice, also known as patrolling monocytes). Once in the vessel wall, Ly-6C(+) monocytes differentiate to a phenotype consistent with inflammatory macrophages and inflammatory dendritic cells. The phenotype of these cells is modulated by lipid uptake, Toll-like receptor ligands, hematopoietic growth factors, cytokines, and chemokines. In addition to newly recruited macrophages, it is likely that resident macrophages also change their phenotype. Monocyte-derived inflammatory macrophages have a short half-life. After undergoing apoptosis, they may be taken up by surrounding macrophages or, if the phagocytic capacity is overwhelmed, can undergo secondary necrosis, a key event in forming the necrotic core of atherosclerotic lesions. In this review, we discuss these and other processes associated with monocytic cell dynamics in the vascular wall and their role in the initiation and progression of atherosclerosis.