Current Sarcoidosis Models and the Importance of Focusing on the Granuloma

Abstract

The inability to effectively model sarcoidosis in the laboratory or in animals continues to hinder the discovery and translation of new, targeted treatments. The granuloma is the signature pathological hallmark of sarcoidosis, yet there are significant knowledge gaps that exist with regard to how granulomas form. Significant progress toward improved therapeutic and prognostic strategies in sarcoidosis hinges on tractable experimental models that recapitulate the process of granuloma formation in sarcoidosis and allow for mechanistic insights into the molecular events involved. Through its inherent representation of the complex genetics underpinning immune cell dysregulation in sarcoidosis, a recently developed in vitro human granuloma model holds promise in providing detailed mechanistic insight into sarcoidosis-specific disease regulating pathways at play during early stages of granuloma formation. The purpose of this review is to critically evaluate current sarcoidosis models and assess their potential to progress the field toward the goal of improved therapies in this disease. We conclude with the potential integrated use of preclinical models to accelerate progress toward identifying and testing new drugs and drug combinations that can be rapidly brought to clinical trials.

Keywords: granuloma; lung; macrophages; modeling; sarcoidosis.

Figure 1

Mechanisms underlying the initiation and evolution of sarcoidosis granuloma formation.

Figure 2

Schematic diagram of the in vitro human granuloma model.

Figure 3

M.tb antigen stimulation induces a unique and divergent transcriptional response in sarcoidosis. Differential gene expression in PBMCs derived from patients with sarcoidosis and healthy control subjects after (A) uncoated and (B) M.tb antigen-coated bead stimulation shown as mva style plots. The x-axis is the log2 average of the gene expression level. All genes with an adjusted P-value of 0.05 and at least a log2-fold change in the magnitude of gene expression (indicated by the two horizontal blue lines) between M.tb antigen and uncoated beads are shaded red.

Figure 4

Key immune cell populations are represented in the in vitro human granuloma model. (A) Immunofluorescence microscopy image showing CD11b⁺ macrophages and CD3⁺ lymphocytes are present in a representative granuloma-like structure formed by M.tb. antigen-simulated PBMCs from a M.tb. naïve sarcoidosis patient. The image is a composite of 3 fluorescent channels: blue, yellow, and cyan channels represent the M.tb. antigen-coated beads, CD11b staining, and CD3 staining, respectively. (B) A differential interference contrast image of the same granuloma-like structure.

Figure 5

CD163 expression is increased in granuloma-like structures formed by antigen-stimulated sarcoidosis PBMCs in the in vitro human granuloma model. Immunofluorescence microscopy imaging demonstrating macrophage CD163 expression upon stimulating healthy control PBMCs with uncoated beads (A) with dramatic loss of expression following 7-days M.tb antigen-stimulation (B). In contrast, abundant CD163 expression was observed on sarcoidosis macrophages after uncoated bead treatment (C) that persisted after M.tb antigen-stimulation (D). A 3D rendered volume of a sarcoidosis granuloma-like structure 7 days after M.tb antigen-stimulation showing centrally clustered CD163-expressing macrophages (E). DAPI and CD163 staining shown in blue and red, respectively.

Figure 6

Pathway analysis of gene expression identifies IL-13 as an important and common upstream mediator of the antigen-dependent sarcoidosis granulomatous response in the in vitro granuloma model (A), human sarcoidosis lymph node tissue (B), and lung biopsies (C).

Figure 7

Integrated and synergistic use of sarcoidosis models. Dashed arrows represent future pathways dependent on animal model advancements.

Figure 8

Different scenarios in which the in vitro human granuloma model can be used to better understand the dynamic, multicellular mechanisms underpinning distinct granulomatous immune responses that occur in humans ranging from (1) self-limited immune reactions; (2) foreign body immune reactions; (3) sarcoidosis granulomas; and (4) TB granulomas. The granuloma model can be used to test mechanistic hypotheses as well as further our understanding of the importance of environmental (e.g., the type of antigen used to promote granulomas) and host factors (e.g., genetics/race, epigenetic factors/sex, etc.) in these reactions. They can also be used for throughput testing of new potential therapies.