Granulomas are an accumulation of immune cells in the tissue, often the result of an overactive immune response. Granulomas contribute to several inflammatory systemic diseases such as sarcoidosis, berylliosis, and rheumatoid arthritis. For the first time, scientists at CeMM, the Medical University of Vienna and the Ludwig Boltzmann Institute for Rare and Undiagnosed Diseases have thoroughly characterized granulomas in the skin in immense detail. The results provide numerous insights into the composition, structure and signaling pathways of granulomas, providing clues for new therapeutic approaches. The study was published in the journal Immunity.

Granulomatous diseases like sarcoidosis pose significant challenges to physicians and researchers. With an unknown origin, sarcoidosis manifests as an inflammatory disorder with an overactive immune response, resulting in the formation of granulomas: clusters of cells that take on a nodular form. These granulomas are most frequently located in the lungs, followed by the skin, and can affect any other organ as well. Out of 100,000 individuals worldwide, between 1 and 35.5 people are diagnosed with sarcoidosis depending on the geographical region. For half of those affected, a spontaneous regression occurs within two years, while around 20% experience a chronic progression.

“Sarcoidosis of the skin can appear in a variety of colors and forms. For example, granulomas in the skin may appear at itchy clusters around old tattoos or scars, after filler procedures, or as a purple discoloration on the tip of the nose and cheeks, a condition also known as lupus perino,” says Anna Redl, PhD student and co-author of the study under project leader Georg Stary, Associate Professor at the Medical University of Vienna and CeMM Adjunct Principal Investigator. Stary adds, “The issue becomes particularly problematic when granulomas spread throughout the body, forming in organs, and causing problems which can lead to organ failure.”

Together with the research group led by CeMM Principal Investigator Christoph Bock, who is also a Professor at the Medical University of Vienna and co-study leader, the research team was able to create a detailed picture of the composition, structure, and components of granulomas from tissue samples for the first time using single-cell sequencing and spatial analysis methods.

The granuloma structure: macrophages, T cells, fibroblasts

According to Christoph Bock, granulomas are composed of various components of our immune system, but they have become disordered. “The cells communicate with each other, but they no longer understand each other – the signaling pathways do not function,” Bock says. “This is how granulomas are formed. More and more cells gather, activate each other, and lead to inflammation, for which there is no logical biological explanation.”

For the study, first authors Thomas Krausgruber and Daniele Barreca, scientists in Bock’s research group, together with Anna Redl, analyzed tissue samples from 12 patients with sarcoidosis and compared the granulomas to seemingly healthy skin. By using single-cell sequencing, the researchers were able to characterize immense detail of the granulomas. As Krausgruber explains, “We see that granulomas are primarily comprised of macrophages that serve as scavenger cells in the body, mainly to defend against unwanted viruses, bacteria, and toxins. In addition, we observed T cells, particularly the Th1 and Th17 subtypes, which are associated with inflammation processes. Around the macrophages and t cells, we see fibroblasts, which play an important role in remodeling connective tissue and give the granuloma its shape.”

Georg Stary adds, “We believe that the fibroblasts remodel collagen around it to allow the granuloma to grow, but the fibroblasts are also trapped within the structure at the same time.” Generally, the study shows that the granulomas on the skin bear a striking resemblance to those found in other regions of the body such as the lungs. Characterizing granulomas in the skin can provide insights into the treatment of sarcoidosis in other organs or granuloma-associated diseases.

Potential target for new therapeutic approaches: tissue enzymes

The protein MMP12, which plays an important role in tissue remodeling, was found to be strongly expressed in macrophages. Thomas Krausgruber explains: “This enzyme seems to play a significant role in the formation of granulomas. Initial in-vivo experiments in mouse models have already shown that inhibiting MMP12 reduces swelling. This is seen as an important starting point for the development of new therapies, but other results of our analysis also show potential for clinical testing.” 

Currently, patients with sarcoidosis are treated with cortisone preparations such as systemic glucocorticoids, which are administered over a long period of time and in high doses. The relapse rate for such treatments is also high. New targeted therapies that are based on the findings of this study, can greatly improve the possibilities and treatment for patients with granuloma-associated diseases.
 

The study “Single-cell and spatial transcriptomics identify aberrant developmental programs driving granuloma formation” was published on 6 February 2023 in the scientific journal Immunology, DOI: 10.1016/j.immuni.2023.01.014

Authors: Thomas Krausgruber*, Anna Redl*, Daniele Barreca*, Konstantin Doberer, Daria Romanovskaia, Lina Dobnikar, Maria Guarini, Luisa Unterluggauer, Lisa Kleissl, Denise Atzmüller, Carolina Mayerhofer, Aglaja Kopf, Simona Saluzzo, Clarice X. Lim, Praveen Rexie, Thomas Weichhart, Christoph Bock**, Georg Stary**;

*shared first authorship

**shared study lead

Funding: The study was funded with support from the Vienna Science, Research and Technology Fund (WWTF), grant number WWTF LS18-058.