ICI-induced Granulomatous Sialadenitis is Responsive to Prednisone

Problem
Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment but frequently cause immune-related adverse events (irAEs). While salivary gland involvement (sialadenitis) occurs in 5–35% of patients receiving ICIs, the specific mechanisms and histological presentations of these toxicities remain poorly understood. Existing literature suggests that ICI-induced sialadenitis can progress to chronic sicca syndrome with irreversible glandular damage, yet disease-modifying therapies are elusive. Furthermore, the interaction between prior targeted therapies (such as BRAF/MEK inhibitors) and subsequent ICI administration in the context of salivary gland toxicity had not been characterized. This study addresses the gap in understanding the pathogenesis of a novel, severe presentation: ICI-induced granulomatous sialadenitis (ICI-GS).

Methodology
The authors present a detailed case study of a male patient in his mid-fifties with BRAF-V600E-mutated papillary thyroid carcinoma. The patient underwent sequential treatment with BRAF/MEK inhibitors (dabrafenib and trametinib) followed by the addition of the PD-1 inhibitor pembrolizumab. Following the onset of severe xerostomia, the patient underwent a comprehensive clinical evaluation including salivary flow rate measurements (unstimulated and stimulated) and minor salivary gland biopsies.

To elucidate the molecular and cellular mechanisms, the study employed an integrative multi-omics approach on biopsy samples obtained before (ICI-GS-pre) and after (ICI-GS-post) a five-week prednisone taper:

1. Histopathology and Immunohistochemistry: Standard H&E staining and immunophenotyping (LCA, CD3, CD4, CD8, PD-1, PD-L1, CD68, CD20, CD138) were used to characterize granuloma structure and cellular composition.
2. Single-Cell RNA Sequencing (scRNA-seq): Dissociated salivary gland tissue was analyzed to quantify cellular composition, identify distinct cell populations (e.g., immune-engaged acinar cells), and assess gene expression pathways related to senescence and immune response.
3. Spatial Transcriptomics (Xenium): This technology was used to map cellular interactions and tissue architecture in situ, revealing the spatial relationship between immune cells and epithelial structures.
4. Bioinformatic Analysis: The authors calculated a composite senescence index based on established markers and performed ligand-receptor interaction analysis to map immune-epithelial crosstalk.

Key Contributions

• First Reported Case: This paper documents the first known case of ICI-associated granulomatous sialadenitis, a distinct histological presentation characterized by non-necrotizing granulomas lacking the dense fibrosis typical of sarcoidosis.
• Two-Hit Pathogenic Model: The study proposes a sequential mechanism where BRAF/MEK inhibitor therapy induces epithelial senescence and a pro-inflammatory microenvironment (the "first hit"), which is subsequently amplified by PD-1 blockade (the "second hit") to trigger cytotoxic T-cell-mediated granuloma formation.
• Molecular Characterization: The authors identify a specific molecular signature involving NOD2 upregulation, NF-κB pathway activation, and a Th1-polarized immune response driven by both CD4+ and CD8+ cytotoxic T cells.
• Therapeutic Response Profiling: The study provides a detailed molecular and histological map of the response to corticosteroids, distinguishing between the resolution of acute inflammation and the persistence of underlying tissue pathology.

Results

• Clinical Outcome: The patient experienced severe xerostomia (CTCAE Grade 3) after three cycles of pembrolizumab. Treatment with prednisone resulted in subjective improvement and a restoration of salivary flow rates (unstimulated flow increased from 0.0 to 3.6 mL/15 min).
• Histological Findings: Pre-treatment biopsies showed extensive, poorly organized non-necrotizing granulomas composed of epithelioid macrophages and T cells. Post-treatment biopsies demonstrated a 68% reduction in granuloma burden and a shift from a balanced CD4:CD8 ratio to CD4+ predominance. However, extensive inter- and intra-lobular fibrosis and epithelial atrophy persisted.
• Single-Cell Insights: Pre-treatment tissue showed a massive expansion of T cells (34% vs. 4% in controls) and M1 macrophages, alongside a significant depletion of acinar cells. A distinct population of "immune-engaged" (IE) acinar cells was identified, exhibiting upregulation of antigen presentation and interferon-stimulated genes.
• Senescence and Fibrosis: The epithelial senescence index was significantly elevated in pre-treatment samples and remained persistently high post-treatment, despite the reduction in inflammatory infiltrate. Spatial transcriptomics revealed that M1 macrophages in pre-treatment samples were actively phagocytosing dying epithelium. Post-treatment, while immune infiltration decreased, fibroblast accumulation increased, indicating ongoing fibrotic remodeling.
• Mechanistic Pathway: Ligand-receptor analysis confirmed extensive crosstalk mediated by TNF, IFNG, and MHC class I interactions. The granulomatous inflammation exhibited a sarcoidosis-like signature with upregulated NOD2 and downstream NF-κB components.

Significance
The paper claims that this case underscores the critical importance of early recognition and intervention in irAEs to preserve glandular function. It highlights that while corticosteroids effectively resolve acute inflammation and improve symptoms, they are insufficient to reverse epithelial senescence or prevent irreversible fibrosis. The authors suggest that future management strategies for ICI-induced salivary toxicity may require combination therapies targeting inflammation, senescence (senolytics), and fibrosis (anti-fibrotics) to maintain both quality of life and the ability to continue life-prolonging cancer therapies. The study demonstrates the utility of integrating single-cell and spatial transcriptomics to elucidate complex irAE mechanisms and guide personalized management.