Type I Interferon Response Identified through Phenotypic and Transcriptional Profiling of Circulating Immune Cells during Idiopathic Multicentric Castleman Disease Flare

Idiopathic multicentric Castleman disease (iMCD) is a rare hematologic illness involving episodic disease flares with polyclonal cytokine-induced lymphoproliferation, systemic inflammation, and life-threatening multi-organ dysfunction. iMCD is further classified by clinical features and the most severe cases of iMCD often fall into the thrombocytopenia, anasarca, fever/elevated C-reactive protein, reticulin myelofibrosis, renal dysfunction, and organomegaly (TAFRO) clinical subtype. A cytokine storm involving interleukin(IL)-6 drives disease pathogenesis in a subset of patients, however only 34% of patients were found to respond to anti-IL-6 therapy with siltuximab in its registrational clinical trial. The identification of next generation therapeutics for iMCD-TAFRO patients has been challenging as the etiology, pathological cell types, and signaling pathways involved in iMCD-TAFRO are largely unknown.

In this study, we aimed to identify cellular drivers and pathophysiological mechanisms of iMCD-TAFRO through the use of an unbiased multi-omics approach. We obtained paired bulk peripheral blood mononuclear cells (PBMCs) from a cohort of ten iMCD-TAFRO patients isolated during disease flare and clinical remission. These paired PBMC samples were utilized for flow cytometry to assess immune cell frequency and phenotype between iMCD-TAFRO flare and remission as well as between flare and age/sex matched healthy donors (n = 10). Three paired iMCD-TAFRO samples were also selected for transcriptional profiling using single-cell RNA sequencing (scRNAseq).

We observed phenotypic differences across the T cell, monocyte, and NK cell compartments. We observed a significant increase in the frequency of CD8 T cells within the T cell compartment as well as an increased frequency of granzyme B and perforin expressing CD8 T cells in iMCD-TAFRO flare compared to healthy donors. We also observed a significant increase in the frequency of CD56+ NK cells within the NK cell compartment and a significant increase in the frequency of CD14+ monocytes within the monocyte compartment during iMCD-TAFRO flare compared to healthy donors. Together, these data suggest activation and involvement of CD8 T cell, NK cell, and monocyte subsets during iMCD-TAFRO flare.

We next utilized Gene Set Enrichment Analysis (GSEA) of our single-cell transcriptomics dataset to ask whether circulating immune cell frequencies display enrichment of the 50 Hallmark gene sets during flare compared to remission across three iMCD-TAFRO patients. We found significant enrichment (FDR < 0.01) of genes within the Interferon Alpha Response gene set in circulating non-naïve CD8 T cells, classical monocytes, nonclassical monocytes, NK cells, and dendritic cells. These data suggest that a number of circulating immune cell populations may be responding to Type I interferon (IFN-I) during iMCD-TAFRO flare. In addition, our group has recently reported mTORC1 activation in iMCD and has characterized in three iMCD-TAFRO patients a clinical response following mTOR inhibition with sirolimus. Within our scRNAseq dataset, we identified mTORC1 signaling to be enriched only in circulating classical and nonclassical monocytes during flare.

Having observed enrichment of both the Interferon Alpha Response Gene set and the mTORC1 signaling gene set in circulating monocytes, we then asked whether the relative expression of IFN-I response genes and mTORC1 signaling genes are correlated within circulating classical and nonclassical monocytes. Indeed, we observed a significant positive correlation between the average relative expression of mTORC1 signaling genes and IFN-I Response genes across classical, but not nonclassical, monocytes from all three iMCD-TAFRO patients (all R2 ≥0.6, p<0.0001).These data suggest a potential link between IFN-I signaling and mTORC1 signaling activation in classical monocytes in iMCD-TAFRO flare. Following further characterization of this association, we propose investigation into the use of Janus kinase (JAK) inhibitors in iMCD, as JAK is a proximal signaling molecule downstream of the IFN-alpha/beta receptor (IFNAR) and upstream of mTORC1 signaling. In summary, this dataset demonstrates involvement of multiple immune cell populations and identifies IFN-I signaling as a novel inflammatory gene program that may contribute to iMCD-TAFRO pathogenesis and drive treatment identification.