Background IgG4-Related Disease (IgG4-RD) is a systemic inflammatory condition characterized by fibrous swelling of affected organs, serum IgG4 elevation, and IgG4+ plasmacells tissue infiltration. 18-Fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) scan is emerging as a promising imaging technique to detect organs involved by IgG4-RD and to assess disease response to treatment. The relationship between FGD-PET findings and immunological perturbations occurring in IgG4-RD has never been evaluated.

Objectives To correlate the intensity and distribution of FDG-PET uptake with clinical and immunological parameters in patients with active untreated IgG4-RD.

Methods Patients with active, untreated, biopsy proven IgG4-RD were included in the study. Disease activity was assessed through clinical (IgG4-RD Responder Index (RI)) and immunological (erythrocyte sedimentation rate (ESR), C reactive protein (CRP), serum IgG4, and circulating plasmablasts) parameters. Plasmablasts, a recently characterized disease biomarker, were identified as CD19+CD20-CD27+CD38bright cells on flow cytometry. FDG-PET/CT was performed in all patients at diagnosis. Quantitative assessment of FDG uptake was measured using the mean Standardized Uptake Value corrected for the Partial Volume Effect (PVC-SUV). Lymph nodes <1 cm of diameter were excluded from the analysis because of the risk of PVC-SUV over/under-estimation. In patients with multiorgan involvement, the IgG4-RD lesion with the highest PVC-SUV was selected to correlate FGD uptake with clinical and serological parameters.

Results We studied 15 patients (7 males, 8 females) with a mean age of 63 years (range, 30–77 years). Twelve (80%) patients had multiorgan IgG4-RD involving lymphnodes (7 patients); aorta (5 patients); parotids glands and pancreas (3 cases each); bones, skin, thyroid, lung, submandibular and lachrymal glands (2 cases each); meninges, nasal cavity, oropharynx, palate, liver, CNS, and retrorbital space (one case each). The median IgG4-RD RI was 9 (range 6–16; normal =0). The median levels of ESR, CRP, serum IgG4 and plasmablasts at baseline were 30 mm/h (range 6–121 mm/h, normal <20 mm/h), 11.0 mg/L (range 0.0–48.0 mg/L; normal <6mg/L), 284.0 mg/dL (range 45–2100 mg/dL, normal <121 mg/dL), and 3870 cells/mL (range 1000–10000 cells/mL, normal <690 cells/mL), respectively. The median PCV-SUV was 6.24 (range 2.48–16.39). Significant positive correlation was found between PVC-SUV and serum plasmablasts levels (r=.84, p=.004). No correlation was found between PVC-SUV and either CRP, ESR, serum IgG4 levels, number of organs involved, and IgG4-RD RI at baseline (p>.05).

Conclusions Our study demonstrates for the first time a positive correlation between circulating plasmablasts and inflammatory activity in IgG4-RD lesions as assessed by PVC-SUV on FDG-PET. Our results further strengthen the utility of circulating plasmablasts as a biomarker of disease activity. Conventional inflammatory markers, serum IgG4 levels, and IgG4-RD RI do not appear to correlate with metabolic activity in IgG4-RD lesions.

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Disclosure of Interest None declared