Bone Marrow Plasma Cell Burden in Gaucher Disease Correlates with Local Macrophage Infiltration and May be Altered By Disease Treatment
Authors: Klimkowska Monika , Machaczka Maciej J. 
Published: November 2019

Introduction
Gaucher disease (GD), caused by deficient activity of lysosomal glucocerebrosidase, results in tissue infiltration by transformed macrophages (Gaucher cells, GC), loaded with undigested glycolipids. Ensuing clinical signs and symptoms include skeletal lesions and cytopenias, and seem to be due to both local tissue replacement by pathological infiltrates but also to accompanying inflammatory response, elicited by engorged macrophages. Bone marrow GC infiltration coincides with increased microvascular density and skewed angiogenic profiles. The basis for increased risk of plasma cell (PC) dyscrasia in GD remains however still unexplained. The presented study aimed to investigate patterns of PC distribution in bone marrow of untreated GD patients, potential their clinical correlates and evolution during treatment.
Methods
Study material includes BM samples from 11 previously untreated GD1 patients (7 males, 4 females), followed at the Hematology Center, Karolinska University Hospital, Stockholm, Sweden. Diagnosis of GD was previously confirmed with proven deficient glucocerebrosidase activity in PB, and demonstrated GBA1 mutation. The study was performed in accordance with Declaration of Helsinki, and institutional ethical permit was obtained. Clinical data were retrieved from patient journals. After BM sampling, 10 patients began disease specific therapy, with 9 subjects receiving enzyme replacement therapy (ERT), and one treated with substrate reduction therapy (SRT).
Bone marrow samples (trephine biopsies or fine needle aspirates) were taken as part of routine hematologic work-up, at baseline and, if needed, during follow-up. Samples were routinely processed at the Department of Clinical Pathology and Cytology at the same hospital. Plasma cell burden and distribution were studied in immunohistochemically stained sections (CD138), scanned at 40x magnification using the NanoZoomer S360 Digital slide scanner (Hamamatsu Photonics, Hamamatsu, Japan). NDPI.view2 Viewing software was used for scan evaluation, and image saving (JPEG format, 461 nm/pix resolution). For each sample, 5 fields with prominent Gaucher cell infiltrates (≥50% field area; GC-rich areas ), and 5 with &lt;50% GC infiltration (non-GC-rich areas) were registered. In the obtained pictures (442x 248 μm) all plasma cells were manually counted.
Results
At baseline, the patients were 21-86 years old (mean 58.8, median 42 yrs). All but one patient carried at least one N370S mutated GBA1 allele. Four were previously splenectomized; only one of the remainder group had no splenomegaly. Three patients had anemia, and 10 thrombocytopenia; one patient was not cytopenic. Ten patients had hyperferritinemia but only 2 had mildly increased IL-6 levels (Tab. 1).
Proteinogram studies identified M-protein in only 2 subjects, with oligoclonal Ig increase in 2, and polyclonal Ig in 3 patients. Decreased Ig levels were found in 2 patients.
Morphological analysis revealed multifocal, compact to dispersed Gaucher cell infiltrates, in all patients, with markedly varying GC burden. Notably, plasma cells were found in higher frequency in GC-rich areas (range 26.2-174.2/HPF), where their usual perivascular distribution pattern was only partially preserved (Fig. 1). Areas dominated by normal hematopoietic tissue demonstrated a significantly lower plasma cell infiltration (range: 2-128.2/HPF; Fig. 2). In patient 5, PC infiltration level was consistent with myeloma, M-protein was at 32 g/L, and difference in PB burden between GC-rich and non-GC areas was in his BM lower than in other patients.
Follow up BM samples were obtained in 5 patients, 12-14 months from treatment onset. In 4 of those, PC burden decreased in GC-rich areas but increased in areas with dominant normal hematopoiesis (Fig. 3). The longest documented follow-up was in the SRT subject (over 6 years, patient 1), where similar trends could be observed. Patient 4 initially increased PC burden in BM, and patient 5 succumbed to myeloma and MDS shortly after study onset.
Conclusion
Bone marrow in GD1 demonstrates increased plasma cell density, particularly in areas affected by the disease. This phenomenon seems to be at least partly modified by disease specific therapy, albeit at a slow pace. N370S mutation was associated with dysimmunoglobulinemia but did not coincide with MGUS in most patients; unexpectedly, the myeloma patient was N370S heterozygous.

Disclosures
No relevant conflicts of interest to declare.