Disturbance of B-Cell Homeostasis In Chronic Graft-Versus-Host Disease of the Lung

Bronchiolitis obliterans syndrome (BOS) characterized by the new development of fixed airflow obstruction after allogeneic hematopoietic cell transplantation (HCT) is an increasingly recognized manifestation of chronic graft-versus-host disease (cGVHD) affecting up to 30% of patients. BOS is associated with increased non-relapse mortality and poor patient outcome after HCT. Its cause is currently unknown but allorecognition of lung antigens is the suspected etiology since BOS has been observed in states of alloimmunity such as cGVHD and after lung transplantation. Currently, the diagnosis of BOS is based on radiologic findings in high resolution expiratory chest CTs (HR-CT) and pulmonary function tests (PFTs). For early diagnosis of BOS biomarkers would be highly desirable allowing efficient treatment of early disease before progression to irreversibility is observed. B-cells are of importance in cGVHD and excess of B-cell activation factor (BAFF), increased CD19⁺CD21^- immature transitional B-cells and dysgammaglobulinemia characterize cGVHD.

We investigated circulating B-cell subpopulations in cGVHD patients to define novel cellular biomarkers for diagnosis of BOS and scoring its severity.

One hundred and thirty-five patients (74 males, 61 females) with a median age of 46 (range, 20–65) years, given myeloablative (n=76) or reduced-intensity (n=59) conditioning for related (n=108) or unrelated donor (n=27) HCT were prospectively evaluated for presence of cGVHD from day 100 after HCT and thereafter serially every 3 months for 2 years. At the same time points PFTs, multicolor flow cytometric analyses of peripheral blood (PB) cells stained for CD19, CD21, CD27, CD10, CD38, IgM, IgD, CD3, CD4, CD8, and CD56, measurements of serum immunoglobulin levels and BAFF were performed. BOS and its severity were defined according to the NIH consensus criteria. All patients with BOS except 2 had HR-CTs of the lung, bronchoalveolar lavage and transbronchial biopsies performed.

After a mean follow-up of 250 (range, 100–640) days after HCT 35 of 135 (26%) patients developed BOS. Other manifestations of cGVHD included skin in 72%, eyes in 58%, oral mucosa in 50%, and liver in 65% of patients. Severity of BOS consisted of NIH score I (FEV1=60-79%) in 14 (40%), NIH score II (FEV1=40-59%) in 16 (46%), and NIH score III (FEV<39%) in 5 (14%) patients, respectively. On day 100 after HCT serum IgG levels (860 vs 540 mg/dl, p=0.001) and CD4⁺ T-cells (17 vs 27%, p<0.0001) were significantly different between patients not experiencing cGVHD (n=63) and patients developing BOS (n=35) during follow-up. Furthermore, a significant difference between serum IgG levels (540 vs 780 mg/dl, p=0.009), CD4⁺ T-cells (27% vs 22%, p=0.04) and CD8⁺ T-cells (34% vs 42%, p=0.04) was observed between patients developing BOS and others experiencing cGVHD without lung manifestations (n=37). In patients with newly diagnosed BOS significantly higher BAFF levels (8.3 vs 4.0 ng/ml, p=0.0003), CD19⁺CD21^- immature transitional B-cells (19.5 vs 9.8%, p=0.01), BAFF/CD19⁺ ratios (0.35 vs 0.05, p=0.008), BAFF/CD21^- ratios (1.33 vs 0.43, p=0.01), CD19⁺CD21^-/CD19⁺CD27⁺ ratios (4.7 vs 1.9, p=0.04) and CD4⁺ T-cells (24.6 vs 19.6%, p=0.03) compared to patients without evidence of cGVHD were observed. Furthermore, patients with newly diagnosed BOS had a significant deficiency of CD19⁺ B-cells (160 vs 254 × 10⁶/L, p=0.027) and CD19⁺CD27⁺ memory B-cells (72 vs 154 × 10⁶/L, p=0.01), and significant lower serum IgG levels (570 vs 1260 mg/dl, p<0.001) compared to patients without cGVHD assessed at the same time point after HCT. A comparison of BOS patients with others presenting cGVHD without lung involvement at the same time point after HCT (day 250) revealed significant differences in BAFF levels (8.3 vs 4.4 ng/ml, p<0.0001), BAFF/CD19⁺ ratios (0.35 vs 0.06, p=0.005) and BAFF/CD21^- ratios (1.33 vs 0.517, p=0.012), respectively. Numbers of B-cell subsets and BAFF levels did not differ between lung severity scores, but mild BOS patients had significantly higher BAFF levels (6.4 vs 2.8 ng/ml, p=0.015) compared to patients with mild cGVHD without lung involvement.

Disturbance of B-cell homeostasis is more pronounced in cGVHD of the lung compared to other main organ manifestations. B-cell subpopulation numbers and excess of BAFF can serve as novel cellular biomarkers for BOS.

No relevant conflicts of interest to declare.