Study Characterizes Pathologic B-Cell Maturation in Crohn’s Disease
FROM CELLULAR AND MOLECULAR GASTROENTEROLOGY AND HEPATOLOGY
, according to investigators.
These findings begin to address a knowledge gap in Crohn’s disease that has been more thoroughly explored in ulcerative colitis, reported lead author Sonja Kappel-Latif, MD, PhD, of Medical University of Vienna, Vienna, Austria, and colleagues.
“Recent studies have investigated the role of B-cell responses in ulcerative colitis, which exclusively affects the colon, whereas data in CD, which mainly affects the terminal ileum, are insufficient,” the investigators wrote in wrote in Cellular and Molecular Gastroenterology and Hepatology. “Granuloma formation within the thickened, inflamed mesentery of patients with CD, however, is associated with significantly worse outcome, and microstructural analysis has suggested increased numbers of B cells in CD mesentery.”
Previous studies have shown that abnormal B-cell development in patients with CD leads to development of IgG targeting commensal — instead of pathogenic — gut bacteria. Yet B-cell receptor sequencing in CD has only been conducted on peripheral blood, despite awareness that anticommensal IgG antibodies can be transported across mucosal barriers in patients with ulcerative colitis, sustaining intestinal inflammation.
To better characterize local B-cell responses in CD, the investigators evaluated paired samples of draining mesenteric lymph nodes (MLNs) from both healthy and adjacently affected intestinal tissue, yielding a range of findings.
First, the investigators noted that CD19+ B cells and CD45+ leukocytes were expanded in affected MLNs, while T cells were reduced. A closer look showed that IgD-CD27- B cells were more abundant among CD19+CD45+ B cells in affected MLNs. Within this CD45+CD19+CD27+IgD- B-cell fraction, CD38- memory B cells were reduced.
The above findings suggest “ongoing antigenic stimulation within affected MLNs,” the investigators wrote.
Further comparison of paired samples showed that germinal centers (within which B cells mature) were significantly larger in affected MLNs, and contained dark and light zones. In contrast, healthy MLNs had smaller, more immature germinal centers.
Due to T-cell dependence during B-cell isotype switching within these germinal centers, the investigators next conducted immunohistochemistry staining for Bcl6, a “master regulator” of T-follicular helper cells expressed in class-switching B cells, and Ki67, which indicates cell proliferation. These analyses shows that both markers were “highly positive” within the germinal centers of affected MLNs.
Next, Dr. Kappel-Latif and colleagues conducted B-cell receptor (BCR) sequencing to characterize differences in class switching. Compared with healthy MLNs, affected MLNs showed decreased use of IGHA and IGHE alongside a significant uptick in IGHG1/2.
Further analyses showed that somatic hypermutation (SHM) frequency was significantly higher in IGHM and IGHA B cells, which was driven by mutations in complementary determining regions (CDRs) and framework regions of IGHA B cells, and mutations in the CDRs of IGHM B cells.
BCR diversity increased in the IGHG1/2 B cells, but remained unchanged in the IGHM or IGHA B cells.
“Overall, our results indicate ongoing class switching within draining MLNs of affected intestinal segments, with a shift toward IGHG1/2 BCRs,” the investigators concluded. “The lack of high SHM rates within IGHG1/2 BCRs, the difference between IGHA and IGHG1/2 BCRs in single MLNs, and increased diversity in IGHG1/2 BCRs suggests that many antigens do not result in long-lasting immunologic stimulation, and IGHA and IGHG1/2 responses may target different pathogens/commensals.”
The study was supported by the Austrian Science Fund and the Major of Vienna. The investigators disclosed no conflicts of interest.