Pregnancy-induced effects on memory B-cell development in multiple sclerosis

Abstract

In MS, pathogenic memory B cells infiltrate the brain and develop into antibody-secreting cells. Chemokine receptors not only define their brain-infiltrating capacity, but also assist in their maturation in germinal centers. How this corresponds to pregnancy, as a naturally occurring modifier of MS, is underexplored. Here, we aimed to study the impact of pregnancy on both ex vivo and in vitro B-cell differentiation in MS. The composition and outgrowth of peripheral B cells were compared between 19 MS pregnant patients and 12 healthy controls during the third trimester of pregnancy (low relapse risk) and postpartum (high relapse risk). Transitional, and not naive mature, B-cell frequencies were found to drop in the third trimester, which was most prominent in patients who experienced a pre-pregnancy relapse. Early after delivery, these frequencies raised again, while memory B -cell frequencies modestly declined. CXCR4 was downregulated and CXCR5, CXCR3 and CCR6 were upregulated on postpartum memory B cells, implying enhanced recruitment into germinal center light zones for interaction with T follicular helper (T_FH) cells. Postpartum memory B cells of MS patients expressed higher levels of CCR6 and preferentially developed into plasma cells under T_FH-like in vitro conditions. These findings imply that memory B- cell differentiation contributes to postpartum relapse risk in MS.

Figure 1

Frequencies of circulating naive and memory B-cell subsets during and early after pregnancy in MS patients and healthy controls. (A) Representative gating of viable CD19⁺ B-cell subsets: transitional (CD38^highCD27^-), naive mature (CD38^-/dimCD27^-IgM⁺), IgM memory (IgM⁺CD27⁺) and IgG memory (IgG⁺CD27⁺) B cells. The percentages of transitional and naive mature B cells (B) as well as IgM and IgD expression (MFI) on these subsets (C) were compared between paired first trimester (T1), third trimester (T3) and postpartum (PP) blood samples of 13 MS patients who did not experience a postpartum relapse (red) and 12 HC (blue). Similar analyses were performed for IgM⁺ and IgG⁺ memory B cells (D,E). Wilcoxon signed-rank test was performed to compared the different gestational periods. * p < 0.05, ** p < 0.01, *** p < 0.001.

Figure 2

Chemokine receptor expression on circulating memory B cells during and early after pregnancy in MS patients and healthy controls. (A) Schematic display of chemokine receptors involved in GC-dependent organization and maturation of B cells. Expression of dark zone-associated CXCR4 and light zone-associated CXCR5, CCR6 and CXCR3 was compared between IgM⁺ (B,D) and IgG⁺ (C,E) memory B cells from first trimester (T1), third trimester (T3) and postpartum (PP) samples from 13 MS patients without a postpartum relapse (red) and 12 HC (blue). Wilcoxon signed-rank test was performed to compare the different gestational periods. Two-way ANOVA with Bonferroni’s multiple comparison test was performed to compare HC with MS patients. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001.

Figure 3

In vitro outgrowth of antibody-secreting cells using memory B cells from paired third trimester and postpartum blood samples. (A) Representative gating of CD138-expressing antibody-secreting cells (CD38^highCD27^high) after culturing of memory B cells under IL-21/CD40L-stimulating (T_FH-like) conditions for 6 days. Fractions of CD38^highCD27^high plasmablasts/plasma cells (B), CD138⁺CD38^highCD27^high plasma cells (C) and IgM⁺ and IgG⁺ CD38^highCD27^high plasmablasts (D; FACS), as well as IgM and IgG secretion (E; ELISA) were compared between cultures with third trimester (T3) and postpartum (PP) memory B cells from 11 MS patients (red) and 5 HC (blue). Wilcoxon signed-rank test was performed to compare third trimester and postpartum samples. Two-way ANOVA was performed to compare MS and HC groups. * p < 0.05, *** p < 0.001.