Fig. 5: A bridging B cell compartment enables epitope spreading across an MHC barrier.

A Experimental setup. 1p = 1 part. B cells B and idiotype+ B cells C in inguinal and mesenteric lymph nodes (IngLN and MesLN), and spleen. D Anti-dsDNA antibodies in sera. E Representative bivariate plot showing gating for I-A^b/b, I-A^d/d, and I-A^b/d cells among B cells (left); a summary graph of these data (second from left); a representative bivariate plot showing gating for GC B cells (middle), and I-A^b/b, I-A^d/d and I-A^b/d gates among GC B cells (second from right); followed by a summary graph of these data (right). F GC B-cell frequencies across lymphoid organs. G Representation of I-A^d cells within GCs relative to their representation in the mature B-cell compartment, across IngLN, MesLN, and spleen. The dashed line through 100% indicates 1:1 representation. H Median fluorescence intensity (MFI) for I-A^b (left) and I-A^d (right), within the I-A^b/b, I-A^b/d, and I-A^d/d B-cell compartments of the chimeras, across IngLN, MesLN, and spleen. I Representative images of GCs in spleen of a bridge chimera. J Zoomed-in representative images of GCs in the spleens of bridge chimeras. K Frequencies of individual GCs dominated by I-A^b/b, I-A^d/d, or both. Quantification based on 6–50 GCs/mouse in 13 mice for a total of 326 GCs. *This population can consist of I-A^b/d positive cells only or any combination of I-A^b/b, I-A^b/d and/or I-A^d/d. L Plasma cell and plasmablast frequencies across IngLN, MesLN, and spleen. M I-A haplotype distribution among plasmablasts (left) and plasma cells (right) across IngLN, MesLN, and spleen. N Schematic interpretation of results. Throughout panels, error bars represent mean ± SD, and n = 27 mice total from two independent experiments. In B, C, F, and G, Kruskal–Wallis test with Dunn’s post-test was used, in H, two-way ANOVA with Tukey’s post-test, and in L, one-way ANOVA with Tukey’s post-test. Images in I–J are crop-outs of tile scans of spleen sections. Color intensities were adjusted uniformly for visual clarity. The data were procured 7–9 weeks post reconstitution. Scale bars represent 100 µm I and 50 µm J.