(A) A heatmap depicting the relative expression of queried receptor-ligand pairs from subset immune cell populations from mouse scRNA-Seq (Figure 3A and Supplemental Figure 4) (B) Chord diagram depicting the predicted lymphotoxin (LT) signaling interactions between sender (LTi cell) and receiver populations of LT family ligand and receptors from MCDA dataset with colors corresponding to the sender population (arrow direction corresponds to target populations, and flat ends correspond to sender population) and pointing to the receivers (outer ring color). (C) Chord diagram from CellChat of all ligand and receptor interactions from LTi cells to other intestinal cell populations (denoted by outer ring color). Arrowhead direction corresponds to predicted receiver. (D and E) IL-7R or LTBR expression in intestinal and fetal liver B cells (CD19⁺B220⁺ live cells) depicted in histogram and quantified MFI across B cell subsets (Figure 2C) (n = 3, 1-way ANOVA, **P < 0.01, ****P < 0.0001; data are shown as mean ± SEM). (F) Representative image from fetal mouse intestinal tissue (E18.5) mice stained with RNA probes Cd19 (white) to mark B cells and Rorc (green) to mark LTi cells, counterstained with DAPI (blue). Yellow box outlines selected ROI depicted to the right. Scale bar: 50 µM. Red arrows denote LTi and B cell interactions. (G) Quantification of B cell and LTi proximity analyzed by determining the percentage of Cd19⁺ cells within 20 μM of Rorc⁺ cells divided by total Cd19⁺ cells. (H) Representative human intestinal tissue section (21 weeks gestational age) stained with RNA probes CD19 (white) and RORC (green), counterstained with DAPI (blue) and a zoomed section outlined in yellow box, depicting LTi and B cell interactions in intestinal tissue. Scale bar: 90 µM. (I) Quantification of B cell and LTi proximity analyzed by determining the percentage of CD19⁺ cells within 20 μM of RORC⁺ cells divided by total CD19⁺ cells.