Galectin-9 bridges human B cells to vascular endothelium while programming regulatory pathways.

Humoral immunity is reliant on efficient recruitment of circulating naive B cells from blood into peripheral lymph nodes (LN) and timely transition of naive B cells to high affinity antibody (Ab)-producing cells. Current understanding of factor(s) coordinating B cell adhesion, activation and differentiation within LN, however, is incomplete. Prior studies on naïve B cells reveal remarkably strong binding to putative immunoregulator, galectin (Gal)-9, that attenuates BCR activation and signaling, implicating Gal-9 as a negative regulator in B cell biology. Here, we investigated Gal-9 localization in human tonsils and LNs and unearthed conspicuously high expression of Gal-9 on high endothelial and post-capillary venules. Adhesion analyses showed that Gal-9 can bridge human circulating and naïve B cells to vascular endothelial cells (EC), while decelerating transendothelial migration. Moreover, Gal-9 interactions with naïve B cells induced global transcription of gene families related to regulation of cell signaling and membrane/cytoskeletal dynamics. Signaling lymphocytic activation molecule F7 (SLAMF7) was among key immunoregulators elevated by Gal-9-binding, while SLAMF7’s cytosolic adapter EAT-2, which is required for cell activation, was eliminated. Gal-9 also activated phosphorylation of pro-survival factor, ERK. Together, these data suggest that Gal-9 promotes B cell – EC interactions while delivering anergic signals to control B cell reactivity. In this report, we analyzed the effects of galectin-9 (Gal-9) on (2) donors A and B. The folllowing groups were analyzed by RNA isolation, cDNA synthesis and RNAseq: 1.) A2 and B2 - Buffer alone; 2.) A3 and B3 - Gal-9; 3.) A4 and B4 - Gal-9 + Lactose; and 4.) A5 and B5 - Lactose alone.