Fig. 3. Serotonin promotes retention and remodeling of DC function through HTR7 signaling and stimulates DC-dependent IgA B cell development.

A HTR7 is the dominant serotonin receptor in lamina propria CD45⁺ cells. FPKM (Fragments Per Kilobase of transcript per Million fragments mapped) values of all serotonin receptors in lamina propria CD45⁺ cells. n = 6 mice. B UMAP plot of single-cell transcriptome data³² of small intestinal lamina propria immune cells showing that expression of the Htr7 gene is restricted to LPDC cells. C Flow cytometry analyses of HTR7 expression in small intestinal lamina propria T cells, B cells, and DC. D Flow cytometry analyses of frequency of DC (Live, CD45⁺, MHC-II⁺, CD64⁻, CD11c⁺) in lamina propria CD45⁺ cells from the small intestine of mice with the indicated genotypes (n = 10 in WT and Tph2^−/− group, n = 9 in Cre+/−; Tph2^fl/fl group). E Differential gene expression in LPDCs treated with serotonin ±HTR7 antagonist SB269970. DESeq2 was used to calculate fold-change and adjusted p-value. Red points represent genes that exhibited significantly increased expression in the serotonin treatment group compared to the group treated with serotonin along with the HTR7 antagonist, while blue points indicate genes that showed significantly decreased expression in the serotonin treatment group compared to the group treated with serotonin along with the HTR7 antagonist. F Gene Ontology pathway analysis of data from E. G Tissue culture assay of IgA production from IgD⁺ B cells that were cocultured with LPDC, serotonin, and a HTR7 antagonist (SB269970) as indicated. The levels of IgA in the supernatants were measured by ELISA after 6 days of coculture (n = 12). Data are representative of three independent experiments. H Schematic of the system used to analyze BMDC homing to the gut in Tph2^fl/fl and Tph2^fl/fl; Hand2-Cre mice. Equal numbers of CFSE-labeled mature WT or CCR7^−/− BMDC were transplanted into Tph2^fl/fl and Tph2^fl/fl; Hand2-Cre mice and the CFSE⁺ cells in different tissues were analyzed 18 h post-transplantation. I Frequency of CFSE⁺ BMDC among CD45⁺ cells in the lamina propria of Tph2^fl/fl and Tph2^fl/fl; Hand2-Cre mice. HTR7 antagonist SB269970 was used to inhibit the HTR7 signaling pathway (n = 9), while CCR7^−/− cells were employed to block CCR7-dependent signaling (n = 6). J Fecal IgA levels in mock and HTR7 antagonist-treated mice. The mice were treated with either i.p PBS or an HTR7 antagonist every other day for 10 days. Fecal IgA measurements were carried out after the 10-day HTR7 antagonist treatment. n = 12 mice per group. K STm CFU burdens in spleens and livers of mock and HTR7 antagonist-treated mice 5 days post oral inoculation. The mice were treated as described in J before oral Salmonella infection. n = 9 mice in the mock and n = 11 mice in the treated group. Data shown are means ± SD. Statistical analysis was performed by a two-tailed Mann–Whitney test in D, G, and I–K.