Multidimensional Analysis of Lung Lymph Nodes in a Mouse Model of Allergic Lung Inflammation following PM2.5<math><mrow><mi>P</mi><msub><mrow><mi>M</mi></mrow><mrow><mrow><mn>2.5</mn></mrow></mrow></msub></mrow></math> and Indeno[1,2,3- cd]pyrene Exposure

Background: Ambient particulate matter with an aerodynamic diameter of ≤2.5μm$\le 2.5\mu m$ (PM2.5${\mathrm{PM}}_{2.5}$) is suggested to act as an adjuvant for allergen-mediated sensitization and recent evidence suggests the importance of T follicular helper (Tfh) cells in allergic diseases. However, the impact of PM2.5${\mathrm{PM}}_{2.5}$ exposure and its absorbed polycyclic aromatic hydrocarbon (PAHs) on Tfh cells and humoral immunity remains unknown. Objectives: We aimed to explore the impact of environmental PM2.5${\mathrm{PM}}_{2.5}$ and indeno[1,2,3-cd]pyrene (IP), a prominent PAH, as a model, on Tfh cells and the subsequent pulmonary allergic responses. Methods: PM2.5${\mathrm{PM}}_{2.5}$- or IP-mediated remodeling of cellular composition in lung lymph nodes (LNs) was determined by mass cytometry in a house dust mite (HDM)-induced mouse allergic lung inflammation model. The differentiation and function of Tfh cells in vitro were analyzed by flow cytometry, quantitative reverse transcription polymerase chain reaction, enzyme-linked immunosorbent assay, chromatin immunoprecipitation, immunoprecipitation, and western blot analyses. Results: Mice exposed to PM2.5${\mathrm{PM}}_{2.5}$ during the HDM sensitization period demonstrated immune cell population shifts in lung LNs as compared with those sensitized with HDM alone, with a greater number of differentiated Tfh2 cells, enhanced allergen-induced immunoglobulin E (IgE) response and pulmonary inflammation. Similarly enhanced phenotypes were also found in mice exposed to IP and sensitized with HDM. Further, IP administration was found to induce interleukin-21 (Il21) and Il4 expression and enhance Tfh2 cell differentiation in vitro, a finding which was abrogated in aryl hydrocarbon receptor (AhR)-deficient CD4+$\text{CD}{4}^{+}$ T cells. Moreover, we showed that IP exposure increased the interaction of AhR and cellular musculoaponeurotic fibrosarcoma (c-Maf) and its occupancy on the Il21 and Il4 promoters in differentiated Tfh2 cells. Discussion: These findings suggest that the PM2.5${\mathrm{PM}}_{2.5}$ (IP)-AhR-c-Maf axis in Tfh2 cells was important in allergen sensitization and lung inflammation, thus adding a new dimension in the understanding of Tfh2 cell differentiation and function and providing a basis for establishing the environment-disease causal relationship. https://doi.org/10.1289/EHP11580.