Residual CXCL13 expression suffices for naive B cell screening of FDCs in virus-challenged PLNs. (A) Example of optical projection tomography images of WT (control, left) and -infected (right) PLN on day 8 post infection. The MECA-79⁺ HEV network is in red, and B220⁺ B follicles are green. Scale bar, 1 mm. (B) CXCL12, CXCL13, Cyb7b1, CH25H, and Ebi2 mRNA levels determined by quantitative polymerase chain reaction on day 8 post infection. Each dot represents an individual PLN. The red bar represents the mean. Levels of CXCL12, CXCL13, and CH25H mRNA are significantly reduced as compared with day 0 (P < .0001), as well as with mRNA levels of Ebi2 (P < .001) and Cyp7b1 (P < .01). (C) Example of 2-P microscopy image of LCMV-infected PLN. Arrowheads depict CXCR5^−/− B cells located at the edge or outside large and small B-cell clusters identified by CD35⁺ FDCs. Scale bar, 30 µm. (D) Percentage of microenvironmental track distribution as determined by 2-P microscopy. (E) Speeds of WT and CXCR5^−/− B cells inside CD35⁺ clusters. Each dot represents an individual track. The red bar represents the mean. Data in D and E are pooled from 4 mice, 7 image sequences, and 1049 WT and 176 CXCR5 ^−/− B-cell tracks. ***P < .001.