Lymphoid follicle-like aggregates are formed within the mucosal tissue of the gastrointestinal and respiratory tract of CCR7^−/− mice. (A) Colons of wt and CXCR5^−/− mice (representative lymphoid follicle indicated by an arrow) exhibited normal numbers of gut-associated lymphoid aggregates compared with CCR7^−/− mice. In CCR7^−/− animals, lymphoid aggregations were larger and more frequent. (B) Mucosal tissue of the lung exhibited enhanced lymphoid aggregates in CCR7^−/− mice (depicted at the age of > 6 months) compared with wt and CXCR5^−/− mice. (C) Mucosal tissue of the stomach of wt and CXCR5^−/− mice showed no lymphoid infiltrates, whereas CCR7^−/− mice harbored a large number of lymphoid follicular-like structures within the submucosa and lamina propria. (D) In the mucosal tissue of the stomach, aberrant lymphoid infiltrates were already detected at the age of 2 weeks in CCR7^−/− mice (indicated by an arrow). At the age of 4 weeks, the onset of follicular-like aggregates was first observed (indicated by arrows). Data are from 1 representative mouse of 6 to 8 mice analyzed per group (A-D). Original magnification, ×100 (A-D); hematoxylin-eosin (HE) staining of paraffin-embedded tissue.