The formation of aberrant lymphoid aggregates is a direct consequence of CCR7 deficiency and not due to inflammatory stimuli. (A) Stable bone marrow chimeras were generated, resulting in a donor chimerism greater than 95%. The phenotype of chimeric mice was analyzed 8 weeks after BM transplantation by hematoxylin staining in paraffin-embedded sections. Transfer of wt bone marrow (CD45.2) (4 × 10⁷) into irradiated congenic wt recipients (CD45.1) did not elicit the formation of lymphoid infiltrates in the mucosal tissue of the stomach (left). Transfer of CCR7-deficient bone marrow cells into wt recipients resulted in the formation of lymphoid aggregates in the gastric mucosa (right). Data are representative of 4 to 6 mice in each group. Original magnification, ×100. (B) CCR7^−/− mice raised under sterile conditions still developed aberrant lymphoid follicular-like structures in the gastric mucosa. Numerous aberrant lymphoid infiltrates were detected within the submucosa and lamina propria (indicated by an arrow) of CCR7^−/− mice raised under vancomycin, metronidazole, and ampicillin treatment over 8 weeks. Histologic sections from 2 representative mice of 8 are shown. Magnification, ×100; HE staining of frozen tissue sections.