GC formation is impaired in Dnmt1 hypomorphic mice. (A-C) C57BL/6 wild type (WT; A), or Dnmt1 R/⁺ (B) and Dnmt1 N/⁺ (C) were immunized with SRBC to induce GC formation. Animals were killed on day 10 and formalin fixed paraffin embedded splenic tissue was stained with anti-B220, PNA, anti-Ki67, and anti-Dnmt1. (D) Quantitative imaging of spleen tissue based on immunohistochemistry staining from Figure 3A revealed no significant difference in the size of primary follicles (*P = .18 and .52, t test), but decreased size of the GCs (E) and the number of proliferating cells per GC (ki67) in the Dnmt1 hypomorph animals (F). The numbers of animals per condition are shown in parenthesis.