B6 PDMVs enable a limited number of normal B6 CD4⁺ T cells to induce GC reactions in CD154^−/− mice depleted of platelets. Platelets were depleted using 10 μg p0p3/4 24 hours before intravenous adoptive transfer of 4 × 10⁶ negatively selected, naive B6 CD4⁺ T cells intravenously and 10⁸ B6 platelets, B6 PDMV from 5 × 10⁸ platelets, or CD154^−/− PDMV from 5 × 10⁸ platelets. Twelve days after injection, spleens were harvested and processed for histologic examination of frozen sections. Blue represents T-cell zone (anti-CD4 and CD8 staining); red, B-cell follicles (anti-B220); green, peanut agglutinin (PNA; GC B cells stain PNA^hi). GCs are visualized as B220 PNA double-positive (green-yellow) areas of cells. Representative histology from the wild-type control consisting of B6 mice immunized with adenovirus alone (A) and CD154^−/− mice immunized with adenovirus and treated with naive B6 CD4⁺ T cells with CD154^−/− PDMV (B) or B6 PDMV (C). GCs were visualized and reported as GCs/PALS (D). A PALS unit consists of the follicular and T-cell zones surrounding a central arteriole. An example of a PALS unit is delineated in panel B with a dashed white line. The experiment was performed with 5 mice per group and repeated once (*2-tailed P = .014; **2-tailed P = .058).