pIV^−/− mice lack MHCII expression on thymic epithelial cells, and passive transfer of MHCII molecules to T cells is abrogated.

(A) Thymic sections were stained (brown) for MHCII, F4/80, CD11c, and epithelial cells (keratin). The counterstain is methylene blue. The strong reduction in MHCII expression in pIV^−/− thymuses is restricted to the cortex (compare upper 2 panels). In pIV^−/− thymuses, the residual patchy MHCII expression in the cortex overlaps with F4/80+ thymic macrophages (compare middle 2 panels). CD11c-positive DCs are restricted to the medulla (bottom left panel). The keratin stain indicates a normal architecture of the stroma in both the cortex and the medulla (bottom right panel). m indicates medulla; c, cortex. Original magnification, × 200 for all images in panels A and B. (B) Immunohistology of thymic medullary areas. MTS10+ and MHCII+ medullary cells appear blue and green, respectively. Double-positive cells appear cyan in the thymic medulla from the pIV^+/− control. No costaining is observed in the pIV^−/− medulla. (C-E) Passive transfer of MHCII molecules to thymocytes does not occur in pIV^−/− mice. (C) MHCII expression was analyzed by FACS on double-positive CD4⁺CD8⁺ thymocytes from pIV^−/−(gray profile) and pIV^+/− mice (open profile). (D) Double-positive CD4⁺CD8⁺ thymocytes derived from pIV^−/− bone marrow progenitors acquire MHCII molecules by passive transfer if they develop in the thymus of an irradiated wild-type recipient (open profile). On the other hand, wild-type thymocytes developing in a pIV^−/− recipient do not acquire MHCII molecules (gray profile). pIV^−/− donor cells (open profile) were identified by gating on Ly5.2+ cells. Wild-type donor cells (gray profile) were selected within the Ly5.1+ gate. (E) MHCII expression on double-positive CD4⁺CD8⁺ thymocytes is compared between pIV^−/− mice (gray profile) and I-Aα^−/−mice (open profile).