p50 and p52 expression in radiation-resistant stromal cells is required for T/B lymphocyte segregation. WT, Nfkb1^+/-Nfkb2^-/-, and LTβR^-/- mice were lethally irradiated and reconstituted with WT or Nfkb1^+/-Nfkb2^-/- bone marrow cells. Spleens from the indicated bone marrow chimeric mice were stained for T cells with Thy1.1 (blue) and B cells with B220 (brown) antibodies 2 to 3 months after reconstitution. At least 20 follicles from each spleen were scored for degree of T/B segregation (complete, partial, and disorganized). WT→WT (A) and Nfkb1^+/-Nfkb2^-/-→WT (C) chimeras showed 100% complete segregation. WT→Nfkb1^+/-Nfkb2^-/- (B) and WT→LTβR^-/- (D) chimeras exhibited 100% disorganized T/B segregation. The difference is very significant (P < .001). Data shown are representative sections from 3 spleens per group.