NK1.1⁺-cell help is required for IgM xenoantibody production by MZB cells, and is deficient in BALB/c nude mice. (A-C) Hamster RBC immunization was performed in naive or TM-β1–treated C57BL/6 nude mice. Serum IgM xenoantibody levels were measured on day 7 after immunization. (A) Bars represent means (± SE) MFI of 3 naive and 9 TM-β1–treated mice (*P < .05 for comparison between groups). (B) Histogram illustrates the MFI of 1 representative animal of both groups. (C) At 1 week before hamster heart xenografting, 6 × 10⁶ NK1.1⁺ or NK1.1⁻ cells from C57BL/6 nude naive mice were transferred to BALB/c nude naive recipients (Table 2; groups G,H). Serum IgM xenoantibody levels were measured at the time of graft rejection, or at day 7. Histograms illustrate 1 representative sample of both groups. (D-H) Electron microscopy of spleen tissue from (D) a lethally irradiated C57BL/6 nude mouse, (E) a lethally irradiated BALB/c nude mouse, (F) a BALB/c nude mouse first given transplants of C57BL/6 nude splenic tissue under the kidney capsule, subsequently given a hamster heart xenograft, and—when in the process of rejecting the xenograft—exposed to a lethal irradiation dose, and (G) a BALB/c nude mouse that first received NK1.1⁺ cells from C57BL/6 nude naive mice, subsequently was given a hamster heart xenograft and finally given lethal irradiation. All pictures were taken at the same original magnification (×11 000). Arrows indicate radioresistant large granular lymphocytes in the white pulp in panels D, F, and G, and these cells are absent in panel E. (H) Detail of a large granular lymphocyte from the same spleen shown in panel G and reveals the dense cytoplasmic granules (←; original magnification, ×19 000). Spleen samples used for electron microscopic examination were taken at 5 days after lethal irradiation. See “Light microscopy, immunohistochemistry, and electron microscopy” for complete image acquisition information.