FIG. 4.

Y. pestis expression of the F1 antigen or Psa in infected mice. Immunohistochemistry of organs from mice challenged i.n. with 10⁵ Y. pestis KIM5 bacteria was performed. Murine tissues were stained with hematoxylin (blue). Bacteria expressing the F1 antigen (in brown) were detected with specific antisera in the spleens of mice that received no iron (A) or 4 mg iron once (B), as well as in the lungs of the latter mice (C). Bacterial numbers in the lungs of the former mice were too low for detection (see Fig. 1B). Similarly, bacteria expressing Psa were detected in the spleens of mice that received no iron (D) or 4 mg iron once (E), as well as in the lungs (F) and bronchial lymph nodes (H) of the latter mice, but not in the lungs of the former mice (not shown). Bacteria expressing Psa were also detected in the lungs of mice treated daily with 4 mg iron dextran (I). No bacteria were visualized in tissues stained with nonspecific antibodies (G). Organs were from day 2.5 (I), 4 (B, C, E, F, and H), or 5 (A, D, and G) p.i. Bars, 50 μm.