Lymph node histomorphology and in situ hybridization in vaccinated monkeys challenged with SHIV-89.6PD viral RNA. A, Representative lymph node of monkey 493 (vaccine protected) . Inguinal lymph nodes were removed at day 70 after challenge, stained with hematoxylin-eosin and subjected to in situ hybridization. The picture shows the conserved lymph-node architecture and the presence of secondary follicles with expanded germinal centers and well-represented paracortical T cell regions. No SHIV viral RNA is detected. B, Representative lymph node of monkey 480 (unprotected). The picture shows the follicular depletion, paracortical atrophy and abundant presence of SHIV viral copies. Reprinted by permission from Macmillan Publishers Ltd: Nature Medicine, Chen, X. et al., 2001, copyright 2001, Nature Publishing Group .