1139 © Lippincott-Raven Publishers From the Division of Immunology, United Medical and Dental Schools of Guy’s and St Thomas’ Hospitals, London and the *Centre for Applied Microbiology and Research, Salisbury, Wiltshire, UK. Sponsorship: This work was supported by the EC Concerted Action against AIDS grant CT962345. Requests for reprints to: Dr L.A. Bergmeier, Division of Immunology, United Medical and Dental Schools of Guy’s and St Thomas’ Hospitals, Guy’s Hospital, London SE1 9RT, UK. Date of receipt: 6 January 1998; revised: 26 March 1998; accepted: 7 April 1998. Antibody-secreting cells specific for simian immunodeficiency virus antigens in lymphoid and mucosal tissues of immunized macaques Lesley A. Bergmeier, Elaine A. Mitchell, Graham Hall*, Martin P. Cranage*, Nicola Cook*, Michael Dennis* and Thomas Lehner Objectives: To examine whether the route of immunization affects the induction of antibody-secreting cells (ASC) in the circulation of macaques. The distribution of ASC in the rectal mucosa and lymphoid tissues following challenge with simian immunodeficiency virus (SIV) was investigated. Design: Macaques were immunized with recombinant SIV gp120 and p27 antigens by the targeted iliac lymph node (TILN) route of immunization or the nasal and rectal route, augmented by intramuscular immunization [naso-rectal intramuscular (NRI)]. The macaques were challenged with live SIV by the rectal route and ASC were assayed in the circulation before and after SIV challenge, and in the tissues removed at post-mortem. Methods: ASC were examined in the circulation by Elispot assay. Mononuclear cells were prepared from peripheral blood, iliac and axillary lymph nodes and spleen. Rectal tissue was treated by enzyme digestion to elute mononuclear cells. Results: TILN and NRI immunization induced circulating IgA and IgG ASC to both gp120 and p27. Following rectal challenge with SIV, TILN macaques were protected from infection whereas NRI route-immunized and unimmunized controls became infected. IgA ASC to p27 were increased significantly in the iliac lymph nodes of the TILN immunized macaques compared with unimmunized controls ( P < 0.05). Only IgA ASC were found in the rectal mucosa of the immunized protected macaques but both IgA and IgG ASC were detected in the unimmunized infected macaques. Overall the number of IgG ASC specific for p27 was significantly higher in the infected NRI and control macaques than in the protected macaques ( P < 0.02). A progressive increase in IgG but not IgA ASC was detected in the peripheral blood mononuclear cells of the unimmunized infected macaques. Conclusions: The results suggest that cells secreting IgA antibodies to p27 in the iliac lymph nodes of the TILN immunized macaques correlate significantly with protection from infection. The unimmunized infected macaques showed a progressive increase in IgG ASC in the peripheral blood after SIV challenge; this was found in the iliac and axillary lymph nodes and also in the spleen, suggesting that it is an immune response to the SIV infection. © 1998 Lippincott-Raven Publishers AIDS 1998, 12:1139–1147 Keywords: Simian immunodeficiency virus, antibody secreting cells, mucosal immunity, immunization route, vaccine