Immunogenic and protective ability of the two developmental forms of Chlamydiae in a mouse model of infertility Sukumar Pal*, Javier Rangel, Ellena M. Peterson, Luis M. de la Maza Department of Pathology, Medical Sciences I, Room D440, University of California, Irvine, Irvine, CA 92697-4800, USA Received 20 October 1998; received in revised form 19 January 1999; accepted 20 January 1999 Abstract To compare the ability of elementary bodies (EB) and reticulate bodies (RB) of the Chlamydia trachomatis mouse pneumonitis (MoPn) biovar to induce a protective immune response, two groups of BALB/c mice were inoculated and boosted twice, with UV-inactivated EB or RB in Freund's adjuvant. Two weeks after the last immunization mice were challenged with C. trachomatis in the ovarian bursa. Vaginal cultures collected for 6 weeks after the intrabursal challenge showed that mice inoculated with EB were signi®cantly protected, while mice inoculated with RB were not. Six weeks after the genital challenge mice were mated. Mice immunized with EB showed signi®cant protection as demonstrated by the number of animals which were fertile and the number of embryos present in the uterine horns. In contrast, no signi®cant protection against infertility was observed in the mice immunized with RB. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Chlamydia trachomatis; Vaccine; Infertility 1. Introduction Several long-term sequelae may occur in women fol- lowing a Chlamydia trachomatis genital infection. Among these, chronic abdominal pain, ectopic preg- nancy and infertility are some of the most devastating complications [1±5]. Chlamydial isolates are susceptible to several types of antibiotics. Unfortunately, a ma- jority of the infections in females are asymptomatic and even in those that are symptomatic implemen- tation of therapeutic measures may be too late to pre- vent the development of sequelae [4]. For these reasons signi®cant eorts have focused on the development of a vaccine [6±9]. Chlamydiae are obligate intracellular pathogens with a characteristic developmental cycle. This cycle alter- nates between two forms: the elementary bodies (EB) which, although metabolically dormant, are the infec- tious form of the organism and the reticulate bodies (RB), which are the replicative form of this bacterium but are not infectious [10]. The EB measure 300 nm in diameter and have a rigid membrane, while the RB are up to 1000 nm in diameter and have a pliable mem- brane. Very little information is available on the rela- tive immunogenic and vaccinogenic ability of the two developmental forms of Chlamydiae. Since most of the growth cycle of the organism occurs while the RB are dividing, an immune response during this stage of the developmental cycle could halt the progression of the infection. Based on this possibility, it was decided to test the immunogenic ability of these two forms of C. trachomatis. 2. Materials and methods 2.1. Organisms The C. trachomatis mouse pneumonitis (MoPn) strain Nigg II was obtained from the American Type Culture Collection (Rockville, MD) and grown in Vaccine 18 (2000) 752±761 0264-410X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0264-410X(99)00032-8 www.elsevier.com/locate/vaccine * Corresponding author. Tel.: +1-949-824-7450; fax: +1-949-824- 2160. E-mail address: spal@uci.edu (S. Pal)