Vaccine 20 (2002) 3709–3719 A modified live Mycoplasma gallisepticum vaccine to protect chickens from respiratory disease L. Papazisi a,b , L.K. Silbart b,c , S. Frasca, Jr. a , D. Rood b,c , X. Liao a,b , M. Gladd a,b , M.A. Javed b,c , S.J. Geary a,b,∗ a Department of Pathobiology and Veterinary Science, The University of Connecticut, Storrs, CT, USA b Center of Excellence for Vaccine Research, The University of Connecticut, Storrs, CT, USA c Department of Animal Science, The University of Connecticut, Storrs, CT, USA Received 14 February 2002; received in revised form 19 June 2002; accepted 23 July 2002 Abstract The aim of this study was to assess the efficacy of a modified live Mycoplasma gallisepticum vaccine (GT5) for the protection of chickens against infection and respiratory disease. GT5 was constructed by the reconstitution of the avirulent high passage R (R high ) strain with the gene encoding the major cytadhesin GapA. GT5 expressed GapA on its surface yet retained the phenotypic characteristics of the parental R high strain. Birds vaccinated with GT5 were protected upon challenge with the virulent low passage R (R low ) strain as evidenced by a complete absence of tracheal lesions 2 and 4 weeks post-challenge, in contrast to sham immunized/challenged control birds. Modest amounts of IgG, and little, if any secretory IgA or IgM anti-M. gallisepticum were found in tracheal washings following vaccination. However, copious amounts of specific IgA were found following challenge, especially in sham immunized birds. This suggests that the tracheal IgG elicited by GT5 vaccination may have been responsible for blocking the initial colonization of R low , thereby resulting in protection. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Mycoplasma gallisepticum; Respiratory disease; Vaccine 1. Introduction Mycoplasma gallisepticum is the primary etiologic agent responsible for chronic respiratory disease (CRD) in chick- ens [1] resulting in substantial economic losses to poultry producers throughout the world. CRD causes significant re- ductions in egg production, hatchability, and the downgrad- ing of carcasses. Transmission can occur through the egg or by inhalation of contaminated airborne droplets, resulting in rapid disease transmission throughout the flock. Killed, whole-cell M. gallisepticum bacterins do not prevent col- onization of the chicken respiratory tract, but can reduce the severity of disease. These vaccines confer protection to chickens challenged with homologous strains, but are fre- quently ineffective against field strains [2–9]. This failure may in part be due to the organism’s capacity to alter the expression of antigenic surface proteins (phenotypic varia- tions), thereby providing a means of evading the host im- mune response. A live attenuated M. gallisepticum vaccine ∗ Corresponding author. Tel.: +1-860-486-0835; fax: +1-860-486-2794. E-mail address: geary@uconnvm.uconn.edu (S.J. Geary). may have a decided advantage over bacterins in this regard, in that similar phenotypic variations may be occurring, ex- posing the host to a wider array of antigens, and eliciting a broader protection against wild-type and variant strains. The R strain of M. gallisepticum, designated “R low ”, is pathogenic in chickens, producing air sacculitis and tracheal lesions. Repetitive in vitro passage of this organism gave rise to a highly attenuated strain “R high ” that produces little if any pathology in chickens, even when used at doses 10,000-fold higher than a pathogenic dose of R low [10]. Previous studies have demonstrated that mutations in M. gallisepticum R high resulted in the loss of expression of three proteins, e.g. the cytadhesin molecule GapA, cytadherence-related molecule A (CrmA), and a component of an ABC transport system, HatA [11]. Reconstitution of the R high strain with the GapA produced a strain (GT5) that expressed GapA on its surface, yet retained many of the phenotypic characteristics of the R high strain, including low levels of in vitro cytadherence. Surface expression of GapA in a non-pathogenic strain is highly desirable since an immune response to this molecule may be important in blocking attachment to the respiratory epithelium [12]. 0264-410X/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0264-410X(02)00372-9