Capsular polysaccharide from Mycoplasma mycoides subsp. mycoides shows potential for protection against contagious bovine pleuropneumonia

Abstract

Contagious Bovine Pleuropneumonia (CBPP) is a severe respiratory disease caused by Mycoplasma mycoides subsp. mycoides (Mmm) which is widespread in Africa. The capsule polysaccharide (CPS) of Mmm is one of the few identified virulence determinants. In a previous study, immunization of mice against CPS generated antibodies, but they were not able to prevent multiplication of Mmm in this model animal. However, mice cannot be considered as a suitable animal model, as Mmm does not induce pathology in this species. Our aim was to induce antibody responses to CPS in cattle, and challenge them when they had specific CPS antibody titres similar or higher than those from cattle vaccinated with the live vaccine. The CPS was linked to the carrier protein ovalbumin via a carbodiimide-mediated condensation with 1-ethyl-3(3-imethylaminopropyl) carbodiimide (EDC). Ten animals were immunized twice and challenged three weeks after the booster inoculation, and compared to a group of challenged non-immunized cattle. When administered subcutaneously to adult cattle, the vaccine elicited CPS-specific antibody responses with the same or a higher titre than animals vaccinated with the live vaccine. Pathology in the group of immunized animals was significantly reduced (57%) after challenge with Mmm strain Afadé compared to the non-immunized group, a figure in the range of the protection provided by the live vaccine.

Introduction

Mycoplasma mycoides subsp. mycoides (Mmm) is the causative agent of Contagious Bovine Pleuropneumonia (CBPP) (OIE, 2008). The infection causes severe respiratory symptoms and death results from respiratory distress caused by lung consolidation in up to 30% of cases in a naïve herd (FAO, 2002, Provost et al., 1987). It is geographically confined in sub-Saharan Africa, north of South Africa (Amanfu, 2009, Muuka et al., 2013) and recognised as a notifiable disease by the World Organisation for Animal Health (OIE) (OIE, 2014). The recommended vaccine T₁44, exhibits relatively poor efficacy both in the field and experimental evaluation (Masiga et al., 1996, Nkando et al., 2012). Use of antibiotic therapy is discouraged since it may promote the formation of carriers attributed to CBPP-infected cattle and emergence of resistant bacteria in the environment (Amanfu, 2007).

The virulence mechanisms involved during Mmm infection, or following vaccination against CBPP, are not fully understood. The enzyme GlpO is known to contribute to host cell death (Pilo et al., 2005), but immunization of cattle with the recombinant protein did not induce protection (Mulongo et al., 2013). Daily treatment of cattle with the immunosuppressive drug cyclosporin appeared to delay events that follow Mmm infection (Scacchia et al., 2007) while immunization with the N-terminal portion of lipoprotein Q significantly enhanced disease pathology (Mulongo et al., 2015). Studies have also linked the capsular polysaccharide (CPS) located on the outer surface or in the plasma membrane to virulence. The CPS, composed of 6-O-β-d-galactofuranosyl-d-galactose (Plackett and Buttery, 1964) is assumed to protect the pathogen from the bactericidal activity of complement and other host defence functions, and to trigger the inflammatory process in the infected host (Pilo et al., 2007). Intravenous injection of CPS to calves produced transient apnoea, increased pulmonary arterial pressure and pulmonary oedema, which may initiate thrombosis (Lloyd and Titchen, 1976). A strain of Mmm that produces low amounts of CPS was shown to be much more sensitive to growth inhibiting antisera and produced a longer duration of bacteraemia in a mouse model (March et al., 2000).

Both humoral and cell mediated responses may contribute to immunity. Studies have shown a correlation between severe disease and lower numbers of IFN- γ-secreting cells (Dedieu et al., 2006, Totte et al., 2013). However, since Mmm is not an intracellular pathogen, it is likely that antibodies are the major protective response. An inverse correlation between specific IgA titres, but not IgG titres, was found with severity of symptoms (Niang et al., 2006). In vitro studies on monoclonal antibody PK-2 specific for Mmm CPS (Rurangirwa et al., 2000) has been shown to inhibit mycoplasma adhesion to bovine lung epithelial cells (Mwirigi et al., 2010) and also block mycoplasma growth (Aye et al., 2015, Waite and March, 2002). Inhibition of mycoplasma adhesion to host cells by monoclonal antibodies has also been demonstrated in other studies (Fiorentin et al., 1998, Svenstrup et al., 2002, Chourasia et al., 2014, Rosales-pérez et al., 2014). This attribute of the CPS makes it a promising vaccine candidate as adhesion may be an important step in the establishment of disease (Aye et al., 2015).

Studies by Waite and March (Waite and March, 2002) have shown that vaccination of mice with a conjugated Mmm CPS significantly increased the antibody responses compared to what was produced by polysaccharide alone, however these responses did not prevent mycoplasma growth in mice. But since monoclonal antibodies against Mmm CPS could block binding to bovine lung epithelial cells (Mwirigi et al., 2010), an important step in the disease process (Aye et al., 2015), we hypothesized that a CPS conjugate vaccine in cattle might still protect against CBPP.