Bacillus cereus var. toyoi enhanced systemic immune response in piglets

Abstract

Probiotic bacteria have been suggested to stimulate the host immune system. In this study we evaluated the immunomodulatory effects of probiotic Bacillus cereus var. toyoi on the systemic immunity of piglets. A pool of 70 piglets was divided into a probiotic or control group. We determined the ratios of peripheral blood mononuclear cell (PBMC) subsets and measured proliferative responses and cytokine production of PBMCs and effects on vaccination responses. Blood samples of probiotic-treated piglets showed a significantly lower frequency of CD8^high/CD3+ T cells and CD8_low/CD3+ T cells and a significant higher CD4+/CD8+ ratio. IL-4 and IFN-γ production of polyclonally stimulated PBMCs was on average higher in the probiotic group. Specific proliferative responses of PBMCs to Influenza vaccination antigens were significantly higher and antibody titers against H3N2 Influenza and Mycoplasma vaccination antigens were on average higher in the probiotic group. In conclusion, B. cereus var. toyoi therefore alters the immune status of piglets as indicated by changes in the ratios as well as functionalities of systemic immune cell populations.

Introduction

The indigenous bacterial flora plays a key role in the development of the gut, including the gut-associated immune system, and thus plays an important role in defense against enteropathogenic bacteria. Prior studies have shown that bacterial colonization of the gut contributes to the development and maintenance of oral tolerance, and therefore not only local immune reactions but also systemic immunity is modulated by the intestinal flora (McGhee et al., 1992, Tanaka and Ishikawa, 2004). Non-pathogenic bacteria appear to possess only moderate immune stimulatory effects on the mature, gut-associated immune system and, under normal physiological conditions, indigenous bacteria should be tolerated by the local immune system of the host (Scharek et al., 2005, Westendorf et al., 2005). However, some probiotics have been shown to affect gut-associated immunity in animals and humans (Mack and Lebel, 2004), and certain probiotic bacteria may even affect systemic immunity (Schultz et al., 2003). It has previously been shown that the probiotic Lactobacillus rhamnosus enhanced the activity of circulating natural killer (NK) cells in healthy elderly (Gill et al., 2001), elevated IFN-γ production of PBMCs in infants with cow's milk allergy (Pohjavuori et al., 2004), and induced T cell hyporesponsiveness, including impaired ex vivo T helper subsets 1 and 2 responses without up-regulation of immune regulatory cytokines (Braat et al., 2004).

Bacillus cereus var. toyoi is used as a probiotic feed supplement in livestock farming, where it has been reported to contribute to higher weight gain, improved feed conversion ratios, a reduction in the incidence of liquid feces and post-weaning diarrhea and lower mortality rates of piglets (Alexopoulos et al., 2001, Baum et al., 2002, Gedek et al., 1993, Kirchgessner et al., 1993, Taras et al., 2005). However, B. cereus strains can be internalized by epithelial cells (Minnaard et al., 2004), and B. cereus var. toyoi, like almost all species of the Bacillus group, produces a hemolysin with lytic activity for epithelial cells (Pruss et al., 1999). In other studies, other Bacillus species were shown to be immunogenic in a murine model marked by enhanced expression of the cytokines TNF-α and IFN-γ (Braat et al., 2004).

Since this bacterium is not a member of the indigenous bacterial flora of swine, we hypothesized an immunomodulatory potential for B. cereus var. toyoi in pigs. In this study, we show that feed supplementation of piglets with B. cereus var. toyoi affects several aspects of the immune system, modulates the T cell status, and appears to possess adjuvant effects in vaccination studies.