Induction of Staphylococcus aureus-specific IgA and agglutination potency in milk of cows by mucosal immunization

Abstract

Lactating cows were immunized with inactivated Staphylococcus aureus strains and concentrated culture supernatants. Application of a repeated mucosal immunization scheme resulted in significant levels of S. aureus-specific IgA in milk of dairy cows. Average IgA titers against whole cell S. aureus increased during the first 10 weeks of immunization after which a plateau level was reached and maintained during lactation. Immune whey agglutinated both bovine and human S. aureus strains including methicillin-resistant S. aureus (MRSA) strains and recognized extracted S. aureus proteins on Western blot. ELISAs to quantify milk IgA reactive with a number of S. aureus virulence proteins (e.g. enterotoxins, microbial surface component recognizing adhesive matrix molecules (MSCRAMMs) and immune modulating proteins) and cell wall components, demonstrated the polyclonality of the IgA. Correlations observed between agglutination and specific IgA titers for whey and for purified IgA suggested functionality of the induced antibodies. Milk from immunized cows may provide a way of producing potentially therapeutic polyclonal antibodies against S. aureus colonization and infection.

Introduction

Staphylococcus aureus is a gram-positive pathogen, responsible for an extensive range of pathologies in humans and animals. In humans, S. aureus is able to cause localized skin infections such as impetigo but also systemic life threatening conditions that include osteomyelitis and infective endocarditis [1]. In cows, S. aureus is the major cause of mammary gland infection, which is causing major economic loss to the dairy industry [2]. Besides being a pathogen, S. aureus is also a commensal, able to colonize the human nares, skin, intestines and other mucosal sites, which may all represent important reservoirs [1], [3], [4]. Colonization with S. aureus is a risk factor for the development of infections and, hence, its reduction may prevent infection [5]. Infection with S. aureus represents a serious and growing public health threat, because of the emergence of antibiotic-resistant strains in the community (community acquired methicillin-resistant S. aureus (CA-MRSA)) [6]. In addition to the current treatment modalities with antibiotics, alternative therapies against infection or colonization need to be developed. A potential alternative to antibiotics for treatment of S. aureus infections is the use of antibodies in the form of passive immuno-therapeutics [7], [8]. Preparations of pooled human intravenous immunoglobulins (IVIG's) have been used in the pre-antibiotic past to treat S. aureus infections and have gained renewed interest with the emergence of MRSA [9], [10]. Also, the development of anti-infective monoclonal antibodies against S. aureus has growing commercial interest, but this has not yet resulted in new therapeutics [9], [11].

Alternatively, methods to induce polyclonal antibodies against several infective and colonizing agents in bovine colostrum or milk have been explored [12], [13] and seem to have potential as a novel therapeutic modality against bacteria, fungi and viruses in the intestines or at other mucosal sites [14], [15], [16], [17]. Immunoglobulin A (IgA), the major class of antibody present in the mucosal secretions of most mammals, represents a first line of defense against invasion by inhaled, ingested or by other means acquired pathogens at vulnerable mucosal surfaces [8]. The IgA produced by the mucosa is secreted mainly in the form of secretory IgA (sIgA), which is a dimer of IgA in complex with a secretory protein component. High concentrations of IgA in human colostrum suggest that IgA plays an important role in the passive immune protection of the newborn [18], [19].

In this paper, a method to induce polyclonal IgA antibodies against S. aureus in milk of cows is explored. We focus on IgA because in human this class of antibody isotype represents the major line of defense of mucosal surfaces and seems most suitable for intestinal or topical therapeutic application. Lactating cows were immunized with an immune stimulant composed of cells of S. aureus strains from human or bovine origin and their culture supernatants. An immunization scheme was developed to induce a mucosal immune response in the mammary gland [16]. Specificities of induced IgA antibodies were assessed by whole cell S. aureus IgA Enzyme-Linked ImmunoSorbent Assay (ELISA), agglutination assays and Western blotting. The IgA response against a selection of S. aureus antigens e.g. microbial surface components recognizing adhesive matrix molecules (MSCRAMMs), staphylococcal enterotoxins (SEs), immune modulating proteins and cell wall components, was quantified by ELISA.