Review of Mycobacterium avium subsp. paratuberculosis antigen candidates with diagnostic potential

Abstract

Mycobacterium avium subsp. paratuberculosis (MAP) is a slow growing bacterium that can infect ruminants and remain latent for years without development of any clinical signs or disease. Diagnosis is often based on detection of MAP antibodies in milk or serum samples or culture of bacteria from faeces; however, these diagnostic tools are often not applicable until years after infection. Detection of MAP specific cell-mediated immune (CMI) responses can serve as an alternative and be implemented in a diagnostic tool. CMI responses can be measured at an early stage of infection, prior to development of antibodies and shedding of detectable amounts of MAP. At present, available diagnostic assays are limited by the lack of MAP specific antigens included in these assays resulting in poor specificity. The objective of this review is to provide a systematic overview of diagnostic MAP antigen candidates described to date with special emphasis on antigen candidates tested for CMI responses. Relevant information on 115 different MAP antigens was systematically extracted from literature and summarized in 6 tables of CMI antigens, secreted antigens, cell wall and membrane antigens, lipoprotein antigens, heat shock antigens and hypothetical antigens. Strategies for evaluation of novel antigen candidates are discussed critically. Relatively few of the described antigens were evaluated for their use in CMI based diagnostic assays and so far, no obvious candidate has been identified for this application. Most of the novel diagnostic candidates were evaluated in few animals and it is recommended that an appropriate sample size is included for evaluation of antigen candidates in future studies.

Introduction

Mycobacterium avium subsp. paratuberculosis (MAP) is the causative agent of paratuberculosis in ruminants. MAP is part of the large and diverse Mycobacterium family that is defined by their acid-fast properties, mycolic acid-containing cell walls, and high genomic C + G content (61–71%). The family currently counts more than 130 established and validated species and subspecies (Turenne et al., 2007). Besides ruminants, MAP can infect a large range of host species including birds, rabbits, foxes and badgers (Beard et al., 2001). Infection of dairy cattle leads to economic losses because of reduced milk yield, premature culling and reduced slaughter value (Ott et al., 1999). Cattle are most susceptible to infection as calves where they are infected while still in the uterus or by ingestion of bacteria through milk, colostrum or faecal matter (Chiodini et al., 1984, Sweeney, 1996). Development of a cell-mediated immune (CMI) response is essential to the host in defence against intracellular MAP and is elicited at an early stage of infection before antibodies are detected. As the disease progress from subclinical to clinical, the CMI response wanes and is replaced by a strong humoral immune response characterized by antibody formation (Chiodini, 1996, Stabel, 2000). Antibodies do not protect the animal against the intracellular pathogen and in the final stages of infection suppression of the immune system and a state of anergy may occur (Waters et al., 1999). Diagnosis of MAP infected animals is frequently based on detection of antibodies in milk or serum, or by cultivation of bacteria from faeces, but these diagnostic methods are usually not applicable until years after infection at an advanced stage of disease (Nielsen and Toft, 2006). Contrary, CMI based diagnostics are applicable at an early stage of infection, prior to antibody formation and bacterial shedding in faeces. Hence, by using various diagnostic test strategies it is possible to detect infection with MAP at different stages of disease. The main limitation of available diagnostic tools is the lack of MAP specificity of included antigens resulting in poor specificity.

The aim of the current review was to provide an overview of MAP antigen candidates that has been used for diagnostic application to date. The emphasis will be on diagnostics measuring CMI, although humoral based approaches and different types of antigen will also be discussed.