BRIEF COMMUNICATIONS and CASE REPORTS Immunohistochemical Detection of Virulence-associated Antigens of Rhodococcus e4ui in Pulmonary Lesions of Foals

Rhodococcus equi was isolated from the lungs of six foals with bronchopneumonia. All isolates expressed 15-1 7-kd antigens by immunoblot analysis and contained a virulence-associated plasmid of 85 or 90 kb. Immunohistochemically, R. equi from all pulmonary lesions showed the expression of 15-17-kd antigens mainly in the phagocytic cells. The specific monoclonal antibody to 15-17-kd antigens of R. equi (MAb 10G5) may be an aid in the diagnosis of R. equi-induced pneumonia. Rhodococcus equi is an important gram-positive pathogen in foals <6 months old that causes suppurative broncho- pneumonia with abscessation, ulcerative enterocolitis, and occasionally other systemic lesions.1,2,5 This ubiquitous dis- ease has a mortality rate of 4O-8O0/o2 and reportedly accounts for more than 3% of all foal deaths4 The pathogenesis of R. equi infection in foals remains unexplained, although there have been several reports about virulence factors associated with R. equi infection.'as Re- cently, 15-1 7-kd antigens of R. equi have been recognized as candidates for virulence factors of R. equi infection be- cause all isolates from independent clinical cases in foals expressed antigens of these molecular masses by immunoblot analysis.8Jfl All isolates expressing 15-1 7-kd antigens con- tained a virulence-associated plasmid and were virulent in mice, whereas isolates without the plasmid and 15-1 7-kd antigens were avir~lent.~~~ Recent experiments with these an- tigens showed they are temperature regulated; when cells were grown at temperatures below 32 C, they did not express the antigens, whereas cells did express these antigens in large amounts when grown at temperatures above 34 C. The an- tigens were expressed on the cell surface, which suggests that they have a possible role in the pathogenesis of the disease.Ifl However, no reports have been published about the expres- sion and localization of the virulence-associated 15-1 7-kd antigens in lesions from affected foals. The purpose ofthis study was to use immunohistochemical methods to determine if the 15-1 7-kd antigens are expressed in lesions in R. equi-induced bronchopneumonia in foals and to ascertain the usefulness of the mouse monoclonal antibody against 15-1 7-kd antigens of virulent R. equi (MAb 1 OG5)' as an diagnostic tool in formalin-fixed, paraffin-embedded sections.