Detection of the 20-kDa virulence-associated antigen of Rhodococcus equi in malakoplakia-like lesion in pleural tissue obtained from an AIDS patient

A malakoplakia-like lesion was detected in a pleural biopsy from an AIDS pa tient presenting clinical and radiologic features of pneumonia. Cultures of bronchoalveolar lavage and pleural fluid evidenced Rhodococcus equi as the causative agent of pleuro-pulmonary infection. Immunochemical characteriza tion of the R. equi isolate showed the presence of a strain similar to the ATCC 33704 reference strain presenting the capsular antigen of serotype 4, and the intermediate virulence-associated antigen of 20-kDa. Histopathology of the patient's pleural biopsy showed plaques of macrophages interspersed with lymphocytes, and intracytoplasmic cocci and bacilli in macrophages, w hich were variably acid-fast positive. Immunohistochemistry of cocci, bacil li and their degradation products resulted strongly positive when stained w ith a mouse monoclonal antibody (MAb) produced against the 20-kDa antigen. This finding could have important implications for the pathogenicity of R. equi for human beings, since we do not know yet all the factors involved in the formation of malakoplakia. Indeed, the results obtained in the present study, taken together with the results obtained for pigs inoculated with R . equi strains of intermediate virulence (Madarame ct al. 1998), raise the possibility that most strains presenting the 20-kDa antigen may be capable of inducing malakoplakia. If this hyphothesis is confirmed by immunohistoch emical analysis of human pulmonary malakoplakia cases due to R. equi, the d etection of this antigen may be extremely helpful in the diagnosis and trea tment of such patients. This is the first report of R. equi infection in hu man beings that suggests a relationship between pleural malakoplakia and th e virulence-associated antigen of 20-kDa.