Studies on the pathogenicity of Rhodococcus equi isolates from patients with AIDS

马红球菌是一种兼性、细胞内、革兰氏阳性球菌，可引起慢性化脓性支气管炎和肠炎，并与1至3个月大的小马驹的高死亡率有关。近年来，马立克氏杆菌已成为免疫抑制患者，尤其是HIV感染者的一种重要的肺部病原体。我们最近报道了马来丝虫15-17 kDa的抗原与马驹和小鼠(致死性所需的10^6个细菌)的毒力有关，并且大质粒(85kb和90kb的毒力质粒)的存在对于毒力和15-17 kDa抗原的表达是必不可少的。最近，我们报道了一些艾滋病分离株通过免疫印迹含有一个20 kDa的抗原，并且在小鼠中是中等毒力的(致死性需要10^7个细菌)。此外，这些分离株含有大小为79至95kb的四个不同的大质粒之一。从马及其环境中经常分离到具有15-17 kDa抗原的马立克次体，但对具有20 kDa抗原的中等毒力马立克斯体的来源知之甚少。然后，我们使用针对这些毒力相关抗原的单抗，通过免疫印迹的方法分析了从猪颌下淋巴结分离的马来丝虫分离株的抗原。免疫印迹显示，几乎所有研究的猪分离株都存在一个20 kDa的抗原，这些分离株对小鼠具有中等毒力。我们还证明了20 kDa抗原的表达及其在小鼠中的致病性与5个大的、不同的、大小为79至95kb的质粒的存在密切相关，其中来自猪分离株的5个质粒中有2个的大小与来自人的分离株的大小相同。需要进一步的研究来阐明来自艾滋病患者的等值R.分离株的致病性，并揭示人类的感染来源和途径。

Rhodococcus equi is a facultative, intracellular, Gram-positive coccobacillus that causes chronic, suppurative bronchopneumonia and enteritis and is associated with high mortality in 1- to 3-month-old foals. Recently, R.equi has emerged as an important pulmonary pathogen among immunosuppressed patients, especially those with HIV infection. We have recently reported that the 15- to 17-kDa antigens of R.equi are associated with virulence in foals and mice (10^6 bacteria needed for lethality), and that the presence of large plasmids (85-kb and 90-kb virulence plasmids) is essential for virulence and expression of 15- to 17-kDa antigens. More recently, we reported that some AIDS isolates contained a 20-kDa antigen by immunoblotting and were intermediately virulent (10^7 bacteria needed for lethality) in mice. Moreover, these isolates contained one of four distinct large plasmids of 79 to 95 kb in size. Virulent R.equi having 15- to 17-kDa antigens has been isolated frequently from horses and their environment, but the source of an intermediately virulent R.equi having a 20-kDa antigen is poorly understood. Then, we analyzed antigens of R.equi isolates from the submaxillary lymph nodes of pigs by immunoblotting using monoclonal antibodies against these virulence-associated antigens. Immunoblots of whole-cell antigen preparations of R.equi pig isolates revealed the presence of a 20-kDa antigen in almost all the pig isolates studied, and these isolates were intermediately virulent for mice. We also demonstrated that the expression of a 20-kDa antigen and its pathogenicity in mice were associated strongly with the presence of five large, distinct plasmids of 79 to 95 kb in size, and two of the five plasmids from pig isolates were the same sizes as those from human isolates.Further studies are needed to elucidate the pathogenicities of R.equiisolates from patients with AIDS,and reveal the source and route of infection in humans.