Gamma Interferon Is Essential for Clearing Mycobacterium genavense Infection

Abstract

Factors determining the in vivo replication of the opportunistic pathogen Mycobacterium genavense are largely unknown. Following intravenous injection of a patient isolate, M. genavense could not be recovered by culture or detected by PCR in the livers or spleens of infected BALB/c mice. In contrast, M. genavense was found to chronically persist and multiply in the livers and spleens of intravenously infected syngeneic gammainterferon-gene-deficient (GKO) mice as evidenced by acid-fast stains of infected tissues and recovery by both PCR and liquid culture following organ homogenization. In GKO mice, M. genavense elicited a chronic inflammatory response, resulting in marked splenomegaly and extensive lymphadenopathy. Granulomatous lesions in the livers of GKO mice were diffuse, were composed of monocytes, neutrophils, and CD3؉ cells, and were histochemically negative for inducible nitric oxide synthase.Mycobacterium genavense is an opportunistic mycobacterium frequently recovered from pet animals (16) that mainly infects immunocompromised patients, particularly those with AIDS (5,22,26). Recently, however, M. genavense has also been detected in the lymph nodes of patients with no apparent primary immune deficiency (2,3,19). M. genavense is difficult to grow in vitro (4,7,17). It has a very low growth rate in the standard liquid cultures used for diagnostic purposes, and growth on solid agar media is almost undetectable (7). Longterm incubation of liquid media, particularly under low oxygen tension (23), followed by acid-fast staining and PCR amplification and sequencing of 16S rRNA is needed to ascertain the presence of M. genavense in diagnostic specimens (1-3, 5). In vitro studies determining susceptibility to various antibiotic agents are severely compromised due to the very limited growth of this organism (4, 7).The pathogenesis of M. genavense infection is not well understood due to the lack of an animal model of infection. For example, it is not known whether an immune deficiency (and if so, which type of deficiency) is truly necessary for infection to become established. In an effort to develop a robust and reproducible model of M. genavense infection in mice, we infected mouse strains (BALB/c or C57BL/6) carrying the susceptible allele of the bcg locus, because these mice are frequently used in studies involving other mycobacterial species, such as Mycobacterium avium (13,14,18). In order to address the question of whether the cytokine gamma interferon (IFN-␥), known to activate macrophages for mycobacteriostasis (6), is important for containing M. genavense infection in vivo, we also infected IFN-␥-gene-deficient (GKO) mice.Recovery of M. genavense from the spleens of infected mice. M. genavense was recovered from the blood of an AIDS patient by liquid culture (BACTEC 13A; Becton Dickinson Microbiology Systems, Cockeysville, Md.). Presence of M. genavense was assessed by acid-fast staining and PCR-mediated detection of mycobacterial 16S rRNA followed by sequencing. PCR for ribosomal DNA was performed by ...

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