Fish monocytes as a model for mycobacterial host-pathogen interactions

Mycobacterium marinum, a relatively rapid-growing fish and human pathogen, has become an important model for the investigation of mycobacterial pathogenesis. M. marinum is closely related to the Mycobacterium tuberculosis complex and causes a disease in fish and amphibians with pathology similar to tuberculosis. We have developed an in vitro model for the study of M. marinum virulence mechanisms using the carp monocytic cell line CLC (carp leukocyte culture). We found that fish monocytes can differentiate between pathogenic and nonpathogenic mycobacterial species. Interestingly, M. marinum enters fish monocytes at a 40- to 60-fold-higher rate than Mycobacterium smegmatis. In addition, M. marinum survives and replicates in fish monocytes while M. smegmatis is killed. We also found that M. marinum inhibits lysosomal fusion in fish monocytes, indicating that these cells may be used to dissect the mechanisms of intracellular trafficking in mycobacteria. We conclude from these observations that monocytic cells from fish, a natural host for M. marinum, provide an extremely valuable model for the identification and characterization of mycobacterial virulence determinants in the laboratory.