Characterization of the host macrophage response to intracellular protozoan pathogens
The ability to reside and proliferate in macrophages is characteristic of several infectious agents that are of major importance to public health. In order for these pathogens to survive in the hostile macrophage environment, they must develop mechanisms to evade the microbicidal activities of the macrophage and to subvert the host immune response. Using broad transcriptional profiling as well as more targeted approaches, we have further elucidated the ways intracellular protozoan pathogens interact with the host macrophage, providing a better understanding of parasite immune evasion.
In order to determine how the macrophage response to intracellular parasites fits into the diverse range of possible macrophage activation programs, we utilized genome-wide microarray analysis to compare the responses of mouse macrophages following infection by the intracellular parasites Trypanosoma cruzi and Leishmania mexicana, the bacterial product lipopolysaccharide (LPS), and various cytokines. We found that infection by L. mexicana produced so few transcriptional changes that the infected macrophages were almost indistinguishable from uninfected cells. Furthermore, the transcriptional response of macrophages infected by the protozoan pathogens more closely resembled the transcriptional response of macrophages stimulated by the cytokines IL-4, IL-10, and IL-17 than macrophages stimulated by Th1 cytokines IFNG, TNF, and IFNB. These observation suggested that infection by L. mexicana may have a suppressive effect on host macrophage activation.
In order to determine if L. mexicana actively suppresses macrophage activation, we compared the response to live vs. heat-killed L. mexicana. Macrophages treated with heat-killed parasites were activated, producing a transcriptional signature that included upregulation of many interferon-stimulated genes, indicating that live L. mexicana actively suppresses this activation. In order to determine the mechanism of suppression, we analyzed the macrophage response to cysteine protease B (CPB)- and cysteine protease A (CPA)-deficient L. mexicana. CPB and CPA are related cathepsin L-like proteases that have been shown to play an important role in the inhibition of Th1 immunity in several mouse models of leishmaniasis. We found that infection with CPB-/- and CPB/CPA-/- L. mexicana also resulted in upregulation of interferon-stimulated genes, suggesting that suppression of host transcriptional responses by L. mexicana is at least partially dependent on CPB and CPA.