Localized skin inflammation during cutaneous leishmaniasis drives a chronic, systemic IFN-γ signature

Author summary Cutaneous leishmaniasis caused by the protozoan Leishmania braziliensis exhibits two dominant inflammatory responses in cutaneous lesions: Interferon-gamma (IFN-gamma)-mediated signaling, which promotes parasite control, and cytolysis mediated by cytotoxic CD8+ T and NK cells, which promotes increased pathology. To determine if these responses were limited to cutaneous lesions, we performed RNA-seq on the blood of cutaneous leishmaniasis (CL) patients, and detected both transcriptional signatures in the peripheral blood. The presence of interferon stimulated genes, as well as circulating IFN-gamma, suggests that protective immune responses are not limited to the lesion site, but are occurring systemically. This may be one mechanism to ensure optimal control of the parasites, both by limiting their systemic spread and by pre-activating cells to kill the parasites prior to entry into the lesions. The cytolytic transcriptional signature was uniquely detectable in the blood of L. braziliensis patients when compared to the blood of patients with tuberculosis (TB) or malaria, further emphasizing the importance of this pathway in cutaneous leishmaniasis. Taken together, these data suggest that this localized infection has a systemic component that may have an impact the development of the disease. Cutaneous leishmaniasis is a localized infection controlled by CD4+ T cells that produce IFN-gamma within lesions. Phagocytic cells recruited to lesions, such as monocytes, are then exposed to IFN-gamma which triggers their ability to kill the intracellular parasites. Consistent with this, transcriptional analysis of patient lesions identified an interferon stimulated gene (ISG) signature. To determine whether localized L. braziliensis infection triggers a systemic immune response that may influence the disease, we performed RNA sequencing (RNA-seq) on the blood of L. braziliensis-infected patients and healthy controls. Functional enrichment analysis identified an ISG signature as the dominant transcriptional response in the blood of patients. This ISG signature was associated with an increase in monocyte- and macrophage-specific marker genes in the blood and elevated serum levels IFN-gamma. A cytotoxicity signature, which is a dominant feature in the lesions, was also observed in the blood and correlated with an increased abundance of cytolytic cells. Thus, two transcriptional signatures present in lesions were found systemically, although with a substantially reduced number of differentially expressed genes (DEGs). Finally, we found that the number of DEGs and ISGs in leishmaniasis was similar to tuberculosis-another localized infection-but significantly less than observed in malaria. In contrast, the cytolytic signature and increased cytolytic cell abundance was not found in tuberculosis or malaria. Our results indicate that systemic signatures can reflect what is occurring in leishmanial lesions. Furthermore, the presence of an ISG signature in blood monocytes and macrophages suggests a mechanism to limit systemic spread of the parasite, as well as enhance parasite control by pre-activating cells prior to lesion entry.

Automated summary

Cutaneous leishmaniasis caused by Leishmania braziliensis exhibits both IFN-gamma-mediated signaling and cytolytic responses in cutaneous lesions. RNA-seq analysis of blood samples from CL patients revealed transcriptional signatures of both responses in the peripheral blood, suggesting systemic immune responses. The presence of ISGs and elevated IFN-gamma levels in the blood indicate a mechanism to control parasite spread and pre-activate cells for parasite killing.