Dysregulation of TNF-α and IFN-γ expression is a common host immune response in a chronically infected mouse model of melioidosis when comparing multiple human strains of Burkholderia pseudomallei

Abstract

Background: Melioidosis is endemic in Southeast Asia and Northern Australia and is caused by the Gram-negative, facultative intracellular pathogen Burkholderia pseudomallei. Diagnosis of melioidosis is often difficult because of the protean clinical presentation of the disease, and it may mimic other diseases, such as tuberculosis. There are many different strains of B. pseudomallei that have been isolated from patients with melioidosis, but it was not clear if they could cause a similar disease in a chronic BALB/c murine model of melioidosis. Hence, we wanted to examine chronically infected mice exposed to different strains of B. pseudomallei to determine if there were differences in the host immune response to the pathogen. Results:We identified common host immune responses exhibited in chronically infected BALB/c mice, although there was some heterogeneity in the host response in chronically infected mice after exposure to different strains of B. pseudomallei. They all displayed pyogranulomatous lesions in their spleens with a large influx of monocytes/ macrophages, NK cells, and neutrophils identified by flow cytometry. Sera from chronically infected mice by ELISA exhibited elevated IgG titers to the pathogen, and we detected by Luminex micro-bead array technology a significant increase in the expression of inflammatory cytokines/chemokines, such as IFN-γ, IL-1α, IL-1β, KC, and MIG. By immunohistochemical and in situ RNA hybridization analysis we found that the increased expression of proinflammatory cytokines (IL-1α, IL-1β, TNF-α, IFN-γ) was confined primarily to the area with the pathogen within pyogranulomatous lesions. We also found that cultured splenocytes from chronically infected mice could express IFN-γ, TNF-α, and MIP-1α ex vivo without the need for additional exogenous stimulation. In addition by flow cytometry, we detected significant amounts of intracellular expression of TNF-α and IFN-γ without a protein transport blocker in monocytes/macrophages, NK cells, and neutrophils but not in CD4 + or CD8 + T cells in splenocytes from chronically infected mice.

Multinucleated Giant Cell Formation as a Portal to Chronic Bacterial Infections

“…This study demonstrated that caseous granulomas are formed during chronic melioidosis, but the response is heterogenous, and also includes non-necrotic granulomas and pyogranulomatous lesions. The severity of the lesion, both microscopically and macroscopically, correlates with the number of bacteria within the lesion, which agrees with more recent studies [ 4 ]. Based on comparisons between laboratory and clinical studies, it is evident that MNGCs are the keystone event of Bpm pathogenesis.…”

“…As previously stated, MNGCs have been found in the granuloma-like structures of chronic Bpm infections but mostly on the perimeter of the central necrotic core that houses most of the bacteria [ 4 , 44 ]. This is consistent with the granuloma structure in M. tuberculosis infection where MNGCs are scattered throughout the periphery of the epithelioid cells, foamy macrophages, and lymphocytes.…”

“…These latent infections are generally manifested as symptomatic or asymptomatic, leading to abscesses in the liver, spleen, or lung. The severity of the abscess seems to correlate with the number of bacteria taking refuge within the lesion, and have a bias for the spleen and lung; however, liver abscesses have also been observed [ 4 ]. These abscesses have been clinically identified as granuloma-like lesions and often are confused with extrapulmonary tuberculosis.…”

Multinucleated Giant Cell Formation as a Portal to Chronic Bacterial Infections

“…The transient re-activation of persister bacteria would shut down the cytoprotection and manipulation exhibited by Bpm and cause the MNGC to undergo cell death, creating foci of necrosis and inflammation, like the ones seen at the center of the mature granuloma-like lesions of chronic melioidosis patients. As previously stated, MNGCs have been found in the granuloma-like structures of chronic infections but mostly on the perimeter of the central necrotic core that houses most of the bacteria [4,37]. This is consistent with granuloma structure in M. tuberculosis infection where MNGCs are scattered throughout the periphery of the epithelioid cells, foamy macrophages, and lymphocytes.…”

“…These latent infections are generally manifested as symptomatic or asymptomatic, leading to abscesses in the liver, spleen, or lung. The severity of the abscess seems to correlate to the number of bacteria taking refuge within the lesion and have a bias for the spleen and lung; however, liver abscesses have also been observed [4]. These abscesses have been clinically identified as granulomalike lesions and often are confused with extrapulmonary tuberculosis.…”