Identification of Piecemeal Degranulation and Vesicular Transport of MBP-1 in Liver-Infiltrating Mouse Eosinophils During Acute Experimental Schistosoma mansoni Infection

Abstract

Eosinophils have been long associated with helminthic infections, although their functions in these diseases remain unclear. During schistosomiasis caused by the trematode Schistosoma mansoni, eosinophils are specifically recruited and migrate to sites of granulomatous responses where they degranulate. However, little is known about the mechanisms of eosinophil secretion during this disease. Here, we investigated the degranulation patterns, including the cellular mechanisms of major basic protein-1 (MBP-1) release, from inflammatory eosinophils in a mouse model of S. mansoni infection (acute phase). Fragments of the liver, a major target organ of this disease, were processed for histologic analyses (whole slide imaging), conventional transmission electron microscopy (TEM), and immunonanogold EM using a pre-embedding approach for precise localization of major basic protein 1 (MBP-1), a typical cationic protein stored pre-synthesized in eosinophil secretory (specific) granules. A well-characterized granulomatous inflammatory response with a high number of infiltrating eosinophils surrounding S. mansoni eggs was observed in the livers of infected mice. Moreover, significant elevations in the levels of plasma Th2 cytokines (IL-4, IL-13, and IL-10) and serum enzymes (alanine aminotransferase and aspartate aminotransferase) reflecting altered liver function were detected in response to the infection. TEM quantitative analyses revealed that while 19.1% of eosinophils were intact, most of them showed distinct degranulation processes: cytolysis (13.0%), classical and/or compound exocytosis identified by granule fusions (1.5%), and mainly piecemeal degranulation (PMD) (66.4%), which is mediated by vesicular trafficking. Immunonanogold EM showed a consistent labeling for MBP-1 associated with secretory granules. Most MBP-1-positive granules had PMD features (79.0 ± 4.8%). MBP-1 was also present extracellularly and on vesicles distributed in the cytoplasm and attached to/surrounding the surface of emptying granules. Our data demonstrated that liver-infiltrating mouse eosinophils are able to degranulate through different secretory processes during acute experimental S. mansoni infections with PMD being the predominant mechanism of eosinophil secretion. This means that a selective secretion of MBP-1 is occurring. Moreover, our study demonstrates, for the first time, a vesicular trafficking of MBP-1 within mouse eosinophils elicited by a helminth infection. Vesicle-mediated secretion of MBP-1 may be relevant for the rapid release of small concentrations of MBP-1 under cell activation.

Eosinophils Control Liver Damage by Modulating Immune Responses Against Fasciola hepatica

“…First, we identified eosinophils during F. hepatica infection and showed that they augment with the time of infection, a feature that also characterizes infections by B. malayi ( 23 ), N. brasiliensis ( 24 ), A. cantonensis ( 25 , 26 ). In line with our results, eosinophils constitute 40–60% of the cells in the peritoneal cavity of B. malayi or S. mansoni -infected mice ( 23 , 30 ). The parasite itself could promote the accumulation of greater number of eosinophils in the peritoneal cavity after invading the intestinal wall that allows the juvenile flukes to reach the peritoneal cavity.…”

“…However, their role during infection is rather controversial. Experimental infections either with the nematodes Brugia malayi ( 23 ), Nippostrongylus brasiliensis ( 24 ), Angiostrongylus cantonensis ( 25 , 26 ), Heligmosomoides polygyrus ( 19 ), Trichinella spiralis ( 27 ), Strongyloides ratti ( 28 ), Trichuris muris ( 29 ), or the trematode Schistosoma mansoni ( 30 ) are characterized by strong eosinophilia. However, only in the case of B. malayi or N. brasiliensis infections, eosinophils seem to mediate unquestionable host protection by reducing damage caused by the parasite ( 17 , 31 – 34 ).…”

Granuloma formation and tissue pathology in Schistosoma japonicum versus Schistosoma mansoni infections

“…While the eosinophil response to S mansoni eggs is much more pronounced than that of neutrophils, the principal role of eosinophils during schistosomiasis is still not fully understood. It remains uncertain whether they act as major effector cells against the worms, or mediators in tissue homeostasis favouring the establishment of the parasites, or merely as facilitators of tissue remodelling and debris clearance during infection 65 . However, in vitro experiments clearly demonstrated that eosinophil activation can occur through antibody‐dependent cytotoxicity to exert damage on S mansoni parasites 66,67 and that degranulation products such as Major Basic Protein‐1 (MBP‐1) are primary eosinophil mediators in this activity 65 …”

Quantitative and functional characteristics of circulating and bone marrow PD-1- and TIM-3-positive T cells in treated multiple myeloma patients

“…The lack of response may be due to the low expression of PD-L1 by a malignant cell clone and/or tumor microenvironment cells in most types of NHL and MM 11 – 14 , unlike HL 9 , 15 . Simultaneously, TIM-3 is considered a perspective target for therapeutic monoclonal antibodies, particularly in combination 16 , 17 .…”