"With a little help from my friends": efferocytosis as an antimicrobial mechanism

Encephalitozoon cuniculi takes advantage of efferocytosis to evade the immune response

Microsporidia are recognized as opportunistic pathogens in individuals with immunodeficiencies, especially related to T cells. Although the activity of CD8+ T lymphocytes is essential to eliminate these pathogens, earlier studies have shown significant participation of macrophages at the beginning of the infection. Macrophages and other innate immunity cells play a critical role in activating the acquired immunity. After programmed cell death, the cell fragments or apoptotic bodies are cleared by phagocytic cells, a phenomenon known as efferocytosis. This process has been recognized as a way of evading immunity by intracellular pathogens. The present study evaluated the impact of efferocytosis of apoptotic cells either infected or not on macrophages and subsequently challenged with Encephalitozoon cuniculi microsporidia. Macrophages were obtained from the bone marrow monocytes from C57BL mice, pre-incubated with apoptotic Jurkat cells (ACs), and were further challenged with E. cuniculi spores. The same procedures were performed using the previously infected Jurkat cells (IACs) and challenged with E. cuniculi spores before macrophage pre-incubation. The average number of spores internalized by macrophages in phagocytosis was counted. Macrophage expression of CD40, CD206, CD80, CD86, and MHCII, as well as the cytokines released in the culture supernatants, was measured by flow cytometry. The ultrastructural study was performed to analyze the multiplication types of pathogens. Macrophages pre-incubated with ACs and challenged with E. cuniculi showed a higher percentage of phagocytosis and an average number of internalized spores. Moreover, the presence of stages of multiplication of the pathogen inside the macrophages, particularly after efferocytosis of infected apoptotic bodies, was observed. In addition, pre-incubation with ACs or IACs and/or challenge with the pathogen decreased the viability of macrophages, reflected as high percentages of apoptosis. The marked expression of CD206 and the release of large amounts of IL-10 and IL-6 indicated the polarization of macrophages to an M2 profile, compatible with efferocytosis and favorable for pathogen development. We concluded that the pathogen favored efferocytosis and polarized the macrophages to an M2 profile, allowing the survival and multiplication of E. cuniculi inside the macrophages and explaining the possibility of macrophages acting as Trojan horses in microsporidiosis.

Mycobacterial Dormancy Systems and Host Responses in Tuberculosis

Tuberculosis (TB) caused by the intracellular pathogen, Mycobacterium tuberculosis (Mtb), claims more than 1.5 million lives worldwide annually. Despite promulgation of multipronged strategies to prevent and control TB, there is no significant downfall occurring in the number of new cases, and adding to this is the relapse of the disease due to the emergence of antibiotic resistance and the ability of Mtb to remain dormant after primary infection. The pathology of Mtb is complex and largely attributed to immune-evading strategies that this pathogen adopts to establish primary infection, its persistence in the host, and reactivation of pathogenicity under favorable conditions. In this review, we present various biochemical, immunological, and genetic strategies unleashed by Mtb inside the host for its survival. The bacterium enables itself to establish a niche by evading immune recognition via resorting to masking, establishment of dormancy by manipulating immune receptor responses, altering innate immune cell fate, enhancing granuloma formation, and developing antibiotic tolerance. Besides these, the regulatory entities, such as DosR and its regulon, encompassing various putative effector proteins play a vital role in maintaining the dormant nature of this pathogen. Further, reactivation of Mtb allows relapse of the disease and is favored by the genes of the Rtf family and the conditions that suppress the immune system of the host. Identification of target genes and characterizing the function of their respective antigens involved in primary infection, dormancy, and reactivation would likely provide vital clues to design novel drugs and/or vaccines for the control of dormant TB.

Mycobacterium tuberculosis ESAT-6 is a leukocidin causing Ca2+ influx, necrosis and neutrophil extracellular trap formation

Mycobacterium tuberculosis infection generates pulmonary granulomas that consist of a caseous, necrotic core surrounded by an ordered arrangement of macrophages, neutrophils and T cells. This inflammatory pathology is essential for disease transmission and M. tuberculosis has evolved to stimulate inflammatory granuloma development while simultaneously avoiding destruction by the attracted phagocytes. The most abundant phagocyte in active necrotic granulomas is the neutrophil. Here we show that the ESAT-6 protein secreted by the ESX-1 type VII secretion system causes necrosis of the neutrophils. ESAT-6 induced an intracellular Ca²⁺ overload followed by necrosis of phosphatidylserine externalised neutrophils. This necrosis was dependent upon the Ca²⁺ activated protease calpain, as pharmacologic inhibition prevented this secondary necrosis. We also observed that the ESAT-6 induced increase in intracellular Ca²⁺, stimulated the production of neutrophil extracellular traps characterised by extruded DNA and myeloperoxidase. Thus we conclude that ESAT-6 has a leukocidin function, which may facilitate bacterial avoidance of the antimicrobial action of the neutrophil while contributing to the maintenance of inflammation and necrotic pathology necessary for granuloma formation and TB transmission.

Effect of apoptotic cell recognition on macrophage polarization and mycobacterial persistence

Intracellular Mycobacterium leprae infection modifies host macrophage programming, creating a protective niche for bacterial survival. The milieu regulating cellular apoptosis in the tissue plays an important role in defining susceptible and/or resistant phenotypes. A higher density of apoptotic cells has been demonstrated in paucibacillary leprosy lesions than in multibacillary ones. However, the effect of apoptotic cell removal on M. leprae-stimulated cells has yet to be fully elucidated. In this study, we investigated whether apoptotic cell removal (efferocytosis) induces different phenotypes in proinflammatory (Mϕ1) and anti-inflammatory (Mϕ2) macrophages in the presence of M. leprae. We stimulated Mϕ1 and Mϕ2 cells with M. leprae in the presence or absence of apoptotic cells and subsequently evaluated the M. leprae uptake, cell phenotype, and cytokine pattern in the supernatants. In the presence of M. leprae and apoptotic cells, Mϕ1 macrophages changed their phenotype to resemble the Mϕ2 phenotype, displaying increased CD163 and SRA-I expression as well as higher phagocytic capacity. Efferocytosis increased M. leprae survival in Mϕ1 cells, accompanied by reduced interleukin-15 (IL-15) and IL-6 levels and increased transforming growth factor beta (TGF-β) and IL-10 secretion. Mϕ1 cells primed with M. leprae in the presence of apoptotic cells induced the secretion of Th2 cytokines IL-4 and IL-13 in autologous T cells compared with cultures stimulated with M. leprae or apoptotic cells alone. Efferocytosis did not alter the Mϕ2 cell phenotype or cytokine secretion profile, except for TGF-β. Based on these data, we suggest that, in paucibacillary leprosy patients, efferocytosis contributes to mycobacterial persistence by increasing the Mϕ2 population and sustaining the infection.

Interaction of Mycobacterium tuberculosis with host cell death pathways

Mycobacterium tuberculosis (Mtb) has coevolved with humans for tens of thousands of years. It is thus highly adapted to its human host and has evolved multiple mechanisms to manipulate host immune responses to its advantage. One central host pathogen interaction modality is host cell death pathways. Host cell apoptosis is associated with a protective response to Mtb infection, whereas a necrotic response favors the pathogen. Consistently, Mtb inhibits host cell apoptosis signaling but promotes induction of programmed necrosis. The molecular mechanisms involved in Mtb-mediated host cell death manipulation, the consequences for host immunity, and the potential for therapeutic and preventive approaches will be discussed.