Intracellular proliferation of Anaplasma phagocytophilum is promoted via modulation of endoplasmic reticulum stress signaling in host cells

Non-canonical activation of the ER stress sensor ATF6 by Legionella pneumophila effectors

Legionella pneumophila secretes toxins into the host cell that induce the non-canonical processing and activation of the ER stress sensor and transcription factor ATF6 via a mechanism that is distinct from the canonical pathway activated by unfolded protein buildup.

The intracellular bacterial pathogen Legionella pneumophila (L.p.) secretes ∼330 effector proteins into the host cell to sculpt an ER-derived replicative niche. We previously reported five L.p. effectors that inhibit IRE1, a key sensor of the homeostatic unfolded protein response (UPR) pathway. In this study, we discovered a subset of L.p. toxins that selectively activate the UPR sensor ATF6, resulting in its cleavage, nuclear translocation, and target gene transcription. In a deviation from the conventional model, this L.p.–dependent activation of ATF6 does not require its transport to the Golgi or its cleavage by the S1P/S2P proteases. We believe that our findings highlight the unique regulatory control that L.p. exerts upon the three UPR sensors and expand the repertoire of bacterial proteins that selectively perturb host homeostatic pathways.

Epidemiological and Clinicopathological Features of Anaplasma phagocytophilum Infection in Dogs: A Systematic Review

Anaplasma phagocytophilum is a worldwide emerging zoonotic tick-borne pathogen transmitted by Ixodid ticks and naturally maintained in complex and incompletely assessed enzootic cycles. Several studies have demonstrated an extensive genetic variability with variable host tropisms and pathogenicity. However, the relationship between genetic diversity and modified pathogenicity is not yet understood. Because of their proximity to humans, dogs are potential sentinels for the transmission of vector-borne pathogens. Furthermore, the strong molecular similarity between human and canine isolates of A. phagocytophilum in Europe and the USA and the positive association in the distribution of human and canine cases in the USA emphasizes the epidemiological role of dogs. Anaplasma phagocytophilum infects and survives within neutrophils by disregulating neutrophil functions and evading specific immune responses. Moreover, the complex interaction between the bacterium and the infected host immune system contribute to induce inflammatory injuries. Canine granulocytic anaplasmosis is an acute febrile illness characterized by lethargy, inappetence, weight loss and musculoskeletal pain. Hematological and biochemistry profile modifications associated with this disease are unspecific and include thrombocytopenia, anemia, morulae within neutrophils and increased liver enzymes activity. Coinfections with other tick-borne pathogens (TBPs) may occur, especially with Borrelia burgdorferi, complicating the clinical presentation, diagnosis and response to treatment. Although clinical studies have been published in dogs, it remains unclear if several clinical signs and clinicopathological abnormalities can be related to this infection.

The Novel Zoonotic Pathogen, Anaplasma capra, Infects Human Erythrocytes, HL-60, and TF-1 Cells In Vitro

Anaplasma capra, a species of the family Anaplasmataceae, is zoonotic tick-borne obligate intracellular bacteria. There have been no reports of human infection with this pathogen since 2015. Therefore, the zoonotic characteristics of A. capra need to be further studied. To verify the ability of A. capra to infect human cells, A. capra were inoculated in human erythrocytes, HL-60, and TF-1 cell lines in vitro. Cell smears were taken after inoculation, using Giemsa staining, transmission electron microscope (TEM), chromogenic in situ hybridization and immunocytochemistry for detection. In the Giemsa staining, many dark colored corpuscles or purple granules were seen in the inoculated erythrocytes, HL-60, and TF-1 cells. The results of chromogenic in situ hybridization show that there were brown precipitates on the surface of most erythrocytes. Immunocytochemistry results show many dark brown vacuolar structures or corpuscles in the cytoplasm of erythrocytes, HL-60, and TF-1 cell lines. The A. capra morulae were seen in the cytoplasm of both HL-60 and TF-1 in TEM, and their diameter was about 295–518 nm. Both dense-cored (DC) and reticulate cell (RC) form morulae could be seen. This study confirmed the ability of A. capra to infect human erythrocytes, HL-60, and TF-1. This study is of profound significance in further verifying the zoonotic characteristics of the pathogen and for establishing an in vitro cultivation model.

Arthropods Under Pressure: Stress Responses and Immunity at the Pathogen-Vector Interface

Understanding what influences the ability of some arthropods to harbor and transmit pathogens may be key for controlling the spread of vector-borne diseases. Arthropod immunity has a central role in dictating vector competence for pathogen acquisition and transmission. Microbial infection elicits immune responses and imparts stress on the host by causing physical damage and nutrient deprivation, which triggers evolutionarily conserved stress response pathways aimed at restoring cellular homeostasis. Recent studies increasingly recognize that eukaryotic stress responses and innate immunity are closely intertwined. Herein, we describe two well-characterized and evolutionarily conserved mechanisms, the Unfolded Protein Response (UPR) and the Integrated Stress Response (ISR), and examine evidence that these stress responses impact immune signaling. We then describe how multiple pathogens, including vector-borne microbes, interface with stress responses in mammals. Owing to the well-conserved nature of the UPR and ISR, we speculate that similar mechanisms may be occurring in arthropod vectors and ultimately impacting vector competence. We conclude this Perspective by positing that novel insights into vector competence will emerge when considering that stress-signaling pathways may be influencing the arthropod immune network.