Chlamydia psittaci Triggers the Invasion of H9N2 Avian Influenza Virus by Impairing the Functions of Chicken Macrophages

Abstract

Simple Summary: Chlamydia psittaci, an obligate, intracellular, Gram-negative bacterium and economically relevant pathogen in poultry and pet bird, could cause psittacosis/ornithosis, and is also a human pathogen causing atypical pneumonia after zoonotic transmission. H9N2 influenza virus, a low pathogenic avian influenza viruses' subtype, has become endemic in different types of domestic poultry in lots of countries, resulting in great economic loss due to reduced egg production or high mortality associated with coinfection with other pathogens. These two pathogens are easily mixed with other pathogens to aggravate the disease, and often cause mixed infection in clinics. Co-infection of C. psittaci with H9N2 commonly induces severe pneumonia and high mortality in specific pathogen-free (SPF) chickens. According to previous studies, we postulated that C. psittaci infection may beneficial for the replication of H9N2 in HD11. Consequently, in this study, we clarify the pathogenic mechanism of coinfection with C. psittaci and H9N2 in the chicken macrophage cell line HD11, which is the first study of the coinfection of C. psittaci and H9N2 in vitro.Abstract: In a pilot study, simultaneous infection with Chlamydia psittaci (C. psittaci) and H9N2 virus induced 20% mortality and severe avian airsacculitis, shedding light on animal models of poultry respiratory diseases. However, the pathogenesis is still unclear. In the current study, we hypothesized that C. psittaci infection execrates macrophage function and facilitates H9N2 infection. To explore the potential mechanism, we studied the effect of C. psittaci and H9N2 on the functions of HD11 cells in vitro by simultaneous infection of C. psittaci and H9N2. At the same time, we used infection with C. psittaci or H9N2 alone as the control groups. The results showed that coinfection with C. psittaci and H9N2 could significantly aggravate the mortality of HD11 cells compared to C. psittaci or H9N2 infection alone. In addition, coinfection with C. psittaci and H9N2 did not induce high C. psittaci loads compared to C. psittaci infection alone at 12-and 24-hours post-inoculation (hpi), but coinfection with C. psittaci and H9N2 could increase the loads of H9N2 compared to H9N2 alone in HD11 cells at 12 hpi. More importantly, inducible nitric oxide synthase (iNOS) expression levels, enzyme activity, nitric oxide (NO) production, and phagocytosis were reduced significantly in the group with C. psittaci and H9N2 coinfection compared to those of H9N2 or C. psittaci alone at 24 hpi. Finally, C. psittaci infection induced robust expressions of type Th2 cytokines interleukin (IL)-4 and IL-10, while interferon gamma (IFN-γ) and tumor necrosis factor-α (TNF-α) displayed a significant decrease compared to H9N2 infection alone at 24 hpi. All the above data indicate that C. psittaci infection can facilitate H9N2 invasion and to aggravate severe avian airsacculitis by impairing macrophage functions.

Case Report: Pneumonia Caused by Chlamydia Psittaci and Cryptococcus Co-Infection

“… 10 We speculate that the patient might have initially been infected with C. neoformans , which has the capacity to induce apoptosis in macrophages via its capsular polysaccharides, predominantly galactoxylomannan and glucuronoxylomannan. 11 , 12 Macrophages play an important role in the immune response against chlamydial infection, 13 a reduction in macrophages could enhance susceptibility to C. psittaci infection. Another study suggested a synergistic interaction between adenovirus and C. psittaci in affected parrots, in which the adenovirus induced immune suppression in the host, resulting in an increase in the C. psittaci bacterial load.…”

“…C. psittaci can compromise the functionality of chicken macrophages, thereby facilitating the invasion of the H9N2 avian influenza virus. 13 In birds co-infected with C. psittaci and H9N2, C. psittaci has been shown to suppress the host’s immune response by inhibiting humoral immunity and altering the Th1/Th2 balance, leading to an elevated mortality rate in the host. 17 Based on these findings, we speculate that co-infections of C. psittaci and Cryptococcus could exacerbate the disease severity, resulting in enhanced damage and an increased mortality rate.…”

The immune system of chicken and its response to H9N2 avian influenza virus

“…Research has shown that IL-4 expression is downregulated after HTC macrophage cell line infection with H9N2, which plays a key role in regulating Th2-type immune responses (Xing et al., 2008 ). However, other studies have reported contrasting results, with IFN-γ and IL-4 significantly upregulated in DH11 cells infected with H9N2 at 12 h and 24 h, respectively (Chu et al., 2020 ). In mammals, alveolar macrophages exhibited polarization toward the M1 phenotype 4 h after H9N2 AIV infection, with the upregulation of M1-associated marker genes STAT1, TNF-α, MCP1, INOS, IL-6, and IL-12.…”

A comprehensive review on avian chlamydiosis: a neglected zoonotic disease

“…Epidemiological findings speculated that the transfer of pathogen to non-avian species like cattle makes them less or no virulence, and some studies suggested that it may be due to less infectious dose. Co-infection of C. psittaci with other pathogens like avian influenza will trigger the infections by damaging macrophages (Chu et al 2020 ).

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