Japanese encephalitis virus infection modulates the expression of suppressors of cytokine signaling (SOCS) in macrophages: Implications for the hosts’ innate immune response
Graphical abstract
Introduction
Arthropod-borne flaviviruses are an emerging and re-emerging threat to mankind and causes endemic and epidemic diseases globally, with significant morbidity and mortality [1]. It has been well documented over the years that these viruses are capable of infecting a wide variety of host cells. As the innate immune facet of the host is the first line of defense against these invasions, the viruses must subvert this process so as to be successful in infecting the host. Macrophages are professional phagocytes which are critical to the host’s innate immunity. Apart from the central depots in secondary lymphoid organs, macrophages reside virtually in all tissues. It has been reported in several studies that macrophages can be effectively infected by arthropod-borne flaviviruses [2], [3], [4], [5]. Upon viral sensing or chemotactic signals released by other infected cells, macrophages produce cyto/chemokines and other inflammatory mediators which serve as the activators of adaptive immune system, leading to the ultimate clearance of the virus from the body [6].
Cytokine binding to cell surface receptors results in the initiation of an intracellular signaling cascade that leads to alteration in the transcriptional profile of that cell. These cascades are inextricably linked to the JAK protein tyrosine kinases and their substrates, the signal transducers and activators of transcription (STAT). The activated (phosphorylated) STAT proteins translocate into the nucleus and activate transcription of a range of cytokine responsive genes [7]. However, like most other signaling cascades in any mammalian system, this process is under the influence of intricate regulatory mechanisms controlled by a family of 8 proteins, collectively known as the suppressors of cytokine signaling (SOCS) [8], [9], [10], [11]. Thus, it is not at all surprising that this regulatory mechanism would be a target of viruses in evading the host’s immune system. It has been reported that mice deficient in SOCS1 are prone to severe inflammatory disease, but on the other hand they are resistant to viral infections [12]. Till date, a number of different viruses have been reported to induce SOCS proteins so as to prevent cytokine signalling and inhibit the initiation of an immune response [13]. Among these, HCV is the only reported flavivirus to modulate the expression of SOCS proteins, even though it is not arthropod-borne.
Japanese encephalitis (JE) virus (JEV) is a mosquito-borne flavivirus that is one of the major causes of encephalitic disease in South-East Asia affecting an estimated 67,900 people annually [14]. Persistence of JEV has been demonstrated in cultured mammalian cells [15], [16], as well as in a mouse model [17], [18], [19]. In JE survivors there have been reports of viral persistence long after the alleviation of symptoms [20], [21]. These observations indicate that JEV may have evolved different mechanisms for immune evasion in the host. However, till date, no theory regarding this is either definitive or conclusive. Efforts to decipher the evasion mechanisms of JEV had mostly been concentrated on their effects in macrophages and dendritic cells, two of the most efficient antigen presenting cells in the body, with relation to their viability and ability to present viral antigens to T lymphocytes [22], [23]. However, there is no report available regarding the modulatory effect of JEV on the expression of SOCS proteins leading to alterations in the anti-viral profile of macrophages. A transcriptome analysis of JEV-infected hamster kidney cell line to test the efficacy of an anti-microbial peptide for therapeutic purposes had shown that SOCS6 was upregulated post-infection [24]. In this current investigation we aimed to study the effect of JEV infection in macrophages over a wide time point and the subsequent changes in the expression of some SOCS proteins. From the obtained data, we wish to correlate the altered expression of these proteins with intracellular survival of the virus that might throw light onto the mechanism of its persistence or clearance from macrophages.
Section snippets
Ethics statement
Animals were handled in strict accordance with good animal practice as defined by the committee for the purpose of control and supervision of experimental animals (CPCSEA), Ministry of Environment and Forestry, Government of India as well as institutional animal and ethics committee (IAEC) of National Brain Research Centre. All animal studies were approved by the IAEC of National Brain Research Centre and experimental protocol approval number is NBRC/IAEC/2011/66.
Viruses and cell
The GP78 strain of the virus
Modulation of proinflammatory cyto/chemokine release from macrophages correlates with SOCS1 and SOCS3 expression in the cells post-infection with the virus
F4/80 + ve macrophages were selected post adherence to tissue culture plates (∼92–95% purity, Supplementary Fig. S1). Cytokine bead array using culture supernatants from non-infected, JEV-infected and boiled JEV-treated macrophages were performed to evaluate the time kinetic secretion of some proinflammatory cyto/chemokines. At 6 h post-infection or treatment, it was observed that there was a significant increase in the levels of all the studied cyto/chemokine as compared to the levels in
Discussion
Viruses have evolved various mechanisms to evade detection and destruction of the infected cells by sentinels of the host’s immune system. In a recent study we had demonstrated that at early time points (6, 12 and 24 h) there is no significant alterations in the expression of MHC-1 or the costimulatory molecules CD80 and CD86 on the surface of JEV-infected macrophages compared to non-infected controls [28]. Hence, it may be assumed that during early infection there is no viral antigen
Conflict of interest
The authors declare no conflict of interest.
Acknowledgments
This work was in part supported by the National Bioscience Award for Career Development from Department of Biotechnology to AB and by the core grants of the National Brain Research Centre to AB. KD was a recipient of research associateship in biotechnology and life sciences from the Department of Biotechnology, Government of India. AN is the recipient of senior research fellowship from Council for Scientific and Industrial Research, Government of India. The authors would like to thank Kanhaiya
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- 1
These authors contributed equally to this work.
- 2
Current address: Centre de Recherche de l’Institut Universitaire en Santé Mentale de Québec, Québec, QC G1J 2G3, Canada.