Influenza as a viral disease was discovered in 1901 and a specific type of pathogenic influenza virus was discovered in 1955, which was then named as avian plague due to great losses it caused. There is no doubt about significance of influenza viruses as a global pathogen in human, domestic animals, and birds. Avian influenza virus is from orthomyxoviridae family, genus A. H9N2 strain of influenza virus A caused large scale loss of birds in Korea and China in 1994. Between 2001 and 2002, H9N2 was widely found in meat and bone marrow of the chickens imported from China to Japan. In March 1999, two separated influenza virus were obtained from girls at age range 1-4 in Hong Kong diagnosed with an influenza like disease. In addition, five H9N2 influenza viruses were extracted from human in 1998 [2].

There are proven evidences of intensification of pathogenic effect of H9N2 strain of influenza virus A separated from chickens in China by co-infections caused by golden staphylococcus and haemophilus paragallinarum [10]. Developing our knowledge about the ways that influenza viruses interact with the host cells at cell surface and the mechanisms and the pathways to induce cell death in the host helps us to find more efficient ways to deal with the disease. It is important to take into account that cell death in most of pathological cases (e.g. damaging viral infections) happens through necrosis; however, recent studies have suggested that many viruses such as influenza induce cell death through apoptosis or programmed cell death [3, 5, 11]. Influenza virus induces pathological changes in different tissues and lymphoid tissues are not an exception. Given the growing prevalence of influenza in human and livestock population, it is imperative to develop our knowledge about the pathogen and its viral strains such as H9N2 in terms of cell damages. The present study is an attempt to examine cell damages caused by avian influenza virus (AIV) serotype H9N2 infection; therefore, it can be helpful for expanding knowledge of some of pathogenic aspects of avian influenza.

Twenty SPF chicken three-weeks old (Valo Lohman Germany) were grouped randomly in groups of treatment and control (n = 10). The treatment group was infected through intranasal inoculation by influenza virus type H9N2 (0.2ml, dilution 1:10; titration EID50 107.5; A/chicken/Iran/772/2000), which was cloned two times in fertilized eggs. Simultaneously, the control group received saline solution of the same volume and through the same method. Three days after inoculation, the chickens were dissected simultaneously and lymphoid tissue samples were collected. The specimens were stabilized in buffer formalin (10%). Following dehydration, impregnation in paraffin, staining, and sectioning, 5-6µm thick layers of the specimens were prepared and marked by hematoxylin eosin and tunel. Based on Cohen’s table (effect coefficient: 0.7 and test power: 90), total sample size was obtained equal to 10. The collected data was analyzed in SPSS (v 13) using Mann-Whitney U test [1, 2, 4].

The major tissue change in Bursa of fabricius was lymphoid depletion, which was due to necrosis and apoptosis of lymphoid cells (Fig 1-2).

Lymphoid depletion due simultaneous incidence of two models of cell death (necrosis and apoptosis) was observed in thymus of the samples infected by influenza virus (Fig 2-3).

Apoptosis in lymphoid cells of spleen was considerable; the cells are demonstrated as condense partitioned nucleus in the samples marked by hematoxylin-eosin and brown in the samples marked by tunel (Fig 4-5).

Statistical analysis using Mann-Whitney U test showed that apoptotic changes of spleen tissue between the control and treatment groups was significant (p<0.001); while the changes were not significant in thymus and bursa of fabricius. In terms of lymphoid depletion and necrosis of cells in spleen, bursa of fabricius, and thymus, the difference between the treatment and control groups was significant. (p<0.001)

Mangkol et al. (2007) noted that influenza virus serotype H5N1 induced apoptosis in lymphocyte cells of spleen red pulp and white pulp in some cases. They also reported that apoptosis was indirectly effective on over activation of immunity responses and discharge of excessive volume of inflammatory cytokines following influenza infection [12]. Roberto et al. (2004) examined the effects of influenza virus on necrotic damages in spleen tissue and other lymphoid organs along with severe bleeding and hyperemia [14]. Perkins et al. (2003) traced influenza virus serotype H5N1 in spleen tissue and reported that in 2 or 5 days after inoculation, viral antigens were cloned in histiocytosis of spleen and induced histiocytosis, endothelial cells, and myocytes of spleen arterial straight muscle death [13]. Mehranpour et al. (2007) studied the effects of H5N1 virus in turkeys and showed that the main damages happened in brain, pancreas and tonsil cecum [10]. The results of the present study were consistent with Mangkol, Roberto, Perkins, and Mehranpour and the role of H9N2 virus in damaging lymphoid tissue was explained. Apoptosis is the main finding in the present study, which might be coincident with several mechanisms. Kunio et al. highlighted hemagglutinin inducing effect of influenza virus and argued that adding the viral factor to sialoglycoprotein at cellular level and expression of inflammatory cytokines (e.g. IL-1β, IL-6, TNF.α, INF.γ, INF β, and GM.CSF) were of the factors effective on apoptosis [7]. However, the most prominent mechanism in this regard is the role of neuraminidase factor mediated by TGF. Β. Free radicals generated by oxidative stressors, which are probably effective on inducing cell death. Kunio mentioned that expression of GST-Д and MN-SOD depended mRNA decreased in presence of influenza infection; therefore, increase of cell oxidative stresses and oxygen free radicals lead to activation of caspases 8 & 9 and caspases 3, 6, 7 consequently and activation of apoptosis pathway [7]. Zhirnov et al. (2002) mentioned that M1 protein or matrix protein of influenza virus attach to caspases 3, 6, and 7 and apoptosis pathway is triggered when this set is attached to caspase 8. They also showed that non-structural protein of the virus (i.e. NS1) may be one of the main initiators of cell death in the case of influenza virus infection [19]. In this regard and along with the effect of NS1 protein, Takizaw et al. (2003) mentioned the role of protein Bcl2, ligand FAS, and activation of kinase protein enzyme and other viral proteins such as neuraminidase and PB1-F2 [18]. Consistently, Zhou et al. (2006) noted that by influencing macrophage cells, influenza virus generates proteins that are depended on TNF.α and TRIAL exclusive receptors and increases sensitivity of DRLs receptor and apoptosis subsequently [20]. Through a comparative study on other serotypes of influenza virus (H7N7, H4H8, H5N2, H5N0) Mo et al. (1997) found that there were moderate damages by serotypes H5N2 and H7N7 in lymphoid tissues in the form of histiocytosis and lymphocytes necrosis [11]. Our results indicated that along with cell necrosis, there was apoptosis in lymphoid tissues cells following experimental infection by avian influenza virus H9N2. Pazani et al. (2008) showed that influenza virus serotype A/Chicken/Tehran/ZMF173/99 (H9N2) induced mild lymphoid depletion, and atrophy of lymph follicle in most of lymphatic tissues. Histopathology studies by the same research team on spleen tissue also showed lymphoid depletion and mild hyperemia in sinusoids tissue. They also reported extreme hyperemia along with necrosis of lymphoid tissue in Thymus tissue and the major damages were reported in thymus and bursa tissues. Results of the present study, however, showed that influenza virus serotype H9N2 (A/chicken/Iran/772/2000) caused the major tissue damages in spleen white pulp mostly in the form of necrosis and apoptosis of lymphocyte and macrophage cells. Pazani et al.’s reported consistent results as to thymus and bursa tissue; while they noted that damage in spleen tissue was more of reactive hyperplasia rather that tissue necrosis. On the other hand, majority of researchers in this field such as Shalaby et al. (1994), Swayne et al. (1995), and To et al. (2001) reported necrosis and lymphocyte depletion of spleen tissue. Clearly, diversity of serotypes of influenza virus may influence diversity of tissue damages [15-17]. Given the above, one can say that cell apoptosis in lymphoid tissues following infection by influenza virus serotype (H9N2) is more probable through the mentioned pathways while detail explanation of the molecular mechanisms in the case of H9N2 serotype needs supplementary studies. One conclusion is that there are several serotypes of influenza virus effective on cell death and H9N2 is not an exemption. In short, the both types of cell death (necrosis and apoptosis) happened following influenza infection in lymphoid tissues of chicken and there is a need for large sale molecular studies on serotype H9N2 to achieve a clearer image of cell death mechanism.

This article is part of thesis work of student of Msc Yusef Doustar. The authors express their gratitude to all their colleagues in research department of the Islamic Azad University.

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