Atrophy of primary lymphoid organs induced by Marek’s disease virus during early infection is associated with increased apoptosis, inhibition of cell proliferation and a severe B-lymphopenia

Abstract

Marek’s disease is a multi-faceted highly contagious disease affecting chickens caused by the Marek’s disease alphaherpesvirus (MDV). MDV early infection induces a transient immunosuppression, which is associated with thymus and bursa of Fabricius atrophy. Little is known about the cellular processes involved in primary lymphoid organ atrophy. Here, by in situ TUNEL assay, we demonstrate that MDV infection results in a high level of apoptosis in the thymus and bursa of Fabricius, which is concomitant to the MDV lytic cycle. Interestingly, we observed that in the thymus most of the MDV infected cells at 6 days post-infection (dpi) were apoptotic, whereas in the bursa of Fabricius most of the apoptotic cells were uninfected suggesting that MDV triggers apoptosis by two different modes in these two primary lymphoid organs. In addition, a high decrease of cell proliferation was observed from 6 to 14 dpi in the bursa of Fabricius follicles, and not in the thymus. Finally, with an adapted absolute blood lymphocyte count, we demonstrate a major B-lymphopenia during the two 1st weeks of infection, and propose this method as a potent non-invasive tool to diagnose MDV bursa of Fabricius infection and atrophy. Our results demonstrate that the thymus and bursa of Fabricius atrophies are related to different cell mechanisms, with different temporalities, that affect infected and uninfected cells.Electronic supplementary materialThe online version of this article (10.1186/s13567-018-0526-x) contains supplementary material, which is available to authorized users.

Transcriptional Profiles Associated with Marek’s Disease Virus in Bursa and Spleen Lymphocytes Reveal Contrasting Immune Responses during Early Cytolytic Infection

“…Like other oncogenic herpes viruses, the course of MDV infection comprises four phases: early productive replication, establishment of latency, reactivation to produce viral progeny, and the transformation phase [37]. The early cytolytic phase of MDV occurs mainly in B lymphocytes of the spleen and bursa within 2-7 dpi [7].…”

Revisiting cellular immune response to oncogenic Marek’s disease virus: the rising of avian T-cell immunity

“…B cells were shown to constitute around 90% of cytolytically infected cells, while CD4 + and CD8 + T cells represented only 8% and 3%, respectively, as determined by pp38 expression when virus replication peaked between 3 and 7 dpi [95,96]. Consequently, B cells became apoptotic and depleted in the bursa, leading to B-cell lymphopenia in the blood [97]. In the interim, the virus is transferred from B cells to T cells, leading to the establishment of latency and transformation [98].…”

“…Although Morimura et al attempted to address the role of CD4 + T cells after CVI988 immunization by depleting CD4 + T cells, their role has not been determined in vaccine-induced protective immunity [112,113], possibly because depletion of CD4 + T cells may also result in the deficiency of lymphoma cells that are transformed from MDV-infected CD4 + T cells after challenge. Although MDV infection induces atrophy of thymus and apoptosis of infected T cells as determined by in situ TUNEL assay [97], CD4 + T cells significantly expand in the periphery after MDV infection [97,115] and the percentage of infected CD4 + T cells (pp38+) is low compared to total infected lymphocytes [95,96,99]. Thus, it is very likely that uninfected CD4 + T cells are activated and participate in host defense against MDV after infection or vaccination.…”

Glutaminolysis and Glycolysis Are Essential for Optimal Replication of Marek’s Disease Virus

“…Lymphocytes (T and B cells) are the principal target of MDV (26), and it has been shown that lymphocytes activated by various ligands, including soluble CD40L and anti-TCR␣␤ antibody, can support MDV replication and transformation of CD4 ϩ T cells in vitro (27). Most MDV-infected immune cells undergo apoptosis in vitro or in vivo (5,28,29), and thus, analysis of metabolic changes in lytic infected immune cells is challenging. Moreover, activation of lymphocytes by soluble CD40L and anti-TCR-␣␤ antibody alters cell metabolism (30), which can potentially mask the effects of MDV infection on metabolism.…”