Aplastic Anemia Rescued by Exhaustion of Cytokine-secreting CD8+ T Cells in Persistent Infection with Lymphocytic Choriomeningitis Virus

Abstract

Aplastic anemia may be associated with persistent viral infections that result from failure of the immune system to control virus. To evaluate the effects on hematopoiesis exerted by sustained viral replication in the presence of activated T cells, blood values and bone marrow (BM) function were analyzed in chronic infection with lymphocytic choriomeningitis virus (LCMV) in perforin-deficient (P0/0) mice. These mice exhibit a vigorous T cell response, but are unable to eliminate the virus. Within 14 d after infection, a progressive pancytopenia developed that eventually was lethal due to agranulocytosis and thrombocytopenia correlating with an increasing loss of morphologically differentiated, pluripotent, and committed progenitors in the BM. This hematopoietic disease caused by a noncytopathic chronic virus infection was prevented by depletion of CD8+, but not of CD4+, T cells and accelerated by increasing the frequency of LCMV-specific CD8+ T cells in T cell receptor (TCR) transgenic (tg) mice. LCMV and CD8+ T cells were found only transiently in the BM of infected wild-type mice. In contrast, increased numbers of CD8+ T cells and LCMV persisted at high levels in antigen-presenting cells of infected P0/0 and P0/0 × TCR tg mice. No cognate interaction between the TCR and hematopoietic progenitors presenting either LCMV-derived or self-antigens on the major histocompatibility complex was found, but damage to hematopoiesis was due to excessive secretion and action of tumor necrosis factor (TNF)/lymphotoxin (LT)-α and interferon (IFN)-γ produced by CD8+ T cells. This was studied in double-knockout mice that were genetically deficient in perforin and TNF receptor type 1. Compared with P0/0 mice, these mice had identical T cell compartments and T cell responses to LCMV, yet they survived LCMV infection and became life-long virus carriers. The numbers of hematopoietic precursors in the BM were increased compared with P0/0 mice after LCMV infection, although transient blood disease was still noticed. This residual disease activity was found to depend on IFN-γ–producing LCMV-specific T cells and the time point of hematopoietic recovery paralleled disappearance of these virus-specific, IFN-γ–producing CD8+ T cells. Thus, in the absence of IFN-γ and/or TNF/LT-α, exhaustion of virus-specific T cells was not hampered.

Animal Models for Acquired Bone Marrow Failure Syndromes

“…6,55 In immune-mediated BM failure, damage to hematopoiesis may be caused by the excessive action of tumor necrosis factor-alpha and interferon-gamma (IFN-γ) secreted by activated CD8 + T cells. 43 This is consistent with findings from human aplastic anemia patients in which activated cytotoxic T lymphocytes infiltrate aplastic BM and produce detectable amounts of IFN-γ mRNA. 56 Murine marrow cells cultured in the presence of stromal cells transduced with a retroviral vector expressing murine IFN-γ had significantly less long-term repopulating stem cell activity in a competitive repopulation assay.…”

“…Depletion of CD8 + T cells could prevent the disease, however, increasing the frequency of lymphocytic choriomeningitis virus-specific CD8 + T cells in T cell receptor transgenic mice accelerates the disease. 43 Infection by human cytomegalovirus is often accompanied by transient neutropenia and thrombocytopenia probably due to one of the following: 1) alteration of accessory cell function by inducing the production of inhibitory cytokines, 2) perturbation of stromal cell function resulting in a decreased production of hematopoietic factors or by altering cell surface adhesion molecule expression, or 3) direct infection of the hematopoietic stem cells or progenitor cells. 44 In a murine model of cytomegalovirus-induced aplastic anemia, functional integrity of the stroma was impaired and the expression of genes encoding the essential hemopoietin stem cell factor, granulocyte colony-stimulating factor, and interleukin-6 was markedly reduced despite the physical integrity of the stromal network and the lack of significant marrow cell loss.…”

Perforin deficiency and susceptibility to cancer

“…Only when challenged with mouse pathogens such as lymphocytic choriomeningitis virus (LCMV) or ectromelia, did the mice rapidly succumb to infections. [37][38][39][40] In response to LCMV, PRF-deficient mice developed a syndrome highly reminiscent of human hemophagocytic lymphohistiocytosis (discussed below). 41 By contrast, loss of individual granzymes, such as mouse granzyme A or B, had no significant effect on the suppression of tested viral pathogens apart from ectromelia, and the absence of both granzymes was needed to show marked susceptibility to these pathogens.…”

Attrition of Virus-Specific Memory CD8+ T Cells During Reconstitution of Lymphopenic Environments

“…The answer, we believe, is during the lymphopenia that is a common feature of severe human and animal viral infections, including measles (18), influenza (19), Ebola (20), Venezuelan equine encephalitis (21), varicella zoster (22), and lymphocytic choriomeningitis viruses (LCMV) 4 (23), among many others. Studies with LCMV have shown that a combination of type I IFN and TNF-␣ can shut down bone marrow function early in infection (24), and there is also a substantial type I IFNdependent apoptosis and loss of mature peripheral CD8 ϩ T cells early in infection, particularly within the CD44 high subset (23). This early apoptosis is mimicked within 24 h of treatment with the IFN-inducing agent poly(I:C) (23) and precedes a sharp increase in the background division of CD44 high cells normally found in mice (25), apparently via the subsequent induction of IL-15 (26).…”