Lymphocytic choriomeningitis of the mouse. IV. Depression of the allograft reaction

Generation and maintenance of Listeria-specific CD8+ T cell responses in perforin-deficient mice chronically infected with LCMV

Disruption of the perforin gene results in primary immunodeficiency and an increased susceptibility to opportunistic pathogens. Perforin-deficient (PKO) mice fail to clear primary lymphocytic choriomeningitis virus (LCMV) Armstrong, resulting in persistent infection and functional exhaustion of virus-specific CD8+ T cells. CD8+ T cell responses to Listeria monocytogenes (LM) challenge within the first week after LCMV infection were diminished in both WT and PKO mice, and correlated with enhanced bacterial clearance. However, bacterial challenge at later time points generated similar CD8 T cell responses in both groups of mice. The phenotype and function of pre-existing LM-specific memory CD8+ T cells were maintained in persistently infected PKO mice. Thus persistent LCMV infection, as a result of perforin deficiency, results in dysfunction of the virus-specific CD8+ T cell response but does not compromise the host's ability to maintain pre-existing memory CD8+ T cells or to generate new memory CD8+ T cell responses against other pathogens.

Visualizing the viral burden: phenotypic and functional alterations of T cells and APCs during persistent infection

Persistent viral infections continue to present major public health problems. Failure to achieve virus control confronts the immune system with a chronic viral burden that may involve immune cells themselves and directly compromise the functionality of effector lymphocytes and APCs. In this study we use the lymphocytic choriomeningitis virus system for persistent viral infection of its natural murine host and use analytical techniques for direct ex vivo visualization of virus-infected immune cells. We report that virtually all cells of the immune system can be infected, but the distribution of the viral burden is differentially allocated to lymphocyte and APC subsets of defined phenotypes. Importantly, the profile of immune cell infection found in the blood is broadly representative for the pattern of cellular infection in most organs and is independent of the presence of Abs or complement. By direct comparison of virus-infected and uninfected cell subsets, we demonstrate that lymphocytic choriomeningitis virus-infected T cells show preferential activation, skewed cytokine profiles, and increased apoptosis. In contrast, increased activation of APCs is generalized and independent of the presence of viral Ag. Our data indicate that specific patterns of immune cell infection are associated with distinct forms of immunostimulatory and immunosuppressive alterations that may provide insights into autoimmune processes associated with infectious disease and offer clues for therapeutic interventions aimed at restoration of complete immunity.

Lymphocytic choriomeningitis virus-induced immune dysfunction: induction of and recovery from T-cell anergy in acutely infected mice

Acute infection of immunocompetent mice by lymphocytic choriomeningitis virus induces a potent cytotoxic T-lymphocyte response that eliminates infectious virus. Concurrently and paradoxically, there is a general suppression of lymphocyte responses to mitogens and to other infectious agents. Splenocytes from infected mice released significant amounts of gamma interferon in response to mitogenic stimulation in vitro, but neither interleukin 2 nor interleukin 4 was similarly elevated relative to the amounts released by control cells. Early T-cell receptor-proximal signaling events were found to be intact, confirming that the cells were viable and had received the mitogenic stimuli in an appropriate manner. Acutely infected adult thymectomized mice regained concanavalin A responsiveness in parallel with euthymic mice, if T cells were left unmanipulated for several weeks after clearance of virus from the mice. Therefore, although acute lymphocytic choriomeningitis virus infection has the effect of disrupting proliferation when the T-cell receptor is ligated, this state is only temporary. In contrast, T cells from persistently infected adult mice reveal long-lasting alterations in concanavalin A responsiveness.

Immunosuppression by lymphocytic choriomeningitis virus infection: competent effector T and B cells but impaired antigen presentation

Lymphocytic choriomeningitis virus (LCMV) may cause a severe immunosuppression in mice. Its pathogenesis is apparently dependent on LCMV-specific CD8 effector T cells that mediate the destruction of virus-infected cells which are normally essentially involved in immune responses. Evaluation of various LCMV isolates in this study established a general correlation between their tropism for lymphohemopoietic cells and immunosuppression. When immune responses were assessed as the capacity of mice to mount an anti-vaccinia virus cytotoxic T cell response or an IgG response to vesicular stomatitis virus (VSV), after a primary LCMV infection, LCMV-Armstrong, WE, Clone 13 and Docile were increasingly immunosuppressive in a dose-dependent fashion with respect to both extent and duration. Analysis of lymphocyte subpopulations showed variable effects of the various LCMV isolates that did not reveal patterns readily explaining immunosuppression. To evaluate whether LCMV infection affected T and/or B cell functions directly or whether antigen presentation was impaired, adoptive transfer experiments were performed. Untreated or irradiated but uninfected normal recipient mice receiving adoptively transferred T or B cells from LCMV-WE or Docile-infected immunosuppressed donor mice responded within 30%-100% of normal ranges in both assay systems. In contrast, when T or B cells from normal donors were transferred to irradiated or non-irradiated LCMV-immunosuppressed recipients, they failed to mount a significant cytotoxic T cell response against vaccinia virus or an IgG response to VSV. Thus, the T and B cells from LCMV-immunosuppressed mice were able to function within normal ranges; in contrast, histologically and functionally, antigen presentation was severely impaired in LCMV-immunosuppressed mice.

Virus-triggered acquired immunodeficiency by cytotoxic T-cell-dependent destruction of antigen-presenting cells and lymph follicle structure

Virus-induced acquired immune suppression in mice infected with lymphocytic choriomeningitis virus is shown here to be caused by the CD8+-T-cell-dependent elimination of macrophages/antigen-presenting cells. Surprisingly, this is associated with severe destruction of the follicular organization of lymphoid organs, indicating a crucial role for dendritic cells and marginal zone macrophages in maintaining follicular structure. Once established, this immunopathology cannot be readily reversed by the elimination of CD8+ effector cells. Such a T-cell-mediated pathogenesis may play a pivotal role in acquired virus-induced immunosuppression and may represent one strategy by which virus escapes immune surveillance and establishes persistent infections in initially immunocompetent hosts.

Lymphocytic choriomeningitis virus-induced immunodepression: inherent defect of B and T lymphocytes

Infection of mice with lymphocytic choriomeningitis virus (LCMV) produces a rapidly induced immuno-suppression manifested by low lymphocyte proliferation in response to lipopolysaccharide (LPS) and concanavalin A (ConA). Analysis of the mechanisms underlying the unresponsiveness to these mitogens was undertaken at the cellular and molecular levels 7 days after infection. The selective elimination of CD8+ T cells and the results of coculture experiments demonstrated that unresponsiveness was not due to suppressor cells. Similarly, the role of inhibitory factors such as prostaglandins was excluded, since indomethacin, which inhibits their production, did not reverse the unresponsiveness. Analysis of different cytokines secreted by ConA-activated macrophages or T cells revealed that interleukin-1 (IL-1), synthesized during the T-dependent activation of macrophages by ConA, was normally produced by cells from LCMV-infected mice. In contrast, IL-2, which is produced by activated CD4+ T cells, was undetectable. Addition of exogenous IL-2 did not restore the proliferative response, although the p55-kilodalton protein of the IL-2 receptor was induced by ConA on CD4+ cells from LCMV-infected mice. Our results can be interpreted as showing that (i) unresponsiveness to mitogens of cells from LCMV-infected mice is not due to altered functions of the macrophages with respect to IL-1 production; (ii) CD4+ cells are activated, since the p55 chain of the IL-2 receptor is induced; (iii) the lack of IL-2 production cannot explain T-cell unresponsiveness, since addition of exogenous IL-2 did not restore the proliferative response. Taken together, these data suggest that T-lymphocyte unresponsiveness should be related to an inherent proliferative defect subsequent to T-cell activation and IL-2 receptor expression.

Generation in vitro of alloreactive lymphocytes is suppressed by the addition of spleen cells from mice infected with lymphocytic choriomeningitis virus

The generation of cytotoxic T cells to alloantigens in mixed lymphocyte culture was suppressed by the addition of spleen cells from adult mice acutely infected with lymphocytic choriomeningitis virus. T cells of surface phenotype Lyt 1+2+ were required for the suppression. Suppressive cells appeared also in lymph nodes, but not in the thymus. In the spleen their number was maximal 6 to 8 days after infection. Infectious virus could not be detected in the suppressive spleen cells. Therefore the virus itself does not seem to cause the effect.

Lymphocytic choriomeningitis virus-induced immunosuppression: evidence for viral interference with T-cell maturation

Acute lymphocytic choriomeningitis virus (LCMV) infection is associated with general immunosuppression which develops during the second week of the infection and persists for several weeks. In the present study, the ability of LCMV-infected mice to mount a cytotoxic T-lymphocyte response was investigated in a transplantation assay, using LCMV-immunized mice as recipients. By this means it was possible to evaluate the T-cell responsiveness of the acutely infected mice separately. Our results revealed a marked depression of the T-cell function temporally related to immunosuppression in the intact mouse. Furthermore, this hyporesponsiveness could not be explained as an effect of suppressor cells. Occurring shortly before these changes were a drastic decrease in cortical thymocytes and a reduction in T-cell progenitors in the bone marrow and spleen. Our findings are consistent with the assumption that a numerical deficiency of immunocompetent T-cells due to viral interference with T-cell maturation plays an important role in LCMV-induced immunosuppression.

Mechanisms of hemopoietic and immunological dysfunction induced by lymphocytic choriomeningitis virus

Sublethal irradiation (500 R) of C3H mice is followed by a gradual replacement of radiosensitive cells in their spleens by surviving stem cells originating in bone marrow. This compensatory hemopoiesis was quantitated by counting the numbers of stem cell-derived colonies appearing on spleen surfaces, as well as those which grew in vitro after marrow cells, suspended in soft agar, were overlaid onto syngenic mouse embryo fibroblast feeder layers. Compensatory colony formation, both in vivo and in vitro, was severely depressed when mice were infected with lymphocytic choriomeningitis virus (LCMV) 1 day before irradiation, although the induction of virus-specific cytotoxic T cells in their spleens was unimpaired. Without irradiation, mice, acutely infected with LCMV, showed a dramatic reduction in the numbers of specific antibody-forming cells generated in their spleens after priming with sheep erythrocytes during week 1 post-infection, yet the ability of their marrow cells to form colonies in vitro remained normal. Therefore, the basis of immunodepression is distinct from that of defective hemopoiesis since the latter is apparent only when LCMV infection is accompanied by irradiation. However, as discussed, both phenomena may be related to alterations induced within the splenic environment by LCMV.

Erythropoietic activity and interferon production in LCM virus-infected nude mice

The distinct haemopoietic lesions induced in ordinary mice during acut LCM virus infection are not mediated either by immunopathological mechanisms or by a direct cytopathogenetic effect of the virus. Medication has recently been artibuted to the high interferon activity found in mice with acute LCM virus infection. In this work, erythropoietic activity and interferon production have been studied in LCM virus-infected nude mice. Compared with ordinary mice, nude mice displayed a more moderate suppression of erythropoiesis and a very poor interferon response. Erythropoietic activity of thymus-transplanted nude mice was not significantly suppressed during the infection, and interferon responsiveness was not restored either by thymus-transplantation or by transfer of large numbers of spleen lymphocytes. The observations support the hypothesis that interferon plays a decisive role in the induction of haemopoietic diorders of mice with acute LCM virus infection. The puzzling question as to why nude mice and reconstituted nude mice are incapable of mounting a normal interferon response is discussed briefly.

Transient impaired cell-mediated tumor immunity after acute infection with lymphocytic choriomeningitis virus

Intraperitoneal injection of nononcogenic lymphocytic choriomeningitis (LCM) virus in adult C3H mice causes a symptomless infection but stimulates specific cell-mediated and humoral immune responses. However, median survival time of virus-infected mice inoculated with syngeneic tumor cells was significantly shortened, and growth of semiallogeneic tumors was significantly enhanced. Cell-mediated cytotoxicity measured as chromium-51 release from labeled tumor cells was significantly suppressed but was recovered within 55 days after infection. The suppressed immune responsiveness could be conferreo on a normal spleen cell population when activated in virus-infected recipients. Chronically LCM virus-infected mice showed an unimpaired cell-mediated immune response to tumor allografts.