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

Making Mouse Models That Reflect Human Immune Responses

Humans are infected with a variety of acute and chronic pathogens over the course of their lives, and pathogen-driven selection has shaped the immune system of humans. The same is likely true for mice. However, laboratory mice we use for most biomedical studies are bred in ultra-hygienic environments, and are kept free of specific pathogens. We review recent studies that indicate that pathogen infections are important for the basal level of activation and the function of the immune system. Consideration of these environmental exposures of both humans and mice can potentially improve mouse models of human disease.

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.

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.

Responsiveness of lymphoid precursors to polyclonal B-cell activators

We have shown previously that in the differentiation of fetal liver cells to mature B cells in irradiated hosts, these cells sequentially gain responsiveness to the polyclonal B-cell activators dextran-sulphate (DxS), lipopolysaccharide (LPS), and purified protein derivative from tuberculin (PPD), in that order. In this paper we show that both fetal liver cells and adult bone marrow cells responded with proliferation to DxS, but not to LPS OR PPD. However, neither fetal liver nor bone marrow cells gave rise to detectable numbers of high-rate antibody-secreting cells on short-term stimulation by polyclonal B-cell activators. The lack of LPS and and PPD responses of fetal liver and bone marrow cells could not be ascribed to the presence of inhibitory cells, and the DxS-induced response in these cell populations was not dependent on adherent cells. However, LPS could inhibit the DxS response of fetal liver cells, possibly indicating that DxS-responsive cells are precursors to B cells. Direct evidence was provided that DxS activated B-cell precursors in bone marrow. Thus, this cell population became responsive to LPS after DxS prestimulation, as measured by DNA synthesis. Bone marrow cells, sequentially stimulated with DxS and LPS, contained increased numbers of cells with surface immunoglobulin, although no significant increase in numbers of antibody-secreting cells was obtained. These data indicate that DxS had the capacity to increase the rate of differentiation of B-cell precursors. Finally, we show that the sequential appearance of responsiveness in B-cell differentiation to polyclonal B-cell activators is not due to lack of accessory cells during early stages in maturation.