Mechanism of depletion of T lymphocytes from the spleen of mice infected with Listeria monocytogenes

Mechanisms and immunological effects of apoptosis caused by Listeria monocytogenes

Infection with Listeria monocytogenes shows an early stage of lymphocyte apoptosis. This is an obligatory stage the extent of which depends on infective dose. Lymphocyte apoptosis occurs early and is rapidly superseded, yet it has a strong biological consequence. The immunological effect of lymphocyte apoptosis following infection is increased susceptibility to L. monocytogenes infection due, in part, to upregulation of IL-10 on macrophages and DC. Lymphocyte apoptosis is dependent on bacterial expression of the pore-forming toxin listeriolysin O (LLO). Also, purified LLO can lead to the induction of death pathways similar to infection, demonstrating that it is a killer agent generated by L. monocytogenes. Signaling through the type I interferon receptor potentiates cell death induced by the bacteria or LLO. Infection with L. monocytogenes also causes death of phagocytic cells, the nature and significance of which is not clear at present. Infection with L. monocytogenes is a tractable model to examine pathogen-induced cell death pathways and their possible immunological consequences in multiple cell types following infection.

Dendritic cells in Listeria monocytogenes infection

Dendritic cells (DCs) represent a unique collection of innate immune cells present throughout the body as distinct subpopulations generally sharing the functions of pathogen recognition, cytokine production, and antigen presentation. A large body of work in recent years has examined DC functions during infection with Listeria monocytogenes (Lm), particularly in the murine model. Here, I review several aspects of DC biology in this model, with particular emphasis on the role DCs play in the establishment of a productive Lm infection and the role of DCs as cytokine producers and antigen-presenting cells in this system.

Lymphocytes serve as a reservoir for Listeria monocytogenes growth during infection of mice

It is widely reported that Listeria monocytogenes can infect virtually all cell types, however, the degree to which this facultative intracellular pathogen can infect lymphocytes has not been well characterized. Previous studies have shown that a subset of lymphocytes, including activated T cells, are susceptible to apoptosis following exposure to L. monocytogenes, but the ability of the bacteria to replicate in the cytosol of lymphocytes prior to cell death was not examined. In this report, we demonstrate that intracellular L. monocytogenes can survive and multiply in vitro in a variety of transformed cell lines of lymphocytic origin. Intracellular L. monocytogenes were also recovered from splenic B cells, T cells, and NK cells following intravenous infection of mice. In fact, lymphocyte-associated L. monocytogenes comprised a substantial portion of the total bacterial burden in the spleen throughout the course of murine infection and B cell-deficient mice had significantly lower titers of bacteria present in the spleen following intravenous infection. These results suggest that lymphocytes can be a reservoir for L. monocytogenes growth in vivo.

Immunological and antitumor effects of IL-23 as a cancer vaccine adjuvant

The promising, but modest, clinical results of many human cancer vaccines indicate a need for vaccine adjuvants that can increase both the quantity and the quality of vaccine-induced, tumor-specific T cells. In this study we tested the immunological and antitumor effects of the proinflammatory cytokine, IL-23, in gp100 peptide vaccine therapy of established murine melanoma. Neither systemic nor local IL-23 alone had any impact on tumor growth or tumor-specific T cell numbers. Upon specific vaccination, however, systemic IL-23 greatly increased the relative and absolute numbers of vaccine-induced CD8(+) T cells and enhanced their effector function at the tumor site. Although IL-23 specifically increased IFN-gamma production by tumor-specific T cells, IFN-gamma itself was not a primary mediator of the vaccine adjuvant effect. The IL-23-induced antitumor effect and accompanying reversible weight loss were both partially mediated by TNF-alpha. In contrast, local expression of IL-23 at the tumor site maintained antitumor activity in the absence of weight loss. Under these conditions, it was also clear that enhanced effector function of vaccine-induced CD8(+) T cells, rather than increased T cell number, is a primary mechanism underlying the antitumor effect of IL-23. Collectively, these results suggest that IL-23 is a potent vaccine adjuvant for the induction of therapeutic, tumor-specific CD8(+) T cell responses.

Selective Depletion of Nonspecific T Cells During the Early Stage of Immune Responses to Infection

Transient T cell depletion occurs before the development of an effective immune response to infection. In this study we show that most T cells, regardless of specificity, are induced to express early activation markers soon after infection with Listeria monocytogenes or lymphocytic choriomeningitis virus. Ag-specific T cells are further activated to display late activation markers and undergo extensive proliferation. As Ag-specific T cells begin to expand, nonspecific T cells are depleted en masse and exhibit no sign of further activation or proliferation before their depletion. This selective depletion of nonspecific T cells is due to in situ death via apoptosis, as visualized by confocal microscopy. Thus, early activation and subsequent depletion of nonspecific T cells are integral parts of the immune response to proinflammatory infections. These results have important implications for our understanding of early events in the development of a robust T cell response.

T-cell activation, proliferation and apoptosis in primary Listeria monocytogenes infection

Listeria monocytogenes infection of mice leads to a rapid expansion of activated T cells, followed by a decline in specific cells once the bacteria are eliminated. In order to define the relationship between T-cell proliferation and activation, and to investigate the role of apoptosis in limiting the expansion, the expression of activation markers, uptake of 5-bromo-2'-deoxyuridine (BrdU) in vivo and the incidence of apoptosis was investigated. Increased numbers of T cells expressing the activated phenotype CD25+, CD44hi and CD62Llo were detected 4 days after infection. Expression of CD25 (IL-2Ralpha chain) on CD4+ and CD8+ T cells peaked at this time and returned to normal by day 7. In contrast, CD44hi and CD62Llo persisted, with the maximum proportion occurring at 7 days after infection. This was accompanied by a burst of in vivo proliferation of CD4+ and CD8+ T cells occurring between day 5 and 7. Apoptosis, which is presumably needed to control this expansion of T cells, also peaked at 7 days after infection. Apoptosis occurred preferentially amongst T cells which had proliferated. Most but not all proliferating T cells had down-regulated their CD62L marker. While most apoptotic T cells were CD62Llo, again not all had down-regulated this marker. Hence, CD25 expression peaked early, but expression of other activation markers, in vivo proliferation and apoptosis coincided after Listeria infection. T cells that had proliferated were over-represented in the apoptotic population.

Listeria monocytogenes as a probe to study cell-mediated immunity

The intracellular bacterium Listeria monocytogenes continues to serve as a model to define general paradigms of cell-mediated immunity. Genetic manipulations of the bacterium and its murine host have allowed us to begin dissecting the intricate interactions between this bacterium and the immune system. As a result, we have gained new insights into the mechanisms of immune surveillance, achieved better understanding of bacterial tactics for immune evasion and developed novel strategies in vaccine development.

Lymphocyte apoptosis during early phase of Listeria infection in mice

During the acute phase of growth of Listeria monocytogenes in spleen and lymph nodes, the infective foci consist of macrophages and neutrophils accompanied by extensive death of lymphocytes. Many of the lymphocytes die by apoptosis. The lesions are found by 48 hours after infection and can regress with time. Depending on the dose, the infected foci can be restricted to the thymus-dependent areas or can occupy the entire lymphoid tissue. The Listeria in the lesions are primarily found inside macrophages, but a few are extracellular amid cellular debris. Lymphocyte death appears to be an obligatory step in primary Listeria infection, the extent of which is controlled by the early restriction of Listeria growth by the innate cellular system.

Studies in listeriosis show the strong symbiosis between the innate cellular system and the T-cell response

Resistance to infection with Listeria monocytogenes involves a series of cellular interactions, many of which are carried out by cytokines. Macrophages, NK cells and neutrophils participate in early stages of Listeria resistance. The neutrophil is specially important for clearance of the liver phase of listeriosis. Macrophages and NK cells interact by way of IL-12 and TNF, which induce the NK cell to produce IFN-gamma. IFN-gamma is the major macrophage-activating cytokine. The CB-17 SCID mouse shows these cellular interactions restricting the growth of Listeria, without its elimination. CD4 and/or CD8 T cells bring about sterilizing immunity. Macrophages influence the lymphocyte response by way of antigen presentation and also by promoting Th 1 differentiation. Thus, elimination of Listeria requires a symbiosis between innate immunity and the T-cell system.

Pathogenesis of lymphoid lesions in murine experimental listeriosis

Adult female Swiss albino mice were infected intraperitoneally or subcutaneously with Listeria monocytogenes Serovar 4b or 1/2a and killed at intervals. Thymus, spleen, Peyer's patches and a variety of lymph nodes, including the jejunal (mesenteric), mediastinal, lumbar, mandibular and superficial inguinal, were examined by histopathology and by immunocytochemistry for detection of L. monocytogenes antigen. Similar results were obtained with both Serovars and by both routes of inoculation used. In the spleen, L. monocytogenes was detected, by immunoperoxidase staining, as soon as 4 h after inoculation, inside phagocytic cells located predominantly in the marginal zone of the white pulp. This was followed by inflammation, necrosis and depletion of lymphoid cells, which extended in extreme cases to the whole organ. Inflammatory lesions diminished progressively at 5 to 6 days after inoculation. In animals dying of the infection, a severe necrotizing splenitis was present. Depletion of lymphoid cells and inflammatory changes were widespread in the lymph nodes and to a lesser extent in the Peyer's patches. An extensive necrotizing lymphadenitis was the prominent lesion in severely affected nodes. Inflammatory lesions and detection of L. monocytogenes antigen started around the venules of high endothelium. A thymus depletion, not associated with the multiplication of bacteria in the organ, was also a constant feature of the infection. This study suggests that L. monocytogenes (1) is transported to the spleen and to the lymph nodes by phagocytes, entering the organs by the marginal sinus in the spleen and by the venules of high endothelium in the lymph nodes; (2) multiplies in these cells as well as in neutrophilic granulocytes (the latter rapidly migrate to the affected zones); and (3) induce a splenitis and lymphadenitis, involving predominantly T cell-dependent areas, with a necrotizing component in severe cases. From our observations it is concluded that infection of the lymphoid system is a major feature in the pathogenesis of murine listeriosis.

Effect of adrenalectomy on ethanol-associated immunosuppression

The alterations in lymphoid cell numbers and lymphocyte function due to administration of ethanol was found to be associated with high levels of circulating corticosteroids. The role of corticosteroids in the ethanol-induced alterations in the immune system was studied by administering ethanol to adrenalectomized rats. The results of these experiments showed that the ethanol-induced loss of cells from the thymus was not completely prevented by adrenalectomy and the ethanol-induced loss of cells from the spleen was not affected by adrenalectomy. Likewise the ethanol-induced decrease in antibody production to the T-cell-dependent antigen sheep erythrocytes were not affected by adrenalectomy. The ability of animals to produce antibodies of the T-cell-independent antigen, TNP-Ficoll, was not affected by ethanol regardless of whether the animals had adrenal glands or not. These data indicate that adrenal corticosteroids are responsible for some but not all of the thymic involution due to ethanol intoxication. Also, adrenalectomized rats did not show as much impairment in lymphocyte proliferation as sham adrenalectomized animals after ethanol administration. However, this loss of cells from peripheral lymphoid organs such as the spleen and the decreased ability to respond to T-cell-dependent antigens is not influenced by adrenalectomy indicating mechanisms other than corticosteroids mediate these effects of ethanol.

Clearance of Cryptococcus neoformans from immunologically suppressed mice

To assess the effects of cryptococcal antigen-induced immunosuppression on a Cryptococcus neoformans infection, CBA/J mice were injected intravenously with saline or suppressive doses of cryptococcal antigen (CneF) at weekly intervals and were then infected with viable C. neoformans cells. By the second week after infection, the cryptococcal antigen-injected mice had suppressed anticryptococcal delayed-type hypersensitivity (DTH) responses compared with the responses of the saline-treated, infected control mice. In addition, the immunosuppressed mice had higher numbers of cryptococcal CFU cultured from their lungs, livers, spleens, lymph nodes, and brains than did the control animals. A direct correlation of suppression of the anticryptococcal DTH response and reduced clearance of cryptococci from tissues was also observed after mice were given a single intravenous injection of CneF and infected. To determine whether or not the cryptococcal antigen was specifically reducing the clearance of C. neoformans or had a more generalized effect, mice were injected with saline or suppressive doses of CneF, infected with Listeria monocytogenes, and then followed daily for 7 days for the clearance of L. monocytogenes from spleens and on day 7 for DTH reactivity to Listeria antigen. There were no differences between the saline- and CneF-treated mice with respect to anti-Listeria DTH responses or clearance of L. monocytogenes from spleens, indicating that CneF was not altering natural resistance mechanisms responsible for early clearance of L. monocytogenes, nor was the CneF influencing the induction of the acquired immune response which was responsible for the late clearance of the bacteria. Together, these data indicate that the specific suppression of this cell-mediated immune response induced by cryptococcal antigen reduces the ability of the animals to eliminate the homologous organism (C. neoformans) but not a heterologous infectious agent, such as L. monocytogenes.

Flow microfluorometry analysis of alterations in T-lymphocyte subsets during murine listeriosis

C57BL/6 mice were infected intravenously with 6 X 10(3) Listeria monocytogenes organisms. As early as day 3 of infection, there was a marked reduction in the number of lymphocytes recovered from the peripheral blood, bone marrow, and thymuses of infected animals. Concomitantly, there was an increase in the number of splenic lymphocytes. By day 14, both the total and differential cell counts were similar in both infected and normal animals. Flow microfluorometric studies comparing the Thy-1.2, Lyt-1, Lyt-2, and surface immunoglobulin (SIg) phenotypes of lymphocytes from normal and infected mice were performed. Between days 3 and 5, there was a decrease in the percentage of Thy-1.2+ cells in the spleens of L. monocytogenes-infected animals. Conversely, the percentages of Lyt-1+, Lyt-2+, and SIg+ cells remained constant. At day 7 of infection, the percentage of Thy-1.2+ splenocytes was within normal limits, and at day 10, the percentage of Thy-1.2+ cells was elevated slightly. The absolute numbers of Thy-1.2+ cells were comparable in both infected and normal animals at early stages (days 3 to 5) of L. monocytogenes infection, but there was a marked elevation of Thy-1.2+ splenocytes at days 7 to 14 of infection. Lyt-1+, Lyt-2+, and SIg+ splenocytes increased in absolute numbers as early as day 3 of infection and were still elevated at day 14. Adrenalectomy before infection had no effect on the results obtained, suggesting that these changes were not mediated by endogenous steroids.

Listeriosis in beige mice and their heterozygous littermates

The ability to resist the facultative intracellular bacterium Listeria monocytogenes was not impaired in the beige mutants of C57BL/6J mice which are known to be deficient in a number of immune functions. The intravenous LD50 of Listeria in beige (bg/bg) mice and their normal heterozygous (bg +) littermates was approximately 5 X 10(5). Growth of Listeria in the spleen and liver during primary and secondary infections was similar in the two groups of mice, and each was able to act efficiently in adoptive transfer of immunity. Histological examination showed a normal accumulation of polymorphonuclear and mononuclear cells at foci of infection in the liver, while in the spleen the previously described depletion of T cells 2-4 days after infection was observed in both groups. In-vitro 18-hr cytotoxicity of peritoneal cells for P815 targets, a function usually attributed to macrophages, was increased 2 days after infection in both bg/bg and bg/+ mice. In contrast, 4 hr cytotoxicity of spleen cells for YAC-1 targets, considered typical of natural killer (NK) cells, was depressed in uninfected bg/bg mice and only slightly raised during infection. This compared with a normal NK activity in uninfected bg/+ mice which was markedly increased during infection.

Recovery from T cell depletion during murine listeriosis and effect on a T-dependent antibody response

During the infection of mice with Listeria monocytogenes, there is a profound depletion of T (Thy-1+ Ig-) lymphocytes between days 1 and 4, followed by an increase in T cells to three times normal levels by day 9. The recovery of T cell numbers required cell proliferation, being sensitive to vinblastin and cyclophosphamide. Adult thymectomy 6 months before infection had no effect on recovery. The repopulating cells were no more sensitive than normal T cells to hydrocortisone. B lymphocytes (Ig+ cells) and null (Thy-1-Ig-) cells increased from day 1 after the injection of either live or (in contrast to T cells) killed Listeria organisms. Their increase was inhibited by vinblastin and cyclophosphamide. Despite T cell depletion, no depression of the antibody response to the T-dependent antigen, sheep erythrocytes, occurred during infection or when spleen cells were adoptively transferred from infected mice to irradiated recipients.