Restriction in adoptive transfer of resistance to Listeria monocytogenes. II. Use of congenic and mutant mice show transfer to be H-2K restricted

MHC class Ib-restricted CTL provide protection against primary and secondary Listeria monocytogenes infection

Infection of B6 mice with the intracellular pathogen Listeria monocytogenes (LM) results in the activation of CD8(+) T cells that respond to Ag presented by both MHC class Ia and class Ib molecules. Enzyme-linked immunospot analysis reveals that these CTL populations expand and contract at different times following a primary sublethal LM infection. Between days 4 and 6 postinfection, class Ib-restricted CTL exhibit a rapid proliferative response that is primarily H2-M3 restricted. The peak response of class Ia-restricted CD8(+) T cells occurs a few days later, after the majority of bacteria have been cleared. Although class Ia-restricted CTL exhibit a vigorous recall response to secondary LM infection, we observe limited expansion of class Ib-restricted memory CTL, even in MHC class Ia-deficient mice (B6.K(b-/-)D(b-/-)). Despite this lack of enhanced expansion in vivo, class Ib-restricted memory CTL retain the ability to proliferate and expand when provided with Ag in vitro. Furthermore, we demonstrate that in vivo depletion of CD8(+) T cells in LM-immune B6.K(b-/-)D(b-/-) mice severely impairs memory protection. Together, these data demonstrate that class Ib-restricted CTL play an important role in clearing a primary LM infection and generate a memory population capable of providing significant protection against subsequent infection.

MHC Class I-Restricted Presentation of Maleylated Protein Binding to Scavenger Receptors

Pathways for loading exogenous protein-derived peptides on MHC class I are thought to be present mainly in monocyte-lineage cells and to involve phagocytosis- or macropinocytosis-mediated antigenic leakage into either cytosol or extracellular milieu to give peptide access to MHC class I. We show that maleylation of OVA enhanced its presentation to an OVA-specific MHC class I-restricted T cell line by both macrophages and B cells. This enhanced presentation involved uptake through receptors of scavenger receptor (SR)-like ligand specificity, was TAP-1-independent, and was inhibited by low levels (2 mM) of ammonium chloride. No peptide loading of bystander APCs by maleylated (maleyl) OVA-pulsed macrophages was detected. Demaleylated maleyl-OVA showed enhanced MHC class I-restricted presentation through receptor-mediated uptake and remained highly sensitive to 2 mM ammonium chloride. However, if receptor binding of maleyl-OVA was inhibited by maleylated BSA, the residual presentation was relatively resistant to 2 mM ammonium chloride. Maleyl-OVA directly introduced into the cytosol via osmotic lysis of pinosomes was poorly presented, confirming that receptor-mediated presentation of exogenous maleyl-OVA was unlikely to involve a cytosolic pathway. Demaleylated maleyl-OVA was well presented as a cytosolic Ag, consistent with the dependence of cytosolic processing on protein ubiquitination. Thus, receptor-specific delivery of exogenous protein Ags to APCs can result in enhanced MHC class I-restricted presentation, suggesting that the exogenous pathway of peptide loading for MHC class I may be a constitutive property dependent mainly on the quantity of Ag taken up by APCs.

Disruption of the cellular inflammatory response to Listeria monocytogenes infection in mice with disruptions in targeted genes

The results of this study to dissect the nature of the acquired immune response to infection with Listeria monocytogenes in mice with targetted gene disruptions show that successful resolution of disease requires the essential presence of alphabeta T cells and the capacity to elaborate gamma interferon. In the absence of either of these entities, mice experience increasingly severe hepatitis and tissue necrosis and die within a few days. The data from this study support the hypothesis that the protective process is the efficient replacement of neutrophils in lesions by longer-lived mononuclear phagocytes; alphabeta-T-cell-knockout mice died from progressive infection before neutrophil replacement could occur, whereas in gammadelta-T-cell-knockout mice this replacement process in the liver has previously been shown to be much slower. In the present study we attribute this delay to reduced production of the macrophage-attracting chemokine MCP-1 in the gammadelta-T-cell-knockout animals. These data further support the hypothesis that gammadelta T cells are important in controlling the inflammatory process rather than being essential to the expression of protection.

Cytokines in the induction and expression of T-cell-mediated granuloma formation and protection in the murine model of listeriosis

Lymphocyte-mediated inflammation is a hallmark of autoimmune diseases, such as multiple sclerosis. Crohn's disease, rheumatoid arthritis and sarcoidosis. However, this type of inflammation probably developed under evolutionary pressure from pathogenic microorganisms, such as mycobacteria and other intracellular infective agents. One such pathogen, the gram-positive bacterium Listeria monocytogenes (L. monocytogenes), induces a cascade of tissue alterations that ultimately results in the eradication of the bacteria associated with a granulomatous response. Consequently, murine listeriosis has been established as a model to analyze not only T-cell-dependent antibacterial protection but also T-cell-mediated mononuclear inflammation in parenchymal organs. Extensive studies of the molecular basis of the latter phenomenon led to the conclusion that the most decisive step from non-specific microabscess formation to granulomatous inflammation is the activation of non-specifically invading CD4+ T cells, which results in high local concentrations of TNF-alpha and IFN-gamma in the presence of IL-2. This in turn induces CD11b-independent mechanisms of intraparenchymal monocyte accumulation. Because any attempt to neutralize the effects of TNF-alpha and IFN-gamma to modulate T-cell-mediated inflammation will also dramatically decrease host resistance, other anti-inflammatory strategies based on the modulation of TNF-alpha and IFN-gamma-induced mechanisms of monocyte accumulation must be developed. Recalling the classical work by Dienes & Schoenheit on the induction of bacterial allergies (1), the cytokine phenotype of granuloma formation also has implications as regards the most potent adjuvant environment for the development of a T-cell response. The murine listeriosis model is the basis for all conclusions in this article on the role of cytokines in the induction and expression of T-cell-mediated inflammation and, as we will show, promises to yield still more insights into the rational design of vaccines.

CD8+ T cell-mediated protection against an intracellular bacterium by perforin-dependent cytotoxicity

Growth of Listeria monocytogenes is mainly controlled by macrophages, which are activated by specific T cells. A potential role of CD8+ T cells by direct lysis of infected cells was investigated in perforin-deficient mice generated by homologous recombination. The absence of perforin-mediated cytotoxicity resulted in delayed clearance of Listeria from the spleen but not the liver after primary infection, overall susceptibility to Listeria however was not increased. Protection against a secondary infection was drastically impaired in perforin-deficient mice. Adoptive transfer of immune spleen cells to recipients revealed that anti-Listeria protection by CD8+ T cells from perforin-deficient versus normal mice was about 10-fold reduced in livers and about 100-fold reduced in the spleen of recipients. CD4+ T cells from immune control and perforin-deficient mice conferred comparable protection. These results indicate that the protective effect of CD8+ T cells against an intracellular bacterium mainly evident in secondary infection is mediated by a perforin-dependent pathway, presumably cytotoxicity, and less by other direct or indirect effector mechanisms.

Role of T-lymphocyte subsets in Rhodococcus equi infection

Rhodococcus equi, a facultative intracellular gram-positive bacterium, can induce life-threatening infections in immunocompromised patients, especially those with AIDS. We have studied the mechanism of acquired immunity to this pathogen in a murine model. Protective immunity was induced by live but not killed bacteria. Adoptive transfer of resistance was obtained with spleen cells but not immune serum from mice immunized intravenously 30 days earlier with live bacteria. In normal mice, an intravenous challenge of 5 x 10(6) CFU of R. equi was cleared from the spleen, liver, and lungs within 3 weeks, whereas athymic nude mice were unable to clear the bacteria. In vivo depletion with monoclonal antibodies showed that both CD4+ and CD8+ T-cell subsets participate in the clearance of bacteria and that CD8+ T cells play the major role.

Dissociation of macrophage cytolysis and ability to transfer anti-listeria resistance by concanavalin A-stimulated spleen cells

In this study, we determined whether spleen cells from Listeria monocytogenes-immunized mice were cytolytic for Listeria-infected macrophages. Spleen cells freshly obtained from immunized donors were unable to lyse Listeria-infected macrophages unless they were first stimulated in vitro for 2-3 days with Concanavalin A (ConA) or L. monocytogenes. Spleen cells from non-immunized mice developed cytolytic activity after incubation with ConA, but not with L. monocytogenes. Cytolytic spleen cells demonstrated an equivalent ability to lyse uninfected and Listeria-infected thioglycollate elicited peritoneal macrophages. Maximal cytolysis required co-incubation of effector and target cells for 18-20 h. Spleen cell culture supernatants did not lyse macrophages, suggesting that cytolysis required direct contact. Preincubation of immune spleen cells with ConA decreased their ability to transfer anti-listeria resistance in the spleens, but not the livers of recipient mice. Depletion of CD4+ or CD8+ cells did not significantly reduce the ability of ConA-incubated Listeria-immune spleen cells to transfer resistance. Despite being cytolytic for Listeria-immune infected macrophages, ConA-stimulated non-immune spleen cells did not transfer anti-listeria resistance. These results indicate that cytolytic cells can be generated by short-term incubation of spleen cells with antigen or mitogen. The dissociation between in vitro cytolytic activity and ability to transfer protection, however, suggests that the two biological activities are not inextricably linked.

Listeriolysin O is a target of the immune response to Listeria monocytogenes

The immunologic mechanism of protective immunity to the intracellular parasite Listeria monocytogenes (Lm) is not well understood, however, antilisterial immunity can be adoptively transferred with T lymphocytes from Lm-immune donors. The Lm-immune cells are believed to produce macrophage-activating lymphokines, which leads to the eventual macrophage-dependent eradication of the bacterium. Increasing evidence suggests that immunity to Lm resides exclusively within the CD8+ T cell subset. It is possible that the Lm-immune CD8+ T cells function to release sequestered Lm from nonprofessional phagocytes to awaiting activated macrophage populations. This study was conducted to determine if listeriolysin O (LLO), which is an essential determinant of Lm pathogenicity, is also a target of the antilisterial immune response. We have found that target cells infected with a LLO+ Lm strain are lysed by Lm-immune cytotoxic cells, whereas target cells infected with a LLO- Lm mutant, or pulsed with a heat-killed Lm preparation, are not lysed by the Lm-immune effector cells. We have used a Bacillus subtilis (Bs) construct that expresses the LLO gene product and found that target cells infected with the LLO+ Bs construct are lysed by antilisterial cytotoxic cells. The antilisterial cytotoxic response is targeted against LLO, in that we have also used a Bs construct that expresses the perfringolysin (PLO) gene product and found that target cells infected with the PLO+ Bs are not lysed by antilisterial cytotoxic effector cells. These data strongly suggest that LLO is a target antigen of antilisterial immunity and may represent the dominant target during the expression of the immune response to Lm.

Use of recombinant interleukin-2 to enhance adoptive transfer of resistance to Listeria monocytogenes infection

In vitro incubation of Listeria-immune spleen cells (LISC) with recombinant interleukin-2 (rIL-2) for at least 3 days increased their ability to transfer antilisteria resistance to recipient mice. This effect was blocked by the in vitro addition of transforming growth factor beta 1. The level of protection afforded by the transfer of rIL-2-incubated LISC was further elevated by the in vivo administration of rIL-2 at a dose that by itself did not significantly increase antilisteria resistance. The antilisteria resistance of recipient mice remained elevated for approximately 7 days and then rapidly declined to undetectable levels by 10 days. After cell transfer, recipient mice were protected against challenge with Listeria monocytogenes but not Salmonella typhimurium, Yersinia enterocolitica, or Streptococcus pyogenes. Flow cytometric analyses revealed an increase in the percentages of CD8+, NK+, and gamma delta T cell receptor+ cells but no change in the percentage of CD4+ cells as a result of LISC coculturing with rIL-2. In vitro depletion of CD4+ cells just prior to transfer had no significant effect on the adoptive transfer of resistance; depletion of CD8+ cells reduced the level of resistance by approximately 25%. Combined depletion of Thy-1.2+, CD4+, and CD8+ cells just prior to adoptive transfer diminished the level of protection in the spleens but not the livers of recipient mice. These data suggest that rIL-2 can be used to augment adoptive immunotherapy for bacterial infection in a manner similar to adoptive immunotherapy of human cancer patients. Although the protective cell population was not definitively identified, it appeared to be independent of CD4+ cells and only partly dependent on CD8+ cells.

Evidence for a role of IFN gamma in control of Listeria monocytogenes in T cell deficient mice

The role of interferon (IFN) gamma in controlling chronic infections of Listeria monocytogenes (Listeria) was studied in athymic C57BL/6 nu/nu mice, and by treating thymectomized C57BL/6 +/+ mice with monoclonal rat CD4 and CD8-specific monoclonal antibodies (Mab). Mice treated with a combination of the two T cell subset antibodies were similar to athymic, nude mice in being able to control Listeria infection, keeping the titers below 3-5 log10 bacteria per organ, but they could not eliminate them completely. Treatment with antibodies to IFN gamma of nude or CD4+ + CD8+ - T cell-depleted mice suffering from chronic Listeria infection caused a marked increase of Listeria titers in liver and spleen. This result implies a role of IFN gamma in maintaining anti-Listeria resistance in mice lacking mature T cells.

Expression of systemic protection and delayed-type hypersensitivity to Listeria monocytogenes is mediated by different T-cell subsets

The relationship between acquired cellular resistance and delayed-type hypersensitivity (DTH) during the immune response to Listeria monocytogenes was investigated. Treatment of concanavalin A-stimulated Listeria-immune spleen cells with anti-CD8 antibody plus complement abrogated the adoptive transfer of systemic antilisterial immunity but had no effect on the transfer of DTH. In contrast, in vitro depletion of the CD4+ T-cell subset eliminated the ability of culture-activated cells to transfer DTH reactivity but did not interfere with the adoptive transfer of protection. In vivo, the infusion of anti-CD8 antibody inhibited the expression of both actively and adoptively transferred protection but did not influence the development of DTH skin test reactivity to L. monocytogenes antigens. In vivo depletion of the CD4+ T-cell subset eradicated the DTH response, with only minor influence of the protective anti-Listeria response. The apparent functional dissociation of the CD4+ (DTH) and CD8+ (protection) T-cell populations was further emphasized by our findings that the adoptive transfer of protection was dependent on a cyclophosphamide-sensitive cell population, whereas DTH reactivity was mediated by a cyclophosphamide-resistant population.

Influence of different regions of the H-2 complex on the rate of clearance of Salmonella typhimurium

The rate of clearance of Salmonella typhimurium from the mouse spleen is under H-2 linked genetic control. The results of the present study, with H-2 recombinant mice on a C57BL/10 background, suggest the involvement of at least two loci, one in the D region and the other in the K-A alpha chromosomal segment.

The in vitro proliferative response of lymphoid cells of mice infected with Salmonella enteritidis 11RX

Intraperitoneal injection of (BALB/c x C57BL/6) F1 mice with live, but not killed Salmonella enteritidis 11RX (11RX) induced T cells in the spleen and peritoneal cavity which were able to proliferate in vitro in response to two different forms of 11RX antigens. The majority of cells which proliferated were L3T4+ T cells and most of the response was restricted by the I-A locus of the H-2 major histocompatibility complex, although a smaller K region restricted response was also detected. T cells able to respond to 11RX antigens could only be demonstrated when non-adherent lymphoid cell suspensions from immunized mice were used, and usually a limited response was obtained unless small numbers of adherent cells present in normal peritoneal cell suspensions were added. Cells cultured in vitro for 3 days were able to mediate local transfer of delayed type hypersensitivity and secondary immunization did not enhance the reactivity of responding cells to 11RX antigens.

T-cell subsets in delayed-type hypersensitivity, protection, and granuloma formation in primary and secondary Listeria infection in mice: superior role of Lyt-2+ cells in acquired immunity

Immunity to Listeria monocytogenes was studied in mice treated with rat monoclonal antibodies (MAbs) specific for the Thy-1.2, L3T4, and Lyt-2 T-cell markers. Three characteristic T-cell-mediated phenomena were investigated. Delayed-type hypersensitivity (DTH) to listerial antigen was totally abolished in mice treated with anti-Thy-1.2 or anti-L3T4 MAbs, whereas anti-Lyt-2 MAb treatment had no effect, regardless of whether the MAb was given during the induction or the expression of DTH. On the other hand, the elimination of bacteria from the spleens of infected animals was inhibited only by the application of either anti-Thy-1.2 MAb or anti-Lyt-2 MAb. This could be shown most impressively during the secondary infection of immune mice with a normally lethal dose of listeriae. In this situation, treatment with anti-Lyt-2 MAb sufficed to completely abolish immunologic memory, whereas anti-L3T4 MAb had only a marginal effect on antibacterial protection. However, the accelerated development of mononuclear cell foci in the livers of immune mice was inhibited by the application of both anti-L3T4 MAb and anti-Lyt-2 MAb. It is concluded that in murine listeriosis, DTH and acquired immunity to reinfection are dissociable phenomena. Although DTH is a function of L3T4+ T lymphocytes, Lyt-2+ T cells are necessary and sufficient for the expression of acquired resistance to L. monocytogenes. The roles of the different T-cell subsets in granuloma formation warrant further investigation.

Role of hemolysin for the intracellular growth of Listeria monocytogenes

Listeria monocytogenes insertion mutants defective in hemolysin production were generated using the conjugative transposons Tn916 and Tn1545. All of the nonhemolytic mutants (hly-) lacked a secreted 58-kD polypeptide, presumedly hemolysin, and were avirulent in a mouse model. An intracellular multiplication assay was established in monolayers of mouse bone marrow-derived macrophages, the J774 macrophage-like cell line, the CL.7 embryonic mouse fibroblast cell line, and the Henle 407 human epithelial cell line. The hly+ strain grew intracellularly in all of the tissue culture cells with a doubling time of approximately 60 min. In contrast, the hly- mutants failed to grow in the murine-derived tissue culture cells, but retained the ability to grow in the human tissue culture cells examined. Hemolytic-positive revertants were selected after passage of the hly- mutants through monolayers of J774 cells. In each case, the hemolytic revertants possessed the 58-kD polypeptide, were capable of intracellular growth in tissue culture monolayers and were virulent for mice.

The role of T cell subpopulations in cell mediated immunity to facultative intracellular bacteria

This brief review summarizes the experimental data which underly the classic concept of antibacterial cell mediated immunity and will integrate recent developments focusing on results obtained by in vivo studies in the model of rodent listeriosis.

H-2 restriction in acquired cell-mediated immunity to infection with Francisella tularensis LVS

The H-2 restriction imposed on the T-lymphocyte-macrophage interaction leading to the expression of acquired cellular immunity was evaluated in an experimental model of infection with the live vaccine strain of Francisella tularensis. Restriction between T cells and macrophages was examined in vitro in cultures containing macrophages from C57BL/10 (B10) mice, T cells from immune B10 H-2 congenic mice, and F. tularensis antigen. The cellular interaction was assayed by the production in the cultures of factors which stimulate thymocyte DNA synthesis. It was observed that homology at the I-A region of the H-2 complex was required for productive T-cell-macrophage cooperation to occur. Restriction was also investigated in an in vivo passive cell transfer system. Spleen cells from immunized B10 mice were injected into naive B10 H-2 congenic mice, which were then challenged with F. tularensis. Enhanced resistance to the challenge infection in recipient mice was used as a marker of a successful T-cell-macrophage interaction. It was found that when the recipient strain shared H-2 I-A region homology with the donor strain, enhanced antitularemic resistance was expressed, whereas homology at the H-2 K or D region was insufficient. Thus, macrophage--T-cell cooperation in immunity to experimental tularemia was restricted at the level of class II determinants.

Passive transfer of acquired resistance to Listeria monocytogenes infection is independent of mononuclear cell granuloma formation

This study documents the formation of leukocyte foci in the livers of mice infused with either normal or immune T cells and then challenged intravenously with Listeria monocytogenes. The results show that the transfer of antilisterial resistance occurred before mononuclear cell granuloma formation and was associated instead with the appearance of foci of infiltrating lymphocytes and neutrophils. Numbers of these foci remained low in mice which received immune cells but increased progressively until death in mice which received normal cells. These findings do not support the previous hypothesis that a major component of acquired resistance against Listeria infection involves the rapid generation of mononuclear cell granuloma formation under the control of immune T cells.

Specific lysis of Listeria monocytogenes-infected macrophages by class II-restricted L3T4+ T cells

Mice were infected with the intracellular bacterium, Listeria monocytogenes, and T cell clones from spleens, lymph nodes and peritoneal exudates were established. The capacity of L3T4+, Lyt2- T-cell clones to specifically lyse L. monocytogenes-infected macrophages was analyzed. As a source of target cells, bone marrow macrophages (BMM phi) after 9 days of culture in hydrophobic teflon bags were used. These BMM phi were totally Ia-; however, significant Ia-expression could be induced by interferon-gamma (IFN-gamma). IFN-gamma-stimulated BMM phi, after priming with live or killed L. monocytogenes organisms were effectively lysed by the vast majority of L3T4+ T cell clones. In the absence of either IFN-gamma stimulation or antigen priming, no lysis occurred. Cytolysis was demonstrable in a conventional 4-h 51Cr-release assay and in an 18-h neutral red uptake assay and was antigen specific and class II restricted. Native T cells from L. monocytogenes-infected mice failed to lyse stimulated, L. monocytogenes-primed BMM phi and gained their cytolytic activity after antigenic restimulation in vitro. These data demonstrate that L. monocytogenes-specific L3T4+ T cells could lyse M phi presenting listerial antigens provided that Ia antigen expression had been induced. L3T4+ T cell clones produced IFN-phi after restimulation with antigen plus accessory cells in vitro and IFN-gamma secretion could be increased by costimulation with recombinant IL 2. These T cell clones conferred significant protection upon recipient mice which was more pronounced in the liver. The possible relevance of lysis by L3T4+ T cells of infected M phi to protection against and pathogenesis of intracellular bacterial infections is discussed.

Function and antigen recognition pattern of L3T4+ T-cell clones from Mycobacterium tuberculosis-immune mice

T-cell clones were established from Mycobacterium tuberculosis-immunized mice. These clones had the phenotype Thy-1+ L3T4+ Lyt-2- and were restricted by the H-2I-A locus. After antigen stimulation, the T-cell clones secreted interleukin-2 and gamma interferon. Factors produced by these T-cell clones activated normal bone marrow macrophages for antimycobacterial activity in vitro. Furthermore, the T-cell clones could adoptively confer delayed-type hypersensitivity on normal recipient mice. These findings indicate that the T-cell clones clones expressed relevant functions of antimycobacterial immunity. The antigen reactivity of the T-cell clones to different mycobacterial species ranged from broad cross-reactivity to stringent specificity, and none of the clones distinguished between M. tuberculosis and M. bovis. Thus, M. tuberculosis-immune helper/inducer T cells of identical phenotype, genetic restriction, and function varied in their antigen specificity. T-cell clones of the type described will facilitate functional characterization of mycobacterial antigens on the T-cell level.

Characterization of three different rat T-cell clones with specificity to Listeria monocytogenes: phenotype, specific proliferation, lymphokine production, and protective capacity in vivo

Splenic T lymphocytes from rats immunized with the facultative intracellular bacterium, Listeria monocytogenes, were cloned by the limiting-dilution technique. From several clones obtained, three have been scrutinized in detail. As demonstrated by their reactivity to the monoclonal antibodies, W3/25 and MRC OX8, the clones RVIIC2 and R23D6 are of helper cell phenotype, whereas cells from clone R30D5 express both the helper and the cytotoxic/suppressor cell markers. Proliferation of all three clones critically depends on antigen-presenting cells, exogenous interleukin 2, Listeria antigen, and on class II-restricted antigen presentation by accessory cells. There are differences between cells from different clones with respect to the degree of production of migration inhibitory and macrophage-activating factors. Thus, T lymphocytes of clones R23D6 and R30D5 are highly active, whereas cells of clone RVIIC2 showed markedly less production of these factors. In vivo studies, analyzing the capacity of cells to transfer systemic protection, showed a positive correlation between the production of migration inhibitory factor, macrophage-activating factor, and systemic protection.

Listeria monocytogenes-reactive T lymphocyte clones with cytolytic activity against infected target cells

Lyt-2+ T cell clones were established from Listeria monocytogenes-infected mice. The clones secreted IFN-gamma after stimulation with spleen cells from L. monocytogenes-infected mice plus IL-2. Spleen cells from normal mice were not stimulatory. Furthermore, cloned T cells lysed L. monocytogenes-infected macrophages. Cytolysis was antigen-specific and H-2K-restricted. These findings suggest a role for specific cytotoxic T cells in the immune response to intracellular bacteria.

Enumeration of Listeria monocytogenes-reactive L3T4+ T cells activated during infection

A limiting dilution system was developed which allows minimal estimates of the number of Listeria monocytogenes-reactive T cells from infected mice. Limiting numbers of T cells were restimulated in vitro with accessory cells in the presence or absence of antigen (heat-killed L. monocytogenes organisms) and proliferative responses determined. The responding T cells resided entirely within the L374+, Lyt2- (helper/inducer) T-cell subset. L. monocytogenes-reactive T cells were not demonstrable in uninfected mice nor during the first 3 or 4 days of infection. In contrast, on days 4 or 5, respectively, of infection approximately 1/1000 T cells showed a response to L. monocytogenes. Their frequency declined only slightly over the subsequent weeks and still was as high as 1/4900 4 weeks after infection when no bacteria were present in the host. The frequency of L. monocytogenes-reactive T cells depended on the number and virulence of the infecting organisms, the highest sublethal dose of virulent bacteria inducing the highest frequency. Chemotherapeutic shortening of infection between days 3 and 4 resulted in a six-fold reduction of reactive T cells. Thus, the frequency of L. monocytogenes-reactive T cells depended on the presence of bacteria in the host during the first 3 to 4 days of infection. These findings may have implications for the rational design of vaccines directed against intracellular bacterial pathogens as they raise the question whether attenuated bacterial strains of low persistence can induce sufficiently high T-cell numbers required for protective immunity.

Local transfer of delayed-type hypersensitivity after Salmonella infection in mice

An adoptive local transfer system has been used to study the mediators of delayed-type hypersensitivity induced in mice by infection with Salmonella enteritidis 11RX. The cells which transfer this state of hypersensitivity to untreated recipients are nonadherent T lymphocytes with the surface phenotype Lyt 1+2-, and successful transfer requires compatibility at the I-A subregion of the H-2 complex. In these and other respects these cells are indistinguishable from those previously found to be responsible for in vitro lymphokine release upon culture with 11RX antigens.

Effective protection against Listeria monocytogenes and delayed-type hypersensitivity to listerial antigens depend on cooperation between specific L3T4+ and Lyt 2+ T cells

Selected L3T4- and Lyt 2- T-cell subpopulations from Listeria monocytogenes-infected mice were transferred into syngenic recipients, and their capacity to adoptively mediate protection against L. monocytogenes and delayed-type hypersensitivity to listerial antigens was determined. Both functions were markedly reduced by pretreatment of cells with either anti-L3T4 or anti-Lyt 2.2 antibodies plus complement, but they could be restored by admixture of the two selected T-cell subsets. Thus, after systemic cell transfer effective protection against L. monocytogenes and delayed-type hypersensitivity to listerial antigens depend on cooperation between specific L3T4+ and Lyt 2+ T cells.

An adoptive transfer system for the investigation of granuloma formation in murine listeriosis

A system is described for studying adoptive transfer of granuloma formation in infection of mice with the facultative intracellular bacterium, Listeria monocytogenes. Intravenous injection of graded numbers of L. monocytogenes-immune peritoneal exudate T-enriched cells (PETLEs) together with 5 X 10(4) living L. monocytogenes resulted in dose-dependent accelerated granuloma formation in the livers of recipient mice. The lymphoid cells conferring granuloma formation were T cells by virtue of their nonadherence to nylon wool and sensitivity to anti-Thy 1.2 antiserum plus complement. Since granuloma formation could not be transferred from C57B1/6J donors to BALB/c recepients it is concluded that adoptive transfer of granuloma formation is genetically restricted.

The mediators of acquired resistance to Listeria monocytogenes are contained within a population of cytotoxic T cells

T cells from peritoneal exudates induced in rats convalescing from a recent infection of Listeria monocytogenes were fractionated into two subsets based on their ability to bind monoclonal antibodies to cell-surface determinants that are expressed on some but not all peripheral T cells. Two phenotypically distinct subsets, one recognized by the antibody MRC OX8 and the other by W3/25, were assayed for their protective capacity in Listeria-challenged recipients, and for their ability to kill unmodified syngeneic fibroblasts in vitro. The two activities were mediated by the OX8+ subset which comprised approximately half the T cells in the exudates.

Antibacterial defence mechanisms

Pathogenic bacteria fall into two groups with regard to their fate within phagocytes: extracellular bacteria are promptly killed after phagocytosis and facultative intracellular bacteria are resistant to intracellular killing unless macrophages are activated. Extracellular bacteria cause purulent infections, and facultative intracellular bacteria granulomatous ones. Humoral immune mechanisms (antibody, complement) deal mainly with extracellular bacteria, while cellular immune mechanisms (T cells, macrophages) deal with facultative intracellular bacteria. The specific and nonspecific factors and their interactions are discussed with respect to their role in the buildup of an effective antibacterial defence.