Administration of purified anti-L3T4 monoclonal antibody impairs the resistance of mice to Listeria monocytogenes infection

N,N,-diethyl-m-toluamide (DEET) suppresses humoral immunological function in B6C3F1 mice

N,N-diethyl-meta-toluamide (DEET) is a particularly effective broad-spectrum insect repellent used commonly in recreational, occupational and military environments. Due to its widespread use and suggested link to Gulf War Illness, this study examined the immunotoxicity of DEET. Adult female B6C3F1 mice were injected sc for 14 days with DEET at 0, 7.7, 15.5, 31, or 62 mg/kg/day. Due to differences in the dermal absorption of DEET between mice and humans, this study eliminated this confounding factor by utilizing sc injection and measured circulating blood levels of DEET to assess bioavailability from sc administration. Effects on lymphocyte proliferation, natural killer cell activity, thymus and spleen weight and cellularity, the antibody plaque-forming cell (PFC) response, and thymic and splenic CD4/CD8 lymphocyte subpopulations were assessed 24 h after the last dose. No effect was observed in lymphocyte proliferation, natural killer cell activity, thymic weight, splenic weight, thymic cellularity, or splenic cellularity. Significant decreases were observed in the percentage of splenic CD4-/CD8- and CD4+/CD8- lymphocytes but only at the 62 mg DEET/kg/day treatment level and not in absolute numbers of these cells types. Additionally, significant decreases in the antibody PFC response were observed following treatment with 15.5, 31, or 62 mg DEET/kg/day. Pharmacokinetic (PK) data from the current study indicate 95% bioavailability of the administered dose. Therefore, it is likely that DEET exposure ranges applied in this study are comparable to currently reported occupational usage. Together, the evidence for immunosuppression and available PK data suggest a potential human health risk associated with DEET in the occupational or military environments assuming similar sensitivity between human and rodent responses.

Oral vaccination with recombinant Listeria monocytogenes expressing human papillomavirus type 16 E7 can cause tumor growth in mice to regress

Listeria monocytogenes is a Gram-positive, facultative intracellular bacterium with the ability to present secreted proteins to the major histocompatibility complex class I pathway to stimulate cell-mediated immune response. In our study, we constructed the recombinant L. monocytogenes encoding human papillomavirus type 16 E7 gene (rLM-E7). When orally administered to syngeneic mice, rLM-E7 could induce a cytotoxic T-lymphocyte (CTL) response. Furthermore, in vitro flow cytometric assay and in vivo immune deficiency assays showed that rLM-E7 could prevent and eradicate tumor growth via CD8+-dependent CTLs. Hence, the potency of rLM-E7 as a therapeutic vaccine for cervical cancer is the result of the induction E7-specific cell-mediated immunity by L. monocytogenes. In addition to potency, this vaccine also offers ease of administration and reduced cost of production compared with other vaccines formulated for injection. Thus, L. monocytogenes encoding HPV-16 E7 may be a useful oral vaccine for cervical cancer treatment.

Quantifying the relationship between multiple immunological parameters and host resistance: probing the limits of reductionism

Although reductionist experimental designs are excellent for identifying cells, molecules, or functions involved in resistance to particular microbes or cancer cells, they do not provide an integrated, quantitative view of immune function. In the present study, mice were treated with either dexamethasone (DEX) or cyclosporin A (CyA), and immune function and host resistance were evaluated. Multivariate statistical methods were used to describe the relative importance of a broad range of immunological parameters for host resistance in mice treated with various dosages of DEX. Multiple regression and logistic regression analysis indicated that changes in 24 immunological parameters explained a substantial portion of the changes in resistance to B16F10 tumor cells or streptococcus group B. However, at least 40% of the change in host resistance remained unexplained. DEX at all dosages substantially suppressed numerous relevant immunological parameters, but significantly decreased resistance to Listeria monocytogenes only at the highest dosage. In contrast, CyA substantially decreased resistance to L. monocytogenes at dosages that caused relatively minor suppression of just a few immunological parameters (unfortunately, CyA data and host resistance data for L. monocytogenes were not suitable for multivariate analysis). These results illustrate that mathematical models can be used to explain changes in host resistance on the basis of changes in immune parameters, and that moderate changes in relevant immunological parameters may not produce the types of changes in host resistance expected on the basis of results from reductionist experimental designs.

Prevention of mortality by in vivo depletion of alpha beta T cells in murine lethal listeriosis and involvement of gamma delta T cells in bacterial elimination

We previously demonstrated that in murine lethal listeriosis, death is mainly due to massive liver necrosis. In the present study we found that in vivo depletion of alpha beta T cells by administration of anti-TCR beta mAb (H57-597) protected recipient mice from acute mortality and converted lethal listeriosis to sublethal infection. Furthermore, our findings suggested that gamma delta T cells were not involved in the liver necrosis in this condition. After depletion of alpha beta T cells, the number of bacteria decreased gradually to the limit for their detection (10(2) CFU) in 4 wks. Depletion of both alpha beta T cells and gamma delta T cells by administration of anti-TCR beta mAb (H57-597) and anti-TCR delta mAb (3A10) resulted in increased multiplication and persistent presence of bacteria in the liver and spleen. These findings suggest that gamma delta T cells play a significant protective role during infection in mice depleted of alpha beta T cells. In these mice, gamma delta T cells appeared with a peak on day 13 in the liver and on day 20 in the lymph nodes. No increase of gamma delta T cells in the spleen was observed throughout the course of infection.

Ethanol ingestion increases susceptibility of mice to Listeria monocytogenes

Excessive consumption of alcohol is associated with an increase in the frequency and severity of infectious diseases. Ethanol adversely affects specific and nonspecific aspects of the immune response. We used a murine model to determine whether ethanol ingestion impairs host mechanisms of resistance to Listeria monocytogenes. Naive mice and mice immune to L. monocytogenes were pair-fed either a Leiber-DeCarli liquid diet containing 7% (v/v) ethanol or an isocaloric control diet for 7 days. Then, nonimmune mice were given a sublethal dose of L. monocytogenes and studied 2 and 5 days after infection, and immune mice were challenged with a lethal dose of L. monocytogenes and studied 5 days after infection. Multifocal liver abscesses developed in nonimmune ethanol-treated and control mice 2 days after infection. Bacterial colony counts in the spleens were similar between the two groups; however, counts in the livers were slightly higher in ethanol-treated mice as compared with those in control mice. Five days after infection the nonimmune ethanol-treated mice had large necrotizing liver granulomas and organ bacterial colony counts 100 to 1000 times higher than those in control mice. Immune ethanol-treated mice had large areas of liver necrosis and inflammation containing numerous Gram-positive bacilli, whereas immune control mice had small, well-formed granulomas and much less necrosis. Organ bacterial colony counts were about 100 times higher in immune ethanol-treated mice as compared with those in immune control mice. Liver enzyme levels and mortality were significantly higher in ethanol-treated immune and nonimmune mice as compared with those in immune and nonimmune control mice. Data support the suggestion that ethanol consumption impairs the development and expression of T cell-mediated immunity of mice to L. monocytogenes, resulting in increased susceptibility to infection with this organism.

Early expression of cytokine mRNA in mice infected with Listeria monocytogenes

Protective immunity first becomes evident at 3 to 4 days after inoculation of mice with a sublethal dose of Listeria monocytogenes. Recent evidence suggests that production of gamma interferon (IFN-gamma) occurs earlier (within the first 24 h of infection). The purpose of this study was to define better the sequence of cytokine mRNA expression during the early stages of L. monocytogenes infection. Cytokine mRNA expression was detected by polymerase chain reaction-assisted amplification of RNA extracted from the spleen cells of individual mice euthanized at 0.5 to 120 h after L. monocytogenes challenge. By using this method, mRNAs for tumor necrosis factor alpha, interleukin-1 alpha (IL-1 alpha), IL-2, IL-4, IL-5, and IFN-gamma were detected in RNA from the spleen cells of uninfected mice. The intensity of the bands for IFN-gamma, however, was increased greatly at 16 h after intravenous injection of 5 x 10(4) CFU (nearly 1 50% lethal dose) of L. monocytogenes. IL-6 and granulocyte-macrophage colony-stimulating factor mRNAs were not detected in spleen cell RNA from uninfected mice but were induced within 30 and 60 min, respectively, after inoculation with L. monocytogenes. Increased amounts of mRNAs for IFN-gamma, IL-6, and granulocyte-macrophage colony-stimulating factor were detected after injection of viable, but not killed, L. monocytogenes. IL-3 mRNA was not detected at any time in RNA extracted from the spleen cells of uninfected or L. monocytogenes infected mice. These results suggest that infection with L. monocytogenes elicits a detectable cytokine mRNA response within the first few hours of infection.

Protection against Chlamydia psittaci in mice conferred by Lyt-2+ T cells

A murine model was used to study the respective roles of L3T4+ and Lyt-2+ T cells in protection against Chlamydia psittaci. Donor mice were intravenously (i.v.) infected with 1 x 10(5) plaque-forming units (PFU) per mice of live C. psittaci. One month after inoculation, splenic cells from donors were transferred into syngenic recipients (5 x 10(7) cells/mouse). As measured by splenic colonization on Day 6 after i.v. challenge (1 x 10(5) PFU/mouse), transfer with primed (untreated) cells conferred a 3 log protection in this model. In vitro treatment, before transfer, of splenic cells with anti-Lyt-2 monoclonal antibody (mAb) and complement, markedly impaired the protection in comparison with control mice transferred with primed untreated cells, whereas treatment with anti-L3T4 mAb did not reduce the transferred protection. Resistance to a reinfection with C. psittaci was also studied after selective in vivo depletion of L3T4+ and Lyt-2+ T cells. One month after primary infection, mice were treated with anti-L3T4 or anti-Lyt-2 mAb and challenged thereafter (i.v., 1 x 10(5) PFU). The splenic colonization on Day 6 after challenge demonstrated that treatment with anti-Lyt-2 mAb impaired resistance against a subsequent infection with C. psittaci. Treatment with anti-L3T4 mAb in vivo had no effect on protection, as previously described in vitro. The mechanisms by which Lyt-2+ T cells could participate in the elimination of bacteria were discussed.

Difference in the functional maturation of T cells against Listeria monocytogenes in lymph nodes and spleen

After subcutaneous immunization of mice with viable Listeria monocytogenes (LM), we evaluated the function of T cells induced in draining lymph nodes (LN) and spleen as determined by the local transfer of delayed-type hypersensitivity (DTH), acquired cellular resistance (ACR) and in vitro lymphokine production. LN cells could transfer specifically DTH but not ACR. In contrast, spleen cells from the same donor mice evoked the DTH response as well as bacterial clearance at the reaction site. Comparison of bacterial counts in spleen and in LN upon subcutaneous inoculation of mice with LM suggested that the lack of bacterial proliferation in LN underlay the failure to induce protective T cells in this lymphoid tissue. Spleen and LN T cells expressed CD4 and CD8 surface antigens equally and DTH response was solely dependent on CD4+ cells. Another major difference between LN and spleen cells was the profile of lymphokines produced in vitro. Upon the in vitro restimulation with killed Listeria, immune spleen cells produced macrophage chemotactic factor (MCF), interleukin-2 (IL-2) and interferon-gamma (IFN-gamma). In contrast, LN cells could produce all of the measured lymphokines but not IFN-gamma. The results provided strong evidence for the dissociation of DTH and ACR. Listerial growth appeared to be the requirement for full maturation of anti-listerial immunity that may coincide with the generation of IFN-gamma-producing T cells.

Dissociated development of T cells mediating delayed-type hypersensitivity and protective T cells against Listeria monocytogenes and their functional difference in lymphokine production

CD4+ T cells mediating both delayed-type hypersensitivity (DTH) and acquired cellular resistance (ACR) were generated in mice after immunization with viable Listeria monocytogenes. In contrast, CD4+ T cells from mice immunized with killed L. monocytogenes in complete Freund's adjuvant were capable of mediating only DTH but not ACR. To determine the functional difference between T cells mediating DTH and T cells mediating ACR, we examined two different populations of T cells for profiles of lymphokine production after stimulation with a specific antigen in vitro. The production of interleukin-2 (IL-2) and IL-3 but not IL-4 was observed in both T cells mediating only DTH and those mediating DTH and ACR. In this respect, both types of T cells could be categorized into the TH1 population, and they produced macrophage chemotactic factor equally well. However, the production of gamma interferon (IFN-gamma) was observed only in T cells capable of mediating both DTH and ACR. This result was confirmed not only by an enzyme immunoassay specific for murine IFN-gamma but also by Northern (RNA) analysis for the detection of IFN-gamma mRNA. These results suggested that the TH1 population may be subdivided further into two distinct subsets and that the ineffectiveness of the killed bacterial vaccine may be partly explained by the dissociated development of T cell function.

Role of CD4+ lymphocytes in resistance to mucosal candidiasis

The role of CD4+ lymphocytes in resistance of N:NIH(S) III bg/bg nu/+ mice to mucosal candidiasis was evaluated. Alimentary tract colonization with a pure culture of Candida albicans induced a population of lymphocytes in both the Peyer's patches and spleens of bg/bg nu/+ mice, but not bg/bg nu/nu mice, that proliferated and produced interleukin-2 (IL-2) in response to C. albicans antigens. The induction of candida-specific lymphocytes correlated with the clearance of C. albicans from the esophagus and tongue of resistant bg/bg nu/+ mice. Isogenic bg/bg nu/nu mice which do not develop candida-reactive lymphocytes were unable to clear C. albicans from their tongues and esophagi. Treatment of bg/bg nu/+ mice with anti-CD4+ monoclonal antibodies depleted their CD4+ lymphocytes and increased their susceptibility to mucosal candidiasis of the tongue and esophagus. In vivo treatment of bg/bg nu/+ mice with anti-IL-2, anti-gamma interferon (IFN-gamma), or both anti-IL-2 and anti-IFN-gamma monoclonal antibodies did not abrogate their resistance to mucosal candidiasis. Furthermore, treatment of C. albicans-susceptible bg/bg nu/nu mice with IFN-gamma and IL-2 did not protect them from mucosal candidiasis. Thus, CD4+ cells apparently play a critical role in resistance to mucosal candidiasis; however, we were unable to demonstrate a role for IL-2 and IFN-gamma in mediating resistance to mucosal candidiasis.

T-cell proliferative response to antigens secreted by Mycobacterium tuberculosis

An infection model of human tuberculosis was established with C57BL/6J mice. The lymphocyte proliferative responses to antigens from Mycobacterium tuberculosis were investigated during the course of infection and compared with results obtained with a group of mice immunized with large amounts of killed bacteria. The two groups responded similarly to a number of mycobacterial antigens, but marked differences in responses against secreted antigens were found; only infected mice responded vigorously to these. The responding lymphocyte subpopulation was made up of L3T4+ T lymphocytes under restriction of the Ia molecule.

Immunity to a pulmonary Cryptococcus neoformans infection requires both CD4+ and CD8+ T cells

The role of CD4+ and CD8+ T cells in mediating pulmonary clearance of a cryptococcal infection was investigated. Intratracheal inoculation of BALB/c and C.B-17 mice with a moderately virulent strain of Cryptococcus neoformans (52D) resulted in a pulmonary infection, which was cleared by a T cell-dependent mechanism. During this clearance, there was a significant influx of both CD4+ and CD8+ T cells into the lungs. Depletion of CD4+ T cells by injections of CD4-specific monoclonal antibody (mAb) prevented pulmonary clearance and also resulted in significant colonization of the brain and spleen of infected mice. CD4 depletion did not prevent the influx of CD8+ T cells into the lungs. Surprisingly, depletion of CD8+ T cells by mAb also ablated pulmonary clearance. CD8-depleted mice also had a small but significant increase in brain and spleen colony-forming unit compared to control mice by the end of the study. CD4+ T cell pulmonary influx was independent of the presence of CD8+ T cells. The lungs of T cell-depleted mice were examined histologically. CD4+ and CD8+ T cells each mediated a degree of inflammatory influx seen in the lungs of infected mice and raised the possibility that CD4+ and CD8+ T cells may synergize to generate the inflammatory response in the lungs. Numerous phagocytized but intact cryptococci were seen in the inflammatory foci of CD8-depleted mice but not in control or CD4-depleted mice. We propose that CD4+ T cells may recruit and activate effector phagocytes while CD8+ T cells predominantly function to lyse cryptococcus-laden unactivated phagocytes similar to the function of CD8+ T cells during listeria and mycobacteria infections.

T cell-mediated immunity in the lung: a Cryptococcus neoformans pulmonary infection model using SCID and athymic nude mice

T cells are important in systemic anticryptococcal defenses, but a role in controlling an initial pulmonary infection has not been demonstrated. A murine model with intratracheal inoculation was developed to study the acquisition and expression of pulmonary T cell-mediated immunity against Cryptococcus neoformans. Infections with four strains of C. neoformans (305, 68A, 613D, and 52D) in two strains of mice (BALB/c and C57BL/6) were examined. Unencapsulated strain 305 and slowly growing strain 68A were readily controlled apparently by nonimmune pulmonary defenses, and no extrapulmonary dissemination was detected. Strain 613D grew progressively in the lungs and disseminated to the brain and spleen. Strain 52D initially grew rapidly in the lungs and disseminated to the spleen, but a clearance mechanism developed in the lungs after day 7 postinfection and in the spleen after day 28. SCID and athymic nude mice were unable to clear a strain 52D pulmonary infection, and a lethal disseminated infection occurred. Pulmonary clearance could be adoptively transferred into SCID mice infected with strain 52D by use of immune T cells from the spleen and lungs and hilar lymph nodes of infected immunocompetent donors. Furthermore, pulmonary clearance was almost 100-fold better in SCID mice that received immune T cells from the lungs and hilar lymph nodes than in those that received immune T cells from the spleen, even though equivalent levels of delayed-type hypersensitivity were transferred by both cell populations. These adoptive transfer studies suggested that the lung and hilar lymph node T cells from immune animals either are enriched in such a way as to mediate protective immunity or home to the lungs better than do splenic T cells.

Effect of two feeding formulas on immune responses and mortality in mice challenged with Listeria monocytogenes

Cell-mediated immunity and natural killer cells play an important role against facultative intracellular organisms. The effect of two commercially available tube feeding formulas used for patients with acute or chronic debilitating and life-threatening illnesses was studied in mice challenged with Listeria monocytogenes. C57BL/6 X DBA/2 F1 hybrid mice were given ad libitum access to one of two formulas or to chow. Sixty mice in each of the feeding groups were challenged with 4.8 X 10(3) organisms intraperitoneally. Mortality was significantly less in animals fed Impact, a formula enriched with arginine, RNA and selected fatty acids. This was associated with reduced number of viable organisms in the spleen on day 7 after challenge. There was no difference in the spleen/body weight index between the different groups. Delayed cutaneous hypersensitivity was slightly higher in the Impact group but this was not statistically significant. Natural killer cell activity was significantly higher in the Impact group compared with the other two feeding regimens. These observations suggest that selective manipulation of the composition of tube feeding formulas may have a significant impact on immune responses and on morbidity and mortality following infectious challenge.

Effects of purified anti-Lyt-2 mAb treatment on murine listeriosis: comparative roles of Lyt-2+ and L3T4+ cells in resistance to primary and secondary infection, delayed-type hypersensitivity and adoptive transfer of resistance

Mice treated with purified anti-Lyt-2 monoclonal antibody (mAb) displayed a delayed ability to eliminate a primary Listeria monocytogenes infection from their spleens. Elimination of listeriae from the liver was unimpaired by anti-Lyt-2 mAb treatment. Treatment with anti-L3T4 mAb, alone or in combination with anti-Lyt-2 mAb, resulted in similar increases in the numbers of listeriae recovered from the spleens at 7 days after challenge. Listeria-infected mice that had been treated with anti-Lyt-2 mAb alone developed a strong delayed-type hypersensitivity (DTH) response, although it was significantly reduced as compared to control listeria-infected mice. In contrast, treatment with anti-L3T4 mAb severely impaired the development of DTH in listeria-infected mice. Treatment with anti-Lyt-2 mAb and anti-L3T4 mAb, singly or in combination, did not prevent mice from developing increased anti-listeria resistance if they were then immunized with a sublethal dose of L. monocytogenes. Treatment of mice with anti-Lyt-2 mAb or anti-L3T4 mAb before immunization, however, reduced the ability of their spleen cells to transfer anti-listeria resistance to recipient mice. These results indicate that Lyt-2+ cells make substantial contributions to the resistance of mice to primary L. monocytogenes infection, and to the ability of spleen cells from listeria-immunized mice to transfer resistance to naive recipients.

Roles of CD4+ and CD8+ cells, and the effect of administration of recombinant murine interferon gamma in listerial infection

Studies were made on the effects of in vivo administration of anti-CD4 mAb, anti-CD8 mAb, or a combination of both mAbs on multiplication of bacteria, the levels of serum transaminases, and mortality in mice infected with Listeria monocytogenes. Results showed that in sublethal infection, CD8+ cells enhanced the peak of bacterial multiplication and liver cell necrosis, and CD4+ cells suppressed CD8+ cell-mediated enhancement. Results also showed that either CD4+ or CD8+ cells were necessary for, and capable of, mediating clearance of the bacteria. CD8+ cells were more efficient than CD4+ cells, but for optimal clearance both were necessary. In lethal listeriosis, treatment of mice with anti-CD8 mAb or a combination of both anti-CD4 and anti-CD8 mAbs, but not anti-CD4 mAb only, protected mice from death by decreasing multiplication of bacteria in the liver and spleen after a peak of approximately 10(8) CFU, and lowering the elevated serum levels of transaminases. These findings indicated that CD8+ cells were responsible for causing irreversible systemic Listeria infection and severe liver necrosis. In lethal listeriosis, administration of rMuIFN-gamma markedly prolonged survival by decreasing multiplication of bacteria and promoting recovery from liver necrosis.

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.

Role of L3T4+ lymphocytes in protective immunity to systemic Candida albicans infection in mice

Protective immunity to lethal Candida albicans challenge in vivo and activation of splenic macrophages with highly candidacidal activity in vitro were detected in mice infected with low-virulence agerminative yeast cells of the variant strain PCA-2, at a time when a strong delayed-type hypersensitivity (DTH) reaction to C. albicans occurred in the footpads of PCA-2-treated mice. The DTH reaction was transferable with spleen cell populations from these animals, and enrichment of splenic lymphocytes in L3T4+ cells significantly increased the footpad swelling. The reactivity transferred by L3T4+ cells was a radiosensitive (2,500 rads in vitro) phenomenon that required collaboration with radioresistant, silica-sensitive syngeneic cells in the host and was inhibited by treatment of recipient mice with antibodies to the L3T4 antigen or murine gamma interferon. In vitro, the PCA-2-immune L3T4+ cells produced various lymphokine activities upon incubation with C. albicans, including gamma interferon and granulocyte-macrophage colony-stimulating factor. Anti-L3T4 monoclonal antibody treatment of PCA-2-infected mice significantly impaired their footpad reaction and resistance to C. albicans, as shown by increased recovery of yeast cells from the kidneys of anti-L3T4-treated mice. These results suggested that the mechanisms of anti-Candida resistance induced by PCA-2 may involve specific induction of a DTH response mediated by inflammatory L3T4+ T cells and lymphokine-activated phagocytic effectors. However, the survival rate of the PCA-2-immune mice challenged with C. albicans was not significantly modified by administration of the anti-L3T4 antibody, thus allowing for the conclusion that compensatory mechanisms lead to considerable anti-Candida resistance when the activity of L3T4+ cells is deficient.

Treatment of mice with human recombinant interleukin-2 augments resistance to the facultative intracellular pathogen Listeria monocytogenes

The effects of exogenously administered human recombinant IL-2 (hrIL-2) on resistance to Listeria monocytogenes infection were examined. Intravenous injection of hrIL-2 significantly enhanced antibacterial resistance in both BDF1 and C3H/HeJ mice. The beneficial effect of hrIL-2 was observed with as little as 0.6 micrograms per mouse, whereas optimum protection occurred at 6 micrograms per mouse, hrIL-2 was equally protective when administered concomitant with the listeriae or up to 24 h prior to infection; it had little effect if given after the bacterial challenge. Kinetic experiments indicated that both the peak bacterial burden and the time lag before L. monocytogenes began to be cleared from the spleen and liver were reduced in hrIL-2-treated mice as compared with control mice. Histopathological examination of spleens and livers confirmed that hrIL-2-treated Listeria-infected mice experienced considerably less damage to these organs than did control mice. Spleen cells from Listeria-infected mice exhibited depressed levels of mitogen-induced proliferation coincident with the peak bacterial burden in the spleen and liver and during the subsequent recovery from the infection. Administration of hrIL-2 to uninfected mice had no effect on spleen cell proliferation in response to mitogens in vitro, nor did hrIL-2 treatment restore normal levels of splenocyte proliferative responses to Listeria-infected mice. In addition, hrIL-2 treatment resulted in attenuated levels of serum colony-stimulating activity in infected mice as compared with control infected mice. Coadministration of both hrIL-2 and human recombinant interleukin-1 alpha at various dose and time combinations had no detectable additive or synergistic effect. Although these data do not suggest an obvious mechanism of action, they clearly demonstrate that hrIL-2 can augment host defense against the facultative intracellular pathogen L. monocytogenes.